Compressor Auxiliaries

Important compressor plant auxiliaries include air receivers, inlet air filters, intercoolers, aftercoolers, and air dryers designed to remove unwanted contaminant's and moisture from the compressed air stream.

An air receiver, typically used to reduce the impact of pulsations from reciprocating compressors, can also be used to provide storage. Storage capacity serves many functions. For example, it can limit distribution pressure swings and reduce problems associated with frequent compressor cycling or capacity modulation. Adequate storage allows compressor plants to respond to high types of filters are used:

• Viscous-impingement or oil-wetted filters include layers of wire mesh, screen, or fibrous filter pads contained in a canister or panel. Debris in the air drawn through the filter medium is retained on the oil-wetted surface. These are relatively low-cost, inefficient filters that are not recommended for non-lubricated compressors or for duty where heavy atmospheric contamination exists.

• Oil bath filters first draw the unfiltered air through an oil reservoir; this is followed by a screen mesh or other medium that scrubs out the oil and dirt particles on the oiled element surfaces. These are more efficient than viscous-impingement types, but also have a higher pressure drop. Due to the oil entrainment in the filtered air, they are also not suitable for non-lubricated compressors.

• Dry-type filters, consisting of densely spaced material that block particle penetration, are usually the most efficient filter option and the best option for non-lubricated compressors. The finer the filter element, the greater the efficiency and the higher the pressure drop. Routine inspection is required to ensure that the filter does not become clogged, possibly allowing a rupture that releases retained debris into the air system. Some elements may be cleaned by blowing them out or scrubbing them. Other less expensive paper-treated elements are disposable types.

The oil bath and dry type are available as filter silencers that also provide sound suppressing characteristics. With viscous-impingement filters, an additional silencing device, such as a pulsation damper, is usually used.

Air always contains some moisture in a vapor state. The maximum amount of moisture that a given volume of air can hold is dependent upon temperature and pressure. As temperature increases, the air is able to hold more moisture, and as pressure increases, the air is able to hold less moisture. For example, in English units, air at 70°F, 50% RH, holds about 4 grains/cf of moisture (1 lbm of water is equal to 7,000 grains). At 100% RH, the air holds about 8 grains/cf. At that point, the air is said to be saturated, i.e., it cannot hold any more moisture. If the absolute pressure is doubled (at constant temperature), the volume is reduced and the air will retain 4 grains/cf in the form of vapor and drop out 4 grains/cf in the form of liquid.

The dewpoint is the temperature at which condensate will begin to form if the air is cooled at constant pressure. Pressurized warm air typically leaves the compressor under c

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