Course Web Portal Environmental Engineering

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COURSE WEB PORTAL ENVIRONMENTAL ENGINEERING Authors: Assoc. Prof. Eng. Eva Janotkova, Ph.D. Eng. Vladimir Krejci Brno 2006 CONTENTS EXTENDED SYLLABI 4 1. LIVING ENVIRONMENT. INDOOR ENVIRONMENT. COMFORT 4 1.1 Living Environment 4 1.2 Indoor Environment and Comfort 5 THERMAL COMFORT AND ITS ASSESSMENT 6 2.1 Thermal Comfort 6 2.2 Assessment of thermal comfort 8 AIR CLEANLINESS AND EFFECTS OF CONTAMINANTS ON A HUMAN BEING 9 3.1 Air Cleanliness in a Room 9 2. 3. 4. 5. 6. 7. 8. 3.2 Hygienic Limits of Substances in Environment 11 3.3 Effects of Contaminants on a Human Organism 11 VENTILATION SYSTEMS. AIR CHANGE CALCULATION. AIR FLOW 12 4.1 Ventilation Systems 12 4.2 Calculation of Air Change 13 4.3 Air Flow with a Ventilated Space 13 FLOW PATTERNS. AIR TERMINAL DEVICES. NATURAL VENTILATION 15 5.1 Flow Patterns 15 5.2 Air Terminal Devices 17 5.3 Natural Ventilation 18 TOTAL AND LOCAL MECHANICAL VENTILATION 21 6.1 Total Mechanical Ventilation 21 6.2 Local Exhaust Ventilation 22 6.3 Local Air Supply 23 AIR CONDITIONING EQUIPMENT. AIR WASHER DESIGN 25 7.1 Components of Air Conditioning Equipment 25 7.2 Air Washer Design 28 AIR CONDITIONING SYSTEMS 29 8.1 All-Air Systems 30 8.2 Air-and-Water Systems 30 8.3 All-Water Systems 31 8.4 Refrigeration Systems 32 2 9. SIZING OF AN AIR CONDITIONING SYSTEM 34 9.1 Illustration of Processes Air Undergoes in Psychrometric Chart 35 10. HEATING SYSTEMS. LOW TEMPERATURE HOT WATER HEATING 38 10.1 Heating Systems 38 10.2 Low Temperature Hot Water Heating 38 10.2.1 Gravity Low Temperature Hot Water Heating System 39 10.2.2 Forced Low Temperature Hot Water Heating System 39 10.2.3 Heating Bodies 42 11. LARGE SURFACE HEATING. MEDIUM TEMPERATURE HOT WATER AND STEAM SYSTEMS 43 11.1 Large Surface Heating 43 11.2 Medium Temperature Hot Water Systems 45 11.3 Steam Heating 45 12. WARM AIR AND RADIANT HEATING. HEAT LOSS CALCULUS 46 12.1 Warm Air Heating 46 12.2 Radiant Heating 47 12.2.1 Suspended Radiant Panel Heating 47 12.2.2 Directly Fired Gas Radiant Heating 48 12.3 Heat Loss Calculus 50 13. NOISE AND VIBRATIONS. NOISE EFFECTS. NOISE PROTECTION 50 13.1 Noise and Vibrations 50 13.2 Physiological Effects of Noise 53 13.3 Noise Sources and Propagation 53 13.4 Means of Noise Protection 53 LITERATURE 54 SOLUTION TO SELECTED EXAMPLES 55 APPENDIX 60 Mollier h–x Chart of Moist Air 60 Table 1 Properties of Dry Air 61 Table 2 Properties of Water 61 Table 3 Properties of Moist Air 62 Table 4 Properties of Saturated Water (Liquid-Vapour) 64 3 EXTENDED SYLLABI 1. LIVING ENVIRONMENT. INDOOR ENVIRONMENT. COMFORT 1.1 Living Environment ‘Living Environment’ as a term is usually understood as the environment which surrounds human beings. The following statement is widely used to define the living environment: “Living environment of a human being is considered to be that part of the world, which interacts with it, or is transformed and exploited by it to fulfil its demands, focusing mainly on the outward where a man lives, works and relaxes.” The environment and a human being cannot be separated and are therefore accepted to make a complex, because of the man’s actions towards the environment not only affect it, but the man has to adapt himself to it as well. The relationship is active as its counterparts cannot be independent due to their mutual interconnection (they are subject to each other) which may be difficult for someone to admit. There are many views to look at living environment of a human being. Location is an argument to divide living environment into indoor and outdoor environment: - Indoor environment is living environment within a building’s interior. - Outdoor environment is living environment outside a building. The main issues of indoor environment, where one spends between 70 % and 80 % of his/her life, are: air cleanliness, thermal comfort, noise and light control, ionizing and electromagnetic radiation etc. Outdoor environment suffers most from air, water, and soil pollution, contamination of the living space of live organisms, soil erosion, change of minerals’ level in soil etc. Outdoor environment is being contaminated by waste of various productive and/or non-productive activities. The waste finishes its life line in a human body which it is transported into via air, water, and/or soil which pass the pollution to nutrient and other cycles of nature. Man’s activity is the prevailing interaction between a man and living environment. To reflect this kind of interaction, it seems useful to divide living environment in accordance to man’s activity giving so called functional subdivision of living environment: - working environment is intended for work and/or accommodates work, e.g. production or office buildings, compartments of transportation vehicles, control cabins, workshops - residential environment is the environment where one lives in or it is serves for short time rest - leisure environment is intended and serves for relaxation and leisure - others – these can be specified by the activity they accommodate, e.g. medical, educational, cultural, communal etc. The particular environment is then inspected, and the parameters and/or properties that influence a human being the most are examined in detail. If atmosphere is the investigated environment, air cleanliness, temperature, humidity, and air flow are the parameters that matter most. The mentioned parameters of the living environment are assigned a term agent of the 4 living environment. The agents’ analysis is the basis for investigation and assessment of the environment conditions and status, but also results in systematic care for living environment. 1.2 Indoor Environment and Comfort Indoor environment, working environment in particular, is an important part of the living environment. Indoor environment (micro-climate) quality is evaluated based on the following micro-climatic agents: 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) air cleanliness air temperature surface temperature of walls and objects air speed air humidity clothing lighting intensity noise, vibrations, and ultra sound ion concentration in air electric and magnetic field intensity ionizing radiation intensity spatial arrangement and aesthetics of environment physique of an individual activity of an individual ability to acclimate climate, racial particularity and people’s habits and routines others like air pressure, mental condition etc. The micro-climatic state which provides healthful conditions for life and for work is the optimum and will be henceforth referred as comfort. A human being senses the comfort as a whole, but there can be a complex comfort or a particular comfort distinguished. For a particular comfort only one of the listed agents or a group of agents is important. Thus there is a comfort from a toxic, aerosol, microbial, odour, thermal, lighting, acoustic, ionizing, electrostatic, electro ionic or psychical point of view. Some of the listed agents (1 to 11) can be controlled with the use of technical equipment like: - equipment for temperature, humidity and air cleanliness control, such as ventilation, airconditioning, heating, dust separating systems - means of vibration and noise protection - lighting systems - equipments to control ionic level in air - equipments to prevent from ionizing and non-ionizing radiation etc. Interaction of the agents 2 to 6, 13 and 14 gives rise to a state called thermal comfort. 5 2. THERMAL COMFORT AND ITS ASSESSMENT 2.1 Thermal Comfort Thermal comfort is usually defined as a feel of satisfaction with thermal state of environment. Biochemical or metabolic processes that take place within a human body cause thermal energy being generated and released. The energy is called metabolic heat and it is transmitted to environment. The amount of the energy released is proportional to physical activity of a human being and its weight. There is only a small portion of the energy being converted into mechanical work; most of it is heat (90 % to 100 %). Temperature within a human body is usually about 36.5 ± 0.5 °C. In order to keep it at this level, metabolic heat has to be transmitted into body’s surroundings. The body exchanges heat with its surroundings by conduction, convection, radiation, evaporation of sweat, and breathing. The rate of heat exchange is controlled by thermostatic control centre and it controls the body temperature chemical, vasomotor and evaporation thermoregulation. The first condition which has to be fulfilled is to balance the energy produced by the body (reduced by the mechanical work that the body undertakes) and the energy lost by a heat transfer into surroundings. Thus the equation of thermal balance reads Q& m (1 − η ) = Q& ved + Q& k + Q& r + Q& v + Q& d [W] (2.1) where Q& m is metabolic rate given by Q& m = q& m S [W], which depends of one’s activity, q&m is specific metabolic heat flux [W·m-2], S body surface area [m2], η mechanical efficiency of a human body, Q& ved , Q& k , Q& r , Q& v , Q& d are heat fluxes passing heat to the body’s surroundings by means of conduction, convection, radiation, evaporation, and respiration [W]. Prevailing conditions make the conductive heat flux very small in the above stated equation causing it to be neglected. The same apply to the mechanical efficiency of a human body. The heat flux transferred by means of convection from a surface of a dressed person is given by Q& k = αS k (t p − t ) [W] (2.2) here, η is heat transfer coefficient [W·m-2·K-1], t p mean temperature of an outer surface of clothing, t temperature of surrounding air, S k = f cl S surface area of a dressed person, S surface area of a naked body (S = 1.9 m2 for an average adult man, S = 1.75 m2 stands for a woman), f cl ratio of a dressed to undressed human body surface areas – depends on a clothing type. The heat flux transferred between a body surface and the surfaces that surround the body is the radiative one and reads ( ) Q& r = εσ 0 S r TP4 − Tr4 [W] (2.3) 6 where ε stands for emissivity coefficient (fraction of the ideal blackbody spectrum energy which a real body actually emits); for a skin and most cloth ε = 0.95, σ0 is Stefan-Boltzmann constant which equals to 5.67·10-8 W·m-2·K-4, S r human body surface area participating in the radiative heat transfer; obviously smaller than the dressed person surface area; S r ≈ 0.71S k , Tr mean radiant temperature [K]. The evaporative heat flux is given by a sum of evaporative heat loss by skin diffusion & Qvs and evaporative heat loss due to regulatory sweating Q& vm Q& v = Q& vz + Q& vm (2.4) The diffusive evaporation is given by Q& vs = 3.05 ⋅ 10 −3 S  p'p'( t ) − p p (t )  [W] k   (2.5) where S is body surface area [m2], p'p'( t ) [Pa] partial pressure of saturated water vapour at k skin temperature tk , p p( t ) [Pa] partial pressure of water vapour at surrounding air temperature t and relative humidity ϕ . The heat flux that removes heat from the body by sweating Q& vm is very important in body thermoregulation mechanism, and it can be varied when needed to keep the body temperature constant. The exhaled air, amount of which depends on the intensity of the activity performed by a human being, gets heated up in lungs to the temperature of 34 °C to 36 °C and saturated by water vapour at the same time. The respiratory heat flux can be expressed by ( ) Q& d = m& c p (t v − t ) + m& l23 x'' − x [W] (2.6) where m& [kg·s-1] is mass flow rate through lungs, c p for specific heat of air at constant pressure ( c p = 1.01kJ ⋅ kg −1 ⋅ K −1 ), t v temperature of the exhaled air ( t v ≈ 34°C ), l 23 latent heat of water ( l23 = 2560kJ ⋅ kg −1 ), x'' , x [kg/kgda] moisture contents in the exhaled (saturated by water vapour) and inhaled air. The convective and radiative heat fluxes that pass across the clothing of a dressed person, are given by S (t − t ) Q& k + Q& r = k p Rcl (2.7) where Rcl [m2·K·W-1] is the clothing thermal resistance. There has been a dimensionless quantity defined, I cl = Rcl / 0.155 for the clothing thermal resistance and it was assigned a unit called ‘clo’. 7 If a body should reach thermal comfort, the first condition be met is the thermal balance equation (2.1). Not only is the thermal balance equation to be satisfied, but also it should be done with minimum intervention by a human thermoregulatory system. For the purpose of thermal comfort assessment, mean skin temperature and heat loss due to sweating are given by t k = 35.7 − 0.0275q& m [°C] (2.8) and Q& vm = 0.42S (q& m − 58) [W] (2.9) These two equations are considered to be the second and third condition of thermal comfort. Solution of the whole set of equations (2.1) to (2.9) results in equation of thermal comfort, which expressed as a function sounds Q& m = f (Rcl , f cl , t , w, t r , ϕ ) (2.10) The equation states relationship between main agents of thermal comfort. The agents stand for: - Q& m person’s activity - Rcl , f cl clothing properties - t , w, t r , ϕ thermal state of environment 2.2 Assessment of thermal comfort Since thermal comfort depends on many agents, derived quantities from these agents have been accepted to simplify the assessment of thermal comfort. Among the derived quantities which express joint action of some or all the agents, operative temperature is the one used most. The calculation procedure of operative temperature based on the thermal conditions of environment and accepted values of it for particular working environment depending on the activity being done (work classification, energy release) and clothing used, are stated in regulations [12] and [13]. Operative temperature, t o [°C], is defined as the integral temperature of a radiantly black enclosure, in which an occupant’s body would exchange the same amount of heat by radiation and convection as it would be the case with the non-uniform temperature distribution in the actual environment. When there are mean radiant, t r [°C], and air, t [°C], temperatures of environment known, the operative temperature is given by to = t + A(t − t r ) (2.11) where A is a function of air speed. Should the air speed be lower than 0.2 m·s-1, the operative temperature can be replaced by globe temperature t g [°C]. The mean radiant temperature at air speeds larger than 0.2 m.s-1 is [ ] t r = (t g + 273) + 2.9 ⋅108 ⋅ w0.6 (t g − t ) 4 0.25 − 273 8 (2.12) where t g is the globe temperature measured by a globe thermometer of 10 cm in diameter, or [ ] t r = (t g + 273) + 2.5 ⋅ 10 8 ⋅ w 0.6 (t g − t ) 4 0.25 − 273 (2.13) where t g is the globe temperature measured by a globe thermometer of 15 cm in diameter. Thermal comfort can be assessed by PMV and PPD indices as well, according to ČSN ISO 7730 [7]. PMV index assigns a 7 point scale to the mean thermal sensation of environment: +3 +2 +1 0 -1 -2 -3 hot warm slightly warm neutral slightly cool cool cold The PMV index can be calculated knowing energy produced by a body, thermal resistance of clothing, air temperature, mean radiant temperature, humidity, and air speed, according to [7]. The PPD index predicts percentage of people dissatisfied with thermal environment (5 % means thermal comfort, 10 % acceptable conditions, 20 % bearable conditions). If the PMV index is known, the PPD index can be determined using a chart or an expression as stated in [7]. The cited regulations give also an expression for ‘draught risk’, DR, calculation. DR index stands for a percentage of people dissatisfied with environment due to draught. It can be determined knowing local temperature, air speed, and turbulence intensity. 