Calculation of airborne cleanliness and air supply rate for non-unidirectional airflow cleanrooms

When designing a cleanroom to achieve a required airborne cleanliness standard or grade, such as specified in ISO 1464411 or the European Union Guidelines to Good Manufacturing Practice (EU GGMP)2, designers have difficulties deciding how much filtered air should be supplied to the cleanroom to achieve the correct airborne cleanliness requirement. Currently, this decision is based on experience and ‘rules of thumb’ and not normally by an analytical method. The consequence of this is that many cleanrooms have excessive air supply that is associated with high capital and running costs, and energy waste. Conversely, a low air supply may result in too high a concentration of contamination, and major remedial work to rectify the problem. It would be useful if an analytical method was available to assist in calculating the air supply rate, as well as making clear what variables affected the calculation, and their relative importance. Prior to the start of manufacturing, personnel will enter an empty cleanroom that has a low airborne concentration of particles and microbe-carrying particles (MCPs). Personnel will then prepare for manufacturing and switch on machinery, and these activities will increase airborne contamination. When manufacturing starts and activity settles, the airborne contamination will fall a little to a fairly constant ‘steady-state’ condition, i.e. the operational condition. The airborne concentration in this condition determines the airborne contamination of products, and several researchers have derived equations to calculate it3–7. However, further investigations are still required into the effect of different designs of ventilation systems, and the method of calculating the reduction in the airborne contamination by the settling of particles from the air onto cleanroom surfaces, i.e. surface deposition. These variables have been incorporated into equations recently derived by Whyte, Lenegan and Eaton8. Practical information on the values of the equation variables are required so that the equations can be used to design actual cleanrooms and these are discussed in this paper. Also investigated are the relative importance of the Equations have been recently derived by Whyte, Lenegan and Eaton for calculating the airborne concentration of particles and microbe-carrying particles in non-unidirectional airflow cleanrooms. These equations cover a variety of ventilation systems, and contain the variables of air supply rate, airborne dispersion rate of contamination from machinery and people, surface deposition of particles from the air, concentration of contamination in fresh make-up air, proportion of fresh air, and air filter efficiencies. The relative importance of these variables is investigated in this present research paper, with particular regard to the removal efficiency, location, and number of air filters. It was shown that air filters were important in ensuring low levels of contamination in cleanrooms but the types of filters specified in current cleanroom designs were very effective in ensuring a minimal contribution to the cleanroom’s airborne contamination especially when a secondary filter is used in addition to a primary and terminal filter. The most important determinants of airborne contamination were the air supply rate and the dispersion rate of contamination within the cleanroom, with a lesser effect from deposition of airborne particles onto cleanroom surfaces. The information gathered confirmed the usefulness of the equation previously used by Whyte, Whyte, Eaton and Lenegan to calculate the air supply rate required for a specified concentration of airborne contamination.