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°R12 = dichlorodifluoromethane, CC12F2, R22 = dichlorodifluoromethane, CHC1F2, R134A = tetrafluoroethane, CF3CH2F, R717 = ammonia, NH3. Source: Reprinted with permission of ASHRAE from the 1990 ASHRAB Handbook-Refrigeration. Ref. 14.

°R12 = dichlorodifluoromethane, CC12F2, R22 = dichlorodifluoromethane, CHC1F2, R134A = tetrafluoroethane, CF3CH2F, R717 = ammonia, NH3. Source: Reprinted with permission of ASHRAE from the 1990 ASHRAB Handbook-Refrigeration. Ref. 14.

The intermediate pressure is selected so that the two compressors will work with the same compression ratio. This means in this case that the intermediate pressure should be about 0.44 MPa, corresponding to a temperature of 0°C.

The Refrigerant

Refrigerants are the vital working fluid in a refrigeration system. They absorb heat from one area and dissipate heat into another. The design of the refrigeration equipment is influenced by the properties of the refrigerant selected.

A refrigerant must satisfy many requirements, some of which do not directly relate to its ability to transfer heat. The environmental consequences of a refrigerant that leaks from a system must also be considered. Because of their stability, fully halogenated compounds, called CFCs, persist in the atmosphere for many years and eventually diffuse into the statosphere. According to several scientists the existence of CFCs in the stratosphere has a detrimental effect on the ozone layer. The ozone layer protects the earth from too much ultraviolet radiation. A depletion of ozone might mean a dangerous increase of uv radiation on earth. This fact, coupled with the CFC contribution to the greenhouse effect, has led most nations to urge the industry to minimize the leakage of CFCs into the atmosphere and gradually to phase out their use.

Several of the large chemical companies are at present heavily engaged in finding good substitutes for the CFCs. One such substitute is refrigerant 134a, which is listed in Table 2.

The present available substitutes do not have the same properties as, for example, R12. The users therefore have to accept the fact that a plant designed for the use of R12 will have some limitations when operated with, eg, R134a, and in most cases modifications are necessary.

Most likely the use of R717 (ammonia) will be more and more frequent, particularly for large industrial applications.

For commercial installations such as those in department stores most likely we will see a central refrigeration plant using ammonia chilling brine being pump-circulated to the various locations where refrigeration is needed. By using a brine the risk of ammonia gas leaking and coming in contact with the public will be minimized.

Compressors

Piston compressors of various sizes dominated in the refrigeration industry until about 25 years ago, when screw compressors were first used also for refrigeration applications.

Today screw compressors are commonly used for industrial applications, while for small systems such as refrigerators and home freezers the use of hermetic piston compressors is still widespread.

Large compressors are often equipped with a capacity-control mechanism, which makes it possible to maintain constant evaporation temperatures even when the demand varies. It should, however, be noted that the efficiency generally decreases when the compressor works with reduced capacity.

The efficiency of refrigeration systems is influenced by many factors other than the compressor. Only when all details are well designed and tuned together can an optimal result be achieved.

Engine Room as Package Unit or Site-Built

In the early days of the refrigeration industry all engine rooms were site-built. Today, however, it is more common for the refrigeration plant to be delivered as a package unit (see Fig. 24). This offers the benefit of quick deliveries with very little work on-site. If the package unit is designed by an experienced supplier, it also means less risk for functional problems during startup. Finally it offers a greater flexibility than a site-built installation, which tends to be more important as the industry must accommodate changing demands.

Economical Operation Conditions

Two basic facts are important for anyone responsible for the operation of an industrial refrigeration plant.

Out of an existing refrigeration installation more refrigeration capacity is achieved if the evaporation temperature is elevated. This is clearly illustrated in Figure 25. Therefore, it is important to make sure that the evaporator surfaces are kept as clean as possible from ice and that good air circulation is obtained over the whole surface. It is also important that the refrigerant be in good contact with every part of the evaporator.

The condition of the condensor is also of importance for a good economy. As can be seen from Figure 26 the electrical power consumption goes down with lowering the condensing temperature. For efficient operation the evaporation temperature should be kept high and the condensing temperature low.

Figure 24. Package refrigeration unit. Refrigeration capacity = 400kWat -40°C evaporation temperature and 35°C condensation temperature. Length = 12 m; width = 2.1 m; height = 3 m.

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