Transcript
APP-508
Application Guidelines Humidifying Equipment Selection Part 1
Table of Contents Purpose ....................................................... 2 Summary ....................................................... 2 Steam Humidifier Specification .................... 2 Evaporative Type Humidifiers ...................... 3 1) Water Spray ..................................... 3 2) Absorbent Medium .......................... 4 3) Evaporative Pan............................... 4 Steam Type Humidifiers ................................ 4 1) Steam Grid or Pipe .......................... 5 2) Steam Cup Type .............................. 5
3) Dry Conditioned Steam ................... 5 a) History ....................................... 6 b) Separator ................................... 6 c) Drain Connection ....................... 7 d) Temperature Switch................... 7 e) Re-evaporation Chamber ........... 8 f) Non-Ducted (Area) Type ........... 9 g) Control Valve Design ................. 9 h) Valve Operator ........................... 11 i) Steam Distribution Manifolds ..... 12 j) Manifold Piping Adaptors .......... 13 k) Steam Discharge Holes .............. 13 l) Steam Traps ................................ 14
©Copyright 1997 Armstrong International, Inc.
HUMIDIFYING EQUIPMENT SELECTION AND SIZING Purpose The job of a humidifier is to introduce water vapor into an air stream or a space to control relative humidity (RH) in a specific space. How the water vapor is introduced is critical to the reliability and exactness of control of RH. This paper will describe the manner of operation, examine the effectiveness of control, and critique the virtues of various types of humidification equipment available for larger scale commercial, institutional, and industrial environment control applications. A properly designed humidifier is an integrated control system composed of several discrete components each with a unique but complimentary function. The components must be specified and provided as a fully integrated and complete system or the humidifier may not control relative humidity properly, and it may deposit water in ducts and/or humidified spaces.
Summary Water spray and water saturated medium types are slow to react to RH changes. They use energy to evaporate water adding to the heating system load. Water absorption may not be complete, so water may collect in the duct or droplets may be carried into the humidified space. Dust from mineral solids in the water may be entrained in the air and deposited in ducts and humidified spaces or deposited on the absorbent medium requiring frequent and costly maintenance. Water lying in ducts or on the medium promotes bacterial and fungal microorganism growth, and thusly aseptic conditions may exist in the air distribution system and be established in the humidified spaces. Evaporative pan types are limited in capacity, slow to respond to RH change, and they use energy from the heating system to evaporate water. Dissolved minerals concentrate in the water and deposit in a hard adherent and insulating layer on pan surfaces. This causes loss of capacity, consequently maintenance cost is high. Microorganisms may grow in the water stored in the pan and be transmitted to the humidified spaces by the air stream thereby causing aseptic conditions. Dry conditioned steam types introduce hot dry steam (water vapor) directly into the air stream or space where it is thoroughly mixed with the air and absorbed rapidly without using heat from the air. The amount of steam injected can be changed immediately and controlled precisely, so response to RH change is quick, thus close RH control is assured. Steam contains no mineral contaminants, so solids are neither entrained nor deposited. Steam temperature is high, therefore microorganisms cannot grow, so only sterile water vapor is added to the air. Installed cost per unit of moisture, and maintenance costs are lowest of all types. Especially where close control of RH is required, dry conditioned steam is best on all counts.
Steam Humidifier Specification The steam humidifier shall have a thick wall cast iron or stainless steel separator with two stages of 180 deg. turn baffles and an integral full flow high pressure steam jacketed re-evaporation chamber containing stainless steel silencing medium.
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The steam control valve shall be mounted integral to the separator, and the valve plug shall be in the high pressure steam space. It shall be a long, C\v" stroke, wide flow rangeability valve with a parabolic modified linear characteristic plug. The valve shall adapt to mounting any available industrial grade operator type (pneumatic, electric motor, hydraulic, or electric solenoid) without disconnecting plumbing. Capacity also may be changed by replacing valve plug and seat without changing or disconnecting plumbing. The steam distribution manifold shall be steam jacketed, have a stainless steel mesh screen full length silencer, and have aerodynamic shape and small cross sectional area to minimize air duct pressure drop. A suitably sized strainer shall be supplied for mounting in the steam inlet to remove solids. A fail open (inverted bucket) steam trap with capacity of 20% of maximum steam flow shall be supplied for mounting at the separator drain connection with a large drain leg to collect and store start-up load and slugs of condensate for drainage by the steam trap. A temperature switch shall be supplied to prevent control valve opening before metal system components are heated to steam temperature.
