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 center 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 the air during compression. The oil is intercepted on the discharge side of the machine, cooled, filtered and then recalculated. 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.
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.
SCREW COMPRESSORS
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 lobes which 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 rotatio causes the line of engagement of the rotors to trael towards the outlets, with the result that the air is displaced from the cells under pressure.
Single stage machines are generally able to generate pressures up to 5 kg/sq.cm. while multistage machines up to 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.
TURBO COMPRESSORS
This term covers two distict types-centrifugal compressors and axial flow compressors. Units of both types are employed for delivery of large volumes of air at moderate pressure (generally up to 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 center 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 fixed and moving blades, gas flow being in an axial direction.
Both type 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.
