There are several types of compressors, as listed in Table 3.1.
Table 3.1 Compressor types.
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. BPHE evaporators and condensers cannot handle such large capacities, so they are not compatible with centrifugal compressors. However, BPHEs may well be used as oil coolers for centrifugal compressors.
In addition to their different working principles, compressors can also be distinguished according to their basic type of construction, as shown in Table 3.2.
Table 3.2 Classification of compressors according to their size.
In an open compressor, the motor and the compressor housing are mounted separately. Because the open compressor lacks a seal around it, there is risk of refrigerant leakage. The advantages are that the compressor components are easily accessible for maintenance and the costs of a shell can be avoided.
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.
The reason for the general size grouping is to show the possibilities for maintaining and repairing expensive compressors, which is less important for small, mass-produced hermetic 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.
The advantage of reciprocating compressors is the relatively simple working principle and construction. The main component, a circular cylinder with a suitable piston, can be manufactured quite easily with good accuracy. A disadvantage of reciprocating compressors is that they have many moving parts, which makes it almost impossible to avoid vibrations. Another disadvantage is the "dead space". When the piston is at its top position, some of the compressed gas will be trapped in the space between the top of the piston and the cylinder roof. The gas in the dead space results in lower volumetric efficiency, because less fresh gas is compressed on each piston stroke than the total volume of the cylinder could actually admit.
The valves controlling the flow of gas to and from the compressor are sensitive to droplets in the gas. If a considerable amount of liquid enters the compressor housing, a very high pressure can be built up when the piston reaches its top position, which may cause severe damage to the valves or crankshaft. This phenomenon is called liquid hammer.
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 singlescrew 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. Unlike reciprocating compressors, screw compressors have no dead space. The refrigerant is fed from the low-pressure to the highpressure side with a continuously decreasing volume, i.e. continuously increasing pressure. Screw compressors therefore have neither suction valves nor pressure valves, only a non-return valve to ensure that there is no return flow of refrigerant when the compressor is stopped.
Screw compressors can work at a high compression ratio because the oil, in addition to its lubrication and sealing functions, also absorbs compression and friction heat during the process. Proper oil cooling is therefore essential in a screw compressor, and can be provided either by the injection of refrigerant into the compressor or by a separate oil cooling system. BPHEs are widely used as oil coolers.
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.
Scroll compressors capture the gas in the volume formed between one fixed and one orbiting scroll. The orbiting scroll is driven by an electric motor, which rotates a shaft. Note that the scrolls perform an orbiting motion. They do not rotate.
Figure 3.4 explains the scroll compressor function. Superheated gas (blue) enters at the outer ends of the spirals and is compressed on its way through the scrolls due to the orbiting motion of one of the spirals. The discharge hole, where high-pressure gas (red) leaves, is located in the center of the scrolls.
Scroll compressors are available in both open and hermetic design. They have several advantages over reciprocating compressors:
- The absence of suction and discharge valves eliminates pressure drops and consequential noise and vibrations.
- The scrolls have no dead space, which results in volumetric efficiencies close to 100%.
- Fewer moving components, leading to a lower failure rate.
- They are relatively insensitive to liquid droplets in the suction gas from the evaporator.