Centrifugal Compressors

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Centrifugal Compressors:

The machine in which velocity and pressure are imparted to the air or gas in a radial direction by one or more impeller – diffuser combination.

Centrifugal compressors are employed in numerous fields, chemical andpetrochemical industries, refineries, fertilizer plants, nuclear reactors and air separation plants, iron and steel plants, production of liquefied natural gas (LNG) and substitute natural gas (SNG), cryogenic and refrigeration plants,
mining, transportation and storage of gas, on-shore and off-shore installations. The range of application can be expanded still further by combining these centrifugal compressors with other compressor type such as axial flow or reciprocating compressors.

The wide range of processes in which centrifugal compressors are employed makes varying demands on these machines. Compressor demand is dependent on such factors as fluid handled, pressure ratio, the volume flow, the number of inter stage coolers, injection and extraction of the medium, and the type of shaft sealing.

Taking all this factors into consideration, the major compressor manufacturers have developed series of centrifugal compressors offering optimum engineering solution implemented by the use of standard components. These series include the two basic types, distinguished by horizontally or vertically split casing, compressors with two or three pairs of main nozzles, and compressors with additional side stream nozzles.

Horizontally split casing permits simple removal of rotor and facilitates the checking of labyrinth clearances and o-rings. As pressure level rise and gas molecular becomes smaller, vertical split casings are employed.

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CHARACTERISTIC DIMENSIONS OF RECIPROCATING COMPRESSOR
CHARACTERISTIC DIMENSIONS :

Suction pressure – is the pressure measured at the compressor cylinder inlet flange.

Discharges pressure – is the total pressure measured at the discharge flange of the compressor

Compression ratio – is the ratio of the absolute discharge pressure to absolute suction pressure.

Top dead centre (TDC) - is the outer (at cylinder end) dead centre of the piston stroke

Bottom dead centre (BDC) – is the inner (crank side) dead centre of the piston stroke

Stroke – It is the distance traversed by the piston in moving between two dead centres (top and Bottom)

Suction volume – is the volume of gas that enters the compressor in each unit of time.

Piston displacement – is the volume of a gas displaced in a stroke

Volumetric efficiency – is the ratio of actual capacity of compressor to displacement (theoretical capacity)

Compression capacity – is ratio of the theoretical horsepower to indicated horsepower.

Mechanical efficiency – is the ratio of the indicated horsepower to the break horsepower delivered at the shaft.

Overall efficiency – is the product of compression efficiency and mechanical efficiency.

Compressor capacity - is expressed in ICFM (intake cubic feet per minute) or ACFM (actual cubic feet per minute) at intake conditions.