Pressure differential is a key ingredient in cleanrooms and clean zones. Differences inpressure can help control airborne contaminants both in clean rooms, clean zones and cleanroom hoods (laminar and non-laminar flow hoods). I am surprised how often I see pressure differential used improperly in some settings. It’s important to understand pressure differential, how and when to use it.
Negative Pressure is acheived when air is evacuated from an area, room or hood. Laminar flow hoods often use negative pressure to sweep away contaminants from a specific processor in areas where particles and/or other contaminants are being generated. The principle is simple, and when designed with the correct airflow, quite effective. Once the air is removed from the area, it’s important to have a clean source of make-up air to replace the air that is being drawn from the area. Evacuated air is either exhausted or run through a series of filters and returned to the room. If the air is exhausted, it is very important to have filtered air returning to the area to avoid pulling in outside contaminants. This ensures a clean work area and contributes to worker safety while cutting down on cross contamination.
Negative pressure hoods or booths are popular in the microelectronics industry and is becoming popular in government mailrooms where they are pre-screening mail for biological contamination.
Positive pressure areas are generally used in cleanrooms as away to keep particle contamination from entering the room or area. The principle of positive pressure is to supply an area with enough clean, filtered air to keep contaminants from entering the room or area. Positive pressure areas are an effective method for reducing cross-contamination and is often misapplied in areas where contaminants are being produced by a process.
The general rule of thumb is to use positive pressure to keep contaminants out of an area, and negative pressure to capture contaminants and keep them from contaminating surrounding areas or worker breathing zones. An important thing to keep in mind when designing or applying pressure differential is to account for the make-up air. For every cubic foot of air that is moved, it will be replaced, if this air needs to be clean, you must filter it.