Essay on Vapour Compression Compressor

The Vapour compression cycle is a refrigeration cycle often known as VCC; the refrigerant used in this system undergoes phase changes in each cycle. VCC is a widely used refrigeration cycle for refrigeration for commercial and domestic applications and air-conditioning of buildings, railways, and automobiles. The Vapour compression system primarily has four equipment, compressor, condenser, expansion unit, and evaporation unit. Compressor amongst all does the work of compressing the vapour refrigerant. Refrigeration compressors are mainly classified into two types, positive displacement compressor and dynamic compressor. Amongst these two, positive displacement compressors are primarily used in many domestic refrigeration and air conditioning applications. The positive displacement pump compresses the low-pressure refrigerant vapour to increase the refrigerant pressure injecting it into a refrigeration circuit. On the other hand, dynamic compressors are generally used in the most extensive refrigeration system and are usually not preferred in domestic applications in contrast to positive displacement compressors (Cape, 2019).

The positive displacement compressor further has three categories, Open compressor, Semi-hermetic compressor, and hermetic compressor. The open compressor always has a risk of refrigerant leakage as there is no seal around it. The open-type compressor is cheaper than the rest, and also it is easy to access the equipment during maintenance. On the other hand, Semi-hermetic has two-piece shell housing in which the motor and compressor are situated. The housings are bolted together, and it can be opened for service; it is usually expensive than the open and hermetic compressor. A shell comprises the motor and the compressor in the hermetic compressor. Although, welding is used to construct a steel shell that offers a complete hermetic seal against the environment. The welded shell cannot be opened for any maintenance and must be scrapped if the compressor or motor is damaged. The primary classification of the compressor is given below (Cape, 2019),

I. Positive displacement compressor (Open, Semi-hermetic, and Hermetic)

• A reciprocating compressor or piston-type compressor
• Screw compressor
• Scroll compressor

II. Dynamic displacement compressor

• Centrifugal compressor

Reciprocating compressor

It is also known as a piston compressor; shown in figure 1 is widely used in the refrigeration market. The operations of reciprocating compressor resemble piston engine in automobile only difference is that a motor powers it. When the piston retracts i.e., superheated refrigerant gas at the bottom of the cylinder enters the compressor through single-acting inlet valves. During the upward movement of the piston, the inlet valve remains closed, which results in increasing refrigerant vapour. Finally, the refrigerant is delivered to the condenser section when the pressure is high enough to open the exit valve. A similar process is repeated in each cycle (Miller & Miller, 2012).

Figure 1: Reciprocating compressor (SWEP, n.d.)

Following are the advantages of the reciprocating compressor,

• Construction and operations of the reciprocating compressor are relatively simple,
• Easy manufacturing of the piston and cylinder,
• The high speed of operation with lightweight and compact units

Screw compressor

The most common type of compressor is the twin-screw compressor. There is no dead space in the screw compressor like a reciprocating compressor. One rotor out of two is male, and the other is a female rotor; the design is such that it decreases the volume between the two rotors from the inlet of the compressor to the outlet continuously. The low-pressure side has an intake through which refrigerant is fed to the compressor, delivered from the high-pressure side. Therefore, the screw compressor does not have a suction and pressure valve but only a non-return valve to ensure no backward refrigerant flow when the compressor is off. The screw compressor can work at extremely high pressure as lubricant oil provides excellent sealing (Miller & Miller, 2012). Figure 2 shows the screw compressor,

Figure 2: Screw compressor (SWEP, n.d.)

Screw compressor offers the following advantages,

• High-speed operations, lightweight and compact dimensions
• Feed rate is practically non-pulsating with no vibrations
• Operation of screw compressor can be controlled through step-less performance control with minor changes in the efficiency
• It can work in humid surroundings due to excellent resistance to water hammer effect
• Highly reliable due to low maintenance throughout its life

Scroll Compressor

Working of the scroll compressor involves trapping of the refrigerant gas between fixed and revolving scroll. The motor drives the revolving scroll. As shown in figure 3, superheated refrigerant gas enters the spirals from the outer end, further compressed through scrolls due to the revolving motion of the spirals. The red refrigerant gas leaves from the discharge hole, which is located at the centre of the scrolls (Miller & Miller, 2012).

Figure 3: Scroll compressor (SWEP, n.d.)

Scroll compressor offers the following advantages,

• The load on the electric motor is drastically reduced, especially at the start of the motor
• Uniform delivery of the refrigerant supply
• High reliability
• Reduction in noise level

Centrifugal compressor

The construction centrifugal compressor consists of the impeller that can handle a high amount of refrigerant gas but relatively lower pressure. Figure 4 shows a centrifugal compressor.

Figure 4: Centrifugal compressor (SWEP, n.d.)

Following advantages offered by the centrifugal compressor,

• Centrifugal compressor is easy to design and manufacture
• Lighter weight
• It is most suitable where a continuous supply of refrigerant is required with considerable low condenser pressure
• Few parts make it reliable
• The rate of refrigerant flow is high
• Energy efficiency is higher as compared to a reciprocating compressor
• The range of rotational speed is wide

Work done equations by the compressor

Work done during isothermal compression

P₁V₁=mRT₁

W= 2.3 m R T₁ log (r)

P₁= Pressure at the compressor inlet, V₁= Volume at the compressor inlet, T₁= Temperature at the compressor inlet, R=Universal gas constant, m= mass of the refrigerant in kg

Work done during polytropic compression

PVⁿ=Constant

R=Universal gas constant, T₁= Temperature at the compressor inlet, T₂= Temperature at compressor outlet, m= mass of the refrigerant in kg

Work done during isentropic compression

W=m Cp (T₂-T₁)

T₁= Temperature at the compressor inlet, T₂= Temperature at compressor outlet, Cp= Specific heat capacity at constant pressure

Compressor recommendation for the R600a

The Temperature of the R600 changes drastically with minor changes in Temperature. Also, Specific heat capacity at a constant pressure of R600a is very high. Therefore, the suitable compressor for R600a is a hermetically sealed reciprocatingcompressor. Because the hermetically sealed compressor has no leakage, it will not affect the R600a due to the external environment. Further, the reciprocating compressors can produce considerably high pressure up to 400 bar and power up to 500hp. Also, the reciprocating compressor does not suffer from oil carry-over.

References

Cape, C. (2019). Different types of Refrigeration Compressors | CAPEDCOM. [online] Capedcom.org. Available at: http://www.capedcom.org/article.php?art_id=189 [Accessed 16 Mar. 2019].

Miller, R. and Miller, M. (2012). Air conditioning and refrigeration. New York: McGraw-Hill.

SWEP (n.d.). 3.2 Compressor Types – SWEP. [online] Swep.net. Available at: https://www.swep.net/refrigerant-handbook/3.-compressors/3.2-compressor-types/ [Accessed 16 Mar. 2019].