T-S and P-h Diagram for VC Refrigeration with Subcooling and Super-heating
Basic Vapor Compression Refrigeration Cycle and Its components
The refrigeration system cannot run without satisfying the second law of thermodynamics (Clausius statement). As the name implies the vapor compression cycle uses a compressor to increase the pressure of the refrigerant or the working fluid. The whole process is explained in brief with the help of this schematic diagram.
1. Low pressure (P-L) refrigerant coming from the evaporator is compressed to a higher pressure P-H.
2. The refrigerant is then passed through a condenser at high-pressure P-H
3. The liquid refrigerant then goes through a throttling device. It is expanded and evaporates at low pressure (P-L) in the evaporator. And the process continues.
T-S and P-h Diagram with Sub-cooling and Super-heating explanation
Reversed Carnot Cycle doesn’t work in practical cases. The reversed Carnot Cycle consists of two isentropic and isotherm process. Theoretical Carnot cycle is completely reversible. It is not possible because of the losses that occur in the condensers, evaporators, and compressors. As a result, a more practical and realistic Rankin cycle is introduced consisting of two isobaric processes, one isentropic compression, and one adiabatic expansion. The Rankin Cycle incorporates some inevitable irreversibilities consequently the COP of Rankin cycle is below the COP of Carnot cycle. To increase the COP of the whole system subcooling and superheating is done. These two concepts are difficult to visualize but very import issue for the VCRS.
What is superheating?
Superheating is the incident where a liquid is heated above its boiling point. After the liquid is leaving the evaporator and heading towards the compressor there should not be any hint of liquid present in the vapor. A tiny amount of liquid can damage the compressor.
What is subcooling?
Subcooling is a process where a liquid cooled down below its condensing temperature (The temperature at which vapor turns in to liquid). Suppose a refrigerant is cooled and turned in to liquid but the cooling will not stop. I will be continued to cool. Without subcooling, the refrigerant flow will not continue and it will not head towards the expansion device.
Comparison of Carnot and Rankin Cycle T-S diagram / Deviation of Real Vapor compression cycle from the ideal one :
The following diagram shows the comparison between the ideal and practical vapor compression cycle.
T-S and P-h Diagram with sub-cooling and superheating
The cycle can be described as follows:
7-1: Evaporation of the liquefied refrigerant (coming from the condenser) taking place at a constant temperature. The process is isothermal. (T1 = T7)
1-2: The vapor coming from the evaporator is superheated and gains the temperature T2 from T1 at constant pressure PL. The process is isobaric.
2-3 The superheated working fluid is compressed. Pressure rises from PL to PH
And the temperature rises from T2 to T3
3-4 The superheated vapor is cooled to the saturated temp T3.
4-5 Isothermal condensation of the saturated vapor at high-pressure PH. and T4=T5
5-6 The liquid refrigerant is subcooled to the temperature T6 from T5 at high-pressure PH.
6-7 The expansion of the refrigerant takes place at constant enthalpy.
Please Read :
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- Mechanical Engineering Interview Questions part – 4 (Refrigeration)
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