Refrigerating Machine

a device used to remove heat from an object that is to be cooled to a temperature below the environmental temperature. Refrigerating machines are used to obtain temperatures ranging from 10°C to –150°C. The temperature range below –150°C is the subject of cryogenic engineering.

The operation of refrigerating machines is based on the principle of a heat pump; that is, such machines remove heat from an object to be cooled and, by expending energy (for example, mechanical or thermal energy), transfer the heat to a cooling medium—usually water or ambient air—that has a higher temperature than the object to be cooled. The operation is characterized by the refrigerating capacity, which, for present-day machines, ranges from several hundred watts to a few megawatts.

The following refrigerating systems are used in refrigerating engineering: vapor-compression, absorption, steam-jet, and air-cycle. The operation of these systems is based on the principle that the working fluid—that is, the refrigerant—completes a reverse thermodynamic cycle, called a refrigeration cycle, owing to the expenditure of work. In vapor-compression, absorption, and steam-jet refrigerating machines, cooling is achieved as a result of the boiling of a liquid that has a low boiling point. In air-cycle refrigerating machines, cooling is accomplished owing to the expansion of compressed air in an expansion valve.

The first refrigerating machines appeared in the middle of the 19th century. One of the oldest such machines is the absorption machine, whose invention and design are associated with J. Leslie (Great Britain, 1810), F. Carré (France, 1850), and F. Windhausen (Germany, 1878). The first vapor-compression machine, which operated on ether, was built by J. Perkins (Great Britain, 1834). Similar machines, which used methyl ether or sulfur dioxide as the refrigerant, were developed later. In 1874, K. Linde (Germany) constructed an ammonia vapor-compression refrigerating machine, which marked the beginning of refrigerating-machine building.

Vapor-compression machines are the most widely used and most versatile refrigerating machines. The main components of such machines (Figure 1) are an evaporator, a refrigeration compressor, a condenser, and an expansion valve. The components, all of which should be well sealed, are connected by piping provided with shutoff, control, and safety equipment. According to the type of compressor used, vapor-compression refrigerating machines are classified as reciprocating, turbine, rotary, or screw-type machines.

Figure 1. Schematic diagram of a vapor-compression refrigerating machine: (1) evaporator, (2) compressor, (3) condenser, (4) heat exchanger, (5) expansion valve

In a vapor-compression refrigerating machine, the refrigerant circulates in a closed cycle. In the evaporator, the refrigerant boils at a low pressure p₀ and a low temperature. The heat required for boiling is removed from the object to be cooled, thus reducing the temperature of the object to the boiling point of the refrigerant. The vapor formed is drawn into the compressor, is compressed to the condensation pressure p_c, and then passes into the condenser, where it is cooled by water or air. Owing to the removal of heat in the condenser, the vapor condenses. The liquid refrigerant thus obtained passes through the expansion valve, where its temperature and pressure are reduced, and returns to the evaporator to be evaporated again, thereby completing the machine’s refrigeration cycle.

In some cases, to improve the efficiency of a refrigerating machine (that is, to reduce the energy expenditure per unit quantity of heat removed from the object to be cooled), the vapor drawn into the compressor is superheated and the liquid is subcooled before expansion. On the basis of this principle, a temperature below –30°C may be obtained by using multistage or cascaded refrigerating machines. In multistage machines, the vapor is compressed in several stages and cooled between the stages. In two-stage refrigerating machines, a refrigerant boiling point of as low as –80°C may be obtained. Cascaded refrigerating machines consist of several refrigerating machines connected in series and operating on different refrigerants, which, according to their thermodynamic properties, are the most appropriate for given temperature conditions. In cascaded machines, a boiling point of as low as –150°C may be obtained.

An absorption refrigerating machine (Figure 2) consists of a boiler, a condenser, an evaporator, an absorber, a pump, and an expansion valve. The working fluid is a binary solution in which the two components have different boiling points at the same pressure. The component with the lower boiling point is the refrigerant; the second component is called the absorbent. In the temperature range from 0°C to –45°C, absorption machines are used in which the working fluid is a solution of ammonia (the refrigerant) and water. At cooling temperatures above 0°C, absorption machines that operate on a solution of water (the refrigerant) and lithium bromide are mainly used.

Figure 2. Schematic diagram of an absorption refrigerating machine: (1) evaporator, (2) absorber, (3) pump, (4) expansion valve, (5) boiler, (6) condenser

In the evaporator of an absorption refrigerating machine, the refrigerant is evaporated by heat removed from the object to be cooled. The vapor thus produced is absorbed in the absorber. The strong solution formed in the absorber is pumped into the boiler, where the refrigerant is vaporized by thermal energy from an external source, and the remaining solution is returned to the absorber. The refrigerant vapor then flows from the boiler to the condenser, where it is condensed; the resulting liquid passes through the expansion valve and to the evaporator, where it evaporates a second time.

The use of absorption machines is very advantageous in industrial enterprises that have secondary energy resources, such as spent steam, hot water, or flue gases from industrial furnaces. Absorption refrigerating machines may be of the single-stage or the two-stage type.

Steam-jet refrigerating machines (Figure 3) consist of an ejector, an evaporator, a condenser, a pump, and an expansion valve. The refrigerant is water, and the energy source is steam at a pressure of 0.3–1 meganewton per m² (3–10 kilograms-force per cm²). The steam enters the ejector nozzle, where it is expanded. As a result, the pressure in the ejector and, therefore, in the evaporator of the machine is reduced to a value that corresponds to a boiling point of water of somewhat above 0°C, usually of the order of 5°C. Owing to partial evaporation, the water that is circulated to the cooling load is chilled in the evaporator. The water vapor drawn from the evaporator and the motive steam from the ejector pass into the condenser, where they become liquid water and give up heat to a cooling medium. Some of the water from the condenser is pumped into the evaporator to replenish the supply of chilled water.

Figure 3. Schematic diagram of a steam-jet refrigerating machine: (1) ejector, (2) evaporator, (3) cooling load, (4) pump, (5) expansion valve, (6) condenser

Air-cycle refrigerating machines are gas refrigerators that use air as the refrigerant. At temperatures above –80°C, the efficiency of air-cycle machines is lower than that of vapor-compression machines. A more efficient type of air-cycle machine is the regenerative air-cycle refrigerating machine, in which the air is cooled in either a counterflow heat exchanger or a regenerator. Depending on the pressure of the compressed air used, air-cycle refrigerating machines are divided into high-pressure machines and low-pressure machines. A distinction is also made between open and closed air-cycle machines.