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shows diagrammatically the general arrangement of a compression refrigerating machine, using ammonia or some other volatile liquid. Assuming that the machine is working with ammonia, the gas from the refrigerating or expansion coils A is drawn into the compressor B through the pipe C and the valve D; on the return stroke of the compressor piston E, the valve D is closed and the gas is compressed and driven through the valve F into the pipe G leading to the condenser H. The ammonia gas in the suction pipe C is at a pressure of about 27 lbs. on the square inch, and at a temperature of 14° F. It contains the heat which has been absorbed from the atmosphere in the cold chamber, and which was necessary to turn the ammonia from a liquid to a gas. By being compressed this gas is decreased in bulk, and the heat contained in it is, so to speak, concentrated, so that the gas in the pipe G is at a temperature much above that of ordinary cold water. The condenser H has cold water trickling over it; this water is at a temperature of about 60° F., and absorbs the heat (representing the latent and specific heat of the ammonia and also the heat due to the energy of the engine) from the gas inside the coils and carries it away. The gas, robbed of its heat, can no longer remain a gas, and becomes liquid in the same way that steam, if cooled, will condense. Though a liquid, it is obvious that the ammonia will be ready to start boiling and again turn to a gas as soon as the pressure upon it is relieved. This is done by passing it through a minute opening in the expansion valve K. When the ammonia has passed through this valve it starts boiling and drops many degrees in temperature. Before it can boil, however, it must absorb heat, and as 1 lb. of ammonia requires, at atmospheric pressure, 555 heat units before it can

turn into a gas, these heat units have to be abstracted from the substance surrounding the expansion coils A, which, as a result, rapidly become covered with snow.

It will be seen that the cycle of operations consists of three parts:

1. Compression or reduction in volume, and increase in temperature.

2. Condensing or giving up heat, and liquefying.

3. Expansion (boiling) or absorbing heat, and so

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gasifying.

In practice the pressure in the expansion coils is kept at from 1 to 30 lbs. (corresponding to a temperature of from 22° to 17°) by the suction of the compressor, which is continually drawing off the gas as it is formed; but on account of the desirability of keeping T, and T2 close together, the pressure, and so the temperature, is kept as high as possible consistent with the work the machine is doing. It is apparent that if the liquid ammonia is constantly fed into the expansion coils A through the valve K, and the gas drawn away by the compressors, a great amount of heat will be absorbed by the boiling ammonia in the coils, and that if these coils are placed in an insulated chamber, as shown in the diagram, that chamber will be rapidly reduced in temperature or refrigerated. The same remark, of course, applies to an icetank, in which the expansion coils are surrounded by brine, which is reduced in temperature, and so freezes the water contained in cans suspended in it.

The cycle above given applies to all of the vapour compression refrigerating machines; the details vary with different makers, and, of course, the size and strength of the various parts vary with the peculiarities of the par

ticular refrigerating agent used. The relative volumes of the compressors are shown in the table previously given, and it is the size of the compressor necessary to produce a certain refrigerating effect that has so largely discounted sulphurous acid as a refrigerating agent. Thus it is that the ammonia and carbonic acid compression machines now practically hold the field.

CHAPTER I.

THE LAWS OF HEAT, FLUIDS, LIQUIDS, GASES, AND VAPOURS.

THE refrigerating machine is essentially a heat-engine reversed, and a proper knowledge of the physical laws that regulate the behaviour, under various conditions, of the gases and vapours which are employed in it is necessary before its action can be properly understood.

HEAT.

All substances are capable of existing in one of three forms, and can be a solid, a liquid, or a gas, the change from one form to another being brought about either by the addition to, or withdrawal from, the substance of heat which is the source of all energy. All material substances, in whichever of these three forms they exist, consist of an infinite number of separate particles or molecules. Heat is a form of motion of these particles, and though the energy of this motion may be transformed and transferred, it can never be destroyed. When a body is in motion, it is said to have kinetic energy, and heat is represented by the kinetic energy or motion of the molecules of a body or substance; the greater the heat, the greater the motion, and vice versa. When, however, a body, by the transference of its heat to some other body, has been reduced to the temperature of

absolute zero (which will be referred to later on), heat is entirely absent, and the particles or molecules of the body are absolutely at rest and without internal energy .of any sort.

Temperature. The degree of heat of a body, or its thermal state, depends on the motion of its molecules, and is known as its temperature. This temperature is measured by the thermometer, the scales of which have been arbitrarily fixed-hot and cold being only relative termsand depends upon the amount of energy the particles of the body possess.

Transference of Heat.-Heat can be conveyed from one body to another in three ways-either by radiation, conduction, or convection. The phenomena connected with the transference of heat are most complicated, and all these three processes may take place singly or at the same time.

Radiation of Heat.-Radiant heat is heat which can pass from one body to another through what is called the ether that is to say, it is transmitted, like the heat of the sun, without the assistance of any material medium. It is always a transference of vibratory motion from a hot body to a cold, and the transference will continue until the source of heat is cut off or the temperatures are equalized. The passage of this radiant heat has no effect upon the temperature of the medium through which it passes, this remaining unaltered. The heat is also capable of being reflected or focussed, and in this respect is governed by exactly the same laws as light. It is, in fact, only when it becomes absorbed by the body upon which it falls that it is, strictly speaking, called heat; in the ether it is radiant energy.

Conduction of Heat.-Heat is conducted when it is

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