In what basis supply and Return grills/diffusers selections are made ?

air flow + Noise + throw + pressure drop

1. Air Flow (CFM / L/s)

2. The  throw , it should reach the room @ least 3/4  of the room.

2. Noise criteria, it must be less than 30NC or 35dba.

3. terminal velocity

4 Pressure drop

1. The  trow , it should reach the room @ least 3/4  of the room.

2. Noise criteria, it must be less than 30NC or 35dba.

3. terminal velocity

4 Pressure drop

5 Architect selection- anodized or baked powder type. 

Thanks hope it will help u.

Robert

hello 

to size a linear grill you must take care about many parameters 

1-CFM per foot 

2-Listed height 

3-friction losses 

4-face velocity 

5-sill and sidewall throw

6-minimum ceiling height 

7-NC

lets imagine that you have a concealed A/C with 1000 CFM connected to only one  sidewall grill 

1-CFM per foot : if the standard length for a sidewall grill is 4' , so you have now 250 CFM/ft , that will lead you to all parameters remaining- from 250CFM/ft you will find that available listed height is 6 , 8 and 10 inches

2-Listed height : so you now have 3 options 6 , 8 and 10 inches , your decision will be according to selected depth of your duct ( you sized the duct according to recommended velocity referred by SMACNA- if this FCU is located in residential application so you can do you calculation in velocity 800 FPM- after sizing you can obtain that the duct depth will be 10'' )  

3-friction losses : now after you have CFM/ft and listed height you can easily find that friction losses will be 0.01 InWg  ( you should accept this value or refused - check blower performance vs external static pressure in A/C catalog )

4-face velocity : and also you will find face velocity 550 FPM ( accept or refused according to SMACNA recommendation )

5-trow : you will find that sidewall trow range from 21 to 32 ft ( accept or refused according to room or space dimensions ) 

6-ceiling height : the ceiling height in this case must me minimum 15 ft 

7-NC : and NC here is 25 ( i would accept 25 NC for residential applications )

all these data from ZamZam ( linear grill)  airoutlets catalog you can find it in internet 

Good luck 

Diffusers and grilles should be selected and calculated according to the following criteria:

• The type and style
• Function
• Air volume requirement
• Throw requirement
• Pressure requirement
• Sound requirement

I agree with Muhammad Arsalan Siddiqui, ROBERT CALIMLIM , and Syed Taqiuddin.

The selection depends on

1. the cfm (the datas are liste in catalogues of their respective brands)

2. the Neck velocity

3. the type of air flow pattern

Air Flow (CFM / L/s), The  throw , Noise criteria(varies), terminal velocity & Pressure drop

air flow rate in CFM or l/s + NC level or db (noise or sound criteria) + distance of throw of air + pressure drop as we require it for ESP calculation.