3. AIR CLEANLINESS AND EFFECTS OF CONTAMINANTS ON A HUMAN BEING 3.1 Air Cleanliness in a Room Air of a room can be polluted by various gaseous contaminants, smell, dust, infectious germs, but also by excessive radiative and convective heat. Gases and vapours leaking from production process make up a large group of industrial contaminants. The best procedure to remove gaseous contaminants from working environment is to extract them at location of their release. Although it may seem satisfactory, there is always a portion of contaminant which escapes from being extracted, thus ends up in envi- 9 ronment. Hence there is a certain level of total ventilation required to keep contaminants concentration bellow permissible limit. At locations occupied by people, CO2 and water vapour are generated. The exhaled air contains about 4 % (by volume) of CO2, 5 % of water vapour, 16 % of oxygen, and 75 % of nitrogen. Increase in CO2 concentration in the air is associated with decrease in O2 concentration. The decrease is not significant from a respiration point of view, because the portion oxygen represents in air (by volume) cannot be lower than 20.5 % if regulations are met so CO2 concentration does not exceed 0.5 %. Hence the statement about vitiated air does not reflect reality. Mostly it is an unpleasant feeling as a result of thermal comfort imbalance due to high relative humidity of air (exhaled water vapour). Odour is a very common cause of indoor air quality deterioration. The most prevailing sources of odour within residential and/or civil building are: kitchens, toilets, laboratories etc. In the occupied meeting rooms, air is contaminated by human odours which originate from epidermis and mucosa disintegration, and dissolution of exudation and other secrete. An odour of cloths and decorative textiles can be also found very irritating. In industry, it is various chemicals used by technology which are common sources of pungent odour. Odour can be removed by means of ventilation and/or spraying scents into air, i.e. air deodorisation. Another component that contaminates air of a room is dust that originates from disintegration and dissolution of organic and inorganic substances. There are branches of industry where a so called industrial dust is produced by production processes. Some of it (siliceous and/or asbestine) are highly noxious. Infectious germs (bacteria and fungi) are present in the rooms where especially large number of people meets. These cannot live on their own in free air but need a carrier (dust or droplets) to survive. The easiest way to reduce their numbers in air is to air the room intensively along with filtering the air. Health service and pharmacy need even more action so air is being disinfected in chemical or physical ways. The chemical disinfection is mostly carried out by spraying antiseptic liquids directly into a room. Exposing supplied air or air in the room to ultra-violet rays is a very efficient means of physical disinfection, which annihilates all the microbes in the air. Last but not least, excessive heat is among the contaminants, which cause high temperatures or high thermal radiation and consequently load vascular system. The following means are used to prevent from high radiant heat load: - lowering intensity of radiation at source (decrease its temperature or change emissivity of its surface) - radiative curtains – mechanical or watery - cooling the worker with the use of air shower, direct water spraying, or cooling radiant panels - thermal insulation of a worker with clothes (multilayer clothing, bright clothing, foil the clothing with aluminium, metalized suit; really hot environments require air chilled suits, e.g. furnace repair) 10 3.2 Hygienic Limits of Substances in Environment Hygienic limits of substances in working environment and means of their assessment are stated in regulations [12] and [13]. Permissible exposure limits, PEL, and maximum permissible concentrations, NPK-P, are stated by the regulations for gases, vapours and aerosols. PEL represent a time averaged mean of a substance concentration or amount in working environment that a worker can be exposed to over an 8 hour shift without harm to its health even if it lasted for the whole life. On the other hand, NPK-P is a limit which must not be trespassed at any moment of a shift. The above mentioned regulations state the limits for dust as well. PELc is a total concentration (inhalable fraction) of particles and PELr stands for a respirable fraction of the particles. The inhalable fraction means all the particles which can be inhaled by nose or mouth. By the respirable fraction a fraction of particles is understood which can penetrate deep into the respiratory system where ciliary epithelium disappears, and to alveoli. Contaminant concentrations are expressed as mass concentrations [mg·m-3] or volumetric [% vol., ppm] where ppm represents parts per million and therefore 10-4 % vol. 3.3 Effects of Contaminants on a Human Organism The noxious effect of particulate and/or gaseous contaminants on a human organism depends on both total amount of the contaminant which enters the organism, and its concentration. The effects can be either local or global (after being absorbed by the body) and their action can be poignant, toxic, or cumulative. The local effect is exhibited at locations of contact or entry to the organism (upper respiratory tract, nasal mucosa, throat, bronchial epithelium, lung tissue, digestive tract, skin, winking membrane etc.). The local effects demonstrate themselves as either chemical or mechanical irritation. Higher concentration of contaminant may lead to inflammation. Sulphur dioxide, ozone, nitrogen oxides, chlorine, fluorine, hydrogen sulphide, aldehydes, ketones, textile particles (cotton, linen, hemp, and synthetic fibres), animal particles (feathers, wool, and fur), and vegetable particles (flour, tee, coffee, spice, and wood). Effects of contaminant after their absorption by a body are more complex. Gaseous contaminants carried by blood are transported to different body parts where they may cause various noxious effects. Some of the absorbed gas can be excreted from the circulating blood in lungs, or kidneys. Harmfulness of solid or liquid contaminants depends not only on their chemical composition and their concentration in air, but also on the size of the particles. Particles of which size is larger than 5 µm are separated at nasal cavity and upper respiratory tract. Particles of a smaller size proceed to lower respiratory tract. Insoluble particles which finish in the lower respiratory tract may lead to pneumoconiosis (black lungs). If the particles are of a fibrous shape (silicon dioxide, asbestos), the lungs suffer from fibrosis and loss of lung function (silicosis and asbestosis). The particles smaller than 0.1 µm are exhaled. The most dangerous are the particles in between 0.1 µm and 5 µm, which can freely penetrate deep the lungs up to alveoli. The soluble particles infiltrate the body via body fluids. Some of them get excreted in kidneys, or intestinal tract. The ones which do not get excreted may gather in body organs and may develop functional and/or structural changes. 11 4. VENTILATION SYSTEMS. AIR CHANGE CALCULATION. AIR FLOW 4.1 Ventilation Systems The purpose of ventilation is to provide clean environment to working, residential and/or civil areas. The task is achieved by replacing stuffy air with fresh outside air. It can be done in regular intervals – intermittent ventilation, or with no pause – continuous ventilation. Fresh air can be delivered to the whole space or its particular zone. According to previously stated, ventilation is classified as: - total ventilation – it is suitable to spaces where no distinction can be made regarding to contaminant source location, or where the sources are uniformly spread over the area, e.g. meeting or assembly rooms - local ventilation – designed to rooms with high intensity sources of contamination which are concentrated at a particular location. The contaminant is then removed from environment by means of local exhaust, or there is a fresh air provided locally to the places occupied by workers (air shower, oasis, and curtain). Ventilation systems can be also divided according to the energy they employ to drive air to the room: - natural ventilation – provides fresh air due to pressure difference between indoor and outdoor environment as a result of difference in densities or wind; the types which belong to this group are: infiltration, interrupted ventilation, aeration, air shaft ventilation - mechanical ventilation – makes use of fans to provide the driving force; the fans can be used either for air supply or removal, or in combination; according to the fan sizing, hence the air flow ratio of supply, V&p , and removal, V&o , flow rates is defined: ε= V&p V& (4.1) o The cases are: - ε = 1 – balanced ventilation: ventilation system does not cause pressure difference between indoor and outdoor; it is employed when there is requirement of no air change between the ventilated space and its surroundings - ε > 1 – overpressure ventilation: the supply air is processed; it is required that no air will penetrate the considered space from the outside (surgery rooms, production of TV screens, control cabins) - ε < 1 – underpressure ventilation: its main purpose is to avoid contaminated air leakage to the outside environment (laboratory rooms, toilets etc.) Nowadays, hybrid ventilation has drawn some attention as it makes use of both natural and mechanical ventilation. It is an intelligent system which decides when to switch between the natural or mechanical system in accordance to favourable conditions. When there is enough driving force for natural ventilation available, it is applied otherwise the mechanical system is run. 12 4.2 Calculation of Air Change The fresh air flow rate for total ventilation purpose can be determined based on mass balance of contaminant inside a ventilated space, which is expressed by the differential equation of ventilation: Odk = V&p k p dτ + S& dτ − V&p k dτ (4.2) where O is the room volume [m3], S& amount of the contaminant generated uniformly within a room [g·s-1], V&p flow rate of supplied air [m3·s-1] (it is assumed the same amount is removed from the room), k p contaminant concentration in supply air [g·m3], k actual instantaneous contaminant concentration [g·m3], dk change of concentration in time interval dτ . Solution of the above equation over a time interval 0 to τ, and for a change in contaminant concentration from k1 to k2, yields the equation for a required air flow rate: V&p = S& O k 2 − k1 [m3·s-1] − k2 − k p τ k2 − k p (4.3) When a long term ventilation is considered with constant amount of contaminant generated, the time interval in equation (4.3) changes to τ = ∞ so to yield V&p = S& [m3·s-1] k2 − k p (4.4) 4.3 Air Flow within a Ventilated Space People’s comfort is highly affected by air flow within a ventilated space. Therefore, not only is the air change in the ventilated space important, but also suitable flow patterns within the space. There is a turbulent flow present in almost all ventilated rooms. The jet of air which enters a relatively large space, not being influenced by walls or objects located in the room, is called a free jet. The jets that are influenced by the walls or any obstacles are called restricted jets. A jet that travels along a wall, consequently it can spread into one direction only, is called an attached jet. There are two basic ways of supplying air to the room from a flow pattern point of view: supply by isolated (individual) outlets, and low-velocity outlets. The fundamentals of a jet flow are explained on a free isothermal round jet, which is schematically depicted in Figure 4.1. Air of the same temperature as it is in the room is enters the room via an air outlet of a circular cross section. The turbulent jet spreads conically as the distance from the air outlet grows. The fluid particles in the jet fluctuate laterally and transmit their momentum to the particles of quiet surroundings; these are then entrained by the jet. As the distance from the jet origin increases, the amount of entrained air grows and consequently the jet cross sectional area increases. Along with the jet growth, the air speed of the jet decreases. The jet boundary is made of two consequential conical surfaces. There are two dis- 13 tinctive regions in the jet. The first one being potential core region and it is limited by a distance xk. The second one is the so called fully developed region where the jet spreading angle is fixed and indicated by 2ϑ . Contour lines of the jet in the fully developed region intersect on the jet axis at a point P, being the jet’s pole. The potential core region is characteristic by the fact the air speed at the jet axis, wx remains constant and equals to the exit velocity at the air outlet, w0 . Velocity decay along the jet axis in the fully developed region is given by S0 wx = Ks w0 x (4.5) where K s is the coefficient of velocity decay with respect to the cross sectional area of the air outlet, S 0 . The coefficient has to be determined experimentally for each particular type of the air outlet. The velocity profile in the fully developed region reads (according to Schlichting)   y 1.5  = 1 −  x   wx   Rx     wx, y 2 (4.6) The free jet generated by a rectangular shaped air outlet resembles the round jet after passing a short distance from the terminal. Hence its rectangular shape diverts gradually to the circular one. Therefore, the equation (4.5) can be applied to calculation of the velocity decay. Figure 4.1 Schematic view of a free isothermal round jet A jet that flows out of an infinity straight slot is called a plain jet. Any jet leaving a slot of length to width ratio greater than 25 can be well considered to be a plain jet. The centre line velocity of a plain jet is given by wx b = Kb 0 w0 x (4.7) 14 where K b is the coefficient of velocity decay of a slot, and b0 is a slot width. When there is a worm or cold air blown into a room, the jet does not exchange its momentum with the surroundings only, but also heat is exchanged. Along with the velocity decay, the temperature profiles across are smeared out to reach the temperature of the surroundings. The dimensionless temperature profiles in the jet are given by the following expression: t x,y − ti w =  x,y t x − ti  wx    PrT (4.8) where t x,y , wx,y are temperature and velocity at location x, y, t x , wx are temperature and velocity at the jet axis at distance x from the air outlet, ti is temperature in the surroundings, and PrT is the turbulent Prandtl number. The measure to decide of the jet isothermality is the ratio of buoyancy forces to the inertial forces which act on a fluid particle and are determined via Archimedes number. The air terminal related Archimedes number sounds: Ar0 = gl0 |T0 − Ti | w02 Ti (4.9) where g is the acceleration of gravity, l 0 is the air terminal characteristic dimension, T0 air temperature at the air terminal, Ti air temperature of surroundings. The jets are considered to be isothermal if Ar0 ≤ 0.001 ; strongly non-isothermal if Ar0 > 0.01 . The most significant effect of buoyancy on a jet is its curvature in vertical direction. The slightly non-isothermal jets can be assumed to spread in the space in a similar way as the isothermal jets do, i.e. in a straight way. 5. FLOW PATTERNS. AIR TERMINAL DEVICES. NATURAL VENTILATION 5.1 Low Patterns A flow pattern represents the image of temperature and velocity distribution across the ventilated space. The air flow within a ventilated room can be of either primary or secondary type. The primary flow is the flow of jets which blow from the air terminals, causing the secondary flow. A jet entrains air from its surroundings, especially in its early stage so that the air flow rate increases. An inlet draws the air from the ventilated space at the same rate as it is supplied to the space if the whole ventilation system is designed to operate as a balanced ventilation system. In that case, the air entrained by the jets just circulates in the room. The flow pattern in the room is determined mainly by: - number, location, and size of the air terminal devices – outlets, jet exit velocity and temperature - location, surface temperature, and strength of the heat or cold sources within the room. The heat sources generate warm plumes which rise above them depending on their 15 strength. The plumes entrain air from their surroundings so they act as chimneys in the room. Windows and walls which are connected with the outside and along which the cold air falls represent the cold sources. Note: machine components, peoples motion, and location of inlets, i.e. exhaust openings, have negligible effect on flow pattern. When speaking of isothermal ventilation, the ideal air change is achieved if the air is supplied at low velocity by the entire wall, and removed by an inlet located opposite the supply wall (see Figure 5.1a). The flow pattern changes dramatically when the supply is realised via a small outlet (see Figure 5.1b). In that case air is entrained by the formed jet, and air circulation happens. a) b) Figure 5.1 Isothermal flow patterns. (a) ideal case, (b) supply outlet Non-isothermal ventilation is influenced by buoyancy forces which deform the jets, especially at low jet exit velocity. If the temperature of supply air t0 is greater than the room temperature ti, the jet deflects upwards and vice versa. If the fresh air is supplied to the mechanically in the vertical direction (using perforated ceiling and a floor grill), the system is of so called top-down or bottom-up scheme, depending on the location air enters and leaves the room. These cases create the ideal flow patterns (see Figure 5.2c, b). A room is ventilated non-uniformly when the warm air is supplied at the floor level (bottom-up scheme), or the other way around the cold air supplied at the ceiling level (top-down scheme); separated warm or cold plumes are generated. Figure 5.2 Non-isothermal flow patterns 16 It can be concluded the right way to ventilate a room non-isothermally using low velocity outlets is to deliver the air against the direction it is driven by nature. 