refrigeration and air conditioning

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 refrigeration cycle to produce require effect i.e. cooling is known as refrigerant.

Heat Pump: Heat pump is a device which is used to maintain a space at 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 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 performance parameter in a refrigerator cycle, called the coefficient of performance.
• Heat pump is a device which operating in a cycle, maintains a body at a temperature higher than the temperature of the surroundings.

Reversed Carnot Cycle:

When the processes of Carnot cycle is 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 extract the heat from one place and release at the other thus maintaining the place at lower temperature.
When refrigerent 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 compressor

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

 h₄  =Enthalpy at the exit of condenser  or inlet of 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

Compressor:

A compressor is used in refrigerator to increase the pressure of gas.

IN VCRS system, we generally prefer reciprocating compressor.

Clearance ratio:

Swept volume:

Volumetric efficiency is given by:

where, D = diameter of compressor

L = stroke length

n = N for 2 stroke

n = N/2 for 4 stroke

N = rotational speed

η_v = volumetric efficiency

k = number of cylinders

V_c = clearance volume

C = clearance ratio 

Vapour Absorption Refrigeration System (VARS):

This system utilizes low grade energy or waste energy to produce 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

Q_g = heat supply to refrigerator in generator

Q_e = heat is absorbed by refrigerator in 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 

Efficiency of Carnot engine 

Here, T_g = generator temperature

T_c = T_o = absorber temperature (Generally surrounding temperature)

T_e = evaporator temperature.

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

Designation of Refrigerant:

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

For a hydrocarbon Chemical formula: C_r H_s F_t Cl_y

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