Evaporative Type Humidifiers These types either spray water mists into the air stream for evaporation or the air stream flows across an exposed surface from which the air evaporates water. Therefore, all use energy from the air stream to evaporate water thus reducing air temperature and increasing heating system loading. 1) Water Spray (Fig. 3-1) Water under pressure flows through a nozzle or many nozzles discharging as a fine mist into an air stream. The air stream evaporates the water, and the resultant water vapor is absorbed into the air. About 1000 Btu of heat is removed from the air to evaporate each pound of water (540 Kcal/Kg). Air dry bulb temperature is reduced, and the heating system is taxed further to replace lost heat. Evaporation is slow, so the system doesn’t respond quickly to RH changes, and control is erratic. All water contains some amount of dissolved mineral solids. Evaporation releases minerals from solution as an adherent solid which may cling to nozzles and ducts or disperse in the air as fine dust.
Heat Taken From Air To Evaporate Moisture
Water Supply Under Pressure
Fig. 3-1. Water Spray Method
Mineral buildup on nozzles may clog reducing flow capacity and forcing expensive shutdown for cleaning. Mineral dust settles on duct surfaces and is transported to humidified spaces to settle on exposed surfaces and be breathed by people causing respiratory problems. Dust increases maintenance cleaning costs. 3
Water not evaporated by the air and dripping from nozzles falls to the duct bottom or coats exposed metal equipment causing corrosion and decay and expensive downtime and parts cost for replacement. Bacteria and algae in the water multiply causing odor and sanitation problems, and bacteria are conveyed into humidified spaces. 2) Absorbent Medium An air stream flows across an expansive exposed absorbent medium surface, from which water is evaporated and absorbed by the air. This method has problems similar to the spray method. However, the surface area necessary for evaporation of large quantities of water vapor is excessive. This method is not practical for high capacity applications. 3) Evaporative Pan (Fig. 4-1) Water Vapor An air stream passes over a Float Steam, Hot Water large area exposed surface Valve or Electricity In of a pan shaped water reservoir. Water in the pan is Heated Water Auto Valve heated by electrodes or heating coils to prepare it Water Pan to be evaporated and absorbed by the air stream. Fig. 4-1. Evaporative Pan Method Since heat is removed from the air for water evaporation, the heating system loading is increased to maintain air temperature. Evaporation is somewhat faster than with water spray, but response to RH change still is quite slow, and precise stable control is not probable. Dissolved solids concentrate above sustainable levels in the pan eventually leaving solution as an adherent mass. This mass clings to and solidifies on electrodes which may result in burn out requiring the system to be shut down periodically for cleaning. Hot water in the pan is a perfect breeding ground for all manner of microorganisms contained in the water supply. Frequent cleaning is required to prevent odors and contamination of ducts and humidified spaces. High capacity applications would require huge pans and heating elements, so this type also is limited to small and medium size systems.
Steam Type Humidifiers Dry conditioned steam humidifiers distribute and mix superheated steam uniformly across a duct or into a space. The water vapor is dry, therefore it is absorbed quickly, its flow rate can be adjusted rapidly and accurately, and already being a vapor, it does not require heat from air for evaporation. Consequently, RH is readjusted promptly and accurately, control is stable, and no heating load is added to the system. Steam contains no minerals, and it is sterile, so no minerals and dust are deposited or distributed and 4
no micro-organic or bacterial growth is supported or conveyed. Maintenance costs are low, and additional health hazards are not promoted. 1) Steam Grid or Pipe (Fig. 5-1) The system includes a separator/header and multiple dispersion tube assembly packaged with control valve, strainer, steam supply drip trap, and one or two header drain traps. The tubes are spaced evenly in the duct or air handler so that steam may be distributed across the duct (or AHU) cross section. Dry steam passes from the header into each of the dispersion tubes and flows through nozzles which extend from the center of each tube, before discharging through orifices into the airstream. Each system is intended to provide uniform distribution and shortened wettable vapor trail. Condensate from the header drain traps cannot be lifted or discharged into a pressurized return system.
