The rule of thumb most used in cleanroom design is air changes per hour. It is useful, but it is not an acceptance criterion. ISO 14644-1 defines classification by airborne particle concentration per unit volume: for ISO Class 5 the concentration of particles ≥0.5 µm is at most 3,520 per m³, for Class 7 it is 352,000 and for Class 8, 3,520,000 [1].
The question being asked, in other words, is not how many times the air changes but whether the particle leaves the room. Two rooms supplied with the same flow rate will fall into quite different classes if their air distribution differs.
Flow rate measures dilution, not cleaning
Air change rate is an average dilution rate. In a real room the flow is not uniform:
- Dead zones. Behind equipment, in corners, under benches. The local change rate there is far below the average and particles accumulate.
- Short-circuiting. Air leaves the supply diffuser and goes straight to the return grille, never passing through the working zone. The flow rate is right on paper; the cleaning is close to zero.
- Source location. Most of the particle burden usually comes from personnel. If the path between source and return passes through the critical zone, the class will not hold.
None of these is visible except in a field solution. A cleanroom is therefore a classic problem in which the distribution, not the mean, is what is being asked. See the note on when CFD is necessary.
Recovery time: measurable and modellable
The most practical way to test a design is its recovery behaviour: the room is given a controlled particle load and the time taken for the concentration to fall to the target level is observed. This is a quantity that can both be measured on site and computed directly by transient analysis — which makes it a convenient matching point for validation. See the verification and validation note.
Modelling decisions
Track particles or solve a tracer. There are two routes: discrete phase tracking (if settling matters) or a passive scalar transport equation (sufficient for fine particles and far cheaper). The choice follows the particle size; below a few microns gravitational settling becomes negligible.
Do not forget thermal buoyancy. Personnel, lighting and equipment loads can set the flow pattern in a low-velocity room. Because the velocities are low, this is often a natural convection problem.
Pressure cascade. The differential between adjacent spaces and door-opening scenarios determine the direction of leakage. A door opening is by definition a transient, moving-boundary problem.
References
- ISO 14644-1:2015. Cleanrooms and associated controlled environments — Part 1: Classification of air cleanliness by particle concentration. International Organization for Standardization, Geneva.
- Morsi, S. A. & Alexander, A. J. (1972). An investigation of particle trajectories in two-phase flow systems. Journal of Fluid Mechanics, 55(2), 193–208. DOI: 10.1017/S0022112072001806