5.2 Air Terminal Devices An air terminal device (ATD) is designed to supply or remove air to/from a ventilated space. Further on, the ATD that supplies air to a space will be referred as the outlet, and the one which removes air the inlet. One way to classify them is according to their location: wall, ceiling, floor. Wall ATDs: Rectangular, slot, and low-velocity outlets are very often used at walls. A rectangular outlet with guiding vanes is very common (see Figure 5.3). These can be either one single or double set (front and rear). The single set ones have only one set of the vanes so the jet direction can be controlled within a plane. The double set ones can completely control the jet direction. A set of opposed blade dampers is used to control the flow rate through the ATD. A slot outlet resembles a very long rectangle which can be further divided by a grill to even smaller slots. Low-velocity outlet consists of multiple boxes which are supplied by air and a perforated plate. These can be of a various shape: fully cylindrical, half cylindrical, quarter cylindrical supply supply exhaust Figure 5.3 Rectangular air outlet. (a) view, (b) flow control by dampers and vanes, (c) adjustment of the front vanes 17 Ceiling ATDs: There are a number of types where diffusers, slot, and low-velocity outlets are among the most widespread. The diffusers can be either circular or square shaped. An example of a circular diffuser is given in Figure 5.4. The location of conical diffusers is adjustable in axial direction which results in the possibility to control flow rate across the device. The low-velocity ceiling outlets consist of perforated panels pressurised by air of which bottom is finely perforated. Suspension cone and inlet connection Set of draw-out conical diffusers Figure 5.4 Circular diffuser Floor ATDs: The usually employed are of square and/or slot shape, and low-velocity outlets. 5.3 Natural Ventilation Natural ventilation provides fresh air due to pressure difference between indoor and outdoor environment as a result of difference in temperatures between them, or dynamic effect of wind on a building; the types which belong to this group are: infiltration, interrupted ventilation, aeration, air shaft ventilation. Infiltration: It is mainly caused by flow through window or door cracks, and porous walls. Pressure distribution over a side walls caused by temperature difference between inner and outer air is depicted in Figure 5.5a. The same but due to wind is shown in Figure 5.5b. Conjugate temperature and wind action sums up. If the internal temperature is larger than the external one, there is overpressure in the upper part of a room and underpressure in the bottom part. The location of zero pressure difference defines location of a neutral plane, n. The pressure difference ∆p at a height h from the neutral plane at zero wind speed is given by 18 ∆p = h( ρe − ρi )g (5.1) where ρe , ρi are density of internal and external air, respectively, and g stands for the acceleration of gravity. Wind acting on a building results in overpressure at the leeward side of a building and underpressure at the windward side. The both can be determined using the following formula: 1 ∆p = Apd = A ρe we2 2 (5.2) where A is a pressure coefficient determined experimentally; 0.9 for the leeward and -0.4 for the windward sides are the average values. Figure 5.5 Pressure distributions over a building’s walls. (a) due to density variation, (b) due to wind Interrupted ventilation: It is a ventilation strategy which makes use of windows with the action being opening and closing them. This intermittent way of ventilation is rather common in residential and public buildings. Short term cross ventilation by large windows is quite energy efficient. Aeration: It is a way of natural ventilation which makes use customisable openings located on walls and the building’s roof (see Figure 5.6). Metallurgical, machine and glass works are the examples of its application. Internal sources of heat load the space at rate greater than 25 W·m-3 in such cases. The plume that rises above the heat source is convected away to the surroundings only partially. There is a part of it which recirculates m& c . It the air in contact with the building envelope where it is cooled down, consequently it moves downward towards the occupied zone. It is mixed with fresh air there and the mixture proceeds to the heat source. Mass flow rate of the air used for ventilation m& , can be determined based on heat balance of the ventilated space: m& = Q& c p (to − te ) (5.3) 19 where Q& = Q& i ± Q& e are the internal and external heat loads, respectively ( − Q& e stands for heat loss), to , te are temperatures of the air removed from the space and the external one. Stack pressure equation (5.1) is the greatest in winter. Hence vent holes (intended for winter use) may be small and located above to the occupied zone for temperature of cold air to increase before it enters the zone. Figure 5.6 Aeration of a warm workplace There was a temperature ratio, B, defined which is used during the stack pressure calculation procedure, and it reads: B= t po − te (5.4) to − te It is determined experimentally and its recommended values depending on the type of technologic process within the considered space can be found in literature. First of all, temperature difference ( to − te ), is calculated, then the stack pressure, according to the equation (5.1). The stack pressure is split between supply and exhaust openings to comply with ∆p = ∆p p + ∆po ∆po ≈ 1 to 2 ∆p p where the following condition is to be fulfilled ∆p p ≤ 5Pa . Air shaft ventilation: It is employed to draw the contaminant away. Effective stack pressure of the shaft is given by 1  h gh( ρe − ρi ) =  λ + ∑ ξ +1 ρi w2 + ( pe − pi )  d 2 20 (5.5) and it is needed to overcome pressure loss of the shaft, generate dynamic pressure at the shaft exit, and overcome pressure difference between the buildings’s exterior and interior. The parameters in the equation (5.5) means the following: λ friction loss coefficient, h shaft height, d shaft diameter, ∑ ξ sum of local pressure losses of duct components, such as bends, junctions etc. 6. TOTAL AND LOCAL MECHANICAL VENTILATION When there is no distinctive location of the contaminant source or its location is not known in prior, or the sources are distributed uniformly over a ventilated space, global ventilation is a good ventilation strategy. The opposite is favourable to the local ventilation, i.e. in such cases when the sources are concentrated at a particular place in the building. The contaminant is removed at location of its release, and/or fresh air is delivered to the occupied zone. 6.4 Total Mechanical Ventilation The total ventilation provides fresh air independent of climate conditions; hence it has a number of advantages compared to the natural ventilation: - intensity of ventilation can be controlled with respect to demands air can be processed (filtration, heating, cooling) heat recovery can be employed can adjust preferable pressure distribution and consequently flow patterns over the ventilated space The mechanical ventilation systems are divided according to the pressure difference between interior and exterior into: balanced, overpressure, underpressure. Ventilating equipment can be represented by a single unit or as a central system. Single unit equipments are installed directly into a ventilated space. The units can both deliver or remove air from the ventilated space. A supply unit consists of a mixing chamber, filter, heat exchanger, fan, and a chamber with an ATD. An air removal designed units is mostly made of a fan. Central ventilation equipment is an assembly of units, henceforth unitary equipment, located at the plant room, and connected with the ventilated space via a ductwork. It usually serves a number of rooms. An example of such unitary ventilation equipment packed with a heat recovery unit is depicted in (see Figure 6.1). This kind of unit would have become air conditioning equipment after being completed by cooling and humidifying units. Ventilation equipments are very demanding from energy point of view. Energy saving are achieved mainly by the use of local exhaust ventilation, i.e. removal of contaminant at the location it is generated. Another means of energy savings is to design the ventilation system which makes use of recirculated air. This means there is portion of exhaust air filtered and delivered back to the ventilated space, used by the system. This is advantageous in winter especially when the fresh air has to be heated up to 30 K or above. Heat recovery can result in the same as well. 21 Figure 6.1 Central ventilation equipment. V – fan unit, F – filter unit, O – air heater unit, K – damper unit, ZZT – heat recovery unit 6.2 Local Exhaust Ventilation Local exhaust ventilation is designed to capture contaminant at location of its generation and remove it out of the ventilated space. It is more energy efficient than the global ventilation for the same source of contaminant on general principle, the reason being the possibility of exceeding hygienic limits of contaminant concentration in the extracted air as it does not come into contact with any one. The equipment for local exhaust ventilation can be either central, or sectional, or a single unit one. The central equipment serves a number of contaminant sources via ductwork interconnected to a single fan unit located outside a workshop. The sectional equipment is featured by separated ducts and a fan unit for a particular contaminant source. Its purpose is to avoid contaminants from mixing into an explosive, flammable or toxic compound. The single unit equipments are prevailing in dusty workshops. They consist of a fan, a dust separator, and a dust collector. They are located into the ventilated room, and the filtered air is delivered back to the room. An important part of the local exhaust ventilation system is, the exhaust hood. It is a device which is designed to capture the contaminant. The types of exhaust hoods are the following: - enclosed box hoods – safety cabinets, paint coating cabins, steel-grit blasting cabins etc. - canopy hoods – roof like hoods (over a heat source), or of another shape adapted to a machine they exhaust contaminant from (metal- or wood-working machines like grinding, or milling machines, powdery matter transportation equipment) - slot hoods – common for surface finishing technology, purging, etching, electrotyping vats) - floor exhaust grills – paint coating, trimming cast parts - exterior exhaust hoods – air terminal devices of round, rectangular or slot shape mounted to the exhaust duct via a flexible duct Sizing of an exhaust hood is based on a potential flow theory; sink approach to be precise. Based on the sink theory, iso surfaces of constant potential, consequently air speed, are of a 22 spherical shape, and perpendicular to the streamlines. The air speed, wr at the distance, r, from the sink is given by wr = V& 4πr 2 (6.1) where V& is volume flow rate extracted by the sink. The slot exhaust flow is derived from a line sink. The velocity its surfaces resemble cylindrical surfaces in this case. Therefore the air speed wr , at the distance, r, from the sink can be calculated using the following expression: wr = V& 2πr (6.2) where V& is a volume flow rate at the sink of a unit length of 1 m. The actual velocity distribution of an exhaust hood differs from the theoretical one; the closer to the hood the interest is focused. Hence, experimentally determined velocity drops are used for the hood’s vicinity area; usually as charts of air speed iso-lines. The equation (6.1) is valid for x / D > 1, the other one (equation (6.2)) for x / b > 2 (D stands for the suction opening diameter, b for suction slot width). Design of an exhaust hood should bear in mind the following: - to locate the hood as close to the contaminant source as possible, enclose the source eventually - to orientate the hood so the plume (contaminant) may flow towards it in a natural way - to locate the hood so it does not interfere with the worker, or the worker cannot place itself in between the source and the hood - it must comply with safety of work regulations 6.3 Local Air Supply Local air supply is intended to change air within a limited zone of a large space. There are three types of the local air supply based systems – air showers, air oasis and air curtains. The air shower protects a worker from a radiant heat load. The air is blown in the direction the heat load affects the worker. The increased air speed enhances the convective heat transfer from the surface of the worker’s suit, hence heat flux is partially diverted from passing across the suit to the body because of the fact the heat is convected away from it. There can be two types of air showers defined. A single unit air shower is mobile or stationary by its design, and does no processing to the air. The mobile air shower is mounted to a support so it can be adjusted vertically, and inclined within a range of ± 30°. Figure 6.2 illustrates such a device, consisting of a mobile support, a fan, and a short duct. A stationary air shower can be mounted to a building structure (like a pillar or so), and used as a permanent shower for a specific workplace. A central air shower is similar to a central ventilation system, as it consists of a plant room where fresh air can be processed (cooled, humidified, filtrated), and a ductwork to deliver air via outlets to workplaces. 