Designing Air-Distribntinn SystemsTe Maximize Cemf ertBy David A. John, P.E., Member ASHRAE

An air-distribution system that provides occupant thermal comfort canbe a compiicated system to predict and analyze. Providing comfortdepends on variables from the obvious thermal conditions in a space, which

include radiant temperature, air speed, air temperature and humidity,

to the less obvious occupant metabolic rate and even choice in clothing.

A system can be successfully designed by understanding what makes us

comfortable and selecting the proper air-distribution products and layout.This article discusses how HVAC de-

signers can select, size, and place outletsusing methods described in the roomair-distribution chapters in the 2009ASHRAE HandbookFundamentals tomaximize occupant thermal comfort asdefined in ASHRAE Standard 55-2010,Thermal Environmental Conditions forHuman Occupancy.

Standard 55-2010 addresses fac-tors that determine human comfort in

20 ASHRAE Journal

a space. Until recently, the air diffuserperformance index (ADPI) as outlinedin Standard 113, Appendix B, was amethod to predict occupant comfort.The cognizant committee for ASHRAEStandard 113-2009, Method of Testingfor Room Air Diffusion, changed thelanguage used in the ASHRAE Hand-book to indicate ADPI is a measure ofpredicted room air thermal mixing, nota direct measure of occupant comfort.

ashrae.org

This article attempts to define thespace comfort as defined by Standard55-2010, noting that this does not tella designer how to select or space air-distribution devices. Using ADPI, adesigner can select, size, and spaceoutlets but can only measure the ther-mal mixing, not the level of occupantcomfort.

Predicting and Quantifying ComfortThe purpose of Standard 55-2010

is to indicate the combination of in-door thermal environmental factorsand personal factors that will producethermal environmental conditions ac-ceptable to a majority of space oc-cupants. The variables that definecomfort in the standard are: metabolicrate, clothing insulation, air tempera-ture, radiant temperature, air speed,and humidity.

About the AuthorDavid A. John, P.E., is general manager, vice pres-ident of A.D.E. Engineered Solutions of Florida, Inc.

S e p t e m b e r 2012

-1-3 Hot

+2 Warm

-1-1 Slightly Warm

0 Neutral

-1 Slightly Cool

-2 Cool

-3 Cold

Figure 1: ASHRAE thermal sensationscale from Sfandard 55-201 0.

The thermal comfort chapter (Chapter 9) in the 2009ASHRAE Handboo/

RELATIVE HUMIDITY80

When applying this Graphic per Section 5.2.1.1, the follov '^ing limitations appiy: Applies to Operative Temperature oniy - cannot be applied based on dry

bulb temperature atohe. See Appendix C for acceptabie approximations. Appiies oniy when requirements of Sections 5.2.3 through 5.2.5.2 are met.

For other compliance paths, see Sectioh 5.2.1.2 for theComputer Modei Method and Section 5.3 for theOptional Method for Naturaiiy Conditioned Spaces.For further compiianoe requirements,see Sections 6 and 7.

Comfort zone moves ieft with:,.-* Higher clothing

Higher metabolic rate Higher radiant temperatureSee Section 5.2.1.2

Computer modei analysis requiredfor humidity ratios above 0.012:See Section 5.5.1.2

Comfort zone moves nght Vi/ith Lower clothing Loviier metabolic rate Loviier radiant temperatureSee Section 5.2.1.2

zone l.5.2.3todetermine\ \ cooiing effect of

elevated air speed

- No lower humidifyrecommendation for graphicalmethod: See Section 5.2.?

.002

OPERATIVE TEMPERATURE ("F){'/ Dry bulb * 'h MRT for still air)

Figure 3: Graphic comfort zone method from Standard 55-201 0.

The metabolic rates ranging from 1.0 to 1.3 is typical ofan office worker in near sedentary physical activity such asworking at a desk in a seated position. More information onmetabolic rate can be found in the Standard 55-2010 Norma-tive Appendix A activity levels. Also, a detailed discussion ofmetabolic rate can be found in Chapter 9, HandbookFun-damentals.

