HEAT VACUUMS- MACHINES
IN THE preceding chapter we dealt with high
temperatures and their employment in melting, molding, and working steel
into useful forms. It will be well for us to pause here to consider
temperatures at the other end of the thermometer scale, how they are
obtained, and the important part they play in modern civilization.
It is not absolutely correct to speak of producing “cold.” We are
apt to forget that cold is merely absence of heat. Strictly speaking,
there is nothing cold on earth. Everything is more or less hot. A piece
of ice at 32 degrees F. is hot compared with a lump of frozen alcohol,
and the latter at its freezing point is hot compared with a lump of
frozen air, while air at its freezing point is hot compared with a lump
of solid helium. In other words, frozen alcohol will be melted by the
heat in the ice; frozen air will be melted by the heat in frozen
alcohol, and frozen helium will be fused by the heat in frozen air.
Everything contains heat, and one object is colder than another only
because it contains less heat.Of course, the temperature of ice may vary. One block of ice may be ten, fifty, or a hundred degrees warmer than another, but ice cannot be heated above 32 degrees F. at the normal pressure of the atmosphere.
[346]
Ice is really a partial heat vacuum, a chamber partially exhausted, into which heat will flow if it gets a chance. We pack it away in sawdust, granulated cork, or other materials through which heat can with difficulty penetrate, and then in hot weather cakes of ice are placed in our household refrigerators, so that the heat that is in our food will have something to flow into. When we place our hands near a cake of ice, they feel cool and it seems as if ice radiated cold just as a stove radiates heat; but, of course, such is not the case. The heat of our hands radiates more rapidly in the direction of the cake of ice than in other directions, because there is a partial heat vacuum for the heat to flow into and the result is a sensation of cold.
We no longer depend upon cold winters for our supply of ice. We have learned how to pump heat and we can make heat vacuums, anywhere and at any time, even in the heart of the tropics, and regions in which no natural ice is ever obtainable have the benefits of refrigeration. Furthermore, we are not dependent upon ice for cooling foods. In many cases it is not necessary or even desirable to reduce temperatures to the freezing point of water. A moderate chilling is all that is required for certain foods. By the proper use of refrigerating machinery any degree of temperature may be obtained and maintained. To-day small refrigerating plants are constructed for domestic purposes, so as to render the housewife independent of the ice man.
With refrigerator cars and refrigerating plants on shipboard, fruit from the far west and from tropical lands may be brought to our breakfast[347] table. Meats from northern slaughterhouses may be transported in perfect condition into hot southern climes. There are also certain industries which are dependent on the use of the low temperatures. In breweries, dairies, margarine factories, etc., refrigeration is of the utmost importance, and refrigerating machinery is used for cooling and drying the air blast for blast furnaces.
In some few places refrigerating machinery is used to cool buildings in warm weather and make life more bearable in summer weather. It is highly probable that refrigeration of dwellings will be more and more extensively developed. In winter time we can make the climate in our houses anything we please. Why should we not control the indoor climate in summer time as well?
ABSOLUTE ZERO
The volume of a gas varies inversely in accordance with the pressure to which it is subjected, and also directly according to the temperature. If we start with a gas at the freezing point (32 degrees F.) and reduce its temperature 1 degree (or to 31 degrees F.), we find that the volume of the gas is reduced 1/492.6 of its original volume, provided, of course, that we do not vary the pressure on it. In fact, for every reduction of 1 degree below the freezing point there is a reduction of 1/492.6 of its volume, and for every degree of increased temperature there is an increase of 1/492.6 of the volume. From this it is assumed that at 492.6 degrees below the freezing point, or 460.6 degrees below zero F., we will reach the absolute zero, or the point at which there is no more heat in the gas.[348]
We have not yet succeeded in reaching the extreme of low temperature, although we have come very near it in laboratory experiments. Helium is liquefied at -448 degrees F., which is very near to the absolute zero. At the other end of the scale we have attained enormously high temperatures. The heat of the electric arc, for instance, is between 6,500 and 7,200 degrees F., and that is the highest degree of temperature that we have been able so far to attain.
Human life occupies a very limited zone in this range of temperatures. We must maintain our blood at a temperature of 98 degrees F. A variation of 8 degrees either way is fatal. By piling on heat insulators, such as fur clothing, to retain the heat of our bodies and keep it from flowing out too rapidly, we can maintain the blood temperature at 98 while the surrounding atmosphere may be 70 or 80 degrees below zero. There are internal fires within us that generate heat which radiates from the body, and by checking this radiation by suitable clothing we can maintain our blood at the normal temperature.