23 Figure 6.2 A mobile air shower The design of an air shower is based on a free round jet theory, where there are the air speed and the jet diameter at the workplace the design parameters. The air speed at the workplace should not exceed 3 m·s-1. The diameter of the jet at the workplace depends on the particular case; the minimum being 1.2 m, assuming the worker does not have to change its place too often. Due to very small velocities near the jet envelope (recall the jet velocity profile), the jet diameter, Dx, is not considered be the same as in the theory but an efficient jet diameter, Dxr, is applied in the design. It can be calculated using the following expression (6.3) D xr = kDx where k = 0.64 if the Schlichting’s velocity profile is assumed. When designing an air shower one should bear in mind the following: - the jet temperature is not to be lower than 3 K below the temperature at the workplace; relative humidity shall not be greater than 70 % - the jet should be directed to hit the side facing radiation - the air shower is not suitable for such activities where the worker walks up and down from a place with high radiant heat load to non-radiant one An air oasis is a device to supply fresh air to a workplace, or a place of rest, to dilute pollution level. It may be also applied to a warm workplace where the convective heat source is the main source contaminant. The oasis makes use of screens to zone the space. The air is delivered with the use of low velocity outlets located above or sidewise the place occupied by workers. The air curtain is meant to lower flow rate across an opening (door usually) which interconnects two zones (areas) of different pressure. The curtain is generated by a plane jet issued from a slot located along the opening side (or both sides). The jet is directed towards the zone of higher pressure. It is the jet’s momentum which acts against the pressure gradient between the zones thus lowering of the flow rate is achieved. The slot location at the opening defines the types of air curtains. Therefore, we distinguish among top, bottom (floor), and side discharge air curtains; the top ones being the most widespread nowadays. The top discharge curtain can be opposed by a floor opening for air recirculation. 24 7. AIR CONDITIONING EQUIPMENTS. AIR WASHER DESIGN 7.1 Components of Air Conditioning Equipment Air conditioning equipment is supposed to be such equipment that controls air temperature, moisture content, and cleanliness on year-round basis. The components of air conditioning equipment process do the particular tasks so altogether can meet the required state of air in the zone of interest. The main components of air conditioning equipment are: air heater, air cooler, air humidifier, refrigerating equipment, fan, screen (filter), and heat recovery. Air heaters and coolers control the air temperature. A flat–fin/round-tube heater is the most common one. The heaters make use of warm or hot water, or steam. The coolers are supplied with water, or a refrigerant. Flat-fin pipes are employed to enhance heat flux on the side where the heat transfer coefficient value is small, i.e. the air side. A flat-fin heat exchanger is made by winding up aluminium flat-fins of a rectangular shape on a copper pipe. The fin thickness is usually about 0.2 mm (2 mm to 3 mm span). The air entering such a heat exchanger has to be filtered to prevent it from being choked. A special circuit has to be designed to protect a fresh air heater from freezing-up. The main purpose of an air humidifier is to control humidity level in the air. The humidifying medium can be either water or steam. An air washer or wet-film humidifier makes use of water. This kind of humidifier as a unit can be a part of a unitary air conditioning equipment. For direct humidification of a space, mechanical or pneumatic water spraying is useful. High comfort air conditioning systems make use of steam humidifiers. Steam is also the only choice when there are strict hygienic constrains required. Evaporation of water vapour from a water droplets surface is the principle applied to an air washer. In this way, the air passing across a humidifying chamber gets humidified. Water is sprayed in the chamber by a set of nozzles. The set are called registers and there can be from 1 to 4 registers in the air washer. There is a water tank for make up and the not-evaporated water at bottom of the chamber which level is controlled with a float. Before leaving the tank into the washer pipes, water is filtered. Due to low efficiency of evaporation mechanism as the amount of actually evaporated water is rather larger than the amount of water delivered to the spraying nozzles, the air washer is quite spacious piece of air conditioning equipment. A wet-film humidifier does not need so much water as an air washer thereby its dimensions are not so limiting. The efficient surface for evaporation is made by a set of plates, or a fibrous fabric layer, which is sprayed with water by a set of nozzles (see Figure 7.1). A widespread type of mechanical spray humidifier makes use of a rotating disc. Water is poured at the disc which revolves at high rate where at the disc orifice water brakes into small droplets entrained by air. Pneumatic humidifiers employ pressurised air (0.03 to 0.07 MPa of gauge pressure) to atomise water and spray it. Steam humidifier consists of a reduction valve, pipes, and nozzles which deliver steam into a humidifying chamber of unitary air conditioning equipment, or directly into ductwork. The steam humidification is easy in design and control (as steam addition to the ventilation air al- 25 most does not change its temperature). The last advantage to be mentioned here is its hygienic operation. Figure 7.1 Wet-film humidifier Refrigerating equipment provides chilled water to air coolers, or the evaporator, being a component of the equipment, is put directly into air conditioning equipment to cool the air. Refrigerant plants of air conditioning equipments are usually large in power output but they operate at full load for a few days a year only. Therefore, good control is required to achieve high efficiency at partial load. The requirement is easily met when the equipment employs a turbocharger or absorption cycle. For small power outputs, suitable refrigerating equipment is the one which uses reciprocating compressors. The most common refrigerating plants are compression refrigerating plants. The only acceptable use of an absorption cycle, which is rather expensive, is when there is a source of heat at low-cost. Fans used in air conditioning equipments are mostly low- or middle-pressure centrifugal ones. The axial fans are very useful when high flow rate at small pressure is required. If there is a need for high flow rate and high pressure axial fans are combined in a series. An axial fan generates more noise than the centrifugal one when it comes to high pressure regime. Filters capture particulate contaminants which are entrained by fresh or recirculated air. The most common are cell and the roll band filters; the cell filters are preferred for low flow rates conditions. Filter media either synthetic or glass fibres made are selected for particular operating conditions according to the class of filtration. The air filter cells can be of many types: plates, pockets etc., and these are mounted to a filter frame and then placed into a filter chamber. The role band filters are employed for high air flow. Gaseous contaminants can be filtered via specially designed cell filters filled with activated carbon. Heat recovery equipment transfers heat between exhaust and fresh air. It makes the use of air conditioning and ventilation equipments more efficient and economical, as a large propor- 26 tion of heat generated or delivered to the building interior would otherwise be exhaust to the exterior without any use. Heat recovery removes the heat from the exhaust air back to the fresh air. Heat recovery equipments employ: - two flat-fin pipe heat exchangers plate heat exchangers heat pipes regenerative heat exchangers, such as heat wheels, enthalpy wheels heat pumps Two flat-fin pipe heat exchangers system consists of two parts (two single heat exchangers) interconnected by pipes where anti-freeze flows as a media. Exhaust air passes through one part, fresh air the other. The exhaust and fresh air are separated so they do not mix hence the distance between the exchangers can be quite large. The heat recovery efficiency can be expressed as η= t p − te (7.1) ti − te where t e , t p are temperatures of fresh air entering and leaving the heat exchanger, respectively, t i is the temperature of exhaust air before passing the exchanger. The efficiency usually takes values in the range from 40 % to 60 %. Plate heat exchangers are made of set of plates 3 mm to 6 mm apart. The flow channels (gaps) can have various surface treatments to enhance heat transfer and lower deposition of particles on the surface. Thus it can be hydraulically smooth, moulded, ribbed etc. The media pass the heat exchanger alternating the channels so fresh air is next to exhaust which is next to another fresh air channel etc. The efficiency achieved by the plate heat exchangers is between 50 % and 70 %. A heat pipe is a device made of finned pipes partially filled with a refrigerant. The most popular ones are the gravitational heat pipes of which example are depicted in Figure 7.2. Warm air while passing the bottom part of pipes heats up the refrigerant to cause its evaporation. The evaporated refrigerant rises to the upper part of the heat pipe where heat is transmitted to the passing by cold air. During this process, the refrigerant condenses and runs down. The efficiency of a heat pipe reaches values between 50 % and 60 %. Regenerative heat exchangers are such exchangers which heat transfer surface of is in contact with both warm and cold media in an alternating way. The warm and cold alternation is done by rotation or reversing. The main part of a rotation employing heat exchanger is a rotating wheel consisting of a large number of honey-comb channels approximately 3 mm in size (Fig. 7.3). Warm exhaust air passes the wheel through its lower part and cold fresh air through its upper part. Not only can heat be transferred by these exchangers but also moisture. If heat is the only one to be exchanged, the material used is aluminium. Should both heat and moisture be exchanged between the media, a material capable of water absorption has to be used. The regenerative heat exchangers are of the highest efficiency among all the exchangers used for heat recovery which can reach up to 90 %. 27 cool air warm air Figure 7.2 Schematic of a gravitational heat pipe fresh air t1 ‘ rotating wheel t 1‘‘ exhaust air t2 ‘ Figure 7.3 Schematic of a rotation wheel (type of a regeneration heat exchanger) A heat pump is a compression refrigeration plant where the active component is the condenser. Warm exhaust air passes the device through its evaporator causing the refrigerant to evaporate as it receives heat from the air. Compressor raises the refrigerant pressure and temperature thus heat can be transmitted to the fresh air at the condenser. The heat pump based system is very expensive from both investment and operation point of view. The most popular heat recovery systems are plate exchangers, two flat-fin heat exchangers, and regeneration rotary heat exchangers. 7.2 Air Washer Design An air washer is a type of a mixing exchanger where both heat and moisture are transmitted. An air washer sprays water into air in a spraying chamber thereby combined heat and mass transfer takes place between the water droplets and air. A wet-film humidifier is based 28 on dousing built-in plates or a drum; water runs down it creating a film on its surface where heat and mass transfer takes place. Design of an air washer is based on conservation of heat and mass for a humidifying chamber and a water circuit. Deduced from the stated assumptions, neglecting heat transfer between the chamber and its surroundings, and omitting a heat exchanger in the water circuit, a so called adiabatic operation of the washer is established with the following governing equation i 2− i 1 x 2− x 1 ≈0 (7.2) where i2 , i1 are enthalpies of moist air entering and leaving the air washer, respectively, x 2 , x1 are moisture contents of air entering and leaving the air washer. The equation suggests the change the air is undergoing while passing through the air washer is isenthalpic as illustrated in psychrometric chart in Figure 7.4. Figure 7.4 Humidification process as it takes place in an adiabatic air washer 8. AIR CONDITIONING SYSTEMS An air conditioning system can be of either central or a single unit type. A central air conditioning system consists of a central plant room and distribution system. There can be single units added to the system to provide supplementary treatment to air of particular rooms. Central air conditioning systems are mostly of unitary type. Each component of such a system is box-like unit that can be easily replaced or combined; the components being filter, mixing, heating, cooling, humidifying, fan, heat recovery units, or a sound damper. Single unit air conditioning equipment is compact box-like equipment which is used to control indoor air quality of a particular room. According to the media employed by an air conditioning system to meet the required comfort or technological conditions, the systems are divided into 29 - all-air systems (the only medium is air) air-and-water (combined) all-water systems refrigeration systems 8.1 All-Air Systems All-air systems that operate at air speed below 12 m·s-1 are called low pressure systems. The systems where air speed can reach up to 25 m·s-1 are called high pressure ones. The all-air systems can be further divided into single or dual (double) duct systems. Low pressure single duct system, as depicted in Figure 8.1, treats air in the same way for all conditioned rooms. The system is simple and cheap. They are not easy to control because only one sensor for the whole system is possible. High pressure single ducts systems differs from the low velocity single duct system in the need to use single duct boxes in the duct to which air terminal devices are mounted. Figure 8.1 Schematic of a low pressure central single duct air conditioning system. O1 – preheat, F – filter, CH – cooler, P – air washer, O2 – air heater, V1,2 - fans for air supply and exhaust High pressure dual duct systems. The ventilation air is treated in two ways by a central plant to produce cold and warm air for each duct (see Figure 8.2). Each room has its own thermostat which controls the proportions of cold and warm air supplied to the room through a mixing chamber so the required state is met. Its advantage over the previous systems is in possibility to control rooms’ conditions individually. On the other hand, the size of the system makes it expensive as spatial requirements are higher than for the previous systems. 8.2 Air-and-Water Systems Air-and-water (combined) systems are of high velocity type. The system allows for individual control of conditions for each served room. The central plant provides conditioned fresh air only which is referred as the primary air. The secondary air is entrained by primary air which is supplied at high velocity to an induction unit where it undergoes the final treatment (see Figure 8.3). Secondary air passes through a finned heat exchanger where heat is transmit30 ted to or out of it, and then mixes with the primary air. Water distribution system to the induction unit can be either two-pipe reversible or four-pipe system, the latter being of higher comfort but more expensive one. air conditioned space Figure 8.2 Schematic of a high pressure dual duct air conditioning system. 1 – warm air, 2 – cold air, 3 – mixing chamber, 4 – exhaust air air conditioned space Figure 8.3 Schematic of high pressure system with induction units. 