Lesson 2. Elementary vapour compression refrigeration cycle

Module 1. Fundamentals of refrigeration

Lesson 2

2.1 Introduction

The principle of refrigeration is based on second law of thermodynamics. It sates that heat does not flow from a low temperature body to a high temperature body without the help of an external work. In refrigeration process, since the heat has to be transferred from a low temperature body to a high temperature body some external work has to be done according to the second law of thermodynamics as shown in (Fig. 2.1).This external work is done by means of compressor.

2.2 Vapour Compression Refrigeration System

The vapour compression refrigeration system is widely used in commercial and domestic refrigeration and air conditioning plants. The working fluid called refrigerant completes its function upon evaporation during which it absorbs the heat in an amount equivalent to its refrigerating effect. Vapour compression refrigeration system using ammonia as refrigerant is widely used in India for industrial refrigeration, air conditioning and cold storages. Refrigeration is very essential requirement for low temperature storage of different food and dairy products. Refrigeration is also very essential for cold chain of handling many agricultural produce especially fruits and vegetables.

Vapour compression system mainly consists of compressor, condenser, receiver, expansion valve and evaporator. The refrigeration system is an enclosed gas tight system of tubes and equipment. It is so constructed that a control quality of refrigerant flows (due to expansion valve) from one necessary steps to another at definite and predetermined pressure. A block diagram of a vapour compression refrigeration system is shown in Fig. 2.2. The liquid refrigerant absorbs the heat from a zone of low pressure (evaporator) by means of its evaporation. The heat is dissipated in a zone of higher pressure (condenser) by means of condensation. The refrigerant like ammonia, R-22 etc. absorbs the heat at evaporator through evaporation. The compressor pumps the vapour refrigerant and the pressure of the refrigerant vapour is raised to a level so that it can be condensed at normal temperature of the cooling medium. Thus, four fundamental steps which are required to complete the mechanical cycles are as under.

1. Evaporation

2. Compression

3. Condensation

4. Pressure reduction or Expansion

The major components required in the system to accomplish the above four operations are (i) Evaporator (ii) Compressor (iii) Condenser (iv) Receiver and (v) Expansion valve.

fig 2.2

Fig. 2.1 Block diagram of vapour compression refrigeration system

2.2.1 Evaporation

The first step in the refrigeration cycle is the evaporation of liquid refrigerant. It is during this phase that the actual work of refrigeration is performed. The equipment in which the evaporation takes place is called the evaporator or cooling coil. The evaporator is the cooling element of the system.

The amount of liquid refrigerant that will evaporate in a given time depends upon the amount of heat transfer in the evaporator. The rate of heat transfer through a given heat exchanger depends on overall heat transfer co-efficient, area of heat transfer and the temperature difference. The refrigeration capacity of any system is based on the amount of cooling effect produced per unit time. The ton of refrigeration, which is common unit of the capacity, is the equivalent of 3000 kcal/h or 12600 kJ/h cooling effect. This is also equivalent to the amount of refrigeration done by melting of one ton (2000 lb.) of ice from and at °C in 24 hour.

2.2.2 Compression

The gas received from the evaporator is compressed by reciprocating compressor. It has two functions. (i) To draw the cold vapour from the evaporator to maintain a pressure in evaporator sufficiently low to achieve the desired evaporating temperature. (ii) To compress the vapour refrigerant and to deliver the compressed gas into the condenser where it can be liquefied at ordinary temperature by means of cooling water or air depending on the type of condenser.

Most of the power required to drive the compressor is absorbed by the vapour in the process of compression and appears as heat which must be taken out by the condenser. The compressor is located between the evaporator and the condenser. The condenser is connected to the discharge a side of the compressor. The compressor is a dividing component that joins the low pressure side to the high pressure side of the system.

2.2.3 Condensation

The purpose of the condensation is to provide a means by which the vapour rejects the heat absorbed at the evaporator as well as the heat of compression. The condenser is an arrangement of the pipe and tube possessing high heat condensing properties and so constructed as to allow for the entrance and exist of the refrigerant. These pipes or tubes are exposed to a cooling media (water or air) which absorbs the heat from refrigerant.

Connected to the outlet of the condenser and in some system forming an integral part of the condenser itself is a tank known as liquid receiver. The purpose of the receiver is to receive the liquid refrigerant upon condensing and to store until it is required by evaporator. The liquid receiver should be large enough to store the entire refrigerant charged in the system in liquid stage.

2.3.4 Pressure reduction or expansion

The refrigerant is in liquid stage under high pressure in the receiver and it is ready for re-use in the evaporator where much lower pressure is maintained. Some means of retaining of pressure difference is required. This is done by arranging a throttling device in the line between receiver and evaporator. This throttling maintains a higher pressure on one side and allows a lower pressure condition to exist on the other side when flow of refrigerant occurs.

The throttling device is often referred to as an expansion valve but its real purpose is to achieve pressure reduction so as to lower the boiling point of the liquid refrigerant.

2.2.5 Receiver

The receiver of the system receives the refrigerant from the condenser and delivers it to the evaporator through expansion valve depending on the requirement of evaporator. This component has important role to collect the entire charge of the system during maintenance work of the plant.

The basic cycle of vapour compression refrigeration is the same for small and large capacity vapour compression refrigeration systems. Refrigerant is a the heat transfer medium in vapour compression refrigeration cycle which absorbs the heat through evaporation at the evaporator and rejects the heat absorbed at evaporator plus the heat of work of compression at the condenser.

Thus, according to second law of thermodynamics,

Heat rejected at condenser, kJ/h = Refrigerating effect, kJ/h + Work of compression, kJ/h.

2.3 Primary Refrigeration

The working fluid of vapour compression refrigeration system is known as primary refrigerant which absorbs the heat through evaporation at the evaporator and again becomes liquid on cooling in the condenser. Primary refrigerants include only those working fluids which pass through the cycle of evaporation, compression, condensation and expansion. These refrigerants have very low boiling point. e.g. boiling point of ammonia is -33.3 ºC at atmospheric pressure. There are many refrigerants which as discussed in another lesion.

2.4 Co-Efficient of Performance (C.O.P.)

The performance of the refrigeration system is expressed as C.O.P. which is the ratio of refrigerating effect produced to the work of compression. The cooling effect (out put of the system) is produced at the evaporator and the refrigerant is compressed by the compressor using the electrical power (input to the system). The various aspects associated with performance of the system are discussed in some another lesson. Higher C.O.P. is always desirable in order to get more cooling effect with less energy in put.

C.O.P=Refrigeration effect, kJ/hWork of compression, kJ/h'

Last modified: Friday, 9 November 2012, 9:56 AM