A refrigerator is a device which is designed to remove heat from a space that is at lower temperature than its surroundings. The same device can be used to heat a volume that is at higher temperature than the surroundings. In this case the device is called a Heat Pump. The distinction between a refrigerator and a heat pump is one of purpose rather than principle. Therefore, this section will concentrate on refrigeration and only make the distinction between the two devices when necessary.
The Clausius statement of the Second Law of Thermodynamics asserts that it is impossible to construct a device that, operating in a cycle, has no effect other than the transfer of heat from a cooler to a hotter body. This means that energy will not flow from cold to hot regions without outside assistance. The refrigerator and heat pump both satisfy the Clausius requirement of external action through the application of mechanical power or equivalent natural transfers of heat.
Continuous refrigeration can be achieved by several processes. Effectively any heat engine cycle, when reversed, becomes a refrigeration cycle. The vapor compression cycle is the most commonly used in refrigeration and air condition applications. The vapor absorption cycle provides an alternative system, particularly in applications where heat is economically available. Steam-jet systems are also being successfully used in many cooling applications while air-cycle refrigeration is often used for aircraft cooling. These cycles are described in detail in Look and Sauer (1988), and in the ASHRAE Handbook of Fundamentals (1993). Refrigeration equipment is described in detail in the ASHRAE Handbook, HVAC Systems and Equipment Volume (1992) and refrigerating systems practices are in the ASHRAE Handbook, Refrigeration Volume (1990).
Mechanical refrigeration processes, of which the vapor compression cycle is an example, belong to the general class of reversed heat engine cycles, Figure 1. This figure represents, schematically, the extraction of heat at rate from a cold body at temperature TC. The process requires the expenditure of work W and the sum is discharged at a higher temperature TH.
The ideal cycle against which any practical reversed heat engine may be compared is the reversible or Carnot Cycle for which, in accordance with the Second Law of Thermodynamics, the following relationship applies:
One measure of the efficiency of such a process is given by:
Clearly the smaller the value of the ratio the more efficient is the process.
It is more usual to describe the efficiency of a reversed heat engine by the inverse of this ratio, known as the Coefficient Of Performance (COP):
It will be observed that the COP may be greater than unity and that it becomes greater as the temperature difference decreases. A real refrigerator or reversed heat engine will have a COP less than that of the ideal Carnot Cycle engine as given by the above equation.
The reversed Carnot Cycle is represented on the Temperature-Entropy (T-S) diagram by a rectangle, Figure 2, and is composed of four reversible processes;
4-1 isothermal expansion, during which heat (the refrigeration load) flows from the cold space to the working fluid.
1-2 adiabatic compression.
2-3 isothermal compression in which heat flows from refrigerant to the hot space.
3-4 adiabatic expansion.
Vapor compression refrigeration, as the name suggests, employs a compression process to raise the pressure of a working fluid vapor (refrigerant) flowing from an evaporator at low pressure PL to a high pressure PH as shown in Figure 3. The refrigerant then flows through a condenser at the higher pressure PH, through a throttling device, and back to the low pressure, PL in the evaporator. The pressures PL and PH correspond to the refrigerant saturation temperatures, T1 and T5 respectively.
The T-S diagram for this real cycle, Figure 4, is somewhat different from the rectangular shape of the Carnot Cycle.
The cycle processes can be described as follows:
7-1 Evaporation of the liquefied refrigerant at constant temperature T1 = T7.
1-2 Superheating of the vapor from temperature T1 to T2 at constant pressure PL.
2-3 Compression (not necessarily adiabatic) from temperature T2 and pressure PL to temperature T3 and pressure PH.
3-4 Cooling of the super-heated vapor to the saturation temperature T4.
4-5 Condensation of the vapor at temperature T4 = T5 and pressure PH.
5-6 Subcooling of the liquid from T5 to T6 at pressure PH.
6-7 Expansion from pressure PH to pressure PL at constant enthalpy.
A further difference between the real cycle and the ideal is that temperature T1 at which evaporation takes place is lower than the temperature TL of the cold region so heat transfer can take place. Similarly the temperature T4 of the heat rejection must be higher than the hot region temperature TH to bring about heat transfer in the condenser.
It is usual for the vapor-compression cycle to be plotted on a pressure-enthalpy (p-h) diagram as shown in Figure 5.
The cycle calculations are described in detail in many textbooks [e.g., Eastop and Mc Conkey (1993) and Rogers and Mayhew (1992)].
Refrigerants are the working fluids in refrigeration systems. They must have certain characteristics which include good refrigeration performance, low flammability and toxicity, compatibility with compressor lubricating oils and metals, and good heat transfer properties. They are usually identified by a number that relates to their molecular composition. The ASHRAE Handbook of Fundamentals (1993) lists a large number of available refrigerants and gives their properties (see Refrigerants.)