1 – central plant, 2 – primary air duct, 3 – induction unit, 4 – heat exchanger, 5 – filter, 6 – input/output of heat transfer medium, 7 – exhaust air duct 8.3 All-Water Systems All-water systems make use of water as the heat transfer medium. Water distribution system is either two-pipe reversible or four-pipe, and it provides water to a fan-coil which comprises of filter, fan, air heater and cooler (see Figure 8.4). The system operates with recirculated air or fresh air provided usually via a hole in a building facade. The all-water system is 31 cheaper than all-air or air-and-water system because of two reasons. First, there is no need for ductwork. Second, cost of operation is less as only the units where there is demand can be in operation. The main drawback is, however, noisy operation. Despite the noise, the all-water systems are becoming more popular so they usually replace the air-and-water systems. Figure 8.4 Fan-coil unit. K – damper, F – filter, V – fan, O – air heater, Ch – air cooler, Č.V. - fresh air, O.V. - recirculated air 8.4 Refrigeration Systems Refrigeration (heat pump) systems are designed to control environment of a single room. A window-mounted air conditioner, mobile air conditioner and split system make use of refrigeration cycle. Windows mounted room air conditioner is a system that makes use of built-in refrigeration plant with an air cooled condenser, located out of the conditioned room. A certain proportion of fresh air can be used by the system, but the system mainly operates with recirculated air. It is designed primarily to be mounted in a window, or through a wall. See Figure 8.5 for the through-wall unit. Mobile air conditioner. The whole system which includes a refrigeration plant is fit in a box. The box is to be located within a conditioned room. The hot air the unit produces in its condenser is carried away by a flexible hose. The condenser is cooled by recirculated air from the room. Split system air conditioner consists of two units. One of the units houses a fan and an evaporator of a refrigeration plant; the unit is located in the conditioned room. Examples of such interior units are given in Figure 8.6. Exterior unit mounted to a facade, or located at the building’s roof, accommodates a condenser and a compressor (see Figure 8.7). The interior and exterior units are interconnected by refrigerant piping. When there is more than one interior unit (up to 40) connected to a single exterior unit, the system is called as multi-split system. Such a system is computer controlled, and heat pump regime is available for winter pe- 32 riod. There can be some units operating as cooling units and some as heaters at the same time. The system is becoming widespread in spite of the fact it does not provide any fresh air. Figure 8.5 Schematic of through a wall mounted room air conditioner. 1 – filter, 2, 3 – fan, 4 – compressor, 5 – evaporator, 6 – condenser, 7 – louver, 8 – damper, 9 – outlet, 10 – electric motor Figure 8.6 Interior units of a split air conditioner. (A) – wall unit, (B) – ceiling unit, (C) – panel unit 33 Figure 8.7 Exterior unit of a split air conditioner 9. SIZING OF AN AIR CONDITIONING SYSTEM Proper size of an air conditioning system is a key factor from hygienic and economic points of view. If the system is insufficient in its size, it cannot cover peak hours. On the other hand, the system having been over estimated is too costly to operate. While sizing of an air conditioning system, one should bear in mind - summer heat load and winter heat loss of the air conditioned space indoor air quality demands of the room (both for summer and winter) parameters of outdoor air (both for summer and winter) type work carried out in the space so contaminant sources can be determined The procedure of determining the summer heat load is standardised by ČSN 730548 [8]. Similarly, the procedure to determine the winter heat loss is stated in ČSN 060210 [9], or ČSN EN 12831 [10]. Parameters of indoor air (temperature and humidity) reflect the type of either desired comfort level in the room, or technological requirements as it is the case in industrial air conditioning. The design parameters (temperature and humidity of outdoor air) are considered with respect to local climatic conditions for summer. The winter design parameters can be considered either according to the local climate or the same as for winter heat load calculation for a particular country region, but the design outdoor temperature is lowered by 3 K. The procedure of sizing of an air conditioning system follows these steps: - select a system type to meet the objective - determine the minimum amount of fresh air needed; using the same procedure as if it was a ventilation system; the best choice is to take into account the amount of contaminant generated within the space and the hygienic limits of the contaminant 34 - draw the process air undergoes for both summer and winter extreme into a psychrometric chart; size the system components For proper sizing of air conditioning equipment, the summer operation is more important. After having been sized for summer operation, the amount of fresh air in the system is kept constant so it uses the same amount for both winter and summer. 9.1 Illustration of Processes Air Undergoes in a Psychrometric Chart Summer operation of an air conditioning system. A low velocity single duct system, as depicted in Figure 8.1, is considered. The following design parameters apply: outdoor air temperature and humidity t e ,ϕ e indoor air temperature and humidity ti ,ϕi total heat load of the air conditioned space Q& i amount of moisture generated in the space m& wi minimum mass flow rate of fresh air m& ve Summer operation of the system is quite simple. The recirculated air leaving the space at state I mixes with outdoor air at state E (see Figure 9.1). The mixture, assigned S here, is then processed by an air cooler to the state P and enters the space. Heat load Q& i , and moisture generated within the space, m& wi cause the air to undergo change, P→I, in its state. The two can be expressed by Q& i = m& v (ii − i p ) (9.1) m& wi = m& v (xi − x p ) (9.2) The room ratio line, direction of the change air exercises between P and I states, is given by δi = ii − i p Q& i = m& wi xi − x p (9.3) where δi is humidification line, and can be found in the circumferential scale of a psychrometric chart. The temperature of supplied air, t p , is determined based on recommended operational temperature difference ∆t prac = ti – tp = between 6 K and 10 K. Knowing the temperature of supplied air, the intersection of tp isotherm with the room ratio line, humidification line drawn from the point I, can be drawn in the psychrometric chart so to give all the parameters of supplied air. The total mass flow rate of air supplied to the air conditioned space is determined from equation (9.1) 35 m& v = Q& i ii − i p (9.4) Figure 9.1 Summer operation of an air conditioning system in psychrometric chart Mass flow rate of recirculated air is then m& vc = m& v − m& ve (9.5) Based on moisture conservation, the following must apply m& ve x e + m& vc xi = (m& ve + m& vc )x s (9.6) thus humidity ratio of recirculated and fresh air xs can be calculated. The actual mixture state, S, is determined by the intersection of join line between points E and I, and humidity ratio line xs . By extending the straight line between points S and P to the saturation curve ( ϕ = 1 ), an intersection, R, can be found yielding the apparatus dew point temperature of the cooling coil. The apparatus temperature decides of the type of the air cooler of which cooling capacity is calculated using the following formula Q& ch = m& v (is − i p ) (9.7) Winter operation of an air conditioning system. The following design parameters apply: outdoor air temperature and humidity te , ϕe indoor air temperature and humidity t i , ϕi 36 heat loss of the air conditioned space Q& i amount of moisture generated in the space m& wi total mass flow rate of supplied air m& v Winter operation of the air conditioning system (see Figure 9.2) treats outdoor air, E, by an air heater (preheater) at first to protect the equipment from water condensation and freezing. The air leaving the heater at state K is mixed with the recirculated air of state I. The mixture, S, is then humidified by an adiabatic air washer to reach state O, which is after another heating supplied to the conditioned space at state P. Figure 9.2 Winter operation of an air conditioning system in psychrometric chart Based on energy conservation (see (9.1)), enthalpy of supplied air is determined i p = ii + Q& z m& v (9.8) Room ratio line is determined by δi = Q& z m& wi (9.9) Knowing the previous two quantities, parameters of the supplied air can be found in the psychrometric chart at point P being the intersection of isoenthalpy line, i p , and room ratio line, δi . The temperature of air leaving the preheater is optional, say 5 °C, giving the state K. The state of air after having been mixed, S, is determined as the intersection of mixture line between points I and K, and humidity ratio line, x s which yields from equation (9.6). The humidification process that air undergoes at air washer is considered to be isenthalpic so the 37 state of air after humidification is at intersection of isenthalpic line is and humidity ratio xp. Efficiency of an adiabatic air washer reads η= xo − xs xoid − xs (9.10) Heating capacities of preheater and air heater after humidification follows Q& o1 = m& ve (ik − ie ) (9.11) Q& o2 = m& v (i p − io ) (9.12) 10. HEATING SYSTEMS. LOW TEMPERATURE HOT WATER HEATING 10.1 Heating Systems Heat for a building’s heating system can be produced locally or centrally. The local heating system employs a heat source which is situated directly in the heated space. The central heating system makes use of a heat source located outside the heated space at a central plant room. Heat is then conveyed to the heated space with a heat transfer media (water, steam, air). According to the media used we distinguish hot water (low, or medium temperature), steam (low or high pressure), and warm air systems. Among all the systems that provide thermal comfort, low temperature hot water ones (temperature below 95 ˚C) are the most common (abbreviated LTHW). It is applied to the most of residential buildings and also where hygienic limits do not allow for high temperature of radiators. A special type of hot water heating system is a large surface system which makes use of hot water at up to 60 ˚C; the heating elements being one of the heated space walls. Low pressure steam system operates at gauge pressure of up to 50 kPa. It is mainly applied to industrial buildings because of its flexibility at intermittent operation of the heating system. High pressure steam or medium temperature hot water systems (with temperature over 110 °C) are used in industry only. High surface temperature is tolerated from a point of view of hygiene, but it is less costly to operate than the low temperature systems. Warm air heating provide thermal comfort to large factory halls and other spacious rooms. Air is heated at a central plant room or at wall mounted units by hot water or steam. Such spaces can be also heated by radiant heating – suspended radiant panels, or local dark or light gas radiators. The media used by the radiant panels is hot water or steam at temperature of up to 160 ˚C. Radiant heating competes favourably with the warm air heating when cost of operation is considered but it is capital-expensive. 10.2 Low Temperature Hot Water Heating It usually operates between 90 ˚C and 70 ˚C (return pipe). The maximum temperature may vary during heating period with respect to outdoor temperature. Nowadays, operational temperatures between 70 ˚C and 50 ˚C are used, as a consequence of improved insulation of a building envelope. There are even buildings of high insulation standard so the temperatures 38 may lower up to 55 ˚C and 45 ˚C. The lower the heating system operational temperature is the higher thermal comfort in the heated space. There can be further distinguishing aspect of the heating systems which reflects the driving force of a hydronic circuit. Thus forced and gravity systems are distinguishable. The gravity system is based on density difference between the supplied and return water. The forced system obviously employs a pump to run water in the system. Another way of the heating system classification is by piping scheme: one-pipe or two-pipe systems. By the use of two-pipe system, supply and return pipes serve the radiator. The one-pipe system makes use single pipe to supply the radiators with water thus the supply pipe gradually turns into a return pipe. The heating system can be further classified again by piping scheme but now it is the distribution system layout which is of interest. If the main horizontal supply pipe is located at the buildings upper part, the system is up-feed; down-feed is the opposite. The last concern is about supply and return pipes orientation so there are horizontal and vertical systems. 10.2.1 Gravity Low Temperature Hot Water Heating System The system is intended for buildings of small ground plan. It is almost not used in today’s practice. The system is usually of two-pipe, with either up-feed or down-feed. The former is a little faster to start due to higher pressure gain than as it is the case with the down-feed system. Yet the up-feed system is more expensive. An example of gravity two-pipe vertical upfeed heating system is given in Figure 10.1. The same but down-feed system is shown in Figure 10.2 (omitting the pump). Figure 10.1 Gravity two-pipe vertical up-feed heating system. K – boiler, SP – vertical supply pipe, HP – horizontal supply pipe, T – radiator, HV – horizontal return pipe, EN – expansion tank, 1, 2, 3 – vertical branches 10.2.2 Forced Low Temperature Hot Water Heating System It is advantageous to the gravity systems due to: - smaller pipe sizes thus lower investment - easier pipe network installation 39 - possibility to connect radiators levelled lower that the boiler - faster start and easier control On the other hand, the drawbacks be grid dependency (need for electricity), greater operation cost, and pump noisiness. Figure 10.2 Forced two-pipe vertical down-feed heating system. K – boiler, SP – vertical supply pipe, HP – horizontal supply pipe, T – radiator, HV – horizontal return pipe, EN – expansion tank, PP, PV – safety pipes (supply and return), 1, 2, 3 – vertical branches, Č – pump, OV – venting valve, OP – venting pipe There is no significant difference between forced and gravity system in this case. The system can be either up-feed or down-feed, two-pipe or one-pipe one. The pump is inserted to the system in return pipe, but supply pipe location is also possible. An example of such a system with the pump in the return pipe is shown in Figure 10.2. The entire system is connected to the expansion tank by a safety pipe at both return and supply pipes. Pipeline blow-off can be performed manually by venting valves which are mounted to the topmost radiators in the system (branch 1), or automatically via a venting pipe (branches 2 and 3). A horizontal system is suitable especially when there is a need to minimise count of vertical branches of the system, or if there is requirement to connect all the radiators of a served flat to a common vertical branch. The system can be either gravity driven or forced. A great variety can be achieved by the use of forced one-pipe system. The distribution system can be either vertical or horizontal, the radiators can be connected to the net directly (continuously) or by-passed. Vertical one-pipe up-feed system (see Figure 10.3) is appropriate to buildings which has a loft. Flow rate through a radiator is controlled via a three-way valve inserted at a junction before the radiator. The radiators further down stream the boiler are fed by water of gradually decreasing temperature; this has to be reflected in their sizing. The down-feed system is suitable for flat roof buildings. 