MECHANICAL THEORY

Refrigeration:Introduction, Vapor Compression Refrigeration Cycle & Vapor Absorption Refrigeration Cycle

Refrigeration-Introduction, Vapor Compression Refrigeration Cycle & Vapor Absorption Refrigeration Cycle: Refrigeration may be defined as a process of removing heat from a substance and pumping it to the surroundings. It also includes the process of maintaining and reducing the temperature of a body below the general temperature of its surroundings.

Thus in a refrigerator heat is prepared from low temperature to high temperature. Theoretically, the refrigerator is a heat pump which pumps heat from a cold body and delivers it to a hot body.

  • Equipment used for removing the heat continuously for maintaining a low temperature in a space is called ‘refrigerator’.
  • Cycles on which it operates are called ‘refrigeration cycles’.
  • The working fluids used for carrying away heat are called ‘refrigerants’ which are used in both refrigeration and air conditioning equipment.
  • Equipment used for air conditioning is called air conditioners.

Applications of Refrigeration:
Refrigeration is extensively used for increasing the storage life of perishable items especially food products, vegetables, fruits, milk, beverages, chilling of water, ice formation, etc.

Industrial applications include chemical manufacturing, petroleum refinery, petrochemical plants, paper, and pulp industry, etc.

  • Ice making
  • Manufacturing and treatment of metals
  • Industrial air-conditioning
  • Transportation of food
  • Chemical and related industries
  • Freezing food products
  • Medical and surgical aids
  • Central air-conditioning

Unit of Refrigeration:

The unit of refrigeration is expressed in terms of “Tonne of Refrigeration” (TR).
One Ton of refrigeration: It can be defined as the amount of refrigeration effect produced by the uniform melting of one tone of ice (1000 kg) from and at 0°C in 24 hrs.
1 Tonne of refrigeration (TR) = (335×1000)/ 24
                              where latent heat of ice = 335 KJ/Kg
                                                   = 13958.333 KJ/hr
                                                   = 13958.333/24 KJ/min
                                                   = 232.6 KJ/min

Coefficient of Performance of a Refrigerator:
The performance of a refrigeration system is expressed by a term known as the coefficient of performance(C.O.P.)
It is the ratio of heat extracted in the refrigerator to the work done on the refrigerant. It is also known as the theoretical coefficient of performance.
Theoretical C.O.P = Heat extracted/ Work done = Q/W

Relative C.O.P = Actual C.O.P / Theoretical C.O.P

Difference between a refrigerator, heat pump and heat engine:

Let’s discuss about Heat Engine first…

heat engine:
In a heat engine, the heat supplied to the engine is converted into useful work. If Q2 is the heat supplied to the engine and Q1 is the heat rejected from the heat engine, then the network done by the engine is given by
We = Q2-Q1
So the performance of the engine or C.O.P is given by
Efficiency = We/Q
                  = Q2-Q1/Q2

Refrigerator:
A refrigerator is a reversed heat engine, where heat is pumped from low temperature (cold body-Q1) to high temperature (hot body-Q2). So, work WR is required to be done on the system.
WR = Q2-Q1
The performance of a refrigerator is the “ratio of the amount of heat taken from the Cold body Q1 to the amount of work to be done on the system WR.

(C.O.P)R = Q1/ WR = Q1 / (Q2-Q1)

heat pump:
Any refrigerating system is a heat pump, which extracts heat from a cold body and delivers it to a hot body.
Thus there is no difference in the operation cycle of a refrigerator and a heat pump.
   

  • The main difference between them is in their operating temperature’s.
  • A refrigerator works between cold body temperature (T1) and atmospheric temp (Ta) whereas the heat pump operates between hot body temp (T2) and the atmospheric temperature (Ta).
  • A refrigerator used for cooling in summer can be used as a heat pump for heating in the winter season.
    so Wp = Q2-Q1

            (C.O.P)hp = Q2/ WR = Q2 / (Q2-Q1)
                             Where hp-heat pump

Methods for Refrigeration:

  • Refrigeration by evaporation
  • Refrigeration by ice
  • Refrigeration by the expansion of air
  • Refrigeration by throttling process
  • Refrigeration by dry ice
  • vapor Refrigeration system

Refrigerants:
Any substance which in the refrigeration system absorbs heat from a low-temperature system and delivers the heat so absorbed to a higher temperature system is referred to as a refrigerant.

Classification of Refrigerants:
1) Primary Refrigerants
2) Secondary Refrigerants

Primary refrigerants: The working mediums or heat carries which directly take part in the refrigeration system and cool the substance by the absorption of latent heat are called as Primary refrigerants.
Eg:- Ammonia, SO2, CO2, Methyl chloride, etc..

Secondary Refrigerants: These are circulating substances which are first cooled with the help of primary refrigerants and then employed for cooling purposes.
Eg:- Ice, Solid CO2, etc..

