Sizing Return Air Grilles

First check the system needs positive pressure or negetive pressure. If positive pressure then size of the return air grill may be less than the area of supply air grill or vise-versa.

Size of the grills depends upon the AHU capacity like Static pr., Air quality like once through etc, temperature condition etc.. OT, ICU, IBU need more filtered air and size needs bigger due to less air flow velocity & quantity.

Any how design the size of grills such a way that 400 cfm TO 600 cfm (Grill size 450 mm x 150 mm with adjustable louvers) need for 01 ton AC load .

If return air grille is at the ceiling, use 600-700 FPM face velocity. In the wall grille should be 300-400 FPM. Based on free area of grille.Figure 65 % free area. Ex: for 100 cfm grille at the ceiling,you would use a grille with free area of 100/600 = 0.167 sf, gross arera of 0.167/.65 = 0.256 sf...a 6 x 6 grille would be used. In the wall grille: 100/300 = 0.333 sf/.65 = 0.51 sf......12 x 6 grille would be used. These are "standard" sizes.

Get a Register and Grille catalog from Titus Co. It contains about all the info you need for sizing grilles and registers......You may find the info on internet........

The smaller the return air grille, the more restriction there is. Why would you want to restrict return air? It does not have to be sized by CFM. Why not use a standard drop-in return air grille made for such ceilings?

You are exactly correct when using a lay-in ceiling consisting of ceiling tile laid into a grid system and the space above the rtn. gr. is a plenum. PatyO didn't indicate a lay-in system, but a drop ceiling, which is a different type ceiling. It is not uncommon to use the drop ceiling concept to contain the supply air ducts, and using that space as a return air plenum.

At the velocities I indicated, the pressure drop through the return air grille is minimal, in the neighborhood of 0.05 - 0.07" H2O.

PatyO states clearly: "Note: This application is using the drop ceiling as the return air plenum."

Anybody knows how to size the return grille (no duct return)?