Cold-air machines are also of two classes :-(1) those in which the same air is used over and over again without coming actually into contact with the substance to be cooled, or the closed cycle machine; and (2) those in which the cold air is exhausted from the machine and circulated through the cold stores in which the goods to be refrigerated are lying. These latter are known as the open cycle machines, and are the only ones now in use, the former class having become obsolete many years ago.

It is undoubtedly largely due to the cold-air machine that modern England owes her abundant and good supply of frozen meat, for, though the trade is now carried on largely by other systems of refrigeration, it was the coldair machine which originated it and made it possible. In the cold-air machine ordinary atmospheric air is compressed to a pressure of about 50 lbs. on the square inch; the air by being compressed is raised in temperature, and the heat is taken out of it by passing it through a cooler, the tubes of which are surrounded by water. From the cooler the air is passed through a dryer, which consists of a series of tubes placed in, and surrounded by, the cold air returning from the cold chamber. In the dryer the compressed air is chilled and any moisture it contains is deposited. The air, still under compression, passes from the dryer to the expansion cylinder, where it expands (and in doing so helps to drive the machine); and, as air expanding while doing work is reduced in temperature, it leaves the cylinder at a temperature of a very considerable number of degrees below freezing-point, and goes away through air-trunks to the cold room. The dryer is a most important part of the apparatus. In the early days the principal difficulty with cold-air machines was the fact that when the air was expanded in the expansion cylinder,

and reduced in temperature, the moisture contained in it was deposited in the form of ice in the cylinder and on the slide valves, choking them or raising them off their seats, and so rendering the cylinder inoperative. The invention and introduction of the dryer overcame this difficulty, and allowed the machine to work smoothly and well.1

Unfortunately the cold-air machine is not economical for large powers, though exceedingly useful for small powers and in small sizes. Professor Ewing 2 states that when examining the subject he was unable to find any case of a cold-air machine having a coefficient of performance greater than three-quarters of a unit, while the coefficient of a modern vapour compression machine is, under ordinary working conditions, between four and five units or about six times as great; which means that for every thermal unit put into the steam end of a cold-air machine only three-quarters of a unit are extracted from the cold room, while with a vapour compression machine from four to five units 3 can be extracted for the same expenditure.

The choice of a refrigerating agent is governed by its boiling-point at ordinary pressures; the latent heat of vaporization per pound; the number of cubic feet of vapour or gas that must be compressed to produce a certain refrigerating effect, or, in other words, the size of the compressor necessary; the pressure required to produce

1 Dryers are not always used, as it is now found that by employing a very large water-cooling surface and by using the air over and over again, the deposition of moisture in expansion cylinder parts is not sufficient to interfere seriously with the working of the machine.

2 Howard Lectures, Society of Arts, 1897.

3 This, of course, depends on the temperature range at which the machine is working.

liquefaction of the gas at certain temperatures; and the specific heat of the liquid.

The qualities of the three chief refrigerating agents are set.out in the following table 1:

All these substances have a boiling-point much below the normal. Ammonia, for instance, at atmospheric pressure becomes liquid at a temperature of -30° F. Before it can be turned from a gas into a liquid, it has to be robbed of its latent heat, and it naturally follows that if its latent heat is restored to it, it will begin to boil and again become a gas. In boiling it will absorb heat from any substance that may be near it, and on being compressed the gas will, in its turn, deliver up the heat so absorbed to the cooling water trickling over the condenser, and again become a liquid.

The class of machine using a volatile liquid is separated into two divisions: (a) the absorption machine, and (b) the compression machine. The absorption machine. depends for its working on two substances that have an affinity for each other. When the machine is

1 J. E. Siebel, Compend of Mechanical Refrigeration.

not at work these substances are united, but their nature is such that on the application of heat their state is changed, and they can be separated from each other. The principal substances used in absorption machines are either sulphuric acid and water or ammonia and water. It is well known that ammonia and water have a great liking for each other, and, if allowed to, will readily unite; but if heat from, say, a steam-coil, is applied to them when in their united condition, they will separate, and the ammonia will be driven off as a vapour under pressure. The vessel containing the ammonia and water and the steam-coil is called the generator, and as the ammonia vapour driven off is not entirely free from water, it has, in order to render it free from water or anhydrous, to be passed through an upright vessel called an analyser, containing a series of iron trays which catch any watery vapour that may have been carried off and return it to the generator, after condensation, as water. The ammonia vapour, now very nearly free from water, passes to the rectifier, where the remaining watery vapour is condensed and separated, and then to the condenser, where it is liquefied. From the condenser it passes through a regulating valve to the refrigerator, where the pressure is relieved and it re-evaporates. In doing so it absorbs heat from the brine, water, or other substance which circulates round it, and which is to be cooled. The ammonia gas which has evaporated is reabsorbed by the water from which it was driven off and is returned to the generator by a small pump. This small pump is the only moving part of an absorption machine, and requires very little power to work it. In some absorption machines even the pump is dispensed with and there are no moving parts.

The absorption machine is more economical than the cold-air machine, but not so economical as the compression machine. Professor Ewing has found that the absorption machine is from two and a half to three and a half times

as efficient as the cold-air machine, and that the compression machine is from five to six times as efficient as the cold-air machine, and from two and a half to three times as efficient as the absorption machine.

Fig. 1