Fig. 5-1. Steam Grid Type
2) Steam Cup Type (Fig. 5-2) Wet steam is injected through a cuplike apparatus in the bottom of the duct. Steam flows into the cup from an inlet on two sides. Moisture may fall to the bottom of the cup for removal, but water usually is entrained in the steam in a mist which does not separate, therefore moist steam enters the duct. The steam flow is not distributed well in the air stream, so the water is not absorbed completely, and it drops out to the bottom of the duct where it causes corrosion, microbe growth and odors. A steam type humidifier must be designed so only perfectly dry steam is discharged into the duct. 3) Dry Conditioned Steam How well any one type of humidifier may perform depends Figure 5-2. Steam Cup Type on the design of the component parts and their functioning reliably, accurately, and responsively as a system. To illustrate the importance of each component and its integration into a system, we compare Armstrong’s design approach with that of two typical competitors. a) History The first dry conditioned steam humidifier was developed by Armstrong to solve a problem associated with the crude separator and steam trap systems used to supply steam for humidifying paper making production and storage spaces. The Separator/Trap system could
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not completely dry steam, and consequent water carryover caused spitting of hot water droplets into the humidified space, onto people, machinery and product. To solve the problem, Armstrong designed a more efficient separator with an integral large volume re-evaporation chamber downstream of the control valve which superheated the full flow of steam assuring that only dry steam was injected into the space. This separator is the heart of any humidifier and the point at which comparisons begin. b) Separator (Fig. 6-1) The purpose of a separator is to ensure that all entrained water is removed so that maximally dry steam is carried to and through the control valve. An effective separator must have a large volume with a wide cross section to reduce steam velocity and effective internal baffles to change flow direction abruptly. Low velocity and abrupt directional change fling water from the steam and direct it to an integral drain at the bottom for removal by a steam trap. Armstrong Design Armstrong features a large volume and wide cross section thick wall cast iron or fabricated stainless steel separator with two baffle stages. One baffle is immediately after the steam inlet. The U shape causes abrupt 180° changes in flow direction and splits the steam into two streams before passing into the large wide separator body. The velocity decrease and direction change cause water droplets to separate from steam and fall to the drain at the bottom.
Fig. 6-1. Armstrong Steam Separator
More condensate forms on the separator walls as heat is lost to outside air, and the amount formed depends on the thickness and thermal conductivity of separator wall material. Thick cast or formed walls minimize heat loss and condensate formation. If volume and cross section of the separator are too small, velocity isn’t reduced sufficiently to permit water to fall or condensate to trickle down the walls. Instead the rapidly flowing steam picks up moisture and carries it to the steam outlet. Even though our separator is large, Armstrong uses a second stage of baffle to reduce carryover to a minimum. Competitive Designs (Fig. 6-2 and 7-1) Both use low cost thin wall formed and welded stainless steel separators with smaller volumes and cross sections and a single stage baffle. In the Spirax-Sarco design, the steam inlet passes 6
Fig. 6-2. Spirax-Sarco Separator
through a nozzle at the bottom, away from the discharge at the top. The steam flow changes direction in order to exit the separator. In the Dri-Steem design, steam enters the separator at the bottom. Directly above is a plate with downward baffle projections. The steam is directed centrifugally across the baffles and out to the walls. The centrifugal flow and agitation of the baffles drop water to the drain. In both separators, most steam is channeled up the side wall where heat flows quickly from the steam through the thin wall to outside air. The resulting condensate must slide down the wall to the drain. Small volume and cross section induce high velocity steam flow up the wall carrying moisture with it to the outlet. If there is not a second outlet baffle stage to help remove this moisture, so it continues to the valve where it wears metal parts.
