The National Air Filtration Association (NAFA) Guide to Air Filtration (1993) describes laminar flow cleanrooms in general.
In the unidirectional air flow type of cleanroom, more commonly known as laminar flow clean rooms, the design intent is that air should make a single pass through the room with a piston-like effect. In one configuration sometimes referred to as vertical laminar flow rooms, cleaned air enters the room through the ceiling and exits through floor grilles or through continuous outlets in the walls at floor level. When returns are in the walls, there are restrictions as to how wide the room can be and still maintain unidirectional flow. Air can also enter through one wall and exit through the opposite wall. This is sometimes referred to as horizontal laminar flow.
In a conventional cleanroom, HEPA filters are usually located in the mechanical room and are the last items in the mechanical equipment sequence. They are downstream of the blower, so that only HEPA-filtered air enters the ductwork connecting the mechanical equipment to the cleanroom. The ductwork is frequently stainless steel, although other non-shedding materials are also used.
In a laminar flow cleanroom, the HEPA filters are located in the wall or ceiling of the room, so that once the air passes through the filter it is in the cleanroom. Intermediate sources of contamination are eliminated. HEPA filters must be sealed into the framing system so that there can be no by-pass of dirty air around them and into the cleanroom. The filters themselves must also be leak-free. Different filter manufacturers have developed framing systems that provide such support.
