Compressors are of two main types : Positive displacement type and Turbo. Positive displacement compressors are either reciprocating or rotary

RECIPROCATING COMPRESSORS

These are most commonly vertical or horizontal and are almost invariably equipped with automatically operating spring loaded inlet and discharge valves. Delivery pressures range from comparatively low pressures of the order of 1.4 kg/sq.cm. to the highest required in practice.

When a gas is compressed under practical conditions, its temperature increases, the difference between delivery and suction temperatures being a function of the compression ratio. When high pressures are required, compression is carried out in stages. Gas temperatures are reduced by cooling the cylinder walls and by passing the gas through intercoolers between successive stages and frequently through an after cooler as well. A compression ration of about 4:1 in each stage is usual. 

Small single-stage trunk piston compressors are frequently splash lubricated, whereas larger units of this type are generally equipped with forced circulation systems. Cylinders of cross head type compressors and multi-stage compressors are generally lubricated by separate oil feeds to the suction pipes or directly to the cylinder walls.

For cylinder lubrication  it is desirable to use oil of high stability. In the presence of air, or even where relatively inactive gases are concerned, a non-stable oil will tend to breakdown under the prolonged influence of elevated temperatures and form gummy deposits which may cause piston rings to stick and may affect the action of valves. It is usual to employ straight mineral oils for cylinders, but if the compressor is dealing with very moist gas a compounded oil may be preferable to counteract the washing and rusting effect of the moisture. Compounded oils are, however, generally unsuitable for crankcase lubrication and in truck-piston machines fully detergent diesel engine oils may be preferable with moist gas. These oils may also be desirable for cylinder lubrication under high temperature conditions. 

Use of viscous oils should be avoided wherever possible, for the reason that they tend to greater drag on the pistons and more deposit formation than thin oils. Oils of relatively low viscosity are suitable under most conditions. In certain multistage compressor, however, where temperatures are consistently above 140 degree centigrade and it is essential that the air or gas discharging from the compressor should be contaminated as little as possible with oil vapours, it may be necessary to use more viscous oils of a special type.

The following important points should be borne in mind in the operation of reciprocating compressors :

- Only high grade oils should be used. The rate of oil feed should be kept to the minimum. If this precaution is not observed there is a danger that oil droplets will accumulate on the discharge valves and form deposits which may interfere with the action of the valves or prevent proper seating. When this happens, wiredrawing of the gas will occur with consequent rapid rise in temperature. The same effect is, of course, produced by badly fitted discharge valves.

- Effective filters should be fitted to prevent the entry of dust, grit and other foreign matter.

- Condensed moisture should be removed regularly from the inter coolers.

- Coolers should be kept free from scale or other deposits which would interface with efficiency and all gas and water connections should be kept tight.

- Piston crowns, piston rings, valves and all passages between the cylinders and receiver should be cleaned periodically.