Heat And Work

Chapter VIII. Heat And Work.


(1) Heat Measurement and Specific Heat

177. Specific Heat.?In the study of density and specific gravity it is made clear that different substances differ widely in the amount of matter contained in equal volumes, e.g., lead is much denser than water. The study of the relative densities of substance is usually considered under the subject of specific gravity.
Specific heat as distinguished from specific gravity is concerned with the capacity for heat possessed by different substances. The definition for specific heat is: The ratio of the amount of heat required to change the temperature of a given mass of a substance 1 C. degree to the amount of heat required to change the temperature of the same mass of water 1 C. degree. By definition, it requires 1 calorie to raise the temperature of the gram of water 1?C. The specific heat therefore of water is taken as one. The specific heat of most substances except hydrogen, is less than that of water, and as a rule, the denser the body the less its specific heat, as may be observed in the following table:

Specific gravity Specific heat
Gold 19.3 0.032
Mercury 13.6 0.033
Copper 8.9 0.093
Brass 8.4-8.9 0.094
Nickel 8.57 0.11
Iron 7.5+ 0.1125
Aluminum 2.67 0.218
Glass 2.5-3.6 0.19
Ice 0.918 0.504
Air 0.00129 0.237
Steam 0.00061 0.480
Hydrogen 0.00009 3.409
[Pg 201]
178. Method of Determining Specific Heat.?The specific heat of a body is usually determined by what is called the method of mixtures.
For example, a definite weight of a substance, say a 200-g. iron ball, is placed in boiling water until it has the temperature of the hot water, 100?C. Suppose that 300 g. of water at 18?C. be placed in a calorimeter, and that the hot iron ball on being placed in the water raises its temperature to 23.5?C. The heat received by the water equals 5.5 ? 300 = 1650 calories. This must have come from the heated iron ball. 200 g. of iron then in cooling 76.5?C. (100?-23.5?) gave out 1650 calories. Then 1 g. of iron in cooling 76.5?C. Would give out 8.25 calories or 1 g. of iron cooling 1?C. would yield about 0.11 calorie. The specific heat of the iron is then 0.11. For accurate determination the heat received by the calorimeter must be considered.
179. Heat Capacity of Water.?The large capacity for heat shown by water is useful in regulating the temperature of the air near lakes and the ocean. In hot weather the water rises slowly in temperature absorbing heat from the warm winds blowing over it. In winter the large amount of heat stored in the water is slowly given out to the air above. Thus the climate near the ocean is made more moderate both in winter and summer by the large capacity of water for heat. This large heat capacity of water may seem to be a disadvantage when one is warming it for domestic purposes since it requires so much heat to warm water to boiling. However, it is this capacity that makes hot-water bottles and hot-water heating effective.
If one takes a pound of ice at 0?C. in one dish and a pound of water at 0?C. in another, and warms the dish of ice by a Bunsen flame until the ice is just melted, and then warms the water in the other dish for the same time, the water will be found to be hot and at a temperature 80?C., or 176?F.
180. The Heat of Fusion of Ice.?This experiment indicates the large amount of heat required to change the ice to water without changing its temperature. As indicated[Pg 202] by the experiment, it requires 80 calories to melt 1 g. of ice without changing its temperature or, in other words, if one placed 1 g. of ice at 0?C. in 1 g. of water at 80?C., the ice would be melted and the water would be cooled to 0?C.
181. Heat Given out by Freezing water.?Just as 80 calories of heat are required to melt 1 g. of ice, so in freezing 1 g. of water, 80 calories of heat are given out.
The fact that heat is set free or given out when a liquid solidifies may be strikingly shown by making a strong solution of sodium acetate. On allowing this to cool quietly it will come to the room temperature and remain liquid. If now a small crystal of sodium acetate is dropped into the liquid the latter quickly becomes a solid mass of crystals, at the same time rising markedly in temperature. The amount of heat now liberated must enter the sodium acetate when the mass of crystals is melted again.
The large amount of heat that must be liberated before water freezes accounts for the slowness of the formation of ice. It is also the reason why the temperature never falls so low in the vicinity of large lakes as it does far inland, the heat given out by the freezing water warming the surrounding air.
The heat that disappears on melting and reappears on solidifying is called the heat of fusion. It is sometimes called latent heat since the heat seems to become hidden or latent. It is now believed that the heat energy that disappears when a body melts has been transformed into the potential energy of partially separated molecules. The heat of fusion therefore represents the work done in changing a solid to a liquid without a change of temperature.
182. Melting of Crystalline and Amorphous Substances.?If a piece of ice is placed in boiling hot water and then removed, the temperature of the unmelted ice is still 0?C. There is no known means of warming ice under[Pg 203] atmospheric pressure above its melting point and maintaining its solid state. Ice being composed of ice crystals is called a crystalline body. All crystalline substances have fixed melting points. For example, ice always melts at 0?C. The melting points of some common crystalline substances are given below:
Melting Points of Some Crystalline Substances
1. Aluminum 658 C.
2. Cast iron 1200 C.
3. Copper 1083 C.
4. Ice 0 C.
5. Lead 327 C.
6. Mercury -39 C.
7. Phenol (carbolic acid) 43 C.
8. Platinum 1755 C.
9. Salt (sodium chloride) 795 C.
10. Saltpeter (potassium nitrate) 340 C.
11. Silver 961 C.
12. Sodium hyposulphite (hypo) 47 C.
13. Zinc 419 C.
Non-crystalline or amorphous substances such as glass, tar, glue, etc., do not have well defined melting points as do crystalline bodies. When heated they gradually soften and become fluid. For this reason glass can be pressed and molded.
183. Change of Volume During Solidification.?The fact that ice floats and that it breaks bottles and pipes in which it freezes shows that water expands on freezing. How a substance may occupy more space when solid than when liquid may be understood when we learn that ice consists of masses of star-shaped crystals. (See Fig. 151.) The formation of these crystals must leave unoccupied spaces between them in the ice. When liquefied, however, no spaces are left and the substance occupies less volume.[Pg 204] Most substances contract upon solidifying. Antimony and bismuth, however, expand on solidifying while iron changes little in volume. Only those bodies that expand, or else show little change of volume on solidifying, can make sharp castings, for if they contract they will not completely fill the mold. For this reason gold and silver coins must be stamped and not cast. Type metal, an alloy of antimony and lead, expands on solidifying to form the sharp outlines of good type. Several important effects of the expansion of water when freezing should be noted. (a) Ice floats, (b) if it sank as soon as formed, lakes and rivers would freeze solid, (c) freezing water is one of the active agents in the disintegration of rocks.
Fig. 151.?Ice crystals.
Fig. 152.?Melting ice by pressure.
Since water expands on freezing, pressure would on compressing ice at 0?C., tend to turn it into water. Pressure does lower the melting point of ice, so that a little ice may melt when it is subjected to pressure. On removing the pressure the water freezes. This may be shown by placing a loop of fine piano wire (see Fig. 152) over a piece of ice[Pg 205] supported so that a weight may be hung upon the wire. The wire will be found to gradually cut through the ice, the melted ice refreezing above the wire.

