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.

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.