Refrigeration and Air-Conditioning : Various Refrigeration System

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:

T_H = High temperature body

T_L = 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, T_H = Constant temperture at which heat is to be rejected

T_L = Constant temperture from where heat is to be extracted

(COP)_HP = (COP)_R + 1

where, Q_L= heat at lower temperature,

Q_H = 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 0^oC 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 T_L and T_H (T_H > T_L) will have fixed COP.
• A reversible heat engine can work like a refrigerator or a heat pump.
• Vapour compression refrigeration cycle
• Entropy:
  
  h₂ = h₂'+C_p (T₂' –T₂)
• For isentropic process:
  S₂ = S₁

Air Refrigeration System

In air refrigeration, Bell Coleman cycle is used which works on reversed Brayton cycle while Brayton cycle is used for gas turbine.

COP of reversed Brayton cycle:

Let pressure ratio: r_p, then:

Where, γ = index, r_p = pressure ratio

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

A schematic diagram of energy transfer in vapour absorption system is shown below

Q_g = heat supply to the refrigerator in generator

Q_e = heat is absorbed by refrigerator in the evaporator

Q_c = heat is rejected to condenser or atmosphere

Coefficient of Performance:

Q_g + Q_e = Q_c

For reversible cycle net entropy change is zero.

COP of Carnot refrigerator=

The efficiency of the Carnot engine

Here, T_g = generator temperature

T_c = T_o = absorber temperature (Generally surrounding temperature)

T_e = evaporator temperature.

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