Michael Kopp (CleanRooms, May, 2003, pg. 34) mentions microelectronics and nanotechnology ISO Class 6 and cleaner rooms “usually require ceiling diffusers, and perforated floors above a return plenum space.” This is usually called raised floors and is an excellent way to achieve true laminar flow across a room; however, it's not the usual way to return air unless it is in an ISO Class 4 or cleaner room.
This is based on two very important issues. First is cost. Adding a raised floor to a room is expensive. The raised floor needs to be purchased with extra supports needed for heavier equipment. Often, frames need to be constructed for machines that normally could sit on the concrete slab.
Then all of the materials need to be installed around exhaust ports and any other process piping in the area. A good example of when raised floors are used is when there is substantial process piping that needs to be close to the machinery in the room, and there is no good place to put it.
The easiest place is in a return space under the work area where it's out of the way and it doesn't matter if it gets dirty.
Second, if there is no distinct benefit to the process, why add it in? Low wall returns are very effective in almost all rooms, as long as they are designed correctly. Michael makes this point well in saying, “Understanding that cleanroom design is much more than controlling particles in the air goes a long way to assuring its success.”
Clean Rooms West Inc.