Cleanroom pressure vs airflow

[Kdouglas]

I'm looking for some advice. I'm an ME but not into HVAC.

I've got a cleanroom of known area/volume, with FFU's of known size and airflow, and we're trying to choose return-air vent sizes that will get us close to a defined pressure rise between it and downstream rooms. We've already calculated what we'll need for heat/cooling and have the AC/Heat units. But I was wondering if there is a basic equation to figure out the area of return air ducts.

Thanks!
Kevin

 

[ChasBean1]

Kevin, manufacturers (e.g., Titus, Tuttle & Bailey) will have register airflow data. The return side tends to be less important as long as ducts are big enough – volume dampering in the ducts can get you the proportions needed for space pressure gradients. Supply is more important due to throw characteristics.

If you know the return flow rates for each space, it might be a good idea to size return ducts keep roughly equal velocity in each branch. 700-1,100 feet per minute is a reasonable range.

 

[MechEngNCPE]

Size the returns for 0.05"wg/100ft. Grilles should have no less than 0.1" pressure drop. Obtaining pressure relations depend on size of doors, how they are constructed, tightness of room, etc. What classification is the room? ISO class 10,000 (EU Grade C) and higher requires low wall returns. Pressurization will be obtained by returning less than is supplied. With a 0.05" delta P across the door you can assume about 300 cfm per 3 foot wide door. If they are rapid rollup contact the manufacturer but beware because they tend to give you leakage rates that are low. Final leakage rates and return cfms will be made during balancing.