Rotary highest efficiency compressor, then two stage scroll, scroll,Reciprocating
A look at how different compressors work
Most cooling systems, from residential air conditioners to large commercial and industrial chillers, employ the refrigeration process known as the vapour compression cycle. At the heart of the vapour compression cycle is the mechanical compressor. A compressor has two main functions: 1) to pump refrigerant through the cooling system and 2) to compress gaseous refrigerant in the system so that it can be condensed to liquid and absorb heat from the air or water that is being cooled or chilled (See the "How it Works" section of the article "Gas Engine Chillers" for an explanation of the vapour compression cycle).
There are many ways to compress a gas. As such, many different types of compressors have been invented over the years. Each type utilizes a specific and sometimes downright ingenious method to pressurize refrigerant vapour. The five types of compressors used in vapour compression systems are Reciprocating, Rotary, Centrifugal, Screw and Scroll.
A reciprocating compressor uses the reciprocating action of a piston inside a cylinder to compress refrigerant. As the piston moves downward, a vacuum is created inside the cylinder. Because the pressure above the intake valve is greater than the pressure below it, the intake valve is forced open and refrigerant is sucked into the cylinder. After the piston reaches its bottom position it begins to move upward. The intake valve closes, trapping the refrigerant inside the cylinder. As the piston continues to move upward it compresses the refrigerant, increasing its pressure. At a certain point the pressure exerted by the refrigerant forces the exhaust valve to open and the compressed refrigerant flows out of the cylinder. Once the piston reaches it top-most position, it starts moving downward again and the cycle is repeated.
In a rotary compressor the refrigerant is compressed by the rotating action of a roller inside a cylinder. The roller rotates eccentrically (off-centre) around a shaft so that part of the roller is always in contact with the inside wall of the cylinder. A spring-mounted blade is always rubbing against the roller. The two points of contact create two sealed areas of continuously variable volume inside the cylinder. At a certain point in the rotation of the roller, the intake port is exposed and a quantity of refrigerant is sucked into the cylinder, filling one of the sealed areas. As the roller continues to rotate the volume of the area the refrigerant occupies is reduced and the refrigerant is compressed. When the exhaust valve is exposed, the high-pressure refrigerant forces the exhaust valve to open and the refrigerant is released. Rotary compressors are very efficient because the actions of taking in refrigerant and compressing refrigerant occur simultaneously.
Screw compressors use a pair of helical rotorsAs the rotors rotate they intermesh, alternately exposing and closing off interlobe spaces at the ends of the rotors. When an interlobe space at the intake end opens up, refrigerant is sucked into it. As the rotors continue to rotate the refrigerant becomes trapped inside the interlobe space and is forced along the length of the rotors. The volume of the interlobe space decreases and the refrigerant is compressed. The compressed refrigerant exists when the interlobe space reaches the other end. (male and female) inside a sealed chamber.
Centrifugal compressors use the rotating action of an impeller wheel to exert centrifugal force on refrigerant inside a round chamber (volute). Refrigerant is sucked into the impeller wheel through a large circular intake and flows between the impellers. The impellers force the refrigerant outward, exerting centrifugal force on the refrigerant. The refrigerant is pressurized as it is forced against the sides of the volute. Centrifugal compressors are well suited to compressing large volumes of refrigerant to relatively low pressures. The compressive force generated by an impeller wheel is small, so chillers that use centrifugal compressors usually employ more than one impeller wheel, arranged in series. Centrifugal compressors are desirable for their simple design and few moving parts.
In a scroll compressor refrigerant is compressed by two offset spiral disks that are nested together. The upper disk is stationary while the lower disk moves in orbital fashion. The orbiting action of the lower disk inside the stationary disk creates sealed spaces of varying volume. Refrigerant is sucked in through inlet ports at the perimeter of the scroll. A quantity of refrigerant becomes trapped in one of the sealed spaces. As the disk orbits the enclosed space containing the refrigerant is transferred toward the centre of the disk and its volume decreases. As the volume decreases, the refrigerant is compressed. The compressed refrigerant is discharged through a port at the centre of the upper disk. Scroll compressors are quiet, smooth-operating units with the highest efficiency ratio of all compressor types. They are commonly used in automobile air conditioning systems and commercial chillers.
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