Stories of Inventors: The Adventures of Inventors and Engineers

by Russell Doubleday

Cold, of course, is but the absence of heat, and so refrigerating machinery is designed to extract the heat from whatever substance it is desired to cool. The refrigerating agent used to extract the heat from the cold chamber must in turn have the heat extracted from it, and so the process must be continuous.

Water, when it boils and turns into steam or vapour, is heated by or extracts heat from the fire, but water vapourises at a high temperature and so cannot be used to produce cold. Other fluids are much more volatile and evaporate much more easily. Alcohol when spilt on the hand dries almost instantly and leaves a feeling of cold—the warmth of the hand boils the alcohol and turns it into vapour, and in doing so extracts the heat from the skin, making it cold; now, if the evaporated alcohol could be caught and compressed into its liquid form again you would have a refrigerating machine.

Alcohol is expensive and inflammable, and many other volatile substances have been discarded for the one or the other reason. Of all the fluids that have been tried, ammonia has been found to work most satisfactorily; it evaporates at a low temperature, is non-inflammable, and is comparatively cheap.

The hold of the supply-ship mentioned at the head of this chapter was a vast refrigerator, but no ice was used except that produced mechanically by the power in the ship. To produce the cold in the hold of the ship it was necessary to extract the heat in it; to accomplish this, coils ran round the space filled with cold brine, which, as it grew warm, drew the heat from the air. The brine in turn circulated through a tank containing pipes filled with ammonia vapour which extracted the heat from it; the brine then was ready to circulate through the coils in the hold again and extract more heat. The heat-extracting or cooling power of the ammonia is exerted continually by the process described below. Ammonia requires heat to expand and turn into vapour, and this heat it extracts from the substance surrounding it. In this marine refrigerating machine the ammonia got the heat from the brine in the tank, then it was drawn by a pump from the pipes in the tank, compressed by a power compressor, and forced into a second coil. The second coil is called a condenser because the vapour was there condensed into a fluid again. Over the pipes of the condenser cool water dripped constantly and carried off the heat in the ammonia vapour inside the coils and so condensed it into a fluid again—just as cold condenses steam into water. The compressor-pump then forced the fluid, ammonia through a small pipe from the condenser coils to the cooling coils in the tank of brine. The pipes of the cooling coils are much larger than those of the condenser, and as the fluid ammonia is forced in a fine spray into these large pipes and the pressure is relieved it expands or boils into the larger volume of vapour and in so doing extracts heat from the brine. The pump draws the heated vapour out, the compressor makes it dense, and the coils over which the cool water flows condenses it into fluid again, and so the circuit continues—through cooler, pump, compressor, and condenser, back into the cooling-tank.

In the meantime, the cold brine is being pumped through the pipes in the hold of the ship, where it extracts the heat from the air and the rows of sides of beef and then returns to the cooling-tank. In the refrigerating plant, then, of the supply-ship, there were two heat-extracting circuits, one of ammonia and the other of brine. Brine is used because it freezes at a very low temperature and continues to flow when unsalted water would be frozen solid. The ammonia is not used direct in the pipes in a big space like the hold of a ship, because so much of it would be required, and then there is always danger of the exposed pipes being broken and the dangerous fumes released.

Opposite as it may seem, heat is required to produce cold—for steam is necessary to drive the compressor and pump of a refrigerating plant, and fire of some sort is necessary to make steam.

The first artificial refrigerating machines produced cold by compressing and expanding air, the compressed air containing the heat being cooled by jets of cool water spirted into the cylinder containing it, then the compressed air was released or expanded into a larger chamber and thereby extracted the heat from brine or whatever substance surrounded it.

It is in the making of ice, however, that refrigerating machinery accomplishes its most surprising results. It was said in the writer's hearing recently that natural ice costs about as much when it was delivered at the docks or freight-yards of the large cities of the North as the product of the ice-machine. Of course, the manufactured ice is produced near the spot where it is consumed, and there is little loss through melting while it is being stored or transported, as in the case of the natural product.

There are two ways of making ice—or, rather, two methods using the same principle.

In the can system, a series of galvanized-iron cans about three and a half feet deep, eight inches wide, by two and a half feet long are suspended or rested in great tanks of brine connecting with the cooling-tank through which the pipes containing the ammonia vapour circulates. The vapour draws the heat from the brine, and the brine, which is kept moving constantly, in turn extracts the heat from the distilled water in the cans. While this method produces ice quickly, it is difficult to get ice of perfect clearness and purity, because the water in the can freezes on the sides, gradually getting thicker, retaining and concentrating in the centre any impurities that may be in the water. The finished cake, therefore, almost always has a white or cloudy appearance in the centre, and is frequently discolored.