Operation of compressed air systems

Refer to figure 6-1, Schematic of a typical compressed air system. Ambient air is pulled through a 5 to 10 micron filter and compressed by the helical rotors of the rotary screw compressor. A safety relief valve is installed in the discharge line and set at the design pressure of the system. The compressed air flows through an after cooler to remove the heat of compression. The compressed air temperature is reduced to within 15 to 20oF of ambient in the aftercooler.

1. The air next flows through a moisture/air separator. This separator removes most of the oil. The oil is cooled and returned to the compressor where it provides sealing, lubrication, and some cooling. From the moisture separator a mixture of condensation water and oil is discharged through a trap to an oil/water separator. This separator removes oil from the condensate down to 2 parts per million (ppm) so that it can be discharged to a sanitary sewer. The oil is accumulated in a filter or collection container, is usually considered hazardous waste and must be properly disposed of.
2. The air may now flow to a receiver with a safety relief valve. Not all compressed air systems incorporate a receiver. The needs of process equipment and the type and size of the compressors determine the need for a receiver. The receiver provides a cushion for compressed air pulses when a reciprocating compressor is used, a storage volume of air to handle peaks of high usage and a storage volume of air to allow operation of systems during periods of power loss. Receivers usually provide one gallon of capacity for each cubic foot of flow. A liquid trap discharges accumulated condensate and oil to the oil/water separator.
3. Downstream of the receiver a prefilter may be installed followed by a dryer and an afterfilter. All of these devices are provided with shutoff valves upstream and downstream and a bypass valve to allow servicing. These devices also are supplied with traps which allow accumulated condensate/oil mixtures to be processed in the oil/water separator. There are several different types of dryers. The most common is a refrigerant dryer, as presented in the typical system. In this dryer a refrigerant cycle cools the compressed air temperature down to about 33 to 40oF. The water content of the cool air is much less than that of hot air. The air is considered dry at this point. Other types of dryers can lower air temperature further for special process requirements. Refrigerant dryers are sufficient for HVAC control systems. The afterfilter removes moisture from the air stream down to 0.5 ppm and filter particulate down to 1 micron. Finer filters can be installed if required by the operating process. Drip legs should be provided in the piping system header and branch lines to collect condensate and should include a trap. The header should be supplied from both ends thereby making a loop of compressed air around the building. Branch lines should be tapped from the top of header lines to prevent condensate and rust accumulation from entering the line. Lines should be sloped toward drip legs.