Fig. 7-1. Dri-Steem Separator
c) Drain Connection Condensation does not flow smoothly at any time. Flow is caused by friction as fast moving steam moves across the water surface pulling it along in surges. Modulating or intermittent valve opening also varies steam flow and causes condensate surges. The drain connection and piping should be large and long enough to store condensate until pressure builds up to increase steam trap capacity on start-up and to handle the surging slugs at all times. Otherwise, condensate may carry over or the trap must be grossly oversized and unreliable during periods of low flow. Armstrong separators have 1, 1Z|v, and 2 inch connections through a range of 5 separator sizes, whereas the competitors provide ¾ inch connection through a range of 5 sizes from Dri-Steem and only two sizes from Spirax-Sarco. In addition, Armstrong suggests a minimum 6 inch (15 cm) vertical drip leg before the takeoff to the horizontally mounted steam trap. d) Temperature Switch At start-up, condensate forms as the many metal system components heat to steam temperature. Total metal system weight doesn’t vary much regardless of separator material, therefore start-up condensate load is about the same with all separator designs. The steam supply piping from the isolation valve to the humidifier must be heated and this often represents the majority of condensate generated during start-up. Spitting of water from the manifold is most likely on start-up if the control valve is allowed to open before the system heats up to steam temperature. Armstrong suggests installing a temperature switch to prevent premature valve opening. All manufacturers offer a switch as an accessory. e) Re-evaporation Chamber Water passing through the control valve should be evaporated before it enters the distribution manifold. This is done best in an integral large volume chamber located at the valve outlet
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inside the separator body. All low pressure steam and condensate from the valve should pass through the chamber which is surrounded by high pressure, higher temperature steam. Heat from the higher temperature steam re-evaporates condensate and superheats all low pressure steam ensuring that only perfectly dry steam enters the distribution manifold. Velocity through a throttled valve is high, consequently humidifier noise level may also be high. Many humidifiers are used in quiet hospitals, buildings, and office spaces in which a noisy whistling valve can be irritating and distracting, so the chamber should be filled with a stainless steel silencing medium. Armstrong Design The chamber is located well within the separator body almost surrounded by high pressure steam. The large chamber at the valve outlet causes decreased velocity, so water is not carried through. All water is evaporated and all steam is superheated, so only dry steam enters the distribution manifold. The chamber is filled with a stainless steel wool silencing medium. Competitive Designs The control valve in both designs is external to the separator, therefore flow from the valve outlet is not directly into and through a re-evaporation chamber. Spirax-Sarco offers no chamber for either the size 150 or 200 separators. It depends entirely on the manifold steam jacket to dry and to superheat the low quality steam before it is injected into the air steam. With high steam velocity at the valve outlet, most water is entrained by turbulent flow through the valve and into the manifold. Low quality steam enters the manifold, and the manifold’s high pressure steam jacket must dry and superheat the mixture before it is discharged into the air. The probability of water spitting into the duct is high. Spirax-Sarco design also does not have silencing medium directly after the valve, therefore valve noise may be transmitted throughout the system. Dri-Steem design doesn’t have a re-evaporation chamber. (See Fig. 7-1). It depends entirely on the manifold steam jacket to dry and to superheat the low quality steam before it is injected into the air stream. The silencer causes a pressure drop in the nearly atmospheric pressure steam entering the manifold. f) Non-Ducted (Area) Types Area type humidifiers discharge directly into warm air in the humidified space, therefore there is no manifold to re-evaporate water or to superheat steam. Any ejected hot water is spit directly into the space to fall on people, equipment, and product. To prevent spitting, humidifier discharge must be dry superheated steam only.
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Armstrong Design (Fig. 9-1) The Armstrong standard duct type design has a reevaporation chamber ensuring that only dry superheated steam is discharged. It does not depend on a steam jacketed manifold to dry steam, therefore no additions or alternations are necessary to use the same separator in area type applications also. Competitive Design (Fig. 9-2) Dri-Steem provides a single baffle plate and a dispersion port in the separator which is mounted after the valve. One separator size is offered for all applications. Dryness should be acceptable. Maximum operating pressure is limited to 15 psig (1 bar).
