Clean Room Pressure Gradient Control

[hamidpia]

Clean rooms of different class are to be maintained at a differential prresure of 12 to15 pa, this is to prevent contaminants travelling from area of higher class to lower class. I had tried to maintain this differential pressure by balancing cfm in and cfm out. with cfm out being less that cfm in room manometer should read some positive pressure but it doesn't. I suspect that i might have to install blower in the return duct. Right now there is one draw through blower installed in the dx Air handling unit. Can some body help me.
Best regards

[Brian_UK]

Yes, you should have a higher air flow from the supply to pressurise the room.

You also need barometric pressure relief dampers fitted to keep the overpressure constant.

If you only have one fan providing the air flow of both supply and extract then it is an impossible task.

[hamidpia]

Thanks Brian, I have also tried use of dampers to reduce return air cfm also but it doesn't help. Differential pressure still shows negative or barely zero. If I am thinking on right track then what should be the capacity of blower in the return duct, should it be same or different , as the origional blower.