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Sizing a Supply/Return Register of Diffuser

There are a number of influences that could affect the choice of a supply register of diffuser. Putting emphasis on

one influence over the others will alter the outcome. Some of the input criteria for making a selection are:

CFM Face velocity Throw and Pressure loss

!ther influences may include:

"oise criteria Mounting location #eating$cooling$both %ooks &vailability Price style Si'e limitation

Material and new$replacement

The type of occupancy or activity in the space under consideration may eliminate some variables. "o single type of

supply register provides optimum air distribution for year(round heating since the preferred outlet for one season

will be compromised for the other season. #owever the success of an air diffusion system is dependent almost

entirely of the location of the supply diffusers. The location of return grills is far less critical. )eturn air flow

affects room air motion only in the immediate vicinity of a return air grille.

&s a general rule ma*im in recommended face velocity for a supply outlet used in residential application is 800

FPM. Ma*imum recommended return grille velocity is 600 FPM. Systems with higher velocities are likely to have

noise problems.

)eturn air filter grille velocities greater than about 400 FPMwill e*ceed the performance of the filter.

Ceiling Diffuser

For a given CFM and chosen register style and location+s, use the chart to select a si'e that delivers the re-uired

CFM at a velocity that is recommended for the particular application of throw re-uirement. "oise increases with

increasing velocities. f CFM is constant an increase in either register si'e or the number of registers of a given si'e

will decrease the velocity throw and related noise.

For this initial discussion of si'ing a supply register or diffuser we will narrow the selection criteria to only CFM

face velocity and throw which will minimi'e unnecessary complications. The location is a ceiling with a circular

or /(way pattern re-uired. & circular ceiling diffuser such as the #art 0 Cooley 123 )ound Ceiling 4iffuser is one

possible choice.

5ith a re-uirement of 267 CFM there are a number of choices for a si'e that can deliver this airflow. & 3(inch

diameter will deliver 267 CFM at a face velocity of 877 feet per minute +FPM, and a radial throw of 9 feet. &n (

inch will also deliver 267 CFM but at a reduced face velocity +and -uieter delivery, of 9;7 FPM and a throw of

appro*imately / feet. &nd lastly a 27(inch will have a face velocity of ;97 FPM and a throw of ; feet.

Face ?elocity * &rea

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The @&reaA +in s-uare feet, for each si'e is the @&BA number listed under each diameter. For instance to find the

face velocity of the 27(inch at our stated 267 CFM re-uirement divide 267 by the area: CFM$&rea > Face ?elocity

or 267$.;/9 > ;/ FPM face velocity.

Engineering Data for a Hart & Cooley !6 Roun" Ceiling DiffuserFa#e $elo#ity %00 400 00 600 '00 800 (00 !000

Pressure loss )006 )0!0 )0!6 )0** )0%! )040 )00 )06*

3A

&B .2;9

CFM

Throw

99

6.9

39

;.7

7

;.9

89

/.7

279

/.9

267

9.7

2;9

9.9

A

&B .669

CFM

Throw

7

6.7

87

;.7

229

;.9

2;9

/.9

237

9.7

27

9.9

677

3.9

669

.7

27A

&B .;/9

CFM

Throw

279

6.9

2/7

;.9

29

/.9

627

9.7

6/7

3.7

69

.7

;27

.7

;/9

.9

26A

&B .977

CFM

Throw

297

;.7

677

/.7

697

9.7

;77

3.7

;97

.9

/77

.9

/97

8.7

977

27.9

2/A

&B .369

CFM

Throw

287

;.9

697

/.9

;29

9.9

;9

3.9

//7

.7

977

8.7

939

27.7

369

22.7

2A

&B 2.7/

CFM

Throw

;27

/.9

/29

3.7

967

.7

369

.9

;7

27.7

;7

22.9

8;9

2;.7

27/7

2/.9

Terminal ?elocity of 97 FPM

Engineering Data for a Hart & Cooley +SR S,uare Return Diffusers

Fa#e $elo#ity 00 600 '00 800 (00 !000 !*00 !400

Pressure loss )0*0 )0*0 )0%0 )040 )00 )060 )0(0 )!*0

8 * 8

&B .;/7

CFM 39 7 87 279 229 2;7 237 287

8 * 8

&B .;/7

CFM 297 27 627 6/7 67 ;77 ;37 /67

26 * 26

&B .3/7

CFM 287 669 639 ;77 ;9 /97 969 377

29 * 29

&B 2.77

CFM 669 67 ;29 ;37 /79 /97 9/7 3;9

2 * 2

&B 2.37

CFM 637 ;29 ;39 /67 /7 969 3;7 ;9