Fig. 9-1. Armstrong Area Type
There is no silencing media in this design so valve noise will be heard in the humidified space. Maximum steam operating pressure is 15 psig (1 bar) in the smallest valves and less in larger sizes. The position of the valve relative to the separator dictates that steam and condensate in the separator is at atmospheric steam pressure. An F&T trap is suggested and the condensate must be drained by gravity to a return line with no back pressure. g) Control Valve Design Just as the separator and re-evaporation chamber are essential to ensure dry steam in the duct or space, a properly designed valve and operator are essential for accurate, stable, and responsive control. Area type applications discharge directly into a huge volume of warm air. Steam mixes with and is absorbed by dry warm air quickly, so acceptable control of RH can be achieved with on-off solenoid type valves. However, duct type humidifiers discharge steam into a limited volume of colder air traveling at high velocity. Temperature can be as low as 55°F (13°C), so steam absorption is slow and unreliable unless absolutely dry steam is introduced in exactly the amount necessary for conditions of the moment. Otherwise, water may drop out and collect in the duct, and RH control will be erratic. Properly placed and designed valves are needed. The valve should be mounted integral to the separator with valve plug and seat surrounded by a high 9
Fig. 9-2. Dri-Steem Area Type
pressure steam jacket and with its discharge direct into the re-evaporation chamber. The main mass of body, valve plug and seat of the integral mount valve is in a high pressure steam space not exposed to outside air, thus added condensate isn’t generated in the valve. Additional condensation occurs in externally mounted valves and the external piping joining them to separator and manifold. The added field piping raises installation cost for external mount valves. Steam flow must be readjusted precisely and Fig. 10-1. Modified Linear repeatedly to control RH accurately and with Valve Characteristics stability. Humidifier maximum capacity is selected for the driest air and highest heating load condition, yet a humidifier must operate most of the time when outside air is warmer and contains more moisture. Consequently, a control valve must control steam flow accurately and with stability despite widely changing requirements. The valve should be reverse acting and normally closed. Steam pressure assists in closing reverse acting valves, hence heavy springs are not needed to aid closure. The valve operator encounters less resistance to movement in either direction, so hysteresis is slight, and positioning accuracy and repeatability are high. The valve should have modified linear output characteristics (Fig. 10-1) rather than equal percentage. Modified linear output changes flow relative to incremental position change as an equal percentage of actual flow rate regardless of valve position. This promotes stable control at all flow rates, whereas equal percentage valves do not have such favorable characteristics at low flow rates. The operation will resemble on-off control at low flows, and RH control will be erratic. Armstrong Design (Fig. 10-2) The valve is mounted integral to the separator with the valve internals surrounded by high pressure steam, and it discharges directly into the large volume full flow re-evaporation chamber. It has extremely wide rangeability because of a specially designed ¾ inch stroke parabolic plug with modified linear characteristics. This coupled with reverse action makes the valve stable and accurate with excellent modulating action over a wide range of flow requirements. 10
Fig.10-2. Parabolic Plug Metering Valve
The valve may be fitted with a wide variety of manufacturer’s valve operators without disconnecting or altering any plumbing. Capacity changes are made simply by replacing only the plug and orifice which also is done without disturbing the plumbing. Competitive Design (Fig. 11-1) Both competing designs employ an external globe type valve field mounted in piping between the separator and manifold, thus both the valve body and the pipe are exposed to surrounding air and subject to condensate formation. The valve is an outside purchased commercial grade direct acting zone control valve with short stroke and equal percentage characteristics. It has limited flow rangeability and controls erratically at low flows. Any greater or more reliable capacity change requires removing the valve and altering piping to accept a different size valve. h) Valve Operator Accurate and stable control of RH depends on quick, accurate, and repeatable change of steam flow in response to RH control signals, and this depends on fast, accurate and repeatable valve positioning. To accomplish this, a pneumatic operator must have a large diaphragm to permit using heavy springs which stabilize positioning and which prevent hysteresis from mechanism drag and flow velocity effects that destroy positioning accuracy and repeatability.
Fig. 11-1. Globe Valve
Fig. 11-2. Pneumatic Operator Characteristics
The positioner stroke must be long to match the valve and preserve its rangeability. Armstrong Design Armstrong has designed its own pneumatic operator with a 12 square inch (34mm2) diaphragm, full ¾ inch stroke, and low drag. It is designed to match and enhance the superb characteristics of its specially designed valve. The combination perfectly compliments the rest of the humidification control system. Competitive Design Both use the inexpensive small diaphragm operator supplied with the standard low grade valve. It has
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Fig. 11-3. Positioning Hysterisis
only ½ inch stroke, and is not powerful enough to overcome valve drag, velocity effects, and the heavy spring of the direct-acting valve. Positioning accuracy and repeatability are poor. Maximum steam operating pressure for the standard pneumatic operator is 35 psig (2.4 bar). i) Steam Distribution Manifolds (Fig. 12-1 & 12-2) A manifold should be steam jacketed to prevent condensation and to provide final drying and superheating of steam before it is injected into the duct. It should have enough holes so that steam is distributed equally across the duct and is not injected as distinct high velocity jet streams which require long open duct lengths to ensure complete mixing and vapor absorption before moisture forms on duct surfaces and other equipment. The manifold should have an internal stainless steel mesh silencer along its full length to prevent high velocity noise transmission. The profile should be aerodynamic, and the face height should be limited to minimize air pressure drop.
