REFRIGERATORS (HOUSEHOLD). Ice and mechan ical refrigerators operate on the same principle, the absorption of heat from the food compartment by a physical change in the refrigerating medium, ice to water in one case, the liquid ref rig erant to a gas in the other.
Both types of refrigerators are similar in finish, porcelain en amel on the inside and porcelain enamel or a durable lacquer on the outside. Insulation varies from two to four inches in thick ness in the different models, depending on price. Amount of in sulation is important since the larger percentage of heat entering the food chamber penetrates through the walls.
The new ice refrigerator is so constructed that the air circu lates only across the lower surface of the ice cake. A compara tively constant temperature—usually 43° to 47° F.—is main tained throughout the food compartment, so long as the rack is covered, even though the layer of ice may be thin. Re-icing is necessary only once in four or five days.
Some ice refrigerators have a damper to regulate the size of the opening in the floor of the ice chamber. The wider this open ing, the greater down-drop of cold air from the ice and the lower the temperature in the food compartment.
In the older type of ice refrigerator, in which air circulates over the entire surface of the ice, there is frequently as much as ten degrees difference in temperature between the coldest and warmest location in the food compartment. When the ice cake has melted to less than one-half its original size the temperature rises rapidly and the box must be re-iced.
Mechanical or automatic refrigerators may be either of the compressor or absorption type. Both have a closed system in which the refrigerant circulates. In the evaporator, the part of the system inside the refrigerator, the liquid refrigerant changes to a gas, removing heat from the food chamber in the process. In the condensing coils it is cooled by air passing over the coils and changed to a liquid again. A thermostat (q.v.) may be regulated to produce the temperature desired.
The compressor system uses a small motor, to horse-power, to start the cycle of operation. It forces the gaseous refrigerant from the evaporator into the condensing coils. From the coils it
passes through a valve into the evaporator. The valve may be the expansion type, through which the refrigerant is sprayed into the evaporator in such a fine stream that it is partially vaporized immediately, or a float type which allows a relatively large amount of liquid refrigerant to collect in the evaporator. The cycle then repeats itself.
The functioning of the absorption system depends upon the fact that water absorbs ammonia gas in the cold but is easily sep arated from it again in the presence of heat.
Heat, usually from a gas or oil flame, is used in the generator to bring about the separation. The ammonia, free from the wa ter, passes into the condensing coils, is liquefied, and flows into the evaporator, where a medium of hydrogen reduces the pressure sufficiently to allow ready evaporation. From the evaporator the ammonia-hydrogen gas combination passes into the abso,rber where a spray of water from the condenser absorbs the ammonia. The insoluble hydrogen returns to the evaporator. The am monium hydroxide flows to the generator to start the cycle again. Heat and cold exchangers complicate the construction of the system, but increase its efficiency.
As has been noted, ammonia, because of its affinity for water, is the refrigerant used in the absorption system. Sulphur dioxide and recently dichlordifluoromethane are most commonly used in the compression system.
Certain characteristics are desirable in a refrigerant. It should be stable under frequent change of state, non-inflam mable, and non-toxic. It should have moderately high latent heat, a low condensing pressure, and no effect on metals or lubri cating oils.
None of the refrigerants used complies with all the require ments, but those mentioned have the necessary latent heat and usable operating pressure to meet the demands of construction in household size refrigerators. Dichlordifluoromethane is non toxic. Ammonia and sulphur dioxide are considered non-toxic in the amounts present in five to eight cubic foot boxes, under usual home ventilating conditions. (See REFRIGERATION AND ICE