Chapter VIII, heat is liable to give trouble in an air compressor,

and sometimes the temperature rises to such a point that there is an explosion of the air and the vapors coming from the oil used to lubricate the machine. The compressed air is therefore cooled by means of water jackets or coils of pipe through which water is passed. In this way the excess heat is carried off. When, however, cooled compressed air is relieved of pressure and allowed to expand again the process is reversed. A partial heat vacuum is formed and heat from surrounding objects flows into the vacuum. In other words, the surrounding objects are cooled.

COLD AIR MACHINES

It is a simple matter to make a machine which will alternately compress, cool, and expand air in such a way as to produce a lowered temperature. Such a machine is indicated diagrammatically in Figure 79. There are two cylinders, _A_ and _B_, and a condenser at _C_. When the piston _a_ in cylinder _A_ descends it compresses the air in the cylinder; this air flows into the condenser _C_. There is a coil of pipe in this condenser through which water circulates. This carries off the heat of compression and then a valve is opened which permits the cooler air to pass off into cylinder _B_. As the air expands in this cylinder it becomes chilled. This chilled air is then forced out of cylinder _B_ by means of piston _b_ and flows into the refrigerator or cold storage room _D_. As the air is liable to take up moisture and to introduce objectionable vapors from oil used to lubricate the pumps, it is usually confined in pipes in the refrigerator and then returned to the cylinder _A_.

This is a type of a refrigerating machine that is used very extensively on ships for chilling perishable foods. However, air has only a very low capacity for heat, and in order to obtain an appreciable amount of refrigeration very large volumes of air must be handled. This means that cold-air machines must be very large and bulky. The efficiency of such machines is low, but they find favor on shipboard because there are no inflammable or poisonous gases to be dealt with. In the standard machine of the United States Navy air is compressed to 260 pounds; and in the expansion is raised to 60 pounds pressure, which is enough to reduce the temperature to between 70 to 90 degrees below zero.

LATENT HEAT

Far more efficient are the machines which utilize latent heat. As explained in a previous chapter, whenever a solid is converted into a liquid or into a gas a certain amount of heat is absorbed and stored up in such a way as not to become apparent to the senses or to a thermometer. Such heat is known as “latent heat.” For instance, we can add a pound of water at 50 degrees temperature to a pound of water at 200 degrees, and the mixture will have a temperature of 125 degrees, or the mean of 200 + 50 degrees. But a pound of ice at 32 degrees mixed with a pound of water at 200 degrees will not give us 116 degrees ((200+32)/2), but only 44½ degrees. In other words, about 143 heat units will be rendered latent in converting solid water into liquid water, reducing the temperature of the water to 57 degrees and then the mean of 57 and 32 is 44.5 (200-143=57, (57+32)/2 = 44½). A more striking experiment is to mix a pound of water cooled to 32 degrees F. with a pound of water at 175 degrees F., and the result will be two pounds of water at 103.5 degrees, but if we mix a pound of chopped ice at 32 degrees F. with a pound of water at 175 degrees F., the result will be two pounds of water cooled to the freezing point.

In passing from a liquid into a gas water absorbs far more heat and renders it latent. For each pound of water converted into steam at atmospheric pressure 970 B. t. u. are absorbed. This storage of latent heat is utilized to good advantage in refrigerating machinery. The vacuum machine invented by Dr. Cullen in 1755 was a latent heat machine.

VACUUM MACHINES

As we have observed before, the boiling point of a liquid depends upon the pressure to which it is subjected. Under the normal atmospheric pressure of 14.7 pounds per inch the boiling point of water is 212 degrees F., but if the pressure be increased the boiling point rises, and if it be reduced the boiling point is lowered. In a partial vacuum of ten pounds absolute pressure the boiling point is 193.2 degrees, at one pound it is 102.1 degrees, and if the pressure is reduced to .089 pound water will boil at 32 degrees, or its normal freezing point. Dr. Cullen, by exhausting the air from a vessel containing water, made the water boil or vaporize at a low temperature. In order to boil it had to absorb heat, and not being supplied with any external heat it had to draw upon itself, thus producing ice.

Dr. Cullen’s machine has been improved upon by using various chemical substances to absorb the water vapor. Such a machine is shown in Figure 80. The vacuum chamber _A_ is partly filled with brine, which may be cooled below the freezing point of pure water without congealing. A pump, _B_, maintains a vacuum in the chamber. In the upper part of the vacuum chamber there is a vessel, _C_, into which sulphuric acid is sprayed from a reservoir, _D_. This acid has a strong affinity for water vapor and hastens the evaporation by absorbing the vapor with which it comes in contact. The mixed sulphuric acid and water flows over into a receiver, _E_. The acid is reconcentrated by steam heat so that it can be used over again. However, this feature of the process is not shown in the diagram. Brine from the chamber _A_ passes through a coil of pipe _F_ in the tank _G_, where the ice is made, and it is returned to the vacuum chamber by an injector _H_, which at the same time introduces fresh water into the chamber to take the place of that absorbed by the acid. The fresh water and brine enter as a spray at _I_, so as to increase the rate of evaporation.

THE ABSORPTION PROCESS

Another form of refrigeration is known as the absorption system, and strangely enough direct heat is applied to the machine in one place in order to abstract heat from it in another. A diagrammatic representation of the first machine of this type (which was invented by Frederick Carré) is shown in Figure 81. Two vessels, _A_ and _B_, are employed, which are connected by a tube _C_. The vessel _A_ contains ammonia solution. A lamp, _D_, is placed under the vessel _A_. Sufficient heat is produced to vaporize the ammonia, whose boiling point is very low, and distill it out of the water in the solution. It passes through tube _C_ into vessel _B_. This vessel is surrounded by a tank, _E_, containing cold water, which condenses the ammonia vapor into liquid ammonia, then the process is reversed.

The circulation of water through tank _E_ is checked and water from pipe _F_ is sprayed on vessel _A_. This cools the contents of vessel _A_, producing a partial vacuum. The ammonia in chamber _B_ boils and its vapors pass back into chamber _A_, where they are reabsorbed by the water in that chamber. The rapid vaporization in chamber _B_ absorbs heat from the immediate surroundings and will freeze water placed in the tank _E_.

In commercial practice the absorption system is very extensively employed. The ammonia is placed in a large cylinder known as a generator and is heated by steam coils. The ammonia vapor passes through an analyzer which traps any water vapor it may contain, and then it goes through a series of condensing coils, which are cooled by water. In these condensing coils the ammonia vapor is liquefied by the pressure in the generator and collects in a receiver, whence it passes through an expansion valve into the cooling coils in the brine tank. From the cooling coils the ammonia gas passes back through an absorber which reverses the operation of the generator. Here weak aqua ammonia is sprayed on the ammonia gas and absorbs it. The rich ammonia solution is then pumped into the generator.

There are several auxiliary apparatus which are omitted in this brief description in the interest of clarity.

THE COMPRESSION SYSTEM

One more type of refrigerating machine remains to be described and that is the compression type. This was invented by Jacob Perkins in 1834, but was not developed into a commercial machine until 1850. This machine is the most important of them all. In some respects it is like the absorption process, but in place of adding direct heat a compressing machine is employed. Figure 82 illustrates the system diagrammatically. At _A_ is the compressor which compresses ammonia gas. The gas which is heated by the compressor is then cooled and liquefied in the condenser _B_. Thence it passes into a brine tank _C_, where it expands and absorbs heat. From this point it is drawn back into the compressor, thus completing the cycle. In other words, the ammonia must absorb as much heat from the brine as is taken out of it at the condenser.