About Compressors
The compressor is considered the heart of the refrigeration system. Its main function is to compress the refrigerant, to change its pressure status, and further to help the refrigerant possible to transit the heat. There are normal types of compressors as follows:
1. Piston or reciprocating type compressor
A Piston compressor works on the two-stroke cycle (see following figure ).Automatic pressure-actuated suction and discharge valves are used. When the piston descends on the suction stroke, the suction valve opens to suction gas from the evaporator. At the bottom of the stroke, this valve will close again as the compression stroke begins. When the cylinder pressure becomes higher than that in the discharge pipe, the discharge valve opens and the compressed gas passes to the condenser. The following figure shows the basic theory of a piston compressor. But actual compressors are normally in multi-cylinder
type. Capacity ranges of piston compressor covers 2.5kw up to 1000kw.
2. Scroll compressor
Scroll compressors are positive displacement machines that compress refrigerants with two inter-fitting spiral-shaped scroll members. One scroll remains fixed whilst the second scroll moves in orbit inside it. The moving scroll does not rotate but orbits with a circular motion. Normally two to three orbits, or crankshaft revolutions, are required to complete the compression cycle. Almost all production scrolls are of the hermetic type. Capacity ranges covers 6.5kw up to 100kw.
3. Screw compressor
The screw compressor can be visualized as a development of the gear pump. For gas pumping the rotor profiles are designed to give maximum swept volume and no clearance volume where the rotors mesh together. The pitch of the helix is such that the inlet and the outlet ports can be arranged at the ends instead of at the side. The solid portions of the screws slide over the gas ports to separate one stroke from the next so that no inlet or outlet valves are needed.
This compressor uses two tapered, machined, screw-type matching gears that squeeze the refrigerant vapor from the inlet to the outlet.
The screw compressor has an open motor instead of the hermetic design. Capacity ranges covers 75kw up to 6000kw.
4. Centrifugal Compressor
Centrifugal compressor is a type of dynamic compressors impart energy to the gas by centrifugal force and then convert this to pressure energy. Suction gas enters axially into the eye of a rotor which has curved blades, and is thrown out tangentially from the blade circumference. The centrifugal compressor has magnetic bearings, variable speeds, and digital electronic technology and achieves high efficiencies.
The smallest available centrifugal compressor for refrigeration duty has a capacity of some 260 kW. Semi-hermetic compressors are made up to 7000 kW and open drive machines up to 21 000 kW capacity. There are no components which require lubrication, with the exception of the main bearings. As a result, the machine can run almost oil free.
5. Compressors mostly used
In HVAC field the scroll compressors and Semi-hermetic screw compressors are mostly used.
Scroll compressors are mostly used for units (chiller or air conditioners) of capacity below 350kw. According to capacity range, the HVAC unit can be one single compressor or multiple compressors. Most popular brands used in China are Emerson, Hitachi, Sanyo, etc.
Semi-hermetic screw compressors are mostly used for chillers capacity range from 350kw up to 4000kw. According to capacity range, the unit can be one single compressor or two compressors. Most popular brands used in China are Bitzer, Hanbell and Fusheng.
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