Desirable Properties of Refrigerants:
Chemical properties:

  • Non-toxic
  • Non-flammable and non-explosive
  • Non-corrosive to metal

Thermodynamic properties:

  • Low freezing point
  • Low boiling point
  • High latent heat of vaporization

Physical Properties:

  • Low specific heat
  • A low specific volume of vapor
  • Low viscosity
    Other properties:
  • Ease of leakage location
  • Availability and low cost
  • High COP

The two cycles which come under Refrigeration are

1.Vapor Compression Refrigeration Cycle

2.Vapor Absorption Refrigeration Cycle

Let’s discuss in detail…

1.Vapor Compression Refrigeration Cycle or Vapor Compression Refrigeration System(VCRS):

Vapor Compression Refrigeration Cycle
Vapor Compression Refrigeration Cycle

This is the most widely used refrigeration system. In this system, the working fluid is a vapor. It readily evaporates and condenses or changes alternatively between the vapor and liquid phase without leaving the refrigerating plant.

During evaporation, it absorbs heat from the cold body and this heat is used as its latent heat for converting it from liquid to vapor whereas in Condensing or cooling, it rejects heat to external bodies, thus creating a cooling effect in the working fluid.

Components of Vapor Compression Refrigeration System:

The figure above consists of the following five essential parts.

  1. Compressor
  2. Condenser
  3. Receiver
  4. Expansion Valve
  5. Evaporator

The detailed explanation of the above parts is as follows.

1.Compressor:

The vapor at low pressure and low temperature enters the compressor from the evaporator where it is compressed to high pressure and high temperature.

This high pressure and temperature vapor refrigerant is discharged into the condenser through the discharge valve.

2.Condenser:

The condenser or cooler consists of coils of pipe in which the high pressure and temperature vapor refrigerant is cooled and condensed.

The refrigerant while passing through the condenser gives up its latent heat to the surroundings condensing medium which is normally air or water.

3.Receiver:

The condensed liquid refrigerant from the condenser is stored in a vessel known as a receiver from where it is supplied to the evaporator through the expansion valve.

4.Expansion Valve:

It is also called a throttle valve. Its function is to allow the liquid refrigerant under high pressure and temperature to pass through it where it reduces its temperature and pressure.

5.Evaporator:

It also consists of coils of pipe in which liquid-vapor refrigerant at low pressure and temperature is evaporated and converted into vapor refrigerant at low pressure and temperature.

Working of Vapor Compression Refrigeration System:

The vapor at low temperature and low pressure enter the compressor where it is compressed isentropically and subsequently, its temperature and pressure considerably increase. This vapor after leaving the compressor enters into the condenser, where it is condensed into high-pressure liquid and is collected in a receiver tank.

From receiver tank, it passes through the expansion valve, here it is throttled down to low pressure and temperature. After finding its way through an expansion valve, it finally passes onto the evaporator, where it extracts heat from the surroundings or circulating fluid and vaporizes to lower pressure vapor.

If expansion takes place without throttling, temperature level drops to a very low level due to which it should undergo sensible heat and latent heat in order to reach the evaporation stage.

The P-h Diagram and T-S Diagrams will be updated soon…

Note:

  • Work is done by the compressor.
  • The heat extracted from the evaporator is the difference of heat entering the evaporator and heat leaving the evaporator.

The coefficient of performance of a simple vapor refrigeration cycle is as follows.

C.O.P. = (heat extracted or refrigerating effect)/Work done.

 C.O.P. = (h2-h1)/(h3-h2)

or

C.O.P. = (h2-h4)/(h3-h2).

[h1=h4, since during throttling expansion 4-1, the total heat content remains unchanged.

This is the explanation of the Vapor Compression Refrigeration Cycle in a detailed way.

Now let’s discuss the Vapor Absorption Refrigeration Cycle.


2.Vapor Absorption Refrigeration Cycle or Vapor Absorption Refrigeration System(VARS):

In the vapor absorption process, the compression process of the vapor compression cycle is replaced by an absorber, a generator and a pump.

The principle of the system is to make use of two substances that can be separated when heated.

The two substances can be

  1. Ammonia(Refrigerant) and
  2. Water (Absorbent).
Vapor Absorption Refrigeration Cycle
Vapor Absorption Refrigeration Cycle

Components of Vapor Absorption Refrigeration Cycle:

The figure above consists of the following six essential parts.

  1. Condenser
  2. Expansion Valve
  3. Evaporator
  4. Absorber
  5. Pump
  6. Generator

Note: The explanation of Condensor, expansion valve and Evaporator does the same work as mentioned above in Vapor Compression Refrigeration Cycle. Therefore I don’t repeat them here and the absorber, pump, a generator will be explained.

Working of Vapor Absorption Refrigeration Cycle:

In this type of refrigeration system, the vapor produced in the evaporator passes into the absorber.

The absorber consists of a homogeneous mixture of ammonia and water known as aqua ammonia.In the absorber, the vapor is absorbed and released from the absorbent,which maintains a constant low pressure.This process takes place at a temperature slightly above that of the surroundings.

In this process, some heat is transferred to the surroundings. The strong ammonia solution is then pumped through a heat exchanger to the generator by a pump where a high pressure and temperature is maintained.

The ammonia vapor enters into the condenser where it gets condensed into liquid ammonia.

The liquid ammonia which is at high pressure is passed through an expansion valve through which wet Ammonia vapor, at low temperature and pressure comes out.

The vapor is then passed through the evaporator. The evaporator absorbs heat and is dried. The dried vapor is again taken to the absorber and thus the cycle is completed.

The Coefficient of Performance of Vapor Absorption Refrigeration System is as follows.

C.O.P = (Heat absorbed during evaporation)/(work done by the pump+heat supplied in the heat exchanger)

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.