But what can we do when the surrounding temperature is higher than blood heat? The outside heat may be kept from flowing in by surrounding ourselves with heat-insulating clothing, but the internal heat then has no means of radiating away from our bodies; it accumulates, and we become overheated. However, Nature provides a cooling system in the perspiration which oozes from our pores, and as this evaporates it cools the skin and enables us to maintain our normal blood heat, although submerged in an atmosphere of a higher temperature. If the air is dry, the evaporation is[349] more rapid and the cooling is greater than in a moist atmosphere. That is why a temperature of 105 degrees on our Western plains may be more endurable than a temperature of 95 degrees in the moist atmosphere of New York. The importance of keeping down the temperature of the blood is particularly appreciated by physicians, and for this reason the earliest attempts at artificial cooling were made by physicians.
EARLY USES OF LOW TEMPERATURES
Very early in his history man discovered fire, learned how to kindle it and how to use it for his good. That discovery placed him immediately on a level far above the beasts. However, it is only in comparatively recent times that he has learned the uses of low temperatures. Nature’s stores of ice were drawn upon, and methods of preserving ice through warm weather were discovered in ancient times. Nero had ice houses built for him in Rome, but he could stock these buildings only with the ice that nature furnished him. Freezing mixtures of salt and ice, such as we use in our ice-cream freezers to-day to obtain temperatures far below the freezing point of water, were probably known in early times, but the ancients did not know how to produce ice.Artificial ice was probably first made in India, where it has long been the practice to produce ice by evaporation. Water is placed in shallow pans and then dry air is circulated over it, causing so rapid a vaporization as to cool the water to the freezing point. The idea of cooling water by evaporation belongs to very ancient times. Water placed in porous earthen vessels was found to be[350] cooler than water kept in water-tight jars. The moisture that escaped through the vessel would evaporate, and in so doing draw heat out of the vessel and its contents. To-day campers keep water cool by putting it in canvas buckets and hanging the buckets in the wind, so that the moisture oozing through the canvas will evaporate quickly.
It was not until 1755 that a mechanical means of producing low temperatures was developed. The inventor was Dr. Cullen, and he used an evaporation system, expediting the evaporation by producing a partial vacuum over the water. But nearly a century elapsed before the first commercially successful refrigerating machine was built. Even then the advantages of artificial refrigeration were not fully realized, and it was not until late in the last century that real progress was made. Since then the development of artificial refrigeration has been truly remarkable.
HEAT AND MECHANICAL ENERGY
There is a definite relation between heat and mechanical energy, in fact the two are mutually convertible. The amount of heat required to raise the temperature of a pound of water 1 degree F. is called a British thermal unit or a B. t. u. This measure is taken at 39.1 degrees F., because at that temperature water is at its densest. Since heat and mechanical energy are mutually convertible, we can express foot-pounds or horsepower in B. t. u. One B. t. u. is equivalent to 778 foot-pounds of energy. In other words, the amount of heat that would raise the temperature of a pound of water 1 degree F. would, if converted into mechanical[351] energy, be sufficient to raise a weight of 778 pounds to a height of one foot, or one pound to a height of 778 feet. A horsepower is equivalent to 2,545 B. t. u. per hour.Heat from burning coal is used to generate steam, and this in turn is used to operate a steam engine and thus heat is converted into mechanical energy (unfortunately most of the original heat units in the coal are wasted, as was pointed out in a previous chapter); but heat will not flow from one body into another of higher temperature without the expenditure of mechanical energy. It always flows from a hot body into a cold one, and not from the cold body into the hot one, unless it is actually pumped up to the higher heat level by some mechanical means. A refrigerating machine is actually a heat pump with which we produce a partial heat vacuum.
Whenever a gas is compressed, heat is generated. Anyone who has operated a tire pump knows how hot the pump becomes from the heat that is seemingly squeezed out of the compressed air. As was noted in 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.
[352]
COLD AIR MACHINES
FIG. 79.—COLD AIR MACHINE
COMPOSITOR AT WORK ON A LINOTYPE MACHINE
THE OPTOPHONE
An instrument which enables the blind to read common print
A BLIND MAN READING WITH HIS EARS
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[354] 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[355] absorb heat, and not being supplied with any external heat it had to draw upon itself, thus producing ice.
FIG. 80.—DIAGRAMMATIC VIEW OF A VACUUM REFRIGERATING MACHINE
FIG. 81.—ORIGINAL ABSORPTION MACHINE
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,[357] 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.
[358]
There are several auxiliary apparatus which are omitted in this brief description in the interest of clarity.
THE COMPRESSION SYSTEM
FIG. 82.—DIAGRAMMATIC VIEW OF A COMPRESSION MACHINE
by A. Russell Bond
Comments
Post a Comment