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building two gigantic ocean liners swifter and larger than any afloat. They are to be 1000 feet long and are to have a horse power of 110,000 and a speed of 30 knots. (See opposite p. 135.)
248. Manufactured ice. In the great majority of modern ice plants the low temperature required for the manufacture of the ice is produced by the rapid evaporation of liquid ammonia. At ordinary temperatures ammonia is a gas, but it may be liquefied by pressure alone. At 80° F. a pressure of 155 pounds per square inch, or about 10 atmospheres, is required to produce its liquefaction. Fig. 192 shows the essential parts. of an ice plant. The compressor, which is usually run by a steam engine,
FIG. 192. Compression system of ice manufacture
forces the gaseous ammonia under a pressure of 155 pounds into the condenser coils shown on the right, and there liquefies it. The heat of condensation of the ammonia is carried off by the running water which constantly circulates about the condenser coils. From the condenser the liquid ammonia is allowed to pass very slowly through the regulating valve V into the coils of the evaporator, from which the evaporated ammonia is pumped out so rapidly that the pressure within the coils does not rise above 34 pounds. It will be noted from the figure that the same pump which is there labeled the compressor exhausts the ammonia from the evaporating coils and compresses it in the condensing coils, for the valves are so arranged that the pump acts as an exhaust pump on one side and as a compression pump on the other. The rapid evaporation of the liquid ammonia under the reduced pressure existing
within the evaporator cools these coils to a temperature of about 5° F. The brine with which these coils are surrounded has its temperature thus reduced to about 16° or 18° F. This brine is made to circulate about the cans containing the water to be frozen. The heat of vaporization of ammonia at 5° F. is 314 calories.
Many thousands of feet of circulating saltwater pipe are laid horizontally and covered with water to be frozen for large indoor skating rinks.
249. Cold storage. The artificial cooling of factories and cold-storage rooms is accomplished in a manner exactly similar to that employed in the manufacture of ice. The brine is cooled precisely as described above, and is then pumped through coils placed in the rooms to be cooled. In some systems carbon dioxide is used instead of ammonia, but the principle is in no way altered. Sometimes, too, the brine is dispensed with, and the air of the rooms to be cooled is forced by means of fans directly over the cold coils containing the evaporating ammonia or carbon dioxide. It is in this way that theaters and hotels are cooled.
QUESTIONS AND PROBLEMS
1. Why is a gas engine called an internal-combustion engine? 2. Why do gasoline engines have flywheels? Why is a one-cylinder engine of the four-cycle type especially in need of a flywheel?
3. How does the temperature of the steam within a locomotive boiler compare with its temperature at the moment of exhaust? Explain.
4. On the drive wheels of locomotives there is a mass of iron opposite the point of attachment of the drive shaft. Why is this necessary? 5. Why does not the water in a locomotive boil at 100° C.?
6. If liquid oxygen is placed in an open vessel, its temperature will not rise above - 182° C. Why not? Suggest a way in which its temperature could be made to rise above - 182° C., and a way in which it could be made to fall below that temperature.
7. How many foot pounds of energy are there in 1 lb. of coal containing 14,000 B. T. U. per pound? How many pounds of iron must be held at a height of 150 ft. to have as much energy as this pound of coal?
} 8. The average locomotive has an efficiency of about 6%. What horse power does it develop when it is consuming 1 ton of coal per hour? (See Problem 7, above.)
9. What amount of useful work did a gasoline engine working at an efficiency of 25% do in using 100 lb. of gasoline containing 18,000 B.T.U. per pound?
10. What pull does a 1000 H. P. locomotive exert when it is running at 25 mi. per hour and exerting its full horse power?
250. Conduction in solids. If one end of a short metal bar is held in the fire, the other end soon becomes too hot to hold; but if the metal rod is replaced by one of wood or glass, the end away from the flame is not appreciably heated.
This experiment and others like it show that nonmetallic substances possess much less ability to conduct heat than do metallic substances. But although all metals are good conductors as compared with nonmetals, they differ widely among themselves in their conducting powers.
THE TRANSFERENCE OF HEAT
Let copper, iron, and German-silver wires 50 cm. long and about 3 mm. in diameter be twisted together at one end as in Fig. 193, and let a Bunsen flame be applied to the twisted ends. Let a match be slid slowly from the cool end of each wire toward the hot end, until the heat from the wire ignites it. The copper will be found to be the best conductor and the German silver the poorest.
In the following table some common substances are arranged in the order of their heat conductivities. The measurements have been made by a method not differing in principle from that just described. For convenience, silver is taken as 100.
FIG. 193. Differences in the heat conductivities of metals
251. Conduction in liquids and gases. Let a small piece of ice be held by means of a glass rod in the bottom of a test tube full of ice water. Let the upper part of the
tube be heated with a Bunsen burner as in Fig. 194. The upper part of the water may be boiled for
some time without melting the ice. Water is evidently, then, a very poor conductor of heat. The same thing may be shown more strikingly aș follows: The bulb of an air thermometer is placed only a few millimeters beneath the surface of water contained in a large funnel arranged as in Fig. 195. If now a spoonful of ether is poured on the water and set on fire, the index of the air thermometer will show scarcely any change, in spite of the fact that the air thermometer is a very sensitive indicator of changes in temperature.
FIG. 194. Water a nonconductor
Careful measurements of the conductivity of water show that it is only about 1200 of that of silver. The conductivity of gases is even less, not amounting on the average to more than that of water.
252. Conductivity and sensation. It is a fact of common observation that on a cold day in winter a piece of metal feels much colder to the hand than a piece of wood, notwithstanding the fact that the tempera- FIG. 195. Burning
ether on the water does not affect the air thermometer
ture of the wood must be the same as that of the metal. On the other hand, if the same two bodies had been lying in the hot sun in midsummer, the wood might be handled without discomfort, but the metal would be uncomfortably hot. The explanation of these phenomena is found in the fact that the iron, being a much better conductor than the wood, removes heat from
the hand much more rapidly in winter, and imparts heat to the hand much more rapidly in summer, than does the wood. In general, the better a conductor the hotter it will feel to a hand colder than itself, and the colder to a hand hotter than itself. Thus, in a cold room oilcloth, a fairly good conductor, feels much colder to the touch than a carpet, a comparatively poor conductor. For the same reason linen clothing feels cooler to the touch in winter than woolen goods.
253. The rôle of air in nonconductors. Feathers, fur, felt, etc. make very warm coverings, because they are very poor conductors of heat and thus prevent the escape of heat from the body. Their poor conductivity is due in large measure to the fact that they are full of minute spaces containing air, and gases are the best nonconductors of heat. It is for this reason that freshly fallen snow is such an efficient protection to vegetation. Farmers always fear for their fruit trees and vines when there is a severe cold snap in winter, unless there is a coating of snow on the ground to prevent a deep freezing.
254. The Davy safety lamp. Let a piece of wire gauze be held above an open gas jet and a match applied above the gauze. The flame will be found to burn above the gauze as in Fig. 196, (1), but it will not pass through to the lower side. If it is ignited below the gauze, the flame will not pass through to the upper side but will burn as shown in Fig. 196, (2).
The explanation is found in the fact that the gauze conducts the heat away from the flame so rapidly that the gas on the other side is not raised to the temperature of ignition. Safety lamps used by miners are completely incased in gauze, so that if the mine is full of inflammable gases, they are not ignited outside of the gauze by the lamp.
FIG. 196. A flame will not pass through wire gauze