Equations for predicting airborne cleanliness in non-unidirectional airflow cleanrooms

When designing a cleanroom to achieve a required airborne cleanliness class or grade, such as specified in the International Organization for Standardization (ISO) 14644-11, or the European Union Guidelines to Good Manufacturing Practice (EU GGMP)2, designers have a problem in deciding how much filtered air should be supplied to the cleanroom. Currently, this is not normally calculated but based on experience or some rule of thumb, and the consequence of this is that designers are over-cautious, and 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 airborne contamination, and major remedial work to rectify the problem. There is, therefore, a need for a method to calculate the air volume supply rate for a required concentration of airborne contamination, and information on such a method has been previously published by the authors3. The airborne cleanliness of a cleanroom is directly related to the amount of filtered air supplied to a cleanroom, and equations derived to calculate airborne cleanliness can be rewritten so that the amount of air required is determined for a specified airborne cleanliness condition. These equations do not consider air mixing and distribution in the cleanroom and this is discussed elsewhere4. A number of equations have been derived to calculate the airborne concentration in a cleanroom5–9. These equations have been mainly derived for a single type of ventilation system and are not applicable to all design configurations, and an investigation is required to determine whether changes in the design of ventilation systems through filter placement, numbers of filters, and methods of mixing fresh and recirculated air within the ventilation plant, will cause differences to the airborne concentrations in the cleanroom. Also, the previously-derived equations have either not considered the effect of surface deposition of particles, or not done so in an analytical way. In addition, some of the previously reported equations have been derived by assuming a transient state, and, therefore, the mathematics is relatively complicated, and it would be useful if the derivation method was simplified. This paper derives equations for a variety of ventilation plants. These equations provide a means of investigating (a) the influence of different designs of ventilation plants, (b) the importance of the different variables that influence the airborne concentration, (c) the possibility of simplification of the equations, and (d) the usefulness of the method suggested by Whyte, Whyte, Eaton and Lenegan3 for calculating the air supply rate for a specified airborne concentration in nonEquations are derived in this paper for predicting the airborne concentration of particles and microbe-carrying particles in non-unidirectional airflow cleanrooms during manufacturing. The equations are obtained for a variety of ventilation systems with different configurations for mixing fresh and recirculated air, air filter placements, and number and efficiency of air filters. The variables in the equations are air supply rate, airborne dispersion rate of contamination from machinery and people, surface deposition of particles from air, particle concentration in fresh makeup air, proportion of make-up air, and air filter efficiencies. The equations are amenable to relatively simple modification for the study of different cleanroom ventilation systems. The use of these equations to investigate the effect of different configurations of ventilation systems and the relative importance of the equation variables on airborne concentrations will be reported in a further paper.