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A BEGINNER’S GUIDE TO: COMPRESSORS


The core of the cooling system


The compressor is an essential piece of cooling kit, and having the knowledge to make the right choices here is crucial. Kevin Glass, managing director of Bitzer UK, shares his insights and expertise with Charlotte Westwood of ACR News.


T


he compressor sits at the very heart of the cooling system, enabling the movement of refrigerant through the system by acting as a


vapour pump.


Bitzer UK’s managing director Kevin Glass acknowledges the indispensable role a compressor plays, likening the nature of its function to that of the engine of a car. He explains: “It does all of the work and makes the system do what it’s supposed to do.


“The aim of a refrigeration system is to take heat from one place and transfer it to another. A compressor does exactly what it says on the tin - it compresses the refrigerant which collects and carries that heat, and pumps it around the system.” Through compressing the refrigerant vapour, the compressor increases the pressure within the sealed system and enables the refrigerant to travel through and reach the different components present within this system.


There is more than one type of compressor, and as a leading manufacturer of compressors used in both refrigeration and air conditioning technology, Bitzer makes reciprocating, screw and scroll models. While they all do the same job, they do it in different ways and, crucially, each one is potentially more suited to a different style of application. For instance, the temperature of the system in question and its corresponding pressure ratio will determine which compressor technology is used. Indeed, each type of compressor can be more efficient than the others in dealing with a certain pressure ratio. When it comes to low temperature refrigeration there is a high pressure ratio, with low pressure on the inlet of the refrigerant and a very high pressure on the discharge.


“With low temperature refrigeration, you’ll normally find reciprocating compressors,” Mr Glass explains. “However, if it’s a very large low


30 November 2017


temperature system, then you’ll probably find screw compressors.”


With a medium temperature system, although the discharge pressure is likely to be the same as that of a low temperature system, the inlet pressure of the refrigerant is higher, resulting in a reduced pressure ratio. In this instance, a screw or reciprocating compressor is going to be the most effective option. Mr Glass adds: “For high temperature refrigeration systems and air conditioning, you really start looking only at scroll and screw compressors.” Best used in instances where there is a high mass flow but low pressure ratio, rotary compressors such as scroll and screw are ideal for moving a large amount of refrigerant.


The size of the system in which the compressor is going to be used is also an important factor to consider. For example, screw compressors are much more likely to be found in a larger system than scroll compressors.


“A big building in London might have a chiller on the roof with two screw compressors installed,” Mr Glass points out. “To do that with scroll compressors, you might need two chillers each with 12 or 16 compressors to do the same amount of cooling.” The type and size of a compressor will also determine the kind of situation in which it is used. As a rule, reciprocating compressors are generally found in commercial plant, while screw compressors are used in larger industrial plant. However, as Mr Glass observes: “There are some quite large reciprocating compressors used in industrial applications and some small screw compressors used in smaller, non-industrial systems.” Both screw and reciprocating compressors are comparable in terms of their construction and the manner in which they are assembled and bolted together. Some are semi-hermetic, with a motor attached to the compressor or internally within


it, while others are open drive. These have an entirely separate motor which is connected to the compressor through a coupling or fly wheel. Regardless of whether a screw or reciprocating compressor is semi-hermetic or open drive, Mr Glass explains that each can be deconstructed down to its individual components. He also stresses the advanced nature of today’s compressors, explaining that if all components remain in balance then very


www.acr-news.com


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