Important Topics

1. Specific heat.
2. Heat of fusion of ice.
3. Crystalline substances have fixed melting points.
4. Expansion on freezing, importance.

Exercises

1. What are two advantages in the high heat of fusion of ice?
2. What are two advantages in the expansion of water while freezing?
3. How much heat will be required to melt 1000 g. of ice and warm the water to 20?C.?
4. How many grams of ice at 0?C. can be melted by 400 g. of water at 55?C.?
5. What are two advantages of the high specific heat of water? Two disadvantages?
6. If the specific heat of iron is 0.1125, how much ice at 0?C. can be melted by a 200-g. ball of iron heated to 300?C?
7. What is the temperature of a hot ball of iron weighing 80 g., if when placed on a piece of ice at 0?C. it melts 90 g. of ice?
8. If 500 g. of copper at 400?C. are placed into 3000 g. of water at 10?C. what will be the resulting temperature?
9. What weight of water at 90?C. will just melt 10 kg. of ice at 0?C.?
10. If the smooth dry surface of two pieces of ice are pressed together for a short time the two pieces will be frozen into one piece. Explain.
11. Tubs of hot water are sometimes placed in vegetable cellars to prevent the vegetables from freezing. Explain.
12. How many B.t.u. are given out when 2 lbs. of water freeze?

