10 air distribution basics

ASHRAE‐Chicago
Air Distribution Selection Basics
June 8, 2010

ASHRAE ‐ Air Distribution Systems
2007 Applications Handbook, Ch. 56
Fully Stratified Partially Mixed Fully Mixed

Upper Zone
Upper Zone Upper Zone
height varies
Fully Mixed

Elevation

Elevation
Elevation
Elevation

Lower Zone Lower Zone Lower Zone

Local Temp. Local Temp. Local Temp. Local Temp.
CO2 Concentration CO2 Concentration CO2 Concentration CO2 Concentration

Partially Mixed Room Air Distribution Systems

Displacement Under Floor Air Distribution G.R.D.

© 2010. All rights reserved

ASHRAE Standard 55‐2004
Occupant Comfort Goal
Dry Bulb = 73-77 °F.
Relative Humidity = 25-60%.
Local Velocities =
Heating < 30 fpm.
Cooling < 50 fpm.
Clo. Rate 0.5 - 1.1
Met. Rates 1.0 - 2.0
Floor level to 66”; < 5.4 °F (3 °C)
Occupied Zone: Floor to 6’0”; except:
(Clear, Near, Adjacent) Zone.


Occupied Zone for Comfort
Mixed Air Outlet

Ceiling

1'
6' F
Occupied Zone

Floor
Underfloor Plenum D.V.
UFAD Outlet


Fully Mixed Air Distribution
Air Patterns
Mapping Room Air Motion
Vertical Projection
ADPI
Perimeter Heating


Air Flow Patterns
High Sidewall – No Ceiling
 Horizontal Pattern
 Deflected (less drop)
 Spread (shorter throw)

High Sidewall with Ceiling
 Longer throw with less drop
High Sidewall near Ceiling
 Deflect up for longer throw/less drop


Supply Sidewall Grilles
Adjustable:
Horizontal
(spread)

Vertical
(deflection)

Duct Mounting
 Velocity & Pressure
 Air Patterns


Return Sidewall Grilles
Return
 Non Adjustable

 Zero Degree Deflection

 30-45 Deflection

 Other


Air Flow Patterns
Radial Air Pattern
 Shorter Throw
 More Induction


Round Pattern Diffusers


Air Flow Patterns
Cross Flow
 Longer Throw

 Increased Drop


Cross Pattern Diffusers


Linear with Plenum or T‐Bar Diffusers
 With Spreader  W/O Spreader

Linear
Plenum

With Without
Spread Spread


Linear‐Plenum Slot Diffusers


Air Flow Patterns
Radial Pattern - Exposed Duct


Air Flow Patterns
Cross Pattern - Exposed Duct


POOR PATTERN EXAMPLE
POORLY ADJUSTED
DIFFUSER

THERMOSTAT


GOOD PATTERN EXAMPLE
PROPERLY ADJUSTED
DIFFUSER

THERMOSTAT


Mapping Room Air Motion
5-step approach
 Primary air
 Total air
 Convection currents
 Room intake
 Room air


Step 1: Identify the Primary Air
Primary Air - mixture  Primary air patterns are
of air supplied to the outlet temperature independent1
and induced room air  Catalog Throw:
within an envelope of
velocities > 150 FPM  150 – 100 – 50
 10 - 17 - 25

Inside Step 1 Exposed
Wall Primary Air Wall

Outlet


Step 2: Identify the Total Air
 Total Air - mixture of  Temperature difference
primary air and room air between total air and room air
which is under the influence produces a buoyancy effect
of the outlet conditions which cause the cool air to drop
and the warm air to rise
 Air has a tendency  Total air has no defined
to scrub surfaces terminal velocity limit

Step 2
Total Air


Step 3: Identify Natural
Convection Currents
 Natural Convection  Affects room air motion
Currents - Caused by and affects the comfort
difference in temperature in the space
between room air and air
in contact with a warm
or cold surface

Step 3
Natural
convection
currents and
stratification
zone


Step 4: Identify the Return Intake
 Return Intake - return  Even natural convection
intake affects only the currents will overcome the
air motion within its draw of the intake
immediate vicinity


Step 5: Identify the Room Air
 Room Air Diagram is complete  Most uniform air motion is
when remaining room air drifts between stratification zone
back towards the primary air and total air
and total air  Highest air motion is in and
 Lowest air motion is near the primary and total air
in stratification zone


Mapping Throw


Mapping Throw

Cooling


Estimating Downward Projection
 Chart provides
vertical throw of a
free jet to 50 fpm for
heating and cooling
with the same
diffuser.
23 40 28
1000 fpm
 Typically, cfm and jet
velocity are known.

