Compressors are of two main types : Positive displacement and Turbo type. Positive displacement compressors are either reciprocating or rotary.
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 ratio 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.
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For cylinder lubrication it is desirable to use an 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 trunk-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 give rise 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 multi-stage compressors, however, where temperatures are consistently above 1400 C 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 :
Positive Displacement Rotary Compressors and Exhausters
- 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 drawing of the gas will occur with consequent rapid rise in temperature. The same effect is, of course, produced by badly fitting 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 interfere with their 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 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.
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These machines are used for supplying relatively large quantities of air or other gas at moderate pressures (upto about 8.8 kg/sq.cm.). Single-stage machines generally deliver at pressures upto 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 decrease to a minimum in the following half revolution. Air or other gas is drawn in by an intake port arranged so that the compartments may fill with 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 casing 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.
Oil is fed to the rings, to the inside of the casing and to the rotor bearings of the
floating-ring types. In the rolling-drum type oil is forced along an axial hole drilled in the rotor shaft centre from which it is thrown out by centrifugal force and reaches all internal parts, including the vane slots. Somewhat thicker oils are required for orthodox sliding-vane machines than for the cylinders of reciprocating machines. This is necessary partly to reduce friction and partly to improve the seal. Heavy oils, sometimes with enhanced antioxidant and detergent properties are required in certain cases where high air temperatures are developed.
In oil-cooled sliding-vane compressors large quantities of oil are injected into the rotor chambers. In this way the oil, in addition to providing effective lubrication and sealing of the moving parts, also ensures efficient cooling of their air during compression. The oil is intercepted on the discharge side of the machine, cooled, filtered and then re-circulated. The operating conditions are severe. During circulation most of the oil is exposed in a finely divided state to large volumes of hot air and although the air discharge temperatures are comparatively low, the oil is subjected to severe oxidizing conditions. Moreover, particularly when the machine is idling, overcooling can lead to the condensation of moisture in the oil receiver where it mixes with the oil to form an emulsion. Should the quantity of condensed moisture become excessive, rusting and consequent blade wear may occur. It is essential, therefore, that the oil should have excellent oxidation stability and good demulsibility. On this basis, the best choice will be an oil of turbine quality.
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These are widely used in the process industry for gas compression. They are similar to the two impeller compressors. They have two intermeshing rotors with helical lobeswhich are designated as male and female. When the male and female rotor lobes engage, they form a cell in which the air is enclosed As the rotors rotate the volume of the cell is reduced and compression begins. Further rotation causes the line of engagement of the rotors to travel towards the outlet, with the result that the air is displaced from the cells under pressure.
Single stage machines are generally able to generate pressures upto 5 kg/sq.cm. while multistage machines upto 12 kg/sq.cm.
Screw compressors present the same lubrication requirements as Blowers. However the bearings of these compressors are usually more heavily loaded and therefore require additive type oils.
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This term covers two distinct types - centrifugal compressors and axial flow compressors. Units of both types are employed for delivery of large volumes of air at moderate pressures (generally upto 7 kg/sq.cm.). Operating speeds are usually in the range of 2000 to 10000 r.p.m.
A centrifugal compressor consists essentially of a casing in which a shaft carrying one or more impellers rotate. Gas enters each impeller at the centre and under centrifugal force, is expelled peripherally at high speed. Pressure increase is obtained by reducing the kinetic energy of the gas in a static diffuser. Multi-stage compression is obtained by directing gas from each diffuser into the eye or hub of a further impeller.
The main application of axial-flow compressors is in conjunction with gas turbines. These compressors resemble reaction turbines in reverse, each stage comprising a row of fixed and moving blades. Compression takes place both in the fixed and moving blades, gas flow being in an axial direction.
Both types of turbo compressors are generally fitted with plain bearings, lubrication being by forced circulation. Where the compressor is coupled to a steam or gas turbine a common circulating oil system is employed. Small centrifugal machines are commonly fitted with ring oiled bearings.
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Two-Impeller Compressors Or Blowers
The commonest application of compressors of this type is for diesel engine supercharging. They are used also in various industrial applications where machines of relatively large capacity, delivering at low gauge pressures are required. Single-stage units generally deliver at pressures below 1 kg/sq.cm. Similar machines with fine operating clearances may be used as vacuum pumps.
A two-impeller compressor consists essentially of a pair of lobed rotors mounted on parallel shafts rotating in opposite directions. The rotors engage each other, clearance between them only a few thousandths of an inch being maintained by accurately cut timing gears. The rotors usually have two or three lobes. Air enters the machine through an inlet port so placed, that as the impellers rotate, air is trapped between adjacent lobes and the casing. On the other side of the casing air is forced out under pressure through the discharge port.
Small machines of this type are fitted with grease-lubricated roller bearings or ring-oiled plain bearings, the timing gears being bath lubricated. Large units may be fitted with a forced circulation system for bearings and gears.
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