(2) Heat and Change of State

Fig. 153.?The black cube in the upper corner represents one cubic inch of water. The entire cube represents the space occupied by the cubic inch of water in the form of steam. The reduced spaces at the bottom and sides show how much short the cube is of being one cubic foot. (American Radiator Co.)
184. Heat of Vaporization.?In our study of evaporation in Art. 174 we considered the more rapidly moving or vibrating molecules in the liquid escaping to the air above[Pg 206] and the slower moving molecules being left behind in the liquid; this means that a loss of heat will result upon evaporation, the liquid remaining becoming cooler as the process continues. Now just as a ball thrown up in the air loses its kinetic energy as it rises, and acquires energy of position or potential energy, so molecules escaping from a liquid lose a certain amount of kinetic energy or heat and acquire a corresponding amount of energy of position or potential energy. Conversely, as the ball returns to the ground its potential energy is changed to kinetic energy. Similarly when vapor molecules return to the liquid condition they lose their energy of position and acquire kinetic energy. In other words, when a liquid evaporates a certain amount of heat disappears, or becomes latent and when the vapor condenses the heat reappears, or becomes sensible heat. The amount of heat that disappears when 1 g. of a substance is vaporized is called the heat of vaporization.[Pg 207] In the case of water at its boiling point, 536 calories of heat disappear when 1 g. of water turns to vapor, and this same amount of heat reappears when the vapor condenses.
The change of volume of water on turning to steam is shown in Fig. 153.
Fig. 154.?Effect of pressure on the boiling point.
185. The Boiling Point.?The boiling temperature depends upon the pressure. The boiling point may be defined as the temperature at which bubbles of vapor are formed within the liquid. These bubbles increase the surface at which evaporation can take place in the liquid, and the principal reason why rapid application of heat to a liquid does not raise its temperature above the boiling point is that as more heat is applied more bubbles form so that the increase of evaporating surface supplies a correspondingly greater surface for cooling. The variation of the boiling temperature with changing pressure may be shown by partly filling a strong 7/8-in. test-tube with water. Close the neck with a one-hole rubber stopper through which passes a glass tube to which is attached a soft rubber tube. (See Fig. 154.) Support the tube by a holder, heat the water and boil until all the air is driven[Pg 208] from the tube, then close the soft rubber tube with a pinch cock and hold the tube in an inverted position. On cooling the end of the tube above the water with cold water or snow, the vapor within is condensed and the pressure upon the water is reduced. Vigorous boiling begins at once. By condensing the vapor repeatedly the water may be made to boil at the room temperature. At the top of Mt. Blanc water boils at 84?C. While in steam boilers at 225 lbs. pressure to the square inch the boiling point is nearly 200?C.
186. Laws of Boiling.?The following statements have been found by experiments to be true.
1. Every liquid has its own boiling point which under the same conditions of pressure is always the same.
2. The temperature of the boiling liquid remains at the boiling point until all the liquid is changed into vapor.
3. The boiling point rises with increased pressure and falls if the pressure is diminished.
4. A boiling liquid and the vapor formed from it have the same temperature. On cooling, a vapor will liquefy at the boiling point.
Fig. 155.?Distilling apparatus.
5. The solution of solid substances in a liquid raises its boiling point, additional energy being needed to overcome the adhesion involved in the solution. The boiling point is also affected by the character of the vessel containing the liquid. In glass the boiling point is 101?.
Fig. 156.?A vacuum pan.
