Naughton ("HVAC Systems... Part 1," 1990) discusses prefiltration in cleanrooms.
Lowering the air-side pressure drop associated with heat transfer coils and filters is a readily available energy-saving option available to the designer. While the use of air filters is the primary means to achieve the desired air cleanliness level, the quantity and quality of prefiltration are normally established by the cleanroom owner's contamination control specialist. Many semiconductor cleanrooms use only minor amounts of prefiltration, typically one set of 30% efficient prefilters. Other semiconductor firms use two stages of prefiltration, a 30% and a 90% efficient filter in series. The primary advantage of two stages of prefilters is to reduce the load on the expensive cleanroom HEPA/ULPA filters. The choice of one-stage filtration, two-stage filtration, or no prefiltration is normally made by the owner. Therefore, the extra operating costs associated with the 0.75 in. WC (186.8 Pa) to 1.0 in. WC [W.G.] (249 Pa) pressure drop required for prefiltration must outweigh the shorter HEPA filter life expectancy. (Prefilter operating costs will vary from $5.00 to $8.00 per year per square foot vs. the HEPA filter cost of $12.00 to $15.00 per square foot.) The expected life of the HEPA/ULPA filters is not the only issue. The potential downtime required to replace the HEPA filters is typically an overriding factor in the decision.
