Harvesting.—The American practice is to lift, dump and fill with water a number of cans at a time. This number may be 5, 6, Io or even 3o cans, in which case a can basket is used for holding the cans securely at all times.
Cooling the Brine.—The brine in the tanks may be cooled by having direct expansion piping in the brine between the cans, or by the use of brine coolers. The old form of direct expansion piping was very inefficient because of the disregard of the princi ples of heat transfer. Improvements have been made of recent years by reducing the length of the coils so as to permit the gas formed by evaporation of the refrigerant to return quickly to the compressor, by the use of the flooded system whereby the coils are kept much more full of liquid refrigerant and by the use of an accumulator or other device to precool the liquid from the condenser to the temperature of evaporation before the liquid enters the evaporating coils. The accumulator acts also as a liquid trap to pre vent liquid returning with the suction gas to the compressor.
Comfort cooling, one of the important ap plications of air conditioning, is the outstanding development of re frigeration. It has made possible the use of auditoriums in the summer with the same degree of comfort, meaning the particular combination of temperature, humidity, and air movement that gives an agreeable condition, as is possible during other periods of the year. It is not required that the temperature be reduced much below that of the outside air, but that the relative humidity must be low enough to prevent uncomfortable conditions due to moist clothing and skin. As a general statement a relative humidity of from 4o to 50%, and a temperature of from 5 to F below atmospheric temperature is de sirable. The movement of the air must be positive without causing drafts, and the air must be washed and filtered enough to prevent odours. "Comfort cooling" also includes the removal of noise.
The usual method of securing comfort cooled conditions involves the use of cool water either in a spray or inside pipes or tubes which may have extended surfaces such as discs or helical fins. In either case the air to be cooled is brought into contact with water, which provides the washing that is usually desirable; and the air temperature is reduced enough below the dew point temperature (the temperature at which the water vapour in the air is just dry and saturated) for the weight of water vapour remaining in the mixture to be reduced to the amount desired. For example, air at standard atmospheric
pressure may have at: According to the laws of gas mixtures, which are approximately true for low pressure steam, the water vapour (steam) occupies space independent of the presence of air. The volume in cubic feet occupied by one pound of dry air, in a mixture of air and steam, is calculated by the gas formula: where is the partial pressure of the air in lb. per sq.in.abs. or p„ is the partial pressure of the saturated water vapour at the temperature of the air (see steam tables p. 51) v is the volume in cu.ft. of the dry air T is the absolute temperature of the air in °F R is the gas constant, 53.34 for air The term "relative humidity" is understood to be the ratio of the weight of water vapour in pounds actually present in one pound of dry air to the weight of water vapour that could be present if the air was saturated. In comfort cooling, the air, cooled to the required water vapour content (absolute humidity) must be mixed with air not cooled, or it must be heated to the required temperature before admission to the space to be conditioned. The entrance temperature of the conditioned air should never be more than 8 to 12° F below the required room temperature. In some applications of air con ditioning requiring refrigeration, and particularly in industrial cases, it is possible to deliver the cooled air at relatively high velocities, and much colder than 12° F below the room temperature, near the ceiling where it mixes quite thoroughly with the air in the space to be con ditioned before contact is made with the occupants of the room.
Milk and Ice after being pasteurized, must be cooled from about i4o° F to with water and then to 4o° by means of brine. The cooling of the milk should be accomplished as proniptly as possible and the method has been to use the Baudalot cooler with the brine inside the pipes and the milk flowing on the outside. In ice-cream (q.v.) making refrigeration is used both in the freezers and in the so-called sharp freezers which are held at about F. The freezers are power-driven and they may be cooled by means of brine or by direct expansion piping. (See also FOOD PRESER VATION ; HEATING AND VENTILATION ; Ant CONDITIONING ; THERMO DYNAMICS.)