Ciotiiing insulationClothing insulation is measured in units of clo. As a refer-

ence, the 0.5 clo is typical for an office environment in thesummer and 1.0 clo is typical for the office environment inthe winter. The Normative Appendix B, Clothing Insulation,in the standard is a good reference for calculating different clovalues for occupants. Clothing insulation is discussed in detailin Chapter 9, HandbookFundamentals.

Radiant Temperature AsymmetryOccupant comfort is also affected by the thermal radiation

field around the body, which can cause discomfort. The radi-ant temperature asymmetry is caused by factors such as hot orcold surfaces or direct sunlight. The radiant temperature may

differ from the dry-bulb temperature in a space. Standard 55-2010 does include allowable radiant temperature asymmetryfor a space. Also, Chapter 9, HandbookFundamentals, is agood reference.

Standard 55-2010 lists four methods for evaluating comfort:1. Graphic Comfort Zone Method for Typical Indoor Envi-

ronments;2. Computer Model Method for General Indoor Applica-

tions;3. Graphical Elevated Air Speed Method; and4. Standard Effective Temperature (SET) Model.All four methods include human factors that determine

comfort such as metabolic rate and clothing insulation, andthermal factors such as space temperature, air velocity, hu-midity, and radiant temperature.

1. Graphic Comfort Zone MethodThe graphical method for predicting comfort in Standard

55-2010 assumes the occupants' metabolic rate is between1.0 and 1.3 met and the clothing worn is between 0.5 and 1.0clo (typical for an office). This method predicts comfort foran acceptance level of 80%. This is based on a 10% PMV-

22 ASHRAE Journal ashrae.org September 201 2

300

TEMPERATURE RISE, 'C1.1 2.2 3.3

0.0 2.0 4.0 6,0TEMPERATURE RISE, "F

8,00,0

OPERATIVE TEMPERATURE (F=)72 76 79 82 90

276

UJ

B tL

IB t9 20 21 22 23 2 25 26 27 28 29 30 31 32OPERATIVE TEMPERATURE (C)

Diffuser Type

High Sidewall Grille

Circular Ceiling Pattern Diffuser

Sill Grille

Ceiling Slot Diffuser

Light Troffer Diffusers

VCross-Flow Pattern Ceiling Diffusers

Characteristic Length L ^

Distance to Wall Perpendicular to Jet

Distance to Closest Wall or Intersecting Air Jet

Length of Room in Direction of Jet Flow

Distance to Wall or Midplane Between Outlets

Distance to Midplane Between Outlets PlusDistance from Ceiling to Top of Occupied Zone

Distance to Wall or Midplane Between Outlets jTable 1: Characteristic room length for several diffusers from 2009 HandbookFundamenfals.

Figure 4 (left): Graphical elevated air speed method. Figure 5 (right): Standard effective temperature (SET) method.

PPD index plus an additional 10% dis-satisfaction that may occur from localthermal discomfort. Air speeds are notgreater than 40 fpm (0.20 m/s). Themethod includes two areas of comfort:one for clothing insulation of 0.5 cloand one for 1.0 clo (Figure 3).

Added to Standard 55-2010 is pre-diction of comfort using elevated airspeeds. The graphical method includesFigure 4 to calculate the required airspeed for applications with both meanand radiant temperatures. This figureallows for elevated air speeds of more than 150 fpm (0.76m/s).

2. Computer Model MethodThe computer model method predicts the PMV and PPD for

a given space. The standard includes computer code (Norma-tive Appendix D) that assumes an average metabolic rate be-tween 1.0 and 2.0 met, and where clo values of the occupantsare 1.5 or less.

