Transcript
ME 425 - Air Distribution & ASHRAE Outlet Selection
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ME 425 Air Distribution & ASHRAE Outlet Selection
Keith E. Elder, P.E.
The Air Distribution System The purpose of air distribution system is to create the proper combination of temperature, humidity and air velocity in the occupied zone of the conditioned room. If not properly designed, the air distribution system has the potential to compromise the comfort conditions it was designed to maintain.
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ASHRAE Air Distribution Design Recommendations Already defined in one lawsuit as “The Acceptable Standard of Care” Two Basic Rules
Cooling - diffuser selection should be based on the ratio of the diffuser’s throw to the length of the area supplied to achieve high ADPI (minimize draft) Heating - diffuser-to-room ∆t should not exceed 15°F to minimize stratification (minimize stratification)
Expanded Comfort Criteria Comfort is maintained through the change in seasons when the following conditions are maintained in the space occupied zone.
Air temp maintained between 73-77°F RH maintained between 25-60% Maximum air motion in the occupied zone 50 fpm cooling 25 fpm heating Maximum temperature gradient 1-2° cooling 4° heating
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Successful Design Achieves: Good ventilation effectiveness Avoids dumping Avoids draft
Occupied Zone 3 inches to 72 inches above the floor level. Two feet in from the walls
2 F t.
2 F t.
O cc u p ied Zone
3 "-6 '-0 " F t.
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Definition of “Draft” Koestel, Tuve and Reinmannn studied the effect of air motion in the 1950’s, measuring the warmth or coolness of a draft above or below a room temperature of 76°F at 30 inches above the floor at room center, with air velocity at 30 fpm. They defined “draft” as:
Any localized feeling of coolness or warmth of any portion of the body due to both air movement and temperature, with humidity and radiation considered constant.
Impact of Draft on Comfort
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Percentage of Occupants Objecting to Drafts
Effective Draft Temperature The physiological effects on a human body due to air temperature & air motion can be described by Effective Draft Temperature (the difference in temperature between any point in the occupied zone and the control condition). E.D.T. can be quantified as:
θ = tx - tc - 0.07(Vx - 30) Where:
θ = effective draft temperature tx = local airstream temperature, °F tc = average room temperature, °F Vx = local airstream velocity, fpm •
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Room Air Distribution Methods Displacement Ventilation Localized Ventilation Mixing Systems
Conventional Mixing Systems
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Entrainment Flow Conventional Mixing
Terminal Velocity A targeted value of interest – usually 50, 100 or 150 fpm
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Terminology Outlet/Inlet Terminals Grille Register Diffuser
Two Other Important Concepts Throw
The distance from the outlet to a point where the maximum velocity in the stream cross section has been reduced to a selected terminal velocity.
Characteristic Room Length
The distance from the diffuser to the nearest boundary wall in the principle horizontal direction of the airflow
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Outlet Classification Group A. Outlets mounted in or near the ceiling that discharge air horizontally. Group B. Outlets mounted in or near the floor that discharge air vertically in a non-spreading jet. Group C. Outlets mounted in or near the floor that discharge air vertically in a spreading jet. Group D. Outlets mounted in or near the floor that discharge air horizontally. Group E. Outlets mounted in or near the ceiling that project primary air vertically.
Group A Outlets
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Group A High Sidewall Grilles
Group A – Ceiling Diffusers
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Neck, face, core & free area
Neck Area & Neck Velocity
Vneck =
Qsa Aneck
Vneck Aneck
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Neck Velocity
Coanda Effect The attachment of a jet flow introduced parallel to a ceiling or other surface. This “surface” effect creates a lower-pressure region above the jet stream than below it, causing the higher pressure in the room to hold the airstream to the ceiling. Conditions necessary for surface effect include:
Angle of discharge < 40° from parallel surface A side wall outlet is within one foot of the ceiling A Floor or sill outlet is within 10 inches of a wall A ceiling outlet discharges along the ceiling
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Coanda Effect
Coanda Effect
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Outlet Discharge Patterns
Circular Diffuser The downward-jetpattern may be of benefit when the outlet is installed in a tall space and HVAC is in heating mode The visualization shows consequences for cooling mode
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Square/Rectangular Diffusers
Square Diffuser Still capable of establishing Coanda adherence to ceiling, but provides flexibility for distributing air in a variety of patterns
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Slot Diffuser
Slot Diffuser Also capable of Coanda adherence, but allows directing the air downward
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Air Diffusion Performance Index ADPI statistically relates the space conditions of local temperatures and velocities to occupant’s thermal comfort Quantitatively, ADPI is the percentage of locations where measurements show that the effective draft temperature, θ, is between - 3 and +2°F and the air velocity is less than 70 fpm.
Isodrafts for a space with an ADPI of 94%.
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Designing to ADPI Criteria ADPI performance can be predicted based on air outlet “throw” data referenced to room “characteristic room length”
ASHRAE Mixed Air System Outlet Selection Steps 1. Determine Air Volume Requirements & Room Size 2. Select diffuser type and location within room 3. Determine room characteristic length, L 4. Select recommended Tv/L ratio 5. Calculate throw distance by multiplying Tv/L ratio by room characteristic length 6. Locate appropriate outlet size from mfgr’s catalog. 7. Check static pressure & noise criteria
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Select diffuser type and location
Ceiling Diffuser Perforated Panel Ceiling Slot Diffuser High Wall Grille 0
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CFM/SF
Determine Characteristic Room Length Type of Outlet High Sidewall Grille Ceiling Slot Diffuser
Characteristic Length Distance to wall perpendicular to jet Distance to wall or midplane between outlets Perforated Ceiling Diffusers Distance to wall or midplane between outlets Circular Ceiling Diffuser Distance to closest wall or intersecting air jet Light Troffer Diffuser Distance to midplane between outlets, plus distance from ceiling to top of occupied zone
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Select Recommended Tv/L ratio
Check noise criteria Design Guidelines for HVAC System Noise in Unoccupied Spaces Private residences, apartments, condominiums Hotels/Motels Individual rooms, Meeting/banquet rooms Halls, corridors, lobbies Office buildings Private offices and Conference rooms Teleconference rooms Open plan offices Circulation and public lobbies Hospitals and clinics Private rooms, Operating rooms Wards Corridors, Public Areas
RC(N) Level Space 25-35 25-35 35-45 25-35 25 (max) 30-40 40-45 25-35 30-40 30-40
From 1995 ASHRAE Handbook, HVAC Applications, Chapter 43 (Abridged for this slide)
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Return Air Grilles
Ideally located in stagnant areas Usually placed in ceiling Low inlet velocities mean placement not
normally the cause of short-circuiting on welldesigned systems
Number Generally one per 4 supply outlets Minimum one per occupied room Air-flows under-specified 5 – 10% relative to supply for pressurization
Outlet Selection Example Select and locate appropriate ceiling air supply diffusers for the space shown below so as to maximize comfort in the occupied space. The ceiling is 11 ft and design supply air requirement is 350 CFM.
12’
24’
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Homework Read 05F33.1 - 33.18 Select and locate appropriate ceiling air supply diffusers on the reflected ceiling plan so as to maximize comfort in the occupied space. The ceiling is 10 feet above the floor and the design supply air requirement is 1400 CFM. The tile dimensions are 2’ x 4’
16’
42’
Reflected Ceiling Plan
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