REFRIGERATION.

The process of refrigeration consists in the abstraction of heat from a substance, and if air, water, or ice is at hand at a lower temperature than it is desired to attain in the body or substance to be cooled the cooling element may be employed to perform the refrigeration directly without the aid of a machine.

If a temperature of 32 degrees and not lower is desired ice can be used directly but if it is necessary to reach a temperature lower than 32 degrees a mixture of salt and ice or other freezing mixture must be used.

By mixing one pound of calcium chloride with 0.7 lbs. of snow a solution is produced which will give a temperature of 67° below zero. But freezing mixtures are too expensive to be used for practical purposes, and it therefore becomes necessary to employ machinery.

The theory and practice of mechanical refrigeration are based upon the two first laws of thermo-dynamics, that is to say first: that mechanical energy and heat are mutually convertible; and second: that an external agent is necessary in order to complete or bring about the transformation.

The generally accepted theory concerning the nature of heat together with definitions of the terms, specific heat, latent heat, the mechanical equivalent of heat, etc., are fully discussed in Chapter 4 of this book and therefore it will not be necessary to enlarge upon these sub

jects in this connection except to state that the phrase commonly used, "heat is generated by compression," is somewhat misleading, because the amount of heat in the Universe is a fixed quantity, and the intrinsic energy possessed by any gas is under given conditions a quantity that can be accurately calculated. Thus if a pound of air at a temperature of 70 degrees Fahrenheit, and at normal atmospheric pressure be taken as an example, the total quantity of energy it possesses is at once known. If this air be placed in a compressor and its volume be reduced to say one half of its original volume, and if this be done so rapidly that there is no time for heat to escape at the end of the compression, that is to say adiabatically or instantaneous compression without transmission of heat, then its energy will have been increased. by the amount of work done upon it. Its statical pressure will be increased, and its temperature will also have risen, by reason of its changed state or condition internally. Now if the temperature be reduced to its former amount, that is to say to 70 degrees Fahrenheit, its volume will contract, so that a small additional quantity of air will have to be forced in in order that the pressure may remain unchanged as the temperature is reduced. It will be seen that there will be now, consequently upon the above, rather more than a pound of air to deal with at the higher pressure, and this is what actually occurs in practice, but is a point which is easily overlooked. Now if this air be allowed to expand in a cylinder, it will give up more of its heat in order to overcome the resistance, and in this way it will lose or part with more heat. The amount of work done is shown by the indicator card, and can be estimated. The mechanical work done by the air in this expansion is exactly the same as that

done upon it during its compression, but there is in addition the further loss of energy, due to the internal work done in the air during the expansion, so that what has been done to the air during the entire process has been to extract some of its original store of heat, thus reducing its temperature; and the cold air is now ready to restore its deficiency at the expense of the surrounding hotter bodies.

It should be borne in mind by the student that all bodies contain more or less heat and that heat can neither be created nor destroyed because it remains a fixed quantity throughout the universe.

Therefore the only method by which the temperature of a body or substance can be reduced is by the transference of more or less of the heat contained in the body to some other body or substance.

The work demanded of a refrigerating machine is to extract heat from a cold body, say from the air in an enclosed space, such as a refrigerating chamber, and by the expenditure of mechanical energy to sufficiently raise the temperature of this heat to admit of its being carried away by a suitable external agent, the latter being most usually water, which is not only the cheapest one available, but also has a greater capacity for heat, weight for weight, than any other known substance, and is taken as the standard of comparison, its specific heat being taken as unity.

A refrigerating or ice-making machine may then properly be defined as a heat-pump for the simple reason that its main function is the abstraction of heat from one body (the body to be cooled) and continuously and dutomatically transferring that heat to the refrigerating or cooling agent.

The various inventions for refrigerating and ice-making that are now in use, can be conveniently classified for the present purpose under the following five principal heads, viz.:

First, those wherein the more or less rapid dissolution or liquefaction of a solid is utilized to abstract heat. This is, strictly speaking, more a chemical process.

Second, those wherein the abstraction of heat is effected by the evaporation of a portion of the liquid to be cooled, the process being assisted by an air-pump. This is known. as the vacuum system.

Third, those wherein the abstraction of heat is effected by the evaporation of a separate refrigerating agent of a more or less volatile nature, which agent is subsequently restored to its original physical condition by mechanical compression and cooling. This is called the compression system.

Fourth, those wherein the abstraction of heat is effected by the evaporation of a separate refrigerating agent of more or less volatile nature under the direct action of heat, which agent again enters in solution with a liquid. This is termed the absorption system.

Fifth, those wherein air or other gas is first compressed, then cooled, and afterwards permitted to expand whilst doing work, or practically by first applying heat, so as to ultimately produce cold. These are usually designated as cold-air machines.

Of the various systems of refrigeration using different refrigerating mediums, only two, namely the ammonia compression system and the ammonia absorption system have come into anything like general use in this country, and these two systems the author proposes to take up and discuss in a practical way beginning with the compression system.

In this system the process of refrigeration is divided. into three distinct stages, viz.:—compression, condensation, and expansion.

Anhydrous ammonia is selected as the refrigerating medium on account of its low boiling point (-28.6° F.), its high latent heat of vaporization, its non-corrosive effect on iron and steel, and because the pressures under which it is used are such as to render it perfectly safe to handle with properly constructed apparatus.

When nitrogen and hydrogen combine to form ammonia one volume of nitrogen unites with three volumes of hydrogen, hence the chemical formula of ammonia is NH. As the atomic weight of nitrogen is 14 and of hydrogen 1, the formula also indicates that 14 parts, by weight, of nitrogen, combine with 3 parts of hydrogen, to create 17 parts of ammonia.

Gaseous ammonia can be liquefied at a pressure of 128 lbs. to the square inch, at a temperature of 70° Fahr., and at a pressure of 150 lbs. at a temperature of 77° Fahr., the pressure required to produce liquefaction rising very rapidly with the temperature. To liquefy by cold it requires to be reduced to a very low temperature, viz.,85.5° Fahr.

The gaseous ammonia is drawn into the ammonia compressor, or pump, and is there compressed to a pressure varying from 125 to 175 pounds per square inch.

During this compression, the latent heat of the vapor (that is, that quantity of heat which was imparted to it to effect its expansion from a liquid to a vapor) is converted into active or sensible heat.

The vapor, under this high pressure, is forced into the condenser, consisting of a series of pipes over which cold water is allowed to flow (atmospheric condenser)