40 Figure 10.3 Vertical one-pipe up-feed system with radiators by-passed Figure 10.4 shows different ways of radiators connection to the distribution system. The continuous horizontal system does not allow for flow rate through an individual radiator to be controlled. When the radiator connection is by-passed, the flow rate through it can be controlled. Control valve or damper in the short circuit pipe is also very popular. Four-way valve and single- or two-point connection belong to the advanced solutions. Figure 10.4 Horizontal one-pipe system. Radiator connections: a) direct (continuous), b) by-passed, c) short circuit control valve, d) short circuit control damper, e) four-way valve and single-point connection, f) four-way valve and two-point connection 41 10.2.3 Heating Bodies There are many types of heating bodies such as panel radiators, sectional radiators, convectors, pipe radiators, baseboard units. Panel radiators are the most commonly used terminal units nowadays. They are made of steel sheets welded together. The sheets can be of smooth or wavy surface. There can be a single panel radiator, but double or triple panel ones are also used; the panel radiator can employ extended surfaces as well (see Figure 10.5). The single panel radiators with no extended surfaces are space-saving, easy to clean, and a large portion of heat is transmitted to the room by radiation of their front surface. The radiant heat contributes to uniform heating of the space. By the use of extended surfaces made of thin metal sheet bellows spot welded to the radiator’s body, increased heat output can be achieved. However, convective heat transfer is enhanced this way and cleanability worsens. Figure 10.5 Panel steel radiators’ design A sectional radiator is generally considered cast-iron elements. The elements make it possible to mount them together so a radiator of any size can be produced. Heat is transmitted to the space both by convection and radiation (33 % of total heat output). Pipe radiators are just bare single pipes or pipe registers. They are mainly applied to heating of stairways, sanitary facilities, or other auxiliary rooms. 42 Convectors consist of pipes (bond with fins) and enclosure which enhances the buoyancy effect so the unit performs as a shaft (see Figure 10.6). Most of the heat is transmitted by convection. A convector is a device of small heat capacity thereby it is flexible but it throws up dust due to increased air circulation within the heated space. Figure 10.6 Convector Base board units are installed at the wall base, and made of cast iron with a significant portion of the front face exposed the heated space, or with a finned tube element in a sheet metal enclosure as seen in Figure 10.7. Figure 10.7 Types of baseboard units 11. LARGE SURFACE HEATING. MEDIUM TEMPERATURE HOT WATER AND STEAM SYSTEMS 11.1 Large Surface Heating Large surface heating is a type of low temperature hot water heating where one of the walls of the room acts as the heat transfer surface. Hence, according to the heating surface there can be ceiling, floor or wall heating. Surface temperature is low; for ceiling heating it is in between 40 °C and 45 °C, for floor heating between 25 °C and 30 °C, and for wall heating it can 43 be between 55 °C and 60 °C. The proportion of heat transmitted by radiation is the highest for the ceiling heating and can reach up to 70 %. The other two have the same maxima which may reach 50 %. It has been just recently the large surface heating become more popular especially in connection with the buildings which comply or even exceed the insulation standards. The ceiling type of heating system can be converted to a cooling one to cool down the rooms in summer. The large surface heating systems make use of a pump to circulate heating medium. Low temperature they employ is achieved by mixing hot water that leaves a boiler with return water with the use of three-way valve. A large surface heating system can be combined with a hot water system and radiators. If the system is to operate correctly from its control point of view (as the thermal mass of both of the systems differs dramatically), the system layout is recommended to resemble that of the Figure 11.1. Figure 11.1 Combination of large surface and radiator heating system The heating surfaces used to be made as an embedded steel piping (coils) in ceilings, floors, and walls of concrete. The steel piping has been replaced by polyethylene one, and the piping is no longer a part of part of a concrete slab but it is mounted to a polystyrene panels attached to a wall nowadays. The piping is fixed to the polystyrene panels with aluminium plates which provide for uniform heat transfer to take place. The system is covered with cement screed. Thermal characteristics of the heated space. Ceiling panel heating systems transmit minor portion of heat by convection (to air) and major part of heat by radiating it to the surfaces that surround the heated space. The surface temperature is thereby somewhat greater than that of air in the space. This feature is preferable from a physiology point of view than the reverse. The warm air heated by the ceiling does not proceed to the space thus no air flow is induced consequently there is no dust thrown up. Vertical air speed distribution is homogene44 ous. If floor heating is considered it creates the most uniform vertical temperature distribution over the space (temperature around the head lower to temperature around the feet difference 2 K). When the ceiling heating is used the same temperature difference is achieved but opposite, i.e. the head is in warmer conditions. Convectional heating heats up air primarily; the air conveys heat to the surfaces of the space hence it is warmer the surfaces. The warm air rises above the radiator and flows to the room ceiling causing cool air to flow towards the radiator along the floor. The flow generated results in quite large temperature difference between floor and ceiling air, e.g. if the temperature set point is at 22 °C, the temperature at floor level is about 19 °C and at the ceiling about 25 °C. 11.2 Medium Temperature Hot Water Systems Medium temperature hot water systems where water temperature exceeds 110 °C are especially useful in cases which do not require high thermal comfort. Thus it is common to apply it in industrial buildings. The effect of high temperature of heating water is in small heating bodies used the same heat output is maintained, piping of smaller size can be used so cost of investment lowered. The system itself resembles the traditional forced low temperature hot water system. The pressure in the system corresponds to the water temperature. The heating bodies associated with the medium temperature system are pipe radiators, finned tubes, or convectors. 11.3 Steam Heating Two types are distinguished: low and high pressure. Low pressure steam heating is used similarly to the medium temperature hot water heating in industry. It is more flexible than the latter. The low pressure steam system does not exceed pressure of 0.15 MPa. The steam is conveyed from the boiler to the radiator by steam piping from where, after turning in water, it returns by condensate piping back to the boiler. The amount of steam supplied to the radiator has to be at such quantity the radiator is fully filled with steam and it is possible for steam to condensate over the entire radiator. If too low amount of steam is supplied, the radiator is filled in its upper part only, and the rest of the radiator fills up with air driven from the condensate piping resulting in heat output deterioration. In this way, low pressure steam system can be controlled directly at the radiators side. Too much of steam supplied to the radiator could cause the condensate piping to be flooded with condensed water so gate valve steam trap is inserted at the radiator’s outlet. Distribution systems can be divided according to the main supply piping into up- and downfeed. The systems can be further subdivided into dry and wet (flooded) condensate piping ones. An example of down-feed dry condensate piping system is depicted in Figure 11.2. Main supply piping is drawn at the building basement ceiling with a piping slope of 5 per thousand stream-wise. An offset bend with a dewatering loop are required when the continuous slope is broken (usually at vertical pipes joints). The condensate piping is connected to free atmosphere via a deaeration pipe. Due to the possibility of exceeding the system maximum pressure, security equipment is mounted to the system. The simplest way to return condensate to the boiler is by gravity drainage. The gravity drainage is only applicable provided 45 the radiators are levelled sufficiently high above the water level in the condensate piping at the ground floor. If this is not the case, the condensate has to be pumped to the boiler. Figure 11.2 Low pressure down-feed steam system with dry condensate piping. K – boiler, PP – main supply piping, KP – collecting condensate piping, KS – dewatering loop, T – radiator, OK – steam trap, O – deaeration pipe, ZZ – security equipment. The main advantages of the low pressure steam system over low temperature hot water system are in its flexibility due to its small thermal mass, reasonable cost of equipment, and the possibility to extend the system arbitrarily. On the other hand, it is very difficult to control centrally, quite high surface temperature of the radiators, fast corroding process of the condensate piping in particular. High pressure steam systems, i.e. systems where steam pressure ranges between 0.15 MPa and 0.3 MPa, are used seldom. It is mainly the industrial processes where steam is required for the technology. The up-feed distribution system is common in the design. Its very difficult control is the system’s main drawback apart from the high surface temperature of the radiators. The control is actually achieved by switching the radiators off. Since it has been replaced by medium temperature hot water systems, the only heating bodies employed by the system are the pipe radiators. 12. WARM AIR AND RADIANT HEATING. HEAT LOSS CALCULUS 12.1 Warm Air Heating Warm air heating supplies warm air to the ventilated space. The warm air is provided either at a central plant room – central warm air heating, or by warm air units located in the ventilated space. The latter does not need ductwork; piping is needed for heat transfer medium (water, steam) or fuel (gas) only. Warm air heating is very flexible, and can be combined with ventilation. It is not as much demanding as the other central heating systems from investment point of view. However it 46 throws up dust, and does not provide radiant heat. It also creates unfavourable vertical temperature distribution with the highest temperatures at the ceiling level. The system is mainly applied to factory buildings and other tall or spacious halls, yet, it has started to appear as a heating system of single-family houses. Spacious and tall halls are usually fitted with local warm air units (unit heaters) which make use of steam or medium temperature hot water system to feed the air heater. The air heater is of a finned tube type. The other unit components are the fan and dampers; there can be even a component for fresh and recirculated air mixing (see Figure 12.1). The units can be classified according to the place they are mounted to. Thus wall, ceiling, and window type of the heater unit. The wall type units are the most widespread; the height they are usually suspended is between 3 m and 4 m above the floor level. The air speed at the unit outlet is required to be high enough for throw of at least 25 m to be reached. Warm air temperature may come up to 70 ˚C. Should the large space be heated properly, a number of units equally positioned over the space are employed. The units may also burn liquid or gas fuel. Figure 12.1 Wall warm air unit. 1 – air heater, 2 – fan, 3 – damper Warm air heating of single-family houses runs mostly with directly fired warm air heaters, usually located at the house entry. The warm air is distributed to the heated rooms by ductwork. Air then leaves the rooming units back to the entry whilst air of kitchen and facilities is discharged to the outside. 12.2 Radiant Heating Not only is the warm air heating used to heat spacious or halls of large height, but also radiant heating employing suspended radiant panels, or directly fired dark or light heaters. 12.2.1 Suspended Radiant Panel Heating Suspended radiant panel heating is made of metal plates (panels) heated by hot water or steam which flows in a grid (see Figure 12.2). The panels are made of a steel or aluminium 47 sheet; the panel width being 500 mm to 1000 mm. The height of panels’ suspension above the floor level is rather large; 5 m being the minimum, usually between 8 m and 12 m. Heat is transmitted down the zone of peoples’ occupation mostly by radiation (75 % to 85 %). Effect of heating is accomplished via heating up surface temperatures of the floor and technological equipment. Figure 12.2 Hall heated by suspended radiant panels Radiant heating, such as suspended radiant panel heating, achieves the required operative temperature by virtue of increase in mean radiant temperature. Therefore the air temperature within the heated space can be lower by 3 K or 8 K than as it is the case with the warm air heating. The mentioned feature can significantly reduce heat loss by ventilation. Total heat consumption of the radiant panel based system may be less by 20 % to 30 % than for the warm air system. Among the other advantages of the suspended radiant panel heating, is the fact there is no fluid flow induced by the heating system hence no dust is thrown up. Moreover the system is noiseless. The main disadvantage is in high cost of investment which is 50 % higher than that of the warm air heating system. The payback period of the radiant panel heating system is between 2 and 4 years in most cases. 12.2.2 Directly Fired Gas Radiant Heating Directly fired gas radiant heating is one of the most efficient ways of heating. By direct fuel firing inside a heated space heat loss of a heat source is minimised. The heat loss of the heat source may reach 15 % for hot water, and 20 % for steam systems. Additionally, the heat loss associated with the distribution system, 5 % for water and 10 % for steam systems, is also out of question. When comparison is made between suspended radiant panels and the directly fired gas radiant heating, the latter is up to 30 % profitable. Regarding the system classification, light and dark infra-red heaters are distinguished. Light directly fired gas infra-red heaters. The source of radiation is a ceramic porous plate which temperature of rises during combustion of the gas to temperatures between 750 ˚C and 900 ˚C. Air needed for gas combustion is taken in by an injector, then it is mixed with gas and the mixture proceeds to chamber of infra-red heater. One of the chamber’s walls is made of the ceramic porous plate through which the mixture passes to burn at the plate surface (see Figure 12.3). The plate surface reaches the red glow state. Very tall halls can be served by the light infra-red heaters (up to 20 m). If a low ceiling room is to be heated by such heaters, there 48 has to be a number of small heat output heaters used at height of 4 m or 5 m above the floor level. Rooms that are low in their height and narrow in their width may accommodate wall suspended heaters at 2.5 m height above the floor and oblique orientated. The heat output can be controlled by throttling the gas inlet. The heaters are also suitable for heating of particular zones of a room or improvement of thermal comfort at open space conditions. Figure 1 Schematic of a light infra-red heater. 