Chillers Energy Efficiency , Types & Classifications

The working principles of displacement compressors and dynamic compressors differ significantly. In positive displacement compressors, a certain volume of gas is trapped in a space that is continuously reduced by the compressing device (piston, scroll, screw or similar) inside the compressor. The reduction in volume increases the pressure of the vapor when the compressor is operating. The principle of a centrifugal compressor, also called a turbo compressor, is different. Here, the gas is compressed by being accelerated by an impeller. The pressure is further increased in the diffuser, where the speed is transformed to pressure. Centrifugal compressors are of interest for very large capacities, where the inlet flows may be approximately 2000 m3/h or more.

In a semi-hermetic compressor, the motor and the compressor housing are located in a two-piece shell. The covers are bolted together, allowing the cover to be opened for servicing, etc. Semi-hermetic compressors are generally a little more expensive than hermetic compressors, due to the bolts and O-rings needed to join the covers.
A hermetic compressor also houses both the motor and the compressor housing inside a shell. However, the steel shell is welded, which provides a true hermetic seal against the surroundings. It is impossible to open the welded shell of a hermetic compressor, and the compressor must therefore be scrapped in the event of damage to the motor or compressor.

Reciprocating compressors
Reciprocating compressors (see Figure 3.2), also called piston compressors, are still widely used but have faced increasing competition from other compressor types in recent decades.
Inside the reciprocating compressor housing, one piston moves up and down in each cylinder. When the piston is at its lowest point, superheated gas enters the compressor through the inlet valves. When the piston moves up, the inlet valve closes and the gas pressure increases, due to the reduced volume. The compressed gas leaves the compressor when the pressure is high enough to open the exit valve. The downward piston action initiates a new intake of gas through the valves.
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Screw compressors
Thanks to the improvements in screw compressors in recent years, they have become more common in air-conditioning and mid-range refrigerant applications. They will probably become even more popular, and replace many large (from 50 kW) reciprocating compressors. Screw compressors are produced in two different configurations: the twin-screw compressor, also called the Lysholm type after its inventor, and the single screw compressor (see Figure 3.3).
The twin-screw, the most common type, is composed of two rotors with complementary profiles referred to as screw and slide rotors, or male and female rotors. The rotor profiles are designed to decrease the volume between them continuously from the inlet to the outlet of the compressor.


Scroll compressors
The advantages of scroll compressors have been known since the early years of the 20th century. The reason for their not being introduced on a large scale until the 80's was the difficulty of producing scrolls, which requires very high precision.

Centrifugal Compressors

A centrifugal compressor is a type of dynamic compressor, or turbo compressor, with a radial design. Unlike displacement compressors that work at a constant flow, dynamic compressors work at a constant pressure and the performance is affected by external conditions such as changes in inlet temperatures.
So, how does a centrifugal compressor work?
Each stage takes up a part of the overall pressure rise of the compressor unit. Depending on the pressure required for the application, a number of stages can be arranged in a series to achieve a higher pressure. This type of multi-stage application is often used in the oil and gas and process industries. Alternately, in wastewater treatment plants, low pressure, single-stage applications are used to achieve the desired pressure ratio.

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