3. Graphical Elevated Air Speed MethodIncluded in Standard 55-2010 is the elevated air speed

method that allows for higher space temperatures and roomair velocities. The comfort level obtained depends on wheth-er or not the occupants have local control of air speed. Thismethod applies to a lightly clothed person with clothing in-sulation between 0.5 and 0.7 clo who is engaged in near sed-entary physical activity with a metabolic rate between 1.0and 1.3 met.

4. Standard Effective Temperature (SET) ModelThis method in Standard 55-2010 uses a thermophysi-

ological simulation of the human body and skin heat lossto predict the occupant's comfort level. This model enablesair velocity effects on thermal comfort to be related acrossa wide range of air temperatures, radiant temperatures, andhumidity ratios (Figure 5).

ASHRAE Thermal Comfort ToolThe ASHRAE Thermal Comfort Tool is a convenient method

to predict PMV/PPV for a space. The tool allows designers topredict PMV based on standard conditions, elevated air speeds,and adaptive method. The program allows users to input air tem-perature, air speed, humidity ratio, mean radiant temperature,activity level (that converts to met) and clothing (that convertsto clo). The program output shows PMV and PPD, as well as in-dicates whether the selection complies with Standard 55-2010.

Selecting Outlets to Maximize ComfortStandard 55-2010 can be used by designers to predict the

comfort level of a space based on PMV, but the standard doesnot indicate to designers where to locate air-distribution de-vices. For an overhead forced air system, the tools availableto the designer to maximize the occupant comfort level is themanufacturers' outlet performance obtained per Standard 70-2006, and the Tf/L ratio to predict the ADPI.

Method of Testing for Room Air DiffusionA method to calculate the ADPI value in a space with over-

head mixing air distribution operating in cooling is outlined inStandard 113-2009, which defines a repeatable method of testingsteady-state air diffiision performance of an air-distribution sys-tem in occupied zones of building spaces. The standard is basedon air velocity and air temperature distributions at specified cool-ing loads and operating conditions. The standard can be applied to

24 ASHRAE Journal ashrae.org September 2012

furnished and unfurnished spaces, actualor laboratory conditions, with or withoutoccupants. The standard is not applicableto naturally ventilated building space.

Appendix B of Standard 113-2009states that the test procedures can be usedto generate the air distribution perfor-mance index for a space. The ADPI is asingle-number rating of the air diffusionperformance of a system of diffusers, asinstalled in a defined space, for a specifiedsupply air delivery rate and space load.ADPI is based only on air speed and effec-tive draft temperature and is not directlyrelated to the wet-bulb temperature or rel-ative humidity. Wet-bulb temperature, hu-midity, and similar effects (such as meanradiant temperature) should be accountedfor according to Standard 55-2010.

The ADPI method requires calculat-ing the eff^ ective draft temperature atmultiple points. The effective draft tem-perature is:

i = ia.-fc-0-07(v,-30)Fwhere

([) = effective draft temperature attest point n

tacn~ corrected temperature at testpoint n

t^^ = average test zone temperaturev^ = time-averaged speed at test

point nADPI = Number of test points that meeteffective draft temperature criteria

(-3F and +2F) %Total number of test points

ADPI is for traditional overhead air-distribution systems under cooling op-eration only. The results can be used asan indicator of occupant comfort in aspace. A high percentage of people willbe comfortable under sedentary condi-tions where the effective draft tempera-ture is between -3F and +2F with anair speed of less than or equal to 70 fpm(-1.7C and +1.1C with an air speedless than or equal to 0.35 m/s). TheADPI is the percent of test points thatmeet these criteria.

Using TQ/L to Select OutletsA designer can use the ratio of T^Q/L

to predict the level of mixing in a zone

rTerminal Device

High SidewallGrilles

Circular CeilingDiffusers

Sill Grille,Straight Vanes

Sill Grille,Spread Vanes

Ceiling SlotDiffusers (for

Light TrofferDiffusers

Cross-FlowIPattern Diffusers

RoomLoad,

Btu/hft280604020

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