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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ME 425 - Air Distribution & ASHRAE Outlet Selection
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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
1
2
3
4
5
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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