Study Notes on Various Vapor and Gas Refrigeration System and Heat Pump Cycles

By Vijay Pratap Singh|Updated : November 25th, 2021

In this article, we are providing you with the Refrigeration and Air-Conditioning study notes for GATE , BARC other Mechanical exams. Refrigeration and Air Conditioning is an important part of Thermodynamics that has a good weightage of the questions to be asked in various mechanical engineering exams.

Refrigeration Cycle

Refrigeration is the process in which heat is removed from a body or enclosed space and its temperature is maintained at a temperature below the surrounding temperature.

Refrigeration Cycle:

Refrigeration is the process in which heat is removed from a body or enclosed space and its temperature is maintained at the temperature below the surrounding temperature.

The working substance used in the refrigeration cycle to produce required effect i.e. cooling is known as refrigerant.

Heat Pump

A heat pump is a device that is used to maintain a space at a higher temperature than that of surroundings, for which external work has to be supplied.

Coefficient of Performance:

Here, the desired effect is to maintain the body at a temperature higher than that of surrounding.

Thus,

where:

TH = High temperature body

TL = Low temperature body

Refrigerator

It is a device which is used to maintain a body at a temperature lower than the temperature of the surrounding.

Where, TH = Constant temperture at which heat is to be rejected

TL = Constant temperture from where heat is to be extracted

(COP)HP = (COP)R + 1

where, QL= heat at lower temperature,

QH = heat at higher temperature

Unit of Refrigeration: It has standard unit of TR (Ton or Refrigeration). 1 TR (one ton of refrigeration) means the capacity to freeze one ton of water from and at 0oC in 24 h.

Key Points

• A refrigerator is a device operating in a cycle, maintains a body at a temperature lower than that of the surroundings.
• There is a performance parameter in a refrigerator cycle, called the coefficient of performance.
• A heat pump is a device that operates in a cycle, maintains a body at a temperature higher than the temperature of the surroundings.

Reversed Carnot Cycle

When the processes of the Carnot cycle are reversed, then it is called reversed Carnot cycle.

Where T2 and T1 are the temperature at section 2 and 1 respectively and S1, S2, S3 and S4 are the entropy at 1, 2, 3 and 4 respectively.

Vapour Compression Refrigeration System

In this system, refrigerant extracts the heat from one place and release at the other thus maintaining the place at lower temperature.
When refrigerant passes through the evaporator, it extracted the heat from the chamber and converts in to vapour and produces refrigerating effect. This process is known as evaporation.

1 → 2 → Isentropic compression process

2 → 3 → condensation process (constant pressure Heat Rejection)

3 → 4 → Isenthalpic Expansion process

4 → 1 → evaporation process (constant pressure Heat Addition)

Where h1 = Enthalpy at the inlet of the compressor

h2 = Enthalpy at the exit of compressor or inlet of the condenser

h4 =Enthalpy at the exit of condenser or inlet of the throttle valve

Key Points

• COP of refrigerator working between two fixed temperature TL and TH (TH > TL) will have fixed COP.
• A reversible heat engine can work like a refrigerator or a heat pump.
• Vapour compression refrigeration cycle
• Entropy:

h2 = h2'+Cp (T2' –T2)
• For isentropic process:
S2 = S1

Vapour Absorption Refrigeration System (VARS)

This system utilizes low-grade energy or waste energy to produce a refrigeration effect.

In vapour absorption system compressor is replaced by a unit comprising an absorber, a pump, and a generator.

COP of an Ideal Vapour Absorption System

Schematic diagram of energy transfer in vapour absorption system is shown below

Qg = heat supply to the refrigerator in generator

Qe = heat is absorbed by refrigerator in the evaporator

Qc = heat is rejected to condenser or atmosphere

Coefficient of Performance:

Qg + Qe = Qc

For reversible cycle net entropy change is zero.

COP of Carnot refrigerator

The efficiency of Carnot engine

Here, Tg = generator temperature

Tc = To = absorber temperature (Generally surrounding temperature)

Te = evaporator temperature.

Refrigerant

Refrigerant is the fluid that flows through the various components of the refrigeration system and absorbs heat by vaporization at the evaporator and rejects it through condensation at the condenser and so produces a refrigerating effect.

Designation of Refrigerant

In International Standards, Refrigerants are designated as R followed by some numeric values.

For a hydrocarbon Chemical formula: Cr Hs Ft Cly

If S+t+y=2r+2, designation of refrigerant is R(r-1)(S+1)t

It S+t+y=2r, designation of refrigerant is R1(r-1)(S+1)t

For an Inorganic Refrigerant

Designation is R(700 + molecular weight)

e.g.

Desirable Properties of a Good Refrigerant

• Low boiling point
• Low freezing point
• High latent heat of vaporization
• High thermal conductivity
• Low viscosity
• Low specific volume in vapour state

Uses and Properties of Some Refrigerant

• In the absorption refrigeration system, we use NH3 – H2O or water lithium bromide or water lithium chloride. In NH3 – H2O system NH3 is refrigerant while H2O is a transport medium. In water lithium bromide or water lithium chloride water if refrigerant.
• In ice plant, the primary refrigerant is NH3 or Freon while the secondary refrigerant is the prime solution.
• The freezing temperature of Freon-12 is – 157.8oC, so it is not used for producing temperatures below the freezing temperature.
• In a dehydrator, silica get (molecular sieve SiO2 + K2O + NO2 + Al2O3) is used with calcium chloride. (Not use sodium chloride).
• A hermetically sealed reciprocating compressor is commonly used in room air conditioners.
• For passenger aircraft air is used as a refrigerant.
• Refrigerant CO2 has the lowest specific volume.

Refrigeration and Air-Conditioning is an important topic of Thermodynamics mainly asked in the GATE ME, ISRO ME, SSC JE ME, ESE IES ME, and other Mechanical exams.

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