how a refrigeration system works

Introduction

Mechanical refrigeration is a thermodynamic process of removing heat from a lower temperature heat source or substance and transferring it to a higher temperature heat sink. This is against the Second Law of Thermodynamics, which states that heat will not pass from a cold region to a warm one. According to Clausius Statement of the Second Law of thermodynamics "It is impossible to construct a device that operates in a cycle and produces no effect other than the transfer of heat from a lower-temperature body to a higher-temperature body ' Therefore in order to accomplish the transfer of heat from low temperature region to high temperature region an "external agent" or energy Input is required and you need a device, like a heat pump or refrigerator, which consumes work. The operating principle of the refrigeration cycle was described mathematically by Sadi Carnot in 1824 as a heat engine. A refrigerator or heat pump is simply a heat engine operating in reverse. Note the direction of arrows: Heat engine is defined as a device that converts heat energy into mechanical energy whereas the heat pump or refrigerator is defined as a device that use mechanical energy to pump heat from cold to hot region. A refrigeration system is a combination of components and equipment connected in a sequential order to produce the refrigeration effect. The most common refrigeration system in use today involves the input of work (from a compressor) and uses the Vapor Compression Cycle. This course is an overview of vapor compression refrigeration cycle, principles of heat generation, transfer and rejection. Since refrigeration deals entirely with the removal or transfer of heat, some knowledge of the nature and effects of heat is necessary for a clear understanding of the subject. You may refer to the basic thermodynamics and glossary of terms at the end  to help you with this Section first time through.

Air cycle refrigeration systems belong to the general class of gas cycle refrigeration systems, in which a gas is used as the working fluid. The gas does not undergo any phase change during the cycle, consequently, all the internal heat transfer processes are sensible heat transfer processes. Gas cycle refrigeration  systems find applications refrigeration system is a combination of components and equipment connected in a sequential order to produce the refrigeration effect. The most common refrigeration system in use today involves the input of work (from a compressor) and uses the Vapor Compression Cycle. This course is an overview of vapor compression refrigeration cycle, principles of heat generation, transfer and rejection. Since refrigeration deals entirely with the removal or transfer of heat, some knowledge of the nature and effects of heat is necessary for a clear understanding of the subject. You may refer to the basic thermodynamics and glossary of terms at the end  to help you with this Section first time through. Air cycle refrigeration systems belong to the general class of gas cycle refrigeration systems, in which a gas is used as the working fluid. The gas does not undergo any phase change during the cycle, consequently, all the internal heat transfer processes are sensible heat transfer processes. Gas cycle refrigeration systems find applications in air craft cabin cooling and also in the liquefaction of various gases. In the present chapter gas cycle refrigeration systems based on air are discussed.

Reversed Carnot cycle employing a gas 

Reversed Carnot cycle is an ideal refrigeration cycle for constant temperature external heat source and heat sinks. Figure 9.1 (a) shows the schematic of a reversed Carnot refrigeration system using a gas as the working fluid along with the cycle diagram on T-s and P-v

coordinates. As shown, the cycle consists of the following four processes:

Process 1-2: Reversible, adiabatic compression in a compressor

Process 2-3: Reversible, isothermal heat rejection in a compressor

Process 3-4: Reversible adiabatic expansion in a turbine

Process 4-1 : Reversible, isothermal heat absorption in a turbine

                                    Schematic of a reverse Carnot refrigeration system

Temperature Limitations of Carnot cycle:

Carnot cycle is an idealization and it suffers from several practical limitations. One of the main difficulties with Carnot cycle employing a gas is the difficulty of achieving isothermal heat transfer during processes 2-3 and 4-1. For a gas to have heat transfer isothermally, it is essential to carry out work transfer from or to the system when heat is transferred to the system (process 4-1) or from the system (process 2-3). This is difficult to achieve in practice. In addition, the volumetric refrigeration capacity of the Carnot system is very small leading to large compressor displacement, which gives rise to large frictional effects. All actual processes are irreversible, hence completely reversible cycles are idealizations only.

BELL COLEMAN  CYCLE

 Schematic of a closed reverse Brayton cycle

This is an important cycle frequently employed in gas cycle refrigeration systems. This may be thought of as a modification of reversed Carnot cycle, as the two isothermal processes of Carnot cycle are replaced by two isobaric heat transfer processes. This cycle is also called as Joule or Bell-Coleman cycle. Figure 9.2(a) and(b) shows the schematic of a closed, reverse Brayton cycle and also the cycle on T-s diagram. As shown in the figure, the ideal cycle consists of the following four processes:

Process 1-2: Reversible, adiabatic compression in a compressor Process

2-3: Reversible, isobaric heat rejection in a heat exchanger Process 3-4:

Reversible, adiabatic expansion in a turbine

Process 4-1 : Reversible, isobaric heat absorption in a heat exchanger

          Reverse Brayton cycle in T-s plane

Process 1-2: Gas at low pressure is compressed isentropically from state 1 to state 2. Applying steady flow energy equation and neglecting changes in kinetic and potential energy, we can write:

Process 2-3: Hot and high pressure gas flows through a heat exchanger and rejects heat sensibly and isobarically to a heat sink. The enthalpy and temperature of the gas drop during the process due to heat exchange, no work transfer takes place and the entropy of the gas decreases. Again applying steady flow energy equation and second T ds equation:

Process 3-4: High pressure gas from the heat exchanger flows through a turbine, undergoes isentropic expansion and delivers net work output. The temperature of the gas drops during the process from T3 to T4. From steady flow energy equation:

Process 4-1: Cold and low pressure gas from turbine flows through the low temperature heat exchanger and extracts heat sensibly and isobarically from a heat source, providing a useful refrigeration effect. The enthalpy and temperature of the gas rise during the process due to heat exchange, no work transfer takes place and the entropy of the gas increases. Again applying steady flow energy equation and second T ds equation:

         Comparison of reverse Carnot and reverse Brayton cycle in T-s plane

a) COP of Brayton cycle approaches COP of Carnot cycle as T 1 approaches T4 (thin cycle), however, the specific refrigeration effect [cp(T1-T4)] also reduces simultaneously.

b) COP of reverse Brayton cycle decreases as the pressure ratio rp increases

Actual reverse Brayton cycle:

The actual reverse Brayton cycle differs from the ideal cycle due to:

i.    Non-isentropic compression and expansion processes

ii.   Pressure drops in cold and hot heat exchangers

Comparison of ideal and actual Brayton cycles T-s plane

                    

Due to these irreversibilities, the compressor work input increases and turbine work output reduces.

thus the net work input increases due to increase in compressor work input and reduction in turbine work output. The refrigeration effect also reduces due to the irreversibilities. As a result, the COP of actual reverse Brayton cycles will be considerably lower than the ideal cycles. Design of efficient compressors and turbines plays a major role in improving the

COP of the system.

In practice, reverse Brayton cycles can be open or closed. In open systems, cold air at the exit of the turbine flows into a room or cabin (cold space), and air to the compressor is taken from the cold space. In such a case, the low side pressure will be atmospheric. In closed systems, the same gas (air) flows through the cycle in a closed manner. In such cases it is possible to have low side pressures greater than atmospheric. These systems are known as dense air systems. Dense air systems are advantageous as it is possible to reduce the volume of air handled by the compressor and turbine at high pressures. Efficiency will also be high due to smaller pressure ratios. It is also possible to use gases other than air (e.g. helium) in closed systems.