About selection of return grille

[lzh007]

When select return grille, what's the rule to follow.
Someone says the speed should be kept under 500 fpm. While according to some product catalogue, if using the lowest NC level to select, the speed can be 800 fpm or even higher.
I am a little confused. The reason to keep the speed is to control noise, if the NC level is low enough, why should we use 500 FPM? (ACCA manual D requires 500 FPM for return grille)

 

[willard3]

Keeping the RA velocity down also keeps the pressure drop down.....

500 fpm is a good rule of thumb.

 

[ChrisConley]

The rule of thumb is 800 fpm maximum through the free area of the grille. As free area varies from grille to grille, 500 fpm through the grille usually translates into an acceptable number.

If you are concerned, you can usually find the actual free area and size the grille accordingly.

 

[chicopee]

Return and supply grills are specified in feet per minute(FPM). Return grills have a recommended face velocity of 500 to 1000 FPM.