76 Fermentation and Biochemical Engineering Handbook Some plants have a separate filter for each sterile vessel. Others place filters in a central group which feeds all the vessels. In this case, one filter for example, might be taken out of service each day, sterilized and put back into service. Ifthere were ten filters in the group, each one would be sterilized every tenth day. This system has the advantage that the filter can be blown dry after sterilization with sterile air before it is put into service again Figure 1. Domnick-Hunter sterile air filter 2.9 Air Compressors It is ideal to have oil-free compressed air. Centrifugal machines generally are available u regarding oil-free screw type compressors, it 40,000 cfim. Oil free"screw air compressors are available in smaller sizes76 Fermentation and Biochemical Engineering Handbook Some plants have a separate filter for each sterile vessel. Others place filters in a central group which feeds all the vessels. In this case, one filter, for example, might be taken out of service each day, sterilized and put back into service. If there were ten filters in the group, each one would be sterilized every tenth day. This system has the advantage that the filter can be blown dry after sterilization with sterile air before it is put into service again. Figure 1. Domnick-Hunter sterile air filter. 2.9 Air Compressors It is ideal to have oil-free compressed air. Centrifugal machines generally are available up to 40,000 cfm. "Oil free" screw air compressors are available in smaller sizes. Regarding oil-free screw type compressors, it