Fig 6.1 Content of this post
Fig 6.2 Vapor Compression System 
Process 1-2(Isentropic Compression)
Low pressure and low temperature refrigerant air passes through compressor. In compressor it converts into high pressure and high temperature refrigerant air. Process shown on T-S and P-H diagram (1-2).
Refrigerant air entering at compressor is at dry saturated state (1), hence point lies on saturated line after isentropic compression air becomes superheated state (2).

Process 2-3(Constant pressure heat rejection)
In this process high temperature and high pressurised refrigerant air reject its heat to surrounding by converting into liquid as shown as 2-3 process. Condenser rejects heat to atmosphere maintaining constant pressure.
In this process high temperature and high pressurised air converted into high temperature and high pressurised liquid refrigerant.
Fig 6.3 T-S and P-H diagram of VCC
Process 3-4 (Throttling -Expansion)
In this process high temperature high pressurised liquid refrigerant converted into low temperature low pressurised liquid refrigerant. This process is generally carried out in capillary tube, throttling valve etc.
Fig shows T-S and P-H diagram showing this process by 3-4. At the end of this process that is at state 4 very cold liquid refrigerant is obtained.
COP calculations
Process 4-1( Heat Extraction)
It is heat absorption process in which heat is absorbed by cold liquid refrigerant and converted into air. This process is carried out in Evaporator as shown by 4-1. Cold refrigerant absorb heat from application and converted into air vapour for compression process in 1-2 and this cycle continues. 

Types of  Compression:
1. Wet compression
2. Dry Compression

Fig 6.4 Difference between Dry and Wet compression
Above figure shows difference between Dry and Wet Compression . In Wet compression refrigerant entered in compressor in liquid state.  State 1 is entry to compressor inlet and it is located inside the saturation curve. 
In Dry compression,  refrigerant enter at vapor state at the entry to compressor, hence state 1 is outside the saturation curve as shown above figure.
Generally dry compression is preferred most of time in system because due to wet compression, corrosion of compressor blades take place and hence life of compressor get reduced.

Effect of Subcooling(undercooling):
first of all we understand the sub cooling process by following diagram as showing below:
Figure shows process 1-2-3-4 is general Vapor compression cycle  and process 1-2-3'-4' is modified cycle this is sub-cooled cycle. we can observe that difference in which cooling of refrigerant in condenser get increased by point 3 to point 3' hence due to which refrigerant effect get increased by 4-1 to 4'-1.
Fig 6.5 Effect of Subcooling
Advantage of subcooling is when subcooling of liquid carried out refrigerant effect increases while work supplied by compressor remains same hence resultant effect is COP get increased.
Following Figure shows COP calculations:
COP Calculations
Comparing this two COP equations of Refrigerant effect is increases in modified cycle by an amount H4 to H4' while work supplied(H2-H1) constant in both cycle hence resultant effect COP increases in subcooling system.

Effect of Superheating:
First of all we understand the superheating process by following diagram.
Figure shows process 1-2-3-4 is general Vapor compression cycle  and process 1'-2'-3-4 is modified cycle this is superheated cycle. we can observe that difference in which cooling effect  increased from 1 to 1' and compressed work also get increased because 2-2' is more than 1-1' process.
Fig 6.6 Effect of Superheating
Advantage of superheating is that refrigerant entering in compressor is in vapor state hence safety of compressor and longer life of compressor.
COP calculation of system as follows:
COP Calculations
Comparing this two COP equations of Refrigerant effect is increases in modified cycle by an amount H1 to H1' while work supplied is tremendously increased (H2'-H1') Hence resultant effect is COP get decreases , but superheating is carried out for the safety working of compressor.

 Effect of increment in discharge pressure:
Fig 6.7 Effect of increment in discharge pressure
Figure shows process 1-2-3-4 is general Vapor compression cycle. Now discharge pressure increased from Pc to Pc’ and new simple vapor compression cycle is shown on T-S and P-H diagram (1-2-2’-3-4-1).

By comparing equation 1 and 2 refrigerating effect reduces as well as work supplied to compressor also increases hence modified VCC resultant COP decreases tremendously.

Effect of decrements in evaporator pressure:
Fig 6.8 effect of decrements in evaporator pressure
Figure shows process 1-2-3-4 is general Vapor compression cycle. Now evaporator pressure decreased from Pe to Pe’ and new simple vapor compression cycle is shown on T-S and P-H diagram (1’-2-3-4’-1).

Refrigerant effect is decreased (h1-h1’) while work supplied also increased by (h1-h1’)and hence resultant effect is COP decreases tremendously.