In recent years, environmental concerns over the use of chlorofluorocarbons (CFCs) as the working fluids in refrigeration and air-conditioning plants have led to the development of alternative fluids. The majority of these fall into two categories, hydrofluorocarbons (HDCs) which contain no chlorine and have zero ozone depletion potential and hydrochlorofluorocarbons (HCFCs), which do contain chlorine, but the addition of hydrogen to the CFC structure allows virtually all the chlorine to be dispersed in the lower atmosphere before it can reach the ozone layer. HCFCs therefore have much lower ozone depletion potentials, ranging from 2 to 10% that of CFCs. Many nations have signed the Vienna Convention which is a treaty intended to control the production of substances known to be depleting the ozone layer. The Montreal Protocol to this treaty in 1987 outlines the means for achieving certain limits in production of particular substances and the timetable for their phasing out. A great deal of research is being carried out to determine the properties of new ozone friendly fluids and mixtures [Sauer and Kuehn (1993)].
Recently interest has been increasing in these cycles because of their potential use as part of energy-saving plants and also because they use more environmentally friendly refrigerants than vapor-compression cycles. A basic vapor-absorption system is shown schematically in Figure 6. The condenser, throttling valve and evaporator are essentially the same as in the vapor compression system (Figure 3). The major difference is the replacement of the compressor with an absorber, a generator, and a solution pump. A second throttling valve is also used to maintain the pressure difference between the absorber (at the evaporator pressure) and the generator (at the condenser pressure).
The refrigerant on leaving the evaporator is absorbed in a low-temperature absorbing medium, some heat, QA, being rejected in the process. The refrigerant-absorbent solution is then pumped to the higher pressure and is heated in the generator, QG. Refrigerants vapor then separates from the solution due to the high pressure and temperature in the generator. The vapor passes to the condenser and the weak solution is throttled back to the absorber. A heat exchanger may be placed between the absorber and the generator to increase the energy efficiency of the system. The work done in pumping the liquid solution is much less than that required by the compressor in the equivalent vapor-compression cycle. The main energy input to the system, QG, may be supplied in any convenient form such as a fuel burning device, electrical heating, steam, solar energy or waste heat. Appropriate refrigerant/absorbent combinations must be selected. One common combination uses ammonia as refrigerant and water as absorbent. An alternative combination is water as refrigerant and lithium bromide as absorbent. There are increasing research activities into finding suitable new combinations [Hodgett (1982)].
Gas-cycle refrigeration, is essentially, a reversed Joule cycle (gas turbine cycle). As the name indicates, the refrigerant in these systems is a gas. The system, as shown in Figure 7, is basically the same as that of the vapor-compression cycle. The main difference is the replacement of the throttling valve by an expander.
The cycle can be described as follows:
1-2 Adiabatic compression.
2-3 Constant pressure cooling.
3-4 Adiabatic expansion.
4-1 Constant pressure heating (cooling effect).
As can be seen from Figure 7, the gas does not receive and reject heat at constant temperature, and, therefore, the gas cycle is less efficient than the vapor cycle for given evaporator and condenser temperatures. Gas-cycle systems are mostly used in air conditioning applications where the working fluid-air can be ejected at T4. A common application is in the air conditioning of aircraft. Air, held from the engine compressor, is cooled in a heat exchanger and then expanded through a turbine. The power from the turbine is used to drive a fan which provides the cooling air for the heat exchanger. Air at T4 is ejected into the cabin to provide the required cooling.
ASHRAE Handbook, (1992) HVAC Systems and Equipment Volume, American Society of Heating, Refrigerating and Air-Conditioning Engineer Inc., Atlanta, GA.
ASHRAE Handbook (1990) Fundamentals Volume, American Society of Heating, Refrigerating and Air-Conditioning Engineer Inc. Atlanta, GA
ASHRAE Handbook (1990) Refrigeration Volume, American Society of Heating, Refrigerating and Air-Conditioning Engineer Inc., Atlanta, GA.
Eastop, T. D. and McConkey, A. (1993) Applied Thermodynamics, Longman Scientific and Technical, Harlow.
Hodgett (1982) Proceeding of Workshop in Berlin, April 14-16, Swedish Council for Building Research, ISSN: 91-54039294.
Look, D. L. and Sauer, H. I. (1988) Engineering Thermodynamics, Van Nostrand Reinhoid (International), Wokingham.
Rogers, G. F. C. and Mayhew, Y. R. (1992) Thermodynamic Work and Heat Transfer, Longman Scientific and Technical, Harlow.
Sauer, H. J. and Kuehn, T. H. (1993) Heat Transfer with Alternative Refrigerants, ASME, HTD-Vol 243, New York.