Figure 3. The inlet pipe of a new refrigeration dryer without prefiltration. Within one month, an emulsion of condensed water and compressor oil formed a mayonnaise-like deposit known as "white sludge" and the air flow through the dryer was excessively restricted.In high purity compressed air systems, a 2- or even 3-stage prefiltration system is required upstream of the air dryer. The first stage filter is an open structure low pressure loss filter designed to remove 90% of the fine dust and oil mist. The final filter has a 99.99998% removal efficiency based on 0.01 µm size particles to prevent contamination of the air dryer. The filters are designed for low pressure loss to prevent excessive costs in manufacturing operations. Older style filters added pressure losses of 5 to 10 psid, but new designs such as the Donaldson Ultra-Filter with lyophobic (liquid repelling) media have been designed for a differential pressure of less than 2 psid.
An example of the need for filtration before a compressed air dryer is shown in Figure 3. A refrigeration type air dryer was installed without the benefit of prefiltration. Within a month, the inlet heat exchanger was plugged with "white sludge," a mayonnaise-like emulsion of aerated compressor oil and water. After retrofitting the dryer with a coalescing prefilter, the problem was eliminated.
High quality compressed air filtration is not a luxury. It is an essential element in a well run, effective, manufacturing operation.
Don White is Chief Engineer and Tina Pocrnich is Product & Engineering Manager at Donaldson Co. Inc., information call them at (952) 887.3131 or visit www.donaldson.com.