 2009 Fundamentals
Chapter 20

1000 cfm
+10 -20 0

ADPI & Comfort
Air Diffusion Performance Index
Statistically related the space conditions of local
or transversed temperature and velocities to
occupants’ thermal comfort
ADPI >= 80 is considered acceptable
Effective draft temperature
 = (tx-tc) – 0.07(Vx-30)
 % of points where -3<= <= +2 = ADPI
 Velocity below 70 fpm


ADPI
The grey area
represents
-3<= <= +2
You can vary
temperature
or velocity to
maintain comfort
15 fpm = 1 °F


Throw vs. Characteristic Length


ADPI Selection Illustration

Point A

ADPI Min 80 ADPI Min
Max
0.6 1.2

20x20 Room
400 cfm = 1 cfm/sq ft
Characteristic Length = 10
Therefore, look for T50 = 12 feet at 400 cfm (Max VAV)
Check Turndown point at .6 T50/L (Min VAV)
Set constant volume systems at Point A.

T50/L


Recommended ADPI Ranges for Outlets
Outlet T50/L Range Calculated T50 & L Data

Sidewall Grilles L 10 15 20 25 30
1.3-2.0 T50 13-20 20-30 26-40 33-50 39-60
Ceiling Diffusers L 5 10 15 20 25
Round Pattern
0.6-1.2 T50 3-6 6-12 9-18 12-24 15-30
Ceiling Diffusers L 5 10 15 20 25
Cross Pattern
1.0-2.0 T50 5-10 10-20 15-30 20-40 25-50
Slot Diffusers L 5 10 15 20 25
0.5-3.3 T50 3-18 5-33 9-50 10-66 15-83
Light Troffer L 4 6 8 10 12
Diffusers

1.0-5.0 T50 4-20 6-30 8-40 10-50 12-60
Sill & Floor Grilles L 5 10 15 20 25
All Types 0.7-1.7 T50 4-9 7-17 11-26 14-34 18-13


ASHRAE Standard 62.1 Table 6.2
Ventilation Air Ez Factor
Ceiling supply of cool air 1.0
Ceiling supply warm air < 15 °F Δt 1.0
150 fpm supply jet reaches 4.5’ AFF
Ceiling supply warm air >15 °F Δt 0.8
Floor supply cooling ceiling return UFAD:
 T 50 > 4.5’ AFF 1.0
 T 50 < 4.5 AFF (RP-1373) 1.2
Displacement Ventilation 1.2


Typical Perimeter Solution
 50/50 throw pattern is
the best compromise for
both heating and cooling
 Half of the air is always
directed in the wrong
direction
 ASHRAE Std. 62.1-2007
 T150 to 4’-5’ AFF
 Max Δt = 15 °F


Auto Changeover Diffuser Solution
Diffuser

Cooling

Window
Air flow does not pause in
intermittent pattern to cause
sustained drafts pop action
of air flow pattern.

Sill
Heating


Auto Changeover Diffuser
 Vertical Pattern For
Heating
 Horizontal Pattern For
Cooling
 30% Increase In Energy
Efficiency
 Room reaches set-point
faster
 Room Stratification for
heating similar to cooling


Sills & Soffits on Perimeter
 Soffits cause air to turn Soffit
up prematurely
 Aim air under the soffit
to hit the glass or cold wall

 Sills deflect cold air
into the room
 Sill height sets the stratification
layer height

Sill


Air Distribution, Partition Effects


Partially Mixed Air Distribution
UFAD – Interior Zone
Perimeter with Series Fan Terminal
Perimeter with Plenum VAV Cooling
Perimeter with Passive Heating


Underfloor Air Distribution (UFAD)

Stratified
Zone


Round Access Floor Diffuser
High induction air flow pattern
Occupant adjustable damper
 Actuated version available

Typical design point is 80-100 cfm
Throw between 4-5 ft.
Low NC


Interior Applications
Use Conventional Components…
 Round (Swirl) Diffusers
 Rectangular Diffusers

In Conventional Applications
 One per worker or area measure


Series Fan Terminal for Perimeter
Used in perimeter and conference rooms
 Any room with varying loads
10 ½”, and 14” tall units for 12” and 16” raised
floors
300 cfm to 1200 cfm
Standard series fan
powered terminal
Primary supply
air damper
 Supply pulls from the floor
 Option for two dampers


