Or another method is to carry the pipe which constitutes the evaporator into the chamber to be cooled. A third way is to dispense with brine and to blow air through the coils of the evaporator, whereby the air is made to carry away the cold to wherever it is needed.
Ice can be made easily in moulds of metal or wood around which brine circulates. If made of ordinary water the ice is likely to be cloudy and opaque, which is quite good enough for many purposes. In cases where it is desired that it should be clear, the water is agitated during freezing, or else distilled water is used. To enable the blocks to be got out of the moulds it is sometimes arranged to circulate warm brine for a few moments.
Ice skating rinks are formed by making, first, an insulating layer of sawdust, slag-wool or something of that sort (those by the way, being the materials generally used for insulating cold chambers) underneath the floor. The floor, too, is made waterproof and then upon it is laid as closely as possible a series of iron pipes. Water is flooded on to the floor until the pipes are covered to a depth of several inches, and then brine is pumped through the pipes. In time the water freezes, and so long as the brine circulates it remains so.
But although the "CO2 process" described above is the simplest illustration of the principle, there are other systems. In one very popular form ammonia gas is the "working fluid." This is liquefied by pressure and cooling with water, being subsequently expanded just as described above.
By permission of Messrs. J. and E. Hall, Ltd., London and Dartford
Machine-made Ice
Here we see a huge block of ice being lifted (it may be on a hot summer day) from the mould in which it has been made
Another much-used system is the "ammonia-absorption" process, in which the ammonia is not liquefied, but when under pressure is absorbed by water, returning to gas again when the pressure is released.
But the degree of cold attained in these commercial machines is as nothing to the extremely intense cold generated on the same principles in the liquid-air machine, which is found in every well-equipped physical laboratory.
Briefly, this consists of a coil of many turns of small tube enclosed in a small double vessel, the space between the inner and outer skins of which is packed with insulating material. A compressor pumps air in at the top of the coil at a pressure of from 150 to 200 atmospheres. An "atmosphere," it may be remarked, is a unit often used in scientific matters, meaning the normal pressure of the atmosphere, which is, roughly speaking, 15 lb. per square inch. Hence 200 atmospheres is about 3000 lb. per square inch.