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.
POSITIVE DISPLACEMENT ROTARY COMPRESSORS AND EXHAUSTERS
Positive displacement rotary compressors are used in applications overlapping those of both reciprocating and turbo types. Similar units are built for service as exhausters.
There are two main classes of positive displacement rotary machines : the sliding - vane and the two-impeller types.
SLIDING VANE COMPRESSORS
These machines are used for supplying relatively large quantities of air or other gas at moderate pressures (up to about 8.8 kg/sq. cm. Single stage machines generally deliver at pressures up to 3.5 kg/sq. cm. Two stage machines are used for higher pressures. Speeds range from about 450 r.p.m. to 4000 r.p.m.
The machine consists essentially of a cylindrical rotor carrying a number of thin rectangular plates or vanes in radial slots. This rotor assembly is supported eccentrically in a cylindrical water - cooled or air cooled casing by ball or roller bearings housed in the end covers. When the rotor revolves the vanes are thrown out by centrifugal force, thereby dividing the space between the rotor and casing into a number of compartments of different sizes. The size of each compartment increases to a maximum during one half revolution and decreases to a minimum in the following half revolution. Air or other gas is drawn in by an intake port arranged so that the compartment s may fill air until they reach their maximum size. A discharge port is arranged further on in the cycle, just before the volume of the compartments reaches the minimum. In some sliding-vane compressors the outward thrust of the vanes is borne by two 'floating rings' fitted in annular recesses in the casting on each side of the inlet and discharge ports. These rings are carried round with the rotor and prevent the blades from scraping over the casing bore. In one type of sliding - vane compressor a cylindrical line or 'rolling drum', mounted on ball bearings is fitted over the whole length of the blades. Gas enters the suction side and leaves the discharge side through holes in the rolling drum.