1 – porous ceramic plate, 2 – chamber, 3 – injector, 4 – gas in, 5 – air in Dark directly fired gas infra-red heaters. An example of such heaters, the indirect tubetype heater, is given in Figure 12.4. The tubes are quite large in diameter (180 mm to 600 mm) and can be placed side-by-side in sets that count between 2 and 6 tubes. The tubes surface temperature may vary from 150 ˚C to 350 ˚C. Air which is used by the heater as the heat transfer medium is warmed up by a direct gas burner. The tubes can be also heated by a mixture of air and flue gases. Air recirculation is achieved by placing a fan into the tube system. Figure 12.4 Indirect tube-type dark infra-red heater The tube-heaters can operate as a closed or vacuum tubes. The system then consists of an air intake which supplies air to the burner’s combustion chamber, sealed radiant tube where 49 the gas fires and a vacuum fan to exhaust the flue gases out of the heated tube as can be seen in Figure 12.5. Figure 12.5 Dark gas infra-red heater with closed burner circuit. (A) – U-type radiant tube, (B) – I-type radiant tube 12.3 Heat Loss Calculus There are two different standards for heat loss calculus valid in the Czech Republic. Both of them can be applied to sizing of radiators and boilers as well. The two standards are: ČSN 06 0210 being in force since 1995 [9], and ČSN EN 12831 [10] being in force since 2005. Despite difference in methodology between them, they are both based on calculation of transmission and ventilation heat losses. Should the calculus follow the first one, ČSN 06 0210, the changes stated in ČSN 73 05540-4 [11] standards, in force since June 2005, have to be taken into account as the former refers to them. 13. NOISE AND VIBRATIONS. NOISE EFFECTS. NOISE PROTECTION 13.1 Noise and Vibrations Noise is an undesirable civilisation product which is becoming an important hygienic factor that affects health of a human being. Humans are often and in the long term exposed to noise level which they are not adapted; humans do not have any means of natural protection against noise. Noise is still underestimated as most of its negative effects do not show themselves either by pain at the time of exposure or functional failure. Noise is any kind of sound that one considers unpleasant, disturbing, or harmfully. Sound is one’s sensation of sound (acoustic) waves as perceived by the sense of hearing. Sound waves are mechanical vibrations of an elastic gaseous, liquid, or solid medium. The gaseous and liquid media allow for longitudinal wave motion only; solids for transverse wave motion only. A 50 sound wave causes a human auditory organ to be sensed if it falls into a frequency range between 16 Hz and 18 000 Hz. Any sound of frequency is above the 18 000 Hz is called ultrasound, below the range it is infrasound. Sound is, thus, a mechanical disturbance of an elastic medium described as medium particles oscillation around their equilibrium locations causing local regions of compression and rarefaction. The pressure oscillations are perceived by an organ of hearing whilst the background pressure (atmospheric) which is orders of magnitude higher than the oscillations is not sensed. Vibrations are generated in the same way as sound is. Human being perceives vibrations within a frequency range 0.2 Hz and 16 000 Hz. Fundamental acoustic quantities are those used to be describing sound waves. It is above all: Sound (acoustic) energy E [J] is energy transmitted by a sound source into the surroundings. Sound (acoustic) power W [W] is the acoustic energy radiated by a sound source per unit time. A noise source may usually radiate between 1·10-8 W (whisper) to 104 W (ramjet). Sound (acoustic) pressure p [Pa] is the easiest to measure out of the acoustic quantities. It is the alternating component of pressure as a result of sound, which causes pressure to oscillate about barometric pressure. For a practical use, the effective pressure is employed given by equation p = p max 2 . The smallest audible acoustic pressure is called acoustic threshold (reference acoustic pressure) and takes value of p0 = 2·10-5 Pa. The highest bearable, not causing pain, acoustic pressure is about 60 Pa. Sound intensity I [W·m-2] is defined as acoustic power per unit area normal to the direction sound travels. Human auditory organ can sense acoustic intensity ranging from its threshold I0 = 10-12 W·m-2 up to 10 W·m-2. Levels. Human sense of hearing is capable of perceiving acoustic pressure and intensity within wide ranges. It has been found out the sound perception does not gain linearly as the acoustic pressure or intensity vary but the change resembles that of logarithm. Therefore a logarithmic (dB) levelling was accepted. The sound intensity level, LI or SIL, is defined as L I = 10 log I [dB] I0 (13.1) and sound pressure level, L or SPL, as L= 20 log p [dB] p0 (13.2) Vibrations can be assessed via vibration acceleration level given by 51 L a = 20 log a [dB] a0 (13.3) where a0 = 10-6 m·s-2 and a is the effective acceleration in m·s-2 at location of interest. There can be three levels assigned to sound; the levels reflecting the frequency range they apply to. The total level refers to the whole frequency band of sound concerned. The octave band levels refer to the each particular octave band of sound. The last one is the third octave band level and it shows the level for each third of the octave band. Sound pressure levels A, B, C. The effort aimed to sense sound as close to the human hearing as possible introduced frequency dependent weights (filters). The weights adjust frequency response of sound level meter to be in accord with that of a human being, i.e. suppress sound level in the frequency range bellow 500 Hz and above 8000 Hz. The suppression levels are depicted in Figure 13.1. Weighted sound pressure levels are assigned LA, LB, LC with the unit being dB. Figure 13.1 Suppression characteristic of filters A, B, C Permissible exposure limits of noise and vibrations. Regulations 502/2000 Sb. [14] and 88/2004 Sb. [15] concerned with health protection against adverse reactions to noise and vibrations, specify maximum permissible noise and vibration levels - in working and out of work environments in residential buildings in civic amenities buildings in open space Permissible exposure limits of noise are expressed by means of maximum permissible equivalent sound pressure level A, LAeq, in dB and maximum permissible equivalent sound pressure level A with time characteristic I, LAIeq, in dB; the latter being used for impulse noise assessment. Permissible exposure limits of vibrations consider mainly weighted vibration acceleration level for third octave bands, Lat, in dB and weighted effective vibration acceleration (rms value) in m·s-2. 52 13.2 Physiological Effects of Noise A human being perceives noise by its organ of hearing, partially by its bones and body surface. The effects of noise on a human organism can be of two types: - specific – caused by direct impact of noise on the organ of hearing - non-specific – effects not connected with hearing Specific health damage by noise is exerted via so called professional hearing impairment which may proceed to functional deafness. It develops when one is regularly exposed to high noise levels. The damage maybe temporary at early stage of exposure but it turns into cloth ears or deafness if the exposure continues. Hearing deficiency cannot be cured. Non-specific effects of noise result in changes in one’s psyche or neural system. Noise at level about 50 dB affects creative, conceptual or managing works in a negative way; it decreases accuracy, lowers concentration, combinatory capabilities. When 65 dB is exceeded, it affects autonomous (vegetative) system, which governs cardiovascular, respiratory, digestive, hormonal, thermoregulatory and other subsystems. It results in hypertension, gastric ulcer, bladder stone, diabetes, aggressiveness, dizziness etc. Further aggravation of neuro-vegetative system has been collocated when there was multiple noise and vibration load. There is no adaptability to noisy surroundings from a physiological point of view. 13.3 Noise Sources and Propagation Noise sources can be equipment and objects, or limited flow regions where acoustic energy is generated and is radiated by acoustic waves into its surroundings. Acoustic waves propagate from the source directly by medium which surrounds the source (mostly air), or by joins to the structure. When the latter is the case, the structure conducts sound by vibrations eventually radiated into air at some part of the structure. Thus noise can be often generated quite far away from the actual noise source. We are mostly concerned with noise propagation by air as it is the medium through which noise enters the human organ of hearing. Acoustic oscillations travelling a space with no obstacles to it create so called free field where there are waves coming directly from the source. The direct waves travelling across an enclosed space are reflected by the space walls causing the acoustic field to be a combination of direct and reflected waves. The acoustic energy when radiated in the free field gradually dissipates with distance from the source. On the other hand, acoustic energy spreads in all directions with the same intensity after reaching reverberant distance within an enclosed space. The diffuse field is such field where there is the same acoustic energy within the space. 13.4 Means of Noise Protection Means of noise protection depend on whether the field one is in is considered to be the free field (direct waves) or reverberant (reflected waves). For the free field, i.e. close to the source of noise, there are the following means applicable: - decreasing of acoustic power output of the source (machine or equipment) - locating of the noisy sources, or the people who operate them into acoustically isolated rooms 53 Operative actions decreasing noise generated by a source follows: - machine design modification to decrease noise production (e.g. use of materials of increased internal noise absorption, minimise mechanical clearance, suitable grease selection, balance rotating parts, reduce possibility to generate turbulence and/or siren effect) - use of casing and baffles. There may be cases when it is impossible to reduce the noise generation by the source. In such cases, a casing is used to house the source or the source is located behind a shielding baffle. The baffle is made of a thin metal sheet. For even better noise damping, the baffle or the casing is covered with a sound absorptive material - use of noise-silencers. It mainly limits aerodynamic noise generation. Reflection-type noise-silencers are applied to pulsation flow in small diameter pipes. Large flow rate machines such as fans are provided with an absorption-type noise-silencer - use vibration insulators to minimise vibration spreading by a structure. These are applied to anchor a machine to its bed slab, to mount or join ducts, to insulate particular structural components etc. - use materials reducing noise radiation from machine or equipment surface, e.g. sound insulation of compressed air or steam piping, thin sheets protection by an anti-vibration paint or three layer sandwich construction - change technology or operation sequence. The technology itself can be a distinctive noise source like it is with riveting, minting, shedding. The noise situation may be improved by change of technology if possible. Locating of the noise sources, or the people who operate them into acoustically isolated rooms. Such rooms have their walls constructed of three layers where the one in the middle is made of a sound insulation. Doors follow the same principle. Windows are triple glazed. The room has to be totally enclosed. For the reverberant field, it is mainly wall facing by sound absorption materials, or suspension of sound absorption bodies in the space. Porous materials are the basic types of sound absorption materials. These materials are applied chiefly as facing and absorb mid- and high frequency sound. Foam rubber (spongy material) and glass or mineral wool (fibrous material) are given as examples. Another group of sound absorbing materials are that which makes use of oscillation membranes – resonance elements. These can cover wide range of frequencies. The absorption materials are applied not only to lower the noise level within the space where noise source is located but also where it penetrates from outdoor. Sound absorption bodies are acoustic bodies like plates, wedge blocks or cones, suspended from 0.5m to 1 m above the noise source. 54 LITERATURE [1] ASHREAE HANDBOOK 1996. HVAC Systems and Equipment. SI Edition [2] Etheridge, D. W., and Sandberg, M.: Building Ventilation. Theory and Measurement. John Wiley and Sons. Chichester 1996 [3] Fan and Ventilation. A Practical Guide. Roles and associates 2005 [4] Goodfellow, H., and Tähti, E.: Industrial Ventilation. Design Guidebook. Academic Press. San Diego 2001 [5] Kreider, J. F., and Rabl, A.: Heating and Cooling of Buildings. Design for Efficiency. McGraw-Hill, Inc. New York 1994 [6] McQuiston, F. C., and Parker , J. D.: Heating, Ventilating, and Air Conditioning. John Wiley and Sons. New York 1994 [7] ČSN EN ISO 7730. Moderate thermal environments – Determination of the PMV and PPD indices and specification of the conditions for thermal comfort [8] ČSN 730548. Calculation of cooling load of air conditioning spaces [9] ČSN 060210. Calculation of heat losses in building with central heating [10] ČSN EN 12831. Heating systems in buildings – Method for calculation of the design heat load [11] ČSN 730540-4. Thermal protection of buildings [12] Decree of the government the Czech Republic no. 178/2001 Sb. – conditions for health protection of employees while at work [13] Decree of the government the Czech Republic no. 523/2002 Sb. – change to the decree no. 178/2001 Sb. [14] Decree of the government the Czech Republic no. 502/2000 Sb. – health protection against noxious effects of noise and vibrations [15] Decree of the government the Czech Republic no. 88/2004Sb. - change to the decree no. 502/2000Sb. 55 SOLUTION TO SELECTED EXAMPLES Example 1 A meeting room is to be ventilated continuously so the CO2 concentration does not exceed permissible limit of 0.1 % in volume. There are 1000 persons there in the room, each one exhaling 7 l/min of air with CO2 concentration of 4 %. CO2 concentration in the supply air is 0.035 %. Determine the required air flow rate of supplied air to meet the limit. The ventilation air flow rate calculus is based on equation (4.4) of the course syllabi which sounds, it is V&p = S& k2 − k p (1) where the nominator stands for the amount of contaminant generated by the source: S& = 1000 × 7 × 0.04 = 280 l/min = 16.8 m 3 /h and V&p = 16.8 = 25850 m 3 /h 0.01(0.1 − 0.035) Example 2 Consider the above mentioned example replacing the CO2 as the contaminant by water vapour. The water vapour is introduced to the room by human respiration. Assume the state of the exhaled air at 36.5 ˚C and 100 % of relative humidity. The air supplied to the room is conditioned to 35 ˚C and 20 % of relative humidity. The room is to be kept at 18 ˚C and 60 % of relative humidity. Determine the air flow rate of supplied air to meet the requirement. Because the room is to be ventilated in the same way is it was with the previous case, the equation (1) applies again. The contaminant concentration is represented by the water vapour density now. Thus the equation of state ρv = pv rvTv (2) yields the following saturated water vapour densities: ρ vs ,exh = pvs ,exh 6106 = = 0.04273 kg/m 3 rvs ,exhTvs ,exh 461.5(273.15 + 36.5) 56 ρ vs ,sup = ρ vs ,req = pvs ,sup = 5623 = 0.03954 kg/m 3 461.5(273.15 + 35) = 2065 = 0.01537 kg/m3 461.5(273.15 + 18) rvs ,supTvs ,req pvs ,req rvs ,reqTvs ,req The actual water vapour densities are then obtained based on relative humidity definition: ρ v = ϕ ρ vs (3) ρ v ,exh = ϕ exh ρ vs ,exh = 1.00 × 0.04273 = 0.04273 kg/m 3 ρ v ,sup = ϕ sup ρ vs ,sup = 0.20 × 0.03954 = 0.007908 kg/m 3 ρ v ,req = ϕ req ρ vs ,req = 0.60 × 0.01537 = 0.009221 kg/m 3 The contaminant production rate is: S& = V&exh ρ v ,exh (4) 60 S& = 1000 × 7 × 0.04273 = 17.95 kg/h 1000 Substituting the above to the equation (1) yields the required flow rate of ventilation air: V&p = 17.95 S& S& = = = 13700 m 3 /h k 2 − k p ρ v ,req − ρ v ,sup 0.009221 − 0.007908 Example 3 Dry saturated steam at 100 ºC is injected at rate of 0.0126×10-4 kg/s into a moist air stream moving at rate of 1.26×10-4 kg/s of dry air, initially at a state of 27 ˚C dry-bulb, 11.4 ˚C wetbulb and 101.325 kPa. Calculate the leaving