Typical Perimeter Installation
Low horsepower fan powered terminal unit
ducted to rectangular floor diffuser plenums


Plenums
Plenum sizes vary
 8” x 16”
 10” x 10”
 12” x 12”

Cores are available in multi-piece
 2-way or 4 way blow


ECM Motor
Electronically Commutated Motor


ECM Motor
Brushless, DC motor
Converts AC to DC for motor
Ultra high efficiency motor
 Efficiencies of up to 70% across the entire operating
range
Energy savings
Control of fan speed through DDC controller


Energy Savings
350 watts savings = .350 kW

250 days/year x 12 hours/day
= 3000 operating hours/year

.350 kW x 3000 hours = 1050 kWh

1050 kWh x $.10 per kWh
= $105 per unit per year energy savings!
 Does not include demand charges


PWM
The ECM motor is controlled by a PWM (Pulse
Width Modulated) signal
Voltage pulses over a specific time period
Voltage is proportional to cfm
Low voltage device
Manual PWM is adjusted like a SCR, with a
screwdriver
Remote PWM is controlled by a 0-10V signal from
the DDC controller


Additional ECM Benefits
Soft start
 Motors are set to start up slowly
No brushes
 No noise associated with brushes
 Titus has found no significant overall sound reduction
with the ECM motor
 No wearing of brushes
ECM motor life is ~90,000 hours (30 years of
typical series fan box life)
Cooler operation


Linear Perimeter System
 (Variable Air Volume)

 (Water Heat)

 (SCR Electric Heat)

 (Return)


Perimeter Linear Diffuser Solution


Transverse Apertures


Perimeter Smoke Demo Video


Interior Zone VAV Cooling

Conference Waiting Area


Underfloor System Christman

Linear Heat
3900 Btu/h
4’ Length


Fin tube application


Fully Stratified Air Systems
Energy & Comfort Optimization (ECO)
Basic system concepts
Displacement diffusers


DV‐ Basic Concepts

Supply Air
TL = 65°F

 Higher supply air temperature
 Air change effectiveness factor of 1.2 Clear
Zone


Displacement Ventilation Applications
Good application
 Room height > 9.5 ft
 Open Plan Offices, Classrooms, & Meeting
Rooms

 Casinos, Restaurants, Theaters, &
Auditoriums


Displacement Ventilation Applications
Poor application
 Room height < 8 ft
 Surplus heat is the main problem – not air
quality

 In combination with chilled beams


System Equipment
Higher equipment efficiency
 Uses warmer supply air
 65°F compared to 55°F
 Energy consumption is often reduced by raising
discharge temperature


System Equipment
Lower horsepower fans
 0.04 in. pressure is required for proper
diffuser performance

 Results in lower horsepower fans required


HVAC System Benefits
Heat sources outside the
stratification layer are not
considered in airflow calculations
 Lighting load is designed as “equipment
load”, but not as “space load”

 Effects cooling load capacity, but not air
distribution capacity


Perimeter Heating
Perimeter heating cannot be
accomplished with the displacement
ventilation system

Separate system is required:


Return Air
Located at ceiling

Allows heat from ceiling light to be
returned before it is able to mix with
occupied zone


Return Air
If 55oF supply air is used for
humidity reasons, return air can be
mixed with plenum air to achieve
65oF air

If 65oF supply air is used, return air
can be brought back to the air
handling unit to be filtered and
dehumidified

Temperature Gradient
ASHRAE Standard 55:
 Occupied Zone
 < 5.4°F

Risk of draft increases
if ∆t > 36°F between
supply and exhaust air

(Displacement ventilation, REHVA)


Ceiling Height 9’-14’

50% Rule is applied

62°F – 72°F – 82°F
10°F - 10°F


33% Rule for > 14’
room heights
33% 67%

64°F – 72°F – 86°F
8°F - 14°F



Adjacent zone
Discharge
Velocity < 80 fpm
Area where air
velocity exceeds
50 fpm
Occupants
should not be
stationed within
the adjacent zone


Comfort with Adjustable Air Patterns

 Standard adjacent zone from
factory

 Modified adjacent zone after
adjusting pattern controllers to
change pattern


Comfort Air Distribution
The ASHRAE 2009 Fundamentals handbook,
Chapter 20, 2007 Applications handbook Chapter
56, and 2008 Systems & Equipment handbook
provides guidance on diffuser selection.
Select throw at Max. & Min. to meet ADPI
guidelines.
It cost less for install a good design than to
replace a bad design.
Employees cost 70 times the cost of Energy.


10 air distribution basics

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