How to use the cleanroom pressurization airflow calculator
Filtration alone does not keep a cleanroom clean. The room has to sit at a higher pressure than the spaces around it so that dirty air cannot drift in through door gaps, and that means supplying more air than you return. This calculator turns a target differential pressure and an effective leakage area into the offset airflow, compares it with the air change requirement, and lays out the supply, return and exhaust quantities.
Leakage follows an orifice relation. Flow is proportional to leakage area but only to the square root of pressure, so doubling the differential pressure raises leakage by about 1.41 times while halving the leakage area cuts it straight in half. Sealing the envelope is almost always cheaper than buying fan capacity.
The larger requirement sets the supply. In most cleanrooms the cleanliness air change rate dwarfs the leakage flow, so it governs the supply air quantity and the leakage becomes the offset that leaves as exhaust instead of returning to the air handler.
The 0.05 in. w.g. target is a widely used practice value, not a number fixed by ISO 14644 or FDA cGMP. Process requirements, door interlocks and interaction with smoke control or process exhaust all matter, so confirm the design against the project basis of design and a qualified engineer.
Frequently asked questions
About 0.05 in. w.g. (roughly 12.5 Pa) relative to the adjacent space is the common practice value. Much less and a door swing or a gust reverses the flow, while more than about 0.12 in. w.g. makes doors heavy and upsets interlocks.
Add up the door undercuts and perimeter gaps, pass-throughs, conduit and pipe penetrations, and ceiling access panels. Use a drawing-based estimate before commissioning, then back-calculate from measured pressure and airflow once the room is built.
Whichever is larger. In most cleanrooms the cleanliness air change rate is far larger, so it sets the supply, and the leakage flow becomes the offset that is exhausted rather than returned.