Fig. 12-2. Armstrong Distribution Manifold
Fig. 12-1. Distribution Manifold
Armstrong’s design incorporates all of these features, and its face height causes 40% less air pressure drop. The competitive designs don’t have aerodynamic cross section, so they contribute additional pressure drop. j) Manifold Piping Adaptors Steam pressure at the manifold inlet is near atmospheric. System capacity is reduced by any restriction between the control valve and the manifold, therefore the transition adaptor should have an ID as large as the manifold pipe. The sealing “O” rings should be on the outside of the manifold tube and should have an external compression fitting which will allow retightening of the joint over many years. Armstrong Design (Fig. 12-3) The manifold inlet pipe slips inside the overly large separator outlet. The “O” ring on the outside of the tube is sealed by a compression nut which screws into a male thread on the separator outlet. The manifold tube is schedule 5 pipe which will not distort from the “O” ring compression and will maintain the same ID from the separator outlet through the manifold 12
Fig. 12-3. Armstrong Adaptors
length. The rigid tube permits retightening of the “O” ring allowing resealing of any leaks over many years. Competitive Design (Fig. 13-1) Dri-Steem uses an adaptor which screws into the control valve outlet. The adaptor has a larger ID than the manifold tube which slips inside it. The manifold tube is sealed by friction as it is slip-fitted inside two “O” rings within the adaptor. The slip-fit assembly procedure may damage the “O” ring initially, nonetheless the construction does not allow retightening of the joint after leaks begin at any time.
Fig. 13-1. Dri-Steam Adaptors
For this reason, the manufacturer suggests changing the “O” ring every 2 to 3 years. To replace the “O” ring, the entire humidifier must be removed from both the steam and return piping. k) Steam Discharge Holes (Fig. 13-2 & 13-3) Discharge from Armstrong’s manifold is through holes drilled into the face, and the manifold is mounted so that steam is ejected into the oncoming air stream. Tests show that the steam is split into two streams, one over the top and one under the bottom of the manifold. The splitting tends to disperse and mix the steam and air quickly enhancing absorption and preventing long vapor trails.
Fig. 13-2. Manifold Discharge
Nozzles used in the past whistled because of steam velocity at high flow rates, and they tended to support separate longer vapor trails. Dri-Steem uses nozzles alternately angled upward and downward. This creates separate distinct vapor trails for each nozzle delaying mixing and complete absorption thereby lengthening the overall vapor trail and increasing the probability of water settling on air system components.
Fig. 13-3. Manifold Discharge
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l) Steam Traps The steam trap should be a fail open mechanical type which will not permit water backup under any condition. Only the Inverted Bucket type qualifies. The trap capacity should be 20% of maximum steam flow. Armstrong supplies an IB trap (Fig. 14-3) sized individually to match the capacity of each size of humidifier. Both competitors supply a ¾ inch F&T trap regardless of the humidifier capacity. (Fig. 14-4) The trap may be too small for the largest humidifier or unnecessarily large for the smallest, and the F&T can fail closed letting water back into the humidifier.
Fig. 14-3. Armstrong Steam Trap
Fig. 14-4. Dri-Steem Steam Trap
APP-508 5/97 3M
Armstrong International, Inc. 816 Maple Street, P.O. Box 408, Three Rivers, MI 49093 - USA Phone: (616) 273-1415 Parc Industriel Des Hauts-Sarts, B-4040 Herstal/Liege, Belgium Phone: (04) 240.90.90 Steam Traps \ Humidifiers \ Steam Coils \ Valves Printed in U.S.A.
14 ©Copyright 1997 Armstrong International, Inc.
