Page:Popular Science Monthly Volume 88.djvu/920

 892

��Popular Science Monthly

���In ammonia machines, high pressure is

avoided by means of an automatic switch

which stops the motor

large amount of heat is absorbed from the remainder of the water, which is thereby cooled to the freezing point. This heat apparently disappears in the vapor, for the vapor is no warmer than the water from which it comes. This heat is said to be latent. Now should the pan containing the cooled water be connected with a pipe-coil located in a refrigerator, the cold water would per- form the same function as ice. In practice, however, water is not used in refrigerating machines, more volatile liquids being used instead; the vapor discharged by the pump is condensed and returned to the evaporator to be again evaporated.

Of the several well-known types of re- frigerating machines, the gas compression and expansion machine is the one most generally used for cooling household re- frigerators. All compression machines are made up of four distinct parts, viz.: a compressor or pump, a condenser, a refrigerating-coil, and an expansion valve between the condenser and the refrigerat- ing-coil. These are connected in a closed cycle so that the compressor can suck out the gas from the refrigerator-coil and discharge it under high pressure into the condenser, where the hot gas is cooled by running water. The combined eflfect of pressure and cooling causes the gas to liquefy. This liquid passes through the

��expansion valve into the refrigerator- coil where it evaporates because of the low pressure maintained by the suction of the compressor and the heat absorbed from the articles being cooled. The func- tion of the refrigerant is that of a heat- carrier; it takes up heat in the refrigera- tor and discharges it into the cooling water. It is able to do this by reason of the work of the compressor which main- tains a high pressure in the condenser, where the heat is discharged, and a low pressure in the refrigerator-coil, where the heat is absorbed. The refrigerants most generally used are ammonia, sul- phur-dioxide, and ethyl-chloride.

The most essential requisite of a household refrigerating machine is that it shall be so nearly automatic that the services of an expert attendant shall not be required.

A near approximation to this require- ment seems to have been attained by the machine shown on page 891. The larger of the two hollow shells encloses the compressor, and the shell itself is the condenser and runs in cooling water, while the smaller shell acts the same as a refrigerator-coil. When completed this device is charged with a suitable amount of sulphur-dioxide and lubricating oil and is then sealed up. As all the moving parts are sealed up, the escape of gas is effectually prevented. By reason of the ingenious manner in which the com- pressor is constructed all danger from high pressure is overcome.

The illustration on the preceding page shows how this machine may be used to cool a refrigerator on the dining-room floor of a home, the machine itself being in the basement.

While ammonia is a most excellent refrigerant in large machines, it is not much used in household machines be- cause of the danger of the high pressure in the condenser and the possibility of leakage. Ammonia machines for house- hold use are usually so arranged that the switch controlling the motor is closed by the pressure of the water flowing to the condenser, and whenever the water fails or is turned off the machine is automatically stopped. In this way dangerously high pressures in the con- denser are avoided. In all ammonia ma- chines the pressure in the condenser

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