state of the air stream. By calculation or from psychrometric tables or a chart: h1 = 31.81 kJ/kg da x1 = 0.001823 kg/kg da From steam tables: hv = 2675.8 kJ/kg 57 Energy and mass balance, m& v hv = m& a (h2 − h1 ) (5a) m& v = m& a ( x2 − x1 ) (5b) From mass balance equation (5b) x2 = x1 + m& v 0.0125 × 10 −4 = 0.001823 + = 0.011823 kg/kg da 1.26 × 10 −4 m& a From energy balance equation (5a) h2 = h1 + m& v 0.00125 × 10 −4 hv = 31.81 + 2675.8 = 58.5 kJ/kg da m& a 1.26 × 10 −4 From enthalpy equation h = c p ,a t + x (l23 + c p , p t ) = 1.00t + x (2501 + 1.82t ) (6) the temperature of moist air is given by t2 = h2 − x2l23 58.5 − 0.0011823 × 2501 = = 28.3 °C c p ,a − xc p , p 1.00 − 0.0011823 × 1.82 Example 4 Determine the total sound intensity level (SIL) of a sound generated by three individual sources. Each of the sources would generate the following SIL at the monitoring location if it was the only source within the space LI1 = 93 dB, LI2 = 98 dB, and LI3 = 90 dB. Equation of the total SIL reads Ii i =1 I 0 n LI = 10 log ∑ (7) Hence, the SIL of an individual sound source is given by Ii I → i = 100.1LIi I0 I0 LIi = 10 log (8) Substituting (8) into (7) yields n LI = 10 log ∑ 100.1LIi (9) i =1 Hence the total SIL is ( ) LI = 10 log 100.1×93 + 100.1×98 + 100.1×90 = 99.7 dB 58 Example 5 Determine the total sound pressure level (SPL) of a sound generated by two sources of the same intensity. The SPL generated by a single source is Li = 60 dB. Applying the same equation (9) for the total SPL as for the SIL yields total SPL ( n ) L = 10 log ∑ 100.1Li = 10 log 2 × 100.1L1 = 10 log 2 + L1 log 10 = 3.01 + 60 = 63 dB i =1 Example 6 What is the total SPL of a 15 supply outlets at the monitoring location placed in the reverberant sound field? The individual acoustic power of the outlets is the same for all of them and it generates SPL of 37 dB at the monitoring location. Rearranging the equation (9) for a number of sources of the same intensity yields ( ) L = 10 log n × 100.1L1 = 10 log n + L1 (10) By substituting the given parameters into the above, the total SPL is obtained L = 10 log15 + 37 = 48.8 dB Example 7 A weaver mill accommodates 100 looms of individual acoustic power causing at the monitoring location SPL of 80 dB. What is the total SPL if all of the looms are in operation? How many looms would have to be in operation if the total SPL of 85 dB was not to be exceeded? The total SPL from (10): L = 10 log n + L1 = 10 log 100 + 80 = 100dB Rearranging (10) gives the number of looms no to exceed 85 dB limit: L = 10 log n + L1 → n = 10 n = 10 85−80 10 Lreq − L1 (11) 10 =3 59 APPENDIX Mollier h-x Chart of Moist Air 60 Table 1 Properties of Dry Air at Pressure p = 98,1 kPa T °C ρ kg.m-3 cp kJ.kg-1.K-1 λ . 102 W.m-1.K-1 a . 105 m2.s-1 η . 106 N.s.m-2 ν . 106 m2.s-1 Pr -180 -150 -100 -50 -20 3,685 2,817 1,984 1,534 1,365 1,047 1,038 1,021 1,013 1,009 0,756 1,163 1,616 2,035 2,256 0,196 0,403 0,800 1,314 1,650 6,672 8,728 11,77 14,61 16,18 1,76 3,10 5,94 9,54 11,93 0,900 0,770 0,742 0,726 0,724 0 10 20 30 40 1,252 1,206 1,164 1,127 1,092 1,009 1,009 1,013 1,013 1,013 2,372 2,454 2,524 2,582 2,652 1,875 2,011 2,128 2,261 2,403 17,16 17,75 18,24 18,73 19,22 13,70 14,70 15,70 16,61 17,60 0,723 0,722 0,722 0,722 0,722 50 60 70 80 90 1,056 1,025 0,996 0,968 0,942 1,017 1,017 1,017 1,021 1,021 2,721 2,803 2,861 2,931 3,000 2,539 2,680 2,828 2,958 3,125 19,61 20,10 20,40 20,99 21,57 18,60 19,60 20,45 21,70 22,90 0,722 0,722 0,722 0,722 0,722 100 120 140 160 180 0,916 0,870 0,827 0,789 0,755 1,021 1,026 1,026 1,030 1,034 3,070 3,198 3,326 3,442 3,570 3,278 3,583 3,917 4,236 4,583 21,77 22,75 23,53 24,12 25,01 23,78 26,20 28,45 30,60 33,17 0,722 0,722 0,722 0,722 0,722 200 250 300 350 400 0,723 0,653 0,596 0,549 0,508 1.034 1,042 1,047 1,055 1,059 3.698 3,977 4,291 4,570 4,849 4,944 5,889 6,889 7,889 9,000 25,89 27,95 29,71 31,48 32,95 35,82 42,80 49,90 57,50 64,90 0,722 0,722 0,722 0,722 0,722 Table 2 Properties of Water T °C c kJ.kg-1.K-1 ρ kg.m-3 λ W.m-1.K-1 η . 106 N.s.m-2 ν . 108 M2.s-1 Pr γ K-1 0 5 10 15 20 4,2257 4,2065 4,1947 4,1868 4,1817 999,9 1000,0 999,7 999,1 998,2 0,558 0,567 0,577 0,587 0,597 1793,6 1534,7 1296,4 1135,6 993,4 179,4 153,5 129,7 113,7 99,6 13,57 11,35 9,42 8,10 6,97 0,000015 0,000090 0,000154 0,000208 25 30 35 40 45 4,1784 4,1763 4,1755 4,1755 4,1763 997,1 995,7 994,1 992,3 990,2 0,606 0,615 0,624 0,633 0,639 880,6 792,4 719,8 658,0 605,1 88,4 79,6 72,4 66,3 61,1 6,08 5,38 4,81 4,34 3,94 0,000256 0,000302 0,000344 0,000386 0,000422 50 55 60 65 70 4,1776 4,1793 4,1816 4,1839 4,1868 988,1 985,7 983,2 980,6 977,8 0,647 0,652 0,658 0,663 0,667 555,0 509,9 471,7 435,4 404,0 56,2 51,8 48,0 44,4 41,3 3,58 3,27 2,99 2,74 2,53 0,000457 0,000490 0,000522 0,000554 0,000584 75 80 85 90 95 4,1901 4,1939 4,1977 4,2019 4,2060 974,9 971,8 968,7 965,3 961,9 0,651 0,673 0,676 0,678 0,680 376,6 352,0 328,1 308,9 292,2 38,6 36,2 33,9 32,0 30,4 2,35 2,19 2,04 1,91 1,80 0,000614 0,000642 0,000670 0,000697 0,000723 100 4,2107 958,4 0,681 277,5 29,0 1,72 0,000749 61 Table 3 Properties of Moist Air T °C Saturated Steam pp“ ρp“. 103 Pa kg.m-3 ρv kg.m-3 Air p = 981 hPa x“. 103 ρ“ -3 kg.m kg/kgda h“ kJ/kgda. -30 37,36 0,333 1,405 1,405 0,236 -29,697 -29 -28 -27 -26 -25 41,48 45,99 51,09 56,68 62,86 0,368 0,406 0,450 0,496 0,548 1,400 1,394 1,388 1,383 1,377 1,400 1,394 1,388 1,383 1,377 0,263 0,291 0,323 0,359 0,398 -28,621 -27,541 -26,452 -25,355 -24,246 -24 -23 -22 -21 -20 69,53 76,78 84,83 93,46 102,97 0,604 0,665 0,731 0,803 0,889 1,372 1,366 1,360 1,355 1,350 1,372 1,366 1,360 1,355 1,350 0,441 0,487 0,538 0,593 0,653 -23,136 -22,010 -20,875 -19,728 -18,573 -19 -18 -17 -16 -15 113,36 124,74 137,00 150,43 165,14 0,967 1,058 1,158 1,267 1,385 1,344 1,338 1,333 1,329 1,324 1,344 1,337 1,332 1,328 1,323 0,719 0,792 0,869 0,955 1,048 -17,396 -16,207 -15,005 -13,787 -12,544 -14 -13 -12 -11 -10 180,93 198,19 217,02 237,42 259,58 1,509 1,649 1,799 1,961 2,136 1,319 1,314 1,309 1,303 1,298 1,318 1,313 1,308 1,302 1,297 1,149 1,259 1,378 1,508 1,650 -11,292 -9,998 -8,692 -7,356 -5,995 -9 -8 -7 -6 -5 283,41 309,59 337,74 368,24 401,19 2,323 2,527 2,747 2,985 3,238 1,294 1,289 1,284 1,279 1,274 1,293 1,288 1,282 1,277 1,272 1,801 1,968 2,148 2,343 2,553 -4,605 -3,178 -1,701 -0,222 1,315 -4 -3 -2 -1 0 436,89 475,62 517,30 562,41 610,76 3,514 3,812 4,131 4,474 4,827 1,270 1,265 1,260 1,255 1,251 1,268 1,263 1,257 1,252 1,248 2,782 3,029 3,297 3,586 3,897 2,897 4,530 6,213 7,946 9,743 1 2 3 4 5 656,6 705,4 757,5 812,9 871,9 5,189 5,555 5,945 6,357 6,793 1,246 1,242 1,237 1,233 1,228 1,243 1,239 1,233 1,229 1,224 4,191 4,505 4,840 5,197 5,578 11,497 13,297 15,152 17,069 19,042 6 7 8 9 10 934,7 1001,3 1072,1 1147,3 1227,7 7,256 7,746 8,265 8,815 9,398 1,224 1,220 1,215 1,211 1,207 1,220 1,216 1,210 1,206 1,201 5,983 6,414 6,373 7,361 7,880 21,081 23,182 25,359 27,608 29,940 11 12 13 14 15 1311,8 1401,6 1496,7 1597,4 1704,1 10,01 10,66 11,34 12,06 12,82 1,202 1,198 1,194 1,190 1,186 1,196 1,191 1,187 1,182 1,178 8,431 9,014 9,632 10,295 10,995 32,351 34,847 37,430 40,130 42,927 16 17 18 19 20 1817,0 1936,4 2062,0 2196,0 2337,0 13,63 14,47 15,36 16,30 17,29 1,182 1,178 1,174 1,170 1,166 1,174 1,169 1,165 1,160 1,155 11,74 12,52 13,36 14,24 15,18 45,845 48,860 52,000 55,266 58,699 62 Properties of Moist Air (Continued) T °C Saturated Steam pp“ ρp“. 103 Pa kg.m-3 21 22 23 24 25 2486 2643 2808 2982 3166 18,33 19,42 20,57 21,77 23,04 1,162 1,158 1,154 1,150 1,146 1,151 1,146 1,142 1,137 1,132 16,17 17,22 18,33 19,50 20,75 62,216 65,942 69,794 73,855 78,042 26 27 28 29 30 3360 3564 3779 4004 4241 24,37 25,76 27,26 28,75 30,37 1,142 1,138 1,135 1,131 1,127 1,127 1,122 1,119 1,114 1,109 22,06 23,45 24,92 26,47 28,11 82,438 87,044 91,858 96,841 102,116 31 32 33 34 35 4491 4753 5029 5318 5622 32,05 33,81 35,65 37,58 39,62 1,123 1,120 1,116 1,112 1,109 1,104 1,100 1,094 1,089 1,085 29,85 31,67 33,61 35,66 37,82 107,601 113,337 119,366 125,688 132,303 36 37 38 39 40 5940 6274 6624 6991 7375 41,72 43,92 46,23 48,64 51,15 1,105 1,102 1,099 1,095 1,091 1,080 1,075 1,071 1,065 1,060 40,09 42,50 45,05 47,74 50,56 139,211 146,496 154,158 162,155 170,486 41 42 43 44 45 7777 8198 8639 9101 9584 53,76 56,49 59,35 62,34 65,44 1,088 1,084 1,081 1,077 1,074 1,056 1,049 1,045 1,039 1,034 53,56 56,73 60,08 63,60 67,34 179,321 188,615 198,371 208,586 219,346 46 47 48 49 50 10088 10614 11163 11736 12335 68,68 72,05 75,57 79,23 83,06 1,071 1,067 1,064 1,061 1,057 1,029 1,023 1,018 1,013 1,007 71,28 75,45 79,87 84,53 89,47 230,693 242,625 255,185 268,416 282,400 51 52 53 54 55 12960 13612 14292 15001 15740 86,96 91,07 95,35 99,80 104,4 1,054 1,051 1,048 1,045 1,042 1,001 0,995 0,990 0,985 0,979 94,73 100,3 106,1 112,3 119,0 297,179 312,880 329,124 346,458 365,131 56 57 58 59 60 16510 17312 18146 19014 19917 109,2 114,2 119,3 124,7 130,2 1,038 1,035 1,032 1,029 1,026 0,972 0,966 0,960 0,954 0,947 125,9 133,4 141,2 149,6 158,6 384,263 405,073 426,635 449,872 474,616 61 62 63 64 65 20860 21840 22850 23910 25010 136,0 142,0 148,2 154,6 161,3 1,023 1,020 1,017 1,014 1,011 0,941 0,935 0,928 0,921 0,914 168,1 178,2 189,0 200,5 213,0 500,783 528,500 558,059 589,543 623,666 66 67 68 69 70 26150 27330 28560 29840 31170 168,2 175,3 182,7 190,3 198,2 1,008 1,005 1,002 0,999 0,996 0,907 0,899 0,891 0,883 0,875 226,1 240,4 255,6 272,0 289,7 659,379 698,400 739,724 784,355 832,336 ρv kg.m-3 63 Air p = 981 hPa x“. 103 ρ“ kg.m-3 kg/kgda h“ kJ/kgda Table 4 Properties of Saturated Water (Liquid – Vapour): Temperature Table v’’ ρ“ kg.m-3 h’ h’’ kJ.kg-1 l23 0,0010002 0,0010001 0,0010004 0,0010010 0,0010018 0,0010030 206,3 147,2 106,42 77,97 57,84 43,40 0,004847 0,006793 0,009398 0,01282 0,01729 0,02304 0 21,05 42,04 62,97 83,90 104,81 2501 2510 2519 2528 2537 2547 2501 2489 2477 2465 2454 2442 0 0,0762 0,1510 0,2244 0,2964 0,3672 9,1544 9,0241 8,8994 8,7806 8,6665 8,5570 0,004241 0,005622 0,007375 0,009584 0,012335 0,0010044 0,0010061 0,0010079 0,0010099 0,0010121 32,93 25,24 19,55 15,28 12,04 0,03037 0,03962 0,05115 0,06544 0,08306 125,71 146,60 167,50 188,40 209,3 2556 2565 2574 2582 2592 2430 2418 2406 2394 2383 0,4366 0,5049 0,5723 0,6384 0,7038 8,4523 8,3519 8,2559 8,1638 8,0753 60 70 80 90 100 0,019917 0,03117 0,04736 0,07011 0,10131 0,0010171 0,0010228 0,0010290 0,0010359 0,0010435 7,678 5,045 3,408 2,361 1,673 0,1302 0,1982 0,2934 0,4235 0,5977 251,1 293,0 334,9 377,0 419,1 2609 2626 2643 2659 2676 2358 2333 2308 2282 2257 0,8311 0,9549 1,0753 1,1925 1,3071 7,9084 7,7544 7,6116 7,4787 7,3547 110 120 130 140 150 0,14326 0,19854 0,27011 0,3614 0,4760 0,0010515 0,0010603 0,0010697 0,0010798 0,0010906 1,210 0,8917 0,6683 0,5087 0,3926 0,8264 1,121 1,496 1,966 2,547 461,3 503,7 546,3 589,0 632,2 2691 2706 2721 2734 2746 2230 2202 2174 2145 2114 1,4184 1,5277 1,6345 1,7392 1,8418 7,2387 7,1298 7,0272 6,9304 6,8383 160 170 180 190 200 0,6180 0,7920 1,0027 1,2553 1,5551 0,0011021 0,0011144 0,0011275 0,0011415 0,0011565 0,3068 0,2426 0,1939 0,1564 0,1272 3,258 4,122 5,157 6,394 7,862 675,5 719,2 763,1 807,5 852,4 2758 2769 2778 2786 2793 2082 2050 2015 1979 1941 1,9427 2,0417 2,1395 2,2357 2,3308 6,7508 6,6666 6,5858 6,5074 6,4318 210 220 230 240 250 1,9080 2,3201 2,7979 3,3480 3,9776 0,0011726 0,0011900 0,0012087 0,0012291 0,0012512 0,1043 0,08606 0,07147 0,05967 0,05006 9,588 11,62 13,99 16,76 19,98 897,7 943,7 990,4 1037,5 1085,7 2798 2802 2803 2803 2801 1900 1858 1813 1766 1715 2,4246 2,5179 2,6101 2,7021 2,7934 6,3577 6,2849 6,2133 6,1425 6,0721 260 270 280 290 300 4,694 5,505 6,419 7,445 8,592 0,0012755 0,0013023 0,0013321 0,0013655 0,0014036 0,04215 0,03560 0,03013 0,02554 0,02164 23,72 28,09 33,19 39,15 46,21 1135,1 1185,3 1236,9 1290,0 1344,9 2796 2790 2780 2766 2749 1661 1605 1542,9 1476,3 1404,2 2,8851 2,9764 3,0681 3,1611 3,2548 6,0013 5,9297 5,8573 5,7827 5,7049 320 340 360 374 11,290 14,608 18,674 22,087 0,001499 0,001639 0,001894 0,002800 0,01545 0,01078 0,006943 0,00347 64,72 92,76 144,00 288,00 1462,1 1594,7 1762,0 2032,0 2700 2622 2481 2147 1237,8 1027,0 719,30 114,70 3,4495 3,6605 3,9162 4,3258 5,5353 5,3361 5,0530 4,5029 T °C P MPa v’ 0,01 5 10 15 20 25 0,0006108 0,0008719 0,0012277 0,0017041 0,002337 0,003166 30 35 40 45 50 Critical Values: 3 m .kg -1 Temperature 374,15 °C Pressure 22,129 MPa Specific Volume 0,00326 m3.kg-1 Specific Enthalpy 2100 kJ.kg-1 Specific Entropy 4,430 kJ.kg-1.K-1 64 s’ s’’ -1 -1 kJ.kg .K Properties of Saturated Water (Liquid – Vapour) – Continued: Pressure Table p MPa T °C v’ 0,001 0,002 0,003 0,004 0,005 0,006 0,007 0,008 0,009 0,010 0,02 0,03 0,04 0,05 0,06 0,07 0,08 0,09 0,10 0,15 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 1,2 1,4 1,6 1,8 2,0 2,2 2,4 2,6 2,8 3,0 3,5 4,0 5,0 6,0 7,0 8,0 9,0 10,0 11,0 12,0 13,0 14,0 16,0 18,0 20,0 22,0 6,92 17,514 24,097 28,979 32,88 36,18 39,03 41,54 43,79 45,84 60,08 69,12 75,88 81,35 85,95 89,97 93,52 96,72 99,64 111,38 120,23 133,54 143,62 151,84 158,84 164,96 170,42 175,35 179,88 187,95 195,04 201,36 207,10 212,37 217,24 221,77 226,03 230,04 233,83 242,54 250,33 263,91 275,56 285,80 294,98 303,32 310,96 318,04 324,63 330,81 336,63 347,32 356,96 365,71 373,7 0,0010001 0,0010014 0,0010028 0,0010041 0,0010053 0,0010064 0,0010075 0,0010085 0,0010094 0,0010103 0,0010171 0,0010222 0,0010264 0,0010299 0,0010330 0,0010359 0,0010385 0,0010409 0,0010432 0,0010527 0,0010605 0,0010733 0,0010836 0,0010927 0,0011007 0,0011081 0,0011149 0,0011213 0,0011273 0,0011385 0,0011490 0,0011586 0,0011678 0,0011766 0,0011851 0,0011932 0,0012012 0,0012088 0,0012163 0,0012345 0,0012520 0,0012857 0,0013185 0,0013510 0,0013838 0,0014174 0,0014521 0,001489 0,001527 0,001567 0,001611 0,001710 0,001837 0,00204 0,00273 v’’ ρ“ kg.m-3 h’ h’’ kJ.kg-1 l23 129,9 66,97 45,66 34,81 28,19 23,74 20,53 18,10 16,20 14,68 7,647 5,226 3,994 3,239 2,732 2,364 2,087 1,869 1,694 1,159 0,8854 0,6057 0,4624 0,3747 0,3156 0,2728 0,2403 0,2149 0,1946 0,1633 0,1408 0,1238 0,1104 0,09958 0,09068 0,08324 0,07688 0,07141 0,06665 0,05704 0,04977 0,03944 0,03243 0,02737 0,02352 0,02048 0,01803 0,01598 0,01426 0,01277 0,01149 0,009318 0,007504 0,00585 0,00367 0,00770 0,01493 0,02190 0,02873 0,03547 0,04212 0,04871 0,05525 0,06172 0,06812 0,1308 0,1913 0,2504 0,3087 0,3661 0,4230 0,4792 0,5350 0,5903 0,8627 1,129 1,651 2,163 2,669 3,169 3,666 4,161 4,654 5,139 6,124 7,103 8,080 9,058 10,041 11,03 12,01 13,01 14,00 15,00 17,53 20,09 25,35 30,84 36,54 42,52 48,83 55,46 62,58 70,13 78,30 87,03 107,3 133,2 170,9 272,5 29,32 73,52 101,04 121,42 137,83 151,50 163,43 173,9 183,3 191,9 251,4 289,3 317,7 340,6 360,0 376,8 391,8 405,3 417,4 467,2 504,8 561,4 604,7 640,1 670,5 697,2 720,9 742,8 762,7 798,3 830,0 858,3 884,4 908,5 930,9 951,8 971,7 990,4 1008,3 1049,8 1087,5 1154,4 1213,9 1267,4 1317,0 1363,7 1407,7 1450,2 1491,1 1531,5 1570,8 1650 1732 1827 2016 2513 2533 2545 2554 2561 2567 2572 2576 2580 2584 2609 2625 2636 2645 2653 2660 2665 2670 2675 2693 2707 2725 2738 2749 2757 2764 2769 2774 2778 2785 2790 2793 2796 2799 2801 2802 2803 2803 2804 2803 2801 2794 2785 2772 2758 2743 2725 2705 2685 2662 2638 2582 2510 2410 2168 2484 2459 2444 2433 2423 2415 2409 2402 2397 2392 2358 2336 2318 2204 2293 2283 2273 2265 2258 2226 2202 2164 2133 2109 2086 2067 2048 2031 2015 1987 1960 1935 1912 1891 1870 1850 1831 1813 1796 1753 1713 1640 1570,8 1504,9 1441,1 1379,3 1317,0 1255,4 1193,5 1130,8 1066,9 932,0 778,2 583 152 m3.kg-1 65 s’ s’’ kJ.kg-1.K-1 0,1054 0,2609 0,3546 0,4225 0,4761 0,5207 0,5591 0,5927 0,6225 0,6492 0,8321 0,9441 1,0261 1,0910 1,1453 1,1918 1,2330 1,2696 1,3026 1,4336 1,5302 1,672 1,777 1,860 1,931 1,992 2,046 2,094 2,138 2,216 2,284 2,344 2,397 2,447 2,492 2,534 2,573 2,611 2,646 2,725 2,796 2,921 3,027 3,122 3,208 3,287 3,360 3,430 3,496 3,561 3,623 3,746 3,871 4,015 4,303 8,975 8,722 8,576 8,473 8,393 8,328 8,274 8,227 8,186 8,149 7,907 7,769 7,670 7,593 7,531 7,479 7,434 7,394 7,360 7,223 7,127 6,992 6,897 6,822 6,761 6,709 6,663 6,623 6,587 6,523 6,469 6,422 6,379 6,340 6,305 6,272 6,242 6,213 6,186 6,125 6,070 5,973 5,890 5,814 5,745 5,678 5,615 5,553 5,492 5,432 5,372 5,247 5,107 4,928 4,591