187. Distillation of Water.?Usually when solids are dissolved in liquids the vapor coming from the liquid contains[Pg 209] none of the dissolved solid. Thus by evaporating salt sea water, and collecting and condensing the vapor, pure water is obtained. Distillation is the process of boiling a liquid and condensing the vapor formed back again into a liquid. (See Fig. 155.) The liquid to be distilled is placed in vessel F and boiled. The vapor is conducted into the tube J which is surrounded by a larger tube containing cold water. The vapor is condensed on the cold walls of the tube. The resulting liquid is collected in the vessel R. Distillation is employed for two purposes: (a) To remove impurities from a liquid (water is purified in this way). (b) Mixtures of different liquids having different boiling points may be separated by distillation. The one having the lower boiling point will be vaporized first. Thus a mixture of alcohol and water, on distillation yields a distillate having a much larger percentage of alcohol than at first. Repeating this process which is called fractional distillation yields alcohol of increasing strength of purity. Distilled liquor such as alcohol, brandy, and whisky are made by distilling fermented liquor, alcohol being made from fermented grains. Gasoline and kerosene are distilled from crude petroleum. Sometimes as in the production of sugar or evaporated[Pg 210] milk the object is to remove the water by evaporation in order to obtain the solid material. Since the two substances named are injured by heating, the syrup, or milk is evaporated under reduced pressure in a vacuum pan, that is in a boiler from which air and vapor are removed by an air pump. (See Fig. 156.)
188. Artificial Cooling.?The fact has been brought out that when a solid is melted, a certain amount of heat, called the heat of fusion, is absorbed or disappears. This absorption of heat is also noticed when a solid is liquefied by dissolving it in a liquid as well as when it is liquefied by simply applying heat. Thus if some table salt is placed in a tumbler of water the temperature of the solution is lowered several degrees below that of the salt and water used. The liquefaction or solution of the salt has been accompanied by an absorption or disappearance of heat. This heat has been taken from the salt and from the water, resulting in a lowered temperature. Sal ammoniac or ammonium nitrate when dissolved in water produce a much more marked cooling effect than does table salt. The dissolving of a crystal in a liquid is something like evaporation, except that the molecules of the liquid attract the molecules of the solid and thus assist the change of state.
189. Freezing Mixtures.?If one attempts to freeze a solution of salt and water, ice will not form at 0?C. but several degrees lower. The ice formed however is pure. Evidently the attraction of the molecules of salt for the water molecules prevented the formation of ice until the motions of the water molecules had been reduced more than is necessary in pure water. As the temperature of freezing water is that of melting ice, ice in a salt solution melts at lower temperature than in pure water. In a saturated salt solution this temperature is -22?C. It[Pg 211] is for this reason that the mixture of ice and salt used in freezing cream is so effective, the salt water in melting the ice, being cooled to a temperature many degrees below the freezing point of the cream. The best proportion for a freezing mixture of salt and ice is one part salt to three parts of finely powdered or shaved ice.
190. Refrigeration by Evaporation.?Intense cold is also produced by permitting the rapid evaporation of liquids under pressure. Carbon dioxide under high pressure is a liquid, but when allowed to escape into the air evaporates so rapidly that a portion of the liquid is frozen into solid carbon dioxide which has a temperature of -80? C. The evaporation of liquid ammonia by permitting it to escape into a pipe, under reduced pressure, is used on a large scale as a means of producing cold in cold storage and refrigeration plants. (See Fig. 157.)
Fig. 157.?Diagram of a refrigerating system.
The essential parts of the refrigerating system employing ammonia is represented in Fig. 157. The compressor exhausts ammonia gas from the coiled pipe in "E" and compresses the gas in "C," where under 150 pounds pressure and the cooling effect of water it condenses to liquid ammonia. This is allowed to pass slowly through the regulating valve, whereupon it[Pg 212] evaporates and expands in the long coiled pipe in "E" on its way back to the compressor. This evaporation and expansion causes a large amount of heat to be absorbed from the brine, cooling the latter below the freezing point of pure water and thus permitting the freezing of cans of water suspended in the brine. The chilled brine may also be sent through pipes in order to cool storage rooms containing meat or other food products. The ammonia absorbs heat when it vaporizes and gives up heat when it is compressed and liquified.

Important Topics

1. Heat of vaporization, of water 536 calories per gram.
2. Boiling point, effect of pressure upon boiling point, laws of boiling.
3. Distillation, artificial cooling, freezing mixtures, refrigeration by evaporation.

Exercises

1. How much heat is required (a) to melt 1 g. of ice at 0?C., (b) to raise the temperature of the water resulting to 100?C., (c) to change this water to steam?
2. If the water leaving a steam radiator is as hot as the steam how is the room warmed?
3. What is the effect of placing salt upon icy sidewalks in cold weather?
4. Is rain water distilled water? Is it perfectly pure?
5. What are two advantages of the high heat of vaporization of water?
6. If the heat from 1 g. of steam at 100?C. in changing to water and cooling to 0?C. could be used in melting ice at 0?C. how much ice would be melted?

(3) Heat and Work

191. Necessity for Heat Energy.?From early times man has been able to transform motion into heat, and has used this ability in many directions as in starting fires and[Pg 213] warming himself by friction. It took man many centuries, however, to devise an effective machine for transforming heat into mechanical energy or to use it in doing work.
The power of a man is small and as long as the work of the world had to be done by man power, progress was retarded. When man began the use of beasts of burden, he took a long step in advance since one man could then employ and direct the power of many men in the animals he controlled. Man also built water-wheels and windmills thus gaining power directly from the forces of nature and these added much to his working ability. But he took the greatest step in gaining control over his surroundings when he learned to use heat energy and to make it drive his machines.
192. Heat Engines.?At the present time there is a great variety of heat engines in use such as steam, hot air, gas, and gasoline engines, all using heat energy to produce motion. The expansive power of steam when confined has been observed for hundreds of years and many different machines have been invented to use it in doing work.
Fig. 158.?Cross-section view of cylinder and steam chest of a steam engine.
Fig. 159.?The steam drives the piston to the left.
Fig. 160.?External view of steam engine.
193. The Steam-engine.?The man who perfected the steam-engine, and devised its modern form was James Watt (1736-1819). The essential parts and the action of the steam engine may be readily understood by studying a diagram. In Fig. 158, S stands for steam chest, C for cylinder, P for piston and v for slide valve. The first two are hollow iron boxes, the latter are parts that slide back and forth within them. The action of the steam engine is as follows: Steam under pressure enters the steam chest, passes into the cylinder and pushes the piston to the other end. The slide valve is moved to its position in Fig. 159. Steam now enters the right end of the cylinder, driving the piston to the left, the "dead" steam in the left end of[Pg 214] the cylinder escaping at E to the air. The slide valve is now shifted to its first position and the process is repeated. It will assist the student to understand this action if he makes a cardboard model of these parts, the piston and slide valve being movable. In practical steam-engines, the piston rod is attached to a crank rod fastened to a crank which turns a wheel. (See Fig. 160.) The back and forth, or reciprocating motion of the piston is by this means transformed into rotary motion, just as in the sewing-machine the back-and-forth motion of the treadle produces rotary motion of the large wheel. Upon the shaft of the steam engine is fastened an eccentric (see Fig. 163) which[Pg 215] moves the slide valve. The steam engine acts continuously as long as steam is supplied to it. Since it shifts the position of the slide valve automatically, it is called an automatic steam engine. And because the team drives the piston both ways, it is called a double-acting steam engine. See Fig. 161 for a length-section of a modern locomotive.
194. The Mechanical Equivalent of Heat.?While watching workmen bore holes in cannon, Count Rumford, 1753-1814, noticed with much interest the large amount of heat produced in the process. He observed that the heat developed seemed to have some relation to the work done upon the drill in boring the holes. Later experiments performed by many men indicated that a definite relation exists between the heat produced by friction and the amount[Pg 216] of work done in overcoming the friction. This discovery indicates that in some way heat is related to energy and that heat is probably a form of energy. Later experiments have confirmed this idea, and it is now considered well established that heat is a form of energy. Many attempts have been made to discover the relation between the units of heat energy and the units of mechanical energy. To illustrate one method employed, suppose one measures a given length in inches and in centimeters; on dividing one result by the other, it will be found that a certain relation exists between the two sets of measurements, and that in every case that 1 in. equals 2.54 cm. Similarly, when the same amount of energy is measured both in heat units and in work units a constant relation is always found between the units employed. One B.T.U. is found equivalent to 778 ft.-lbs. 1 calorie being equivalent to 42,700 g. cm. (427 g. m.). This relation is called the mechanical equivalent of heat, or in other words it represents the number of work units equivalent to one heat unit.
Fig. 162.?Apparatus for determining the mechanical equivalent of heat.
William Gilbert (1540-1603), "Father of magnetic philosophy." Especially noted for his experiments and discoveries in magnetism; first to use the word "electricity." First man to practically emphasize experimental science.

Dr. William Gilbert
(Popular Science Monthly)
James Prescott Joule (1818-1889), England, determined the mechanical equivalent of heat; discovered the relation between an electric current and the heat produced; first proved experimentally the identity of various forms of energy.

James Prescott Joule
(Popular Science Monthly)
One of the first successful experiments in determining the relation between work units and heat units was devised by Joule in England. (See portrait p. 217.) The experiment consisted in taking a can of metal containing water (Fig. 162) in which was placed a thermometer, and a rod carrying paddles. The rod was turned by a cord connected through suitable apparatus to heavy weights, W and W. The energy represented by the downward[Pg 217]
[Pg 218]
[Pg 219]
motion of the weights through a given distance was compared with the heat energy developed in the water as shown by its rise in temperature. Careful experiments showed that when 778 ft.-lbs. of work had been done by the moving weights the heat produced at the same time would warm one pound of water 1 Fahrenheit degree. If the experiment was performed using metