Room pressure gradient calculations

[Dani8]

Hi everyone

I was a bit confused on the matter of room pressurization in pharmaceutical facilities. We are currently designing an area where we have to create a pressure gradient across 4 rooms. The inner most being at 65 Pa (gauge) then the others are all at a differential of 15 Pa each until we get to the outer corridor which is at 5 Pa. The air changes requirement are different as the production area is class C alongwith the adjacent room and the other 2 are class D. Now I'm confused as to how much external air do i need to pump into the system to maintain the desired pressure gradient in an "operating condition". When the rooms are also taking up return air and are being supplied with new air all the time, if I add a certain amount of air per cycle, wouldnt that mean that the pressure in the room will keep on increasing? I have accomplished this practically by an iterative check method which is very time consuming and difficult. I set the supply using CAVs and I squeezed the return air until the pressure built up to my desired value. But i want to know if there is a theoretical calculation I can do to preset the amount of supply air and offset the return air value by some amount that would enable me to accomplish this task much more easily.

Thank you

 

[lilliput1]

In our firm (in USA) we don't set specific pressure. Instead we indicate pressure by +++, ++, + 0, -, --, -- etc from highest pressure room to lowest. Then we set pressurization air CFM through doors usually 100 CFM per door to next lower pressure room but within the control accuracy range of supply & return or exhaust air boxes serving the space and set corresponding control CFM differential on them.