COMPRESSOR

A. Definition of Compressor
Machines for repeated compression of gases are termed compressor. Since compression of the aspirated gas requires energy from some external source, these devices are working machine. By their action, compressors convert mechanical work into heat and therefore belong to the class of thermal machines.

B. Classification of Compressor
The pressure of a gas or vapor may be increased either by decreasing its volume (as in the case of positive displacement compressor) or by imparting to it a high kinetic energy which is converted into pressure energy in diffuser (dynamic compressor).
Positive displacement compressor is further sub divided into piston compressor in which the gas volume changes due to the action of one or two reciprocating pistons moving axially in a cylinder, membrane compressors, in which the volume variations are effected by deflection of an elastic partition, and rotary compressors of various types.
The dynamic principle is utilized in multi-blade compressors which are further sub divided into centrifugal and axial compressors, and in jet type devices.
In centrifugal compressors, a high peripheral velocity is imparted to the gas being compressed by the blades of the impeller which rotates at a speed of 3000 – 27000 rpm (in special cases speed in excess of 100000 rpm are reached). The centrifugal force developed in this way partially compresses the gas. Further compression occurs after the gas has left the impeller by reducing its velocity in the diffuser. The flow of gas here is approximately radial. In the other type of multi-blade compressors the gas streams mainly in the direction of the axis (hence the name axial compressors). The gas flow is effected by impellers having aerodynamically shaped blades and appropriately directed at the outlet from each set of blades by guide vanes, which act as diffusers increasing the pressure of the gas at the expense of its kinetic energy.
The essential parts of a jet compressor are the Laval nozzle in which the driving vapor attains a high supersonic velocity, the mixing chamber where the vapor is mixed with the gas or vapor to be compressed, and the diffuser into which the mixture enters at high velocity. Here kinetic energy is converted into pressure energy.
In other case, water or air is used as a driving agent in jet compressors. For water a convergent nozzle is used.
With piston compressors may be grouped hydraulic compressors in which the task of the piston is carried out by a water column falling through the compression tube.
Another criterion for the classification of compressors is the suction or delivery pressure. The ratio of delivery pressure to suction pressure is termed the compression ratio (or pressure ratio). This must be distinguished from the compression ratio as used in the design of internal combustion engines, where it denotes the ratio of the total cylinder volume including clearance volume to the clearance volume alone.
Machines operating with suction below atmospheric pressure and compressing to atmospheric or slightly higher pressure are termed vacuum pumps (multi-blade machines are usually term exhausters, jet type devices working under similar conditions are called ejectors).
Multi-blade machines overcoming only the resistance which arise in the flow of gases (up to 1200 mm of water column) are termed fans.
Machines with suction at approximately atmospheric pressure and having a compression ratio less than 3 may be termed blowers. Only machines for a final pressure of 2 to 500 atg are compressors in the narrower sense of the word. They may be further sub-divided into a) low pressure, b) medium pressure, and c) high pressure compressors. Up to the present, the interpretation of these terms has been rather inconsistent. For this reason, the limits of their application have been recently standardized on the basis of their final pressures as under:
1. Low pressure compressors for a final pressure of up to 25 atg
2. Medium pressure compressors for a final pressure of from 25 to 100 atg
3. High pressure compressors for a final pressure of from 100 to 500 atg
A classification of compressors according to size, based on the quantity of free air delivered is as follows:
1. Small – piston compressors handling up to 160 m3/hr
2. Medium – compressors handling 160 to 4000 m3/hr
3. Large – compressors handling more than 4000 m3/hr
For compressors having a high compression ratio the compression is divided into or more stages. This makes for improved safety and can also produce a reduction in the power input. Depending upon the fluid pumped and the size of the machine the maximum pressure ratio for single-stage compression may be from 5 to 8.
Multistage reciprocating machines for delivery pressures exceeding 500 atg are termed hypercompressors. They have up to 7 stages.
For compressors with a high suction pressure (e.g. 270 atg) and low compression ratio (e.g. 1,1) the term circulation pump or circulator has been adopted.
A further criterion of compressor classification, important from the point view of the designer, is the medium to be compressed. This may be air, oxygen, hydrogen, nitrogen, acetylene, lighting gas, and vapors and gases used in refrigeration cycles such as methyl chloride, carbon dioxide, the fluorocarbons, ammonia, etc. Their physical and chemical properties exert a considerable influence upon the design of compressors.
Further differentiation may depend upon type of drive, method of cooling, arrangement of cylinders, etc.
Each type of compressor has its own operation characteristics which limit its efficient application.
In multi-blade compressors leakage losses are significant. For this reason these compressors are suitable for low compression ratios and the compression of large gas volume. In the case of positive displacement machines, rotary compressors are suitable for low compression ratios and small or medium gas volumes, while reciprocating compressors are used for medium and high compression ratio irrespective of the volume to be compressed. Membrane compressors, in which the absence of leakage and of oil contamination are the main assets, are used only for small volumes of gas.
Jet compressors are capable of operating at low pressure and can handle the largest volumes of vapors or gases to be compressed.