Then, to complete the process, we let the gas expand again. Now just as compressing a gas heats it, letting it expand cools it. If we compressed it and then expanded it again we should be just as we were to commence with. But since, in between the two operations we extract a quantity of heat by means of the cooling water, we get at the end a very much lower temperature than that with which we started.
We cannot cool the gas without compressing it, because heat will only flow from one body into another when the second is cooler than the first. But by making the gas hot temporarily by compression we enable the water to draw some heat from it, and then, allowing it to sink back to its original state, we get practically the old temperature, less what the water has extracted. The principle is really absurdly simple when one once gets to understand it. The application is not so simple as far as the designer of the machine is concerned, for he has to adjust the various parts to exactly the right shape and dimensions, so that they may work well with one another and produce the desired result with the minimum expenditure of power.
To the observer, however, and to the user too, the finished machine is wonderful in its simplicity. The principle is illustrated diagrammatically in Fig. 5.
In the centre is the compressor. Its action forces the gas along the pipe to the right and down into the condenser. As it flows downwards through the coil there cold water enters at the bottom of the tank, flows upward past the coil and escapes again at the top. Thus the coil is kept in contact with cold water.
Passing then through the bottom of the tank the gas travels from right to left through the "regulating valve" and into an arrangement almost exactly similar to the condenser but called the evaporator. Here the gas expands and suffers a great fall in temperature. This cold is communicated to liquid circulating in the tank which forms a part of the evaporator, and this liquid can be circulated through pipes into any rooms to be cooled or around vessels of water which it is desired to freeze. This liquid, which acts as the carrier of the cold, is called "brine," and is water to which is added calcium chloride to keep it from freezing.
Fig. 5.—This diagram shows the working of the Refrigerating Machine. The pump compresses the gas and drives it through the coil in the condenser, where it is cooled by water. It passes thence through the coil in the evaporator, where it expands and cools the surrounding brine.
Now the observant reader may have noticed that there is no apparent reason for the name of the left-hand vessel. It will be quite clear, however, when I explain that although I have spoken of the working fluid all along as gas, I have only done so to avoid bringing in too many explanations at once. It is actually liquid for a good part of its journey. Carbonic acid gas liquefies at a very moderate temperature and pressure, and so while it leaves the compressor as a gas it becomes liquid in the condenser and remains so until it has passed the regulating valve. Then it begins to expand into gas once more, and in that state it passes back to the compressor.
There is a pressure-gauge on the pipe leaving the compressor and another on the one entering it. A comparison of the readings on these two tells how the apparatus is working. The difference between them indicates how much compression is being given to the gas. Assuming that the compressor is working at a constant speed, this compression can be regulated to a nicety by the valve: close it a little and the compression will increase: open it a little and the compression will decrease. By this means the degree of cold produced can be varied at will.
This is the way in which many ships are enabled to carry cargoes of frozen meat. The chambers in which the meat is stowed are insulated—that is to say, their walls are made as impervious as possible to heat. Then the brine is carried into the chambers in pipes, cooling them much as the hot-water pipes heat an ordinary public building.