WHAT'S IN THE ICE BOX

Cue (NZERS), Issue 33, 15 October 1945, Page 34

WHAT'S IN THE ICE BOX

/T has often been said that the rich man's pleasures of today arc the poor man’s necessities of tomorrow. Ten years ago this could rightly have been applied to the refrigerator, for, with the -increasing research in the field of bacteriology, it has become evident that if disease is to he successfully combated then the food of the working man must he kept clean and wholesome. Pasteur in his struggle against tuberculosis brought home to the masses the importance of bacteria-free food. As yet, prices and war-time conditions have denied the majority of New Zealand housewives this kitchen necessity, but with the cessation of hostilities the day may not be far distant when refrigerators will be almost as common among domestic appliances as the iron or the saucepan. The refrigerator, or, as it is sometimes called, the ’’ice-box”, is to most people the place where perishable foodstuffs are kept— stop to think juist how it works. In the commercial sense ’’refrigeration” is the science of maintaining a space at a temperature lower than that of its surroundings. The science of refrigeration came into prominence when, pi 1928, a Food Act came into force prohibiting the use of all chemical food preservatives. Certain specified exceptions, however, were made. This left but one other method of arresting the breeding of bacteria in food, and that was sterilisation. However, this method is not always convenient and may affect the flavour of the food. Immediately after sterilising, food must be completely

sealed off from the air, as bacteria would be introduced into the warm food —an ideal breeding ground. Refrigeration does not kill the bacteria, but keeps them in a comatose condition. At 50 degrees Fahrenheit breeding practically ceases and becomes progressively slower as the temperature falls. There are four types of domestic refrigerator—water cooled safes, iceboxes, compression—type refrigerators and absorption-type refrigerators. The basic principle on which they all operate is that when any liquid evaporates heat is absorbed, a phenomenon which is met with in many ways in our daily life. Any soldier who has removed his ’’tin hat’’ from his head after a hot route march and then stood in the breeze will have noticed how much cooler the breeze seems on the moist area of the face than on the dry. The explanation for this is that the evaporation of the sweat absorbs heat rapidly from the skin and lowers the temperature. The simplest type of refrigerator to be found in New Zealand homes is the water-cooled safe, consisting of a box with porous sides covered with material such as flannel, which will absorb moisture. The flannel is kept wet by a water reservoir or a sprinkler system and as the water evaporates the temperature inside the cabinet falls below that of the surrounding air. This type of refrigerator works

best if erected in a draughty place. Most electrically-operated refrigerators are of the compression type. To put it technically, they operate by the alternate compression and expansion of a ’’refrigerant”— ’’refrigerant” is a substance which exists as a gas at ordinary atmospheric temperatures pressures, but which, when compressed, becomes a liquid. Exposed to lower temperatures the liquid will immediately evaporate, thus becoming a gas again. This evaporation is the crucial point of the whole process, because, as in the case of the perspiring soldibr exposing his humid brow to the breeze, evaporation absorbs heat from the surrounding surfaces. • The whole process may be followed in Figure 1. When the electric motor is - started it works the pump which compresses the refrigerant to a pressure of about four or five atmospheres.

The compressed gas, which is warm, now passes in the direction of the arrow until it reaches the condenser, a scries of pipes with fins over which air can circulate. The gas now cools and becomes a liquid which is pushed along to a receiver and then to the expansion valve. This is a small hole on the outlet side of which is a large container—the evaporator. The pressure in the evaporator is much lower because the pump is sucking from it, so that as soon as the liquid passes the expansion valve it evaporates in the evaporator. The evaporator is the only working part inside the cabinet, and constitutes the cooling unit. The evaporating liquid, being very cold, absorbs heat from the air surrounding the cooling unit. The gas in the evaporator is sucked into the compressor and, recompressed, and the whole process continues automatically as long as the motor is running. The motor is switched on and off by a thermostatic switch, which keeps the temperature inside the cabinet around 50 degrees Fahrenheit. The absorption-type refrigerator, the principle of which is depicted in Figure 2, makes use of the application of heat instead of a compressor. Gas or oil may be used to produce a flame which, by boiling water in which the refrigerant (ammonia) is dissolved, drives off the refrigerant under pressure. After cooling in the condenser the gas, because of its high pressure of about eight atmospheres, becomes liquid. The liquid ammonia, without passing through an expansion valve, now flows into the evaporator inside the cabinet. The ammonia in the evaporator is able to evaporate because of the presence of hydrogen and so absorb heat from the cabinet. A mixture of ammonia vapour and hydrogen flows into the absorber where it is separated. The liberated hydrogen returns to the evaporator and the cycle is repeated,