There are two general types of refrigerating systems, the compression and the absorption type. In the usual compression type a small compressor, operated by an electric motor, is used to compress the vapor coming from the refrigerating coils to a pressure sufficiently high to cause it to liquefy at ordinary temperatures, when heat is removed either by water or air. The liquid is then readmitted through an expansion valve or equivalent device to the refrigerating coils, where its evaporation at a low pressure produces the cooling for which the machine is designed.

In the absorption type the vapor from the refrigerating coils is absorbed in a suitable substance, such as water, or other liquid, or by a solid which is capable of absorbing large quantities of vapor. Subsequently the substance containing the absorbed vapor is heated, either electrically or by a gas flame, and the vapor is driven off, then cooled and condensed to a liquid, which is returned to the refrigerating coils. Machines of this type have few or no moving parts, practically all of them are almost noiseless in operation and, in contrast with many machines of the compression type, require connection to a water supply for cooling. Some of the machines using a liquid absorber are continuous in operation, the heat being applied always to one part, while the liquid is caused to circulate. Others are of the intermittent type, the heat being supplied for a time to one part, then to another part, or to one part at intervals. Nearly all of the machines now on the market are designed to provide for freezing ice cubes, and since this feature is so very generally included, no further consideration of it is required here.

A very large number of makes of refrigerating machines of the compression type have been put on the market. These have included such variations as direct drive, belt drive, and gear drive; reciprocating single or multiple cylinder compressors, various types of rotary compressors; various refrigerants such as sulphur dioxide, methyl chloride, ethyl chloride, ammonia, volatile hydrocarbons, etc.; air or water cooling; refrigeration by direct expansion or by the use of brine tanks, etc. Completely self-contained and sealed machines of the compression type have also been made. It is impracticable to discuss here the various merits and demerits of the features which are often emphasized out of all proportion to their importance, in advertising and by salesmen. The user of a machine is not so much concerned with the kind of drive, refrigerant or absorbent used, type of compressor or system of refrigeration as he is in the kind of service the machine will give and what the service may cost over a period of years. For example, there is no outstanding advantage in a machine with a brine tank as compared with one of the direct expansion type, but the success or failure of either will depend upon the quality of the whole machine and not upon such a detail of design.

1 From "Electric and Gas Refrigerators" (Letter Circ. 255; U.S. Dept. of Commerce, Bureau of Standards, 1929).

Knowledge of details of design of this kind is of value to the expert in judging whether the machine is designed and made so that it can be expected to have a reasonably long life and give satisfactory service during its life. The fact that a machine has one or several features of design which seem superior does not necessarily indicate that it will prove to be superior to other machines having other features of design. For example, the refrigerant used is a factor of minor importance as regards efficiency, since machines can be designed to use any of the ordinary refrigerants effectively. Similarly either compression or absorption machines can give very satisfactory service.

There have been instances where refrigerants which constituted a distinct hazard to life or health have been used, but this does not apply to the refrigerants now in general use. Again a poorly designed machine might introduce a distinct fire or accident hazard. The purchaser of a machine should, therefore, take into consideration evidence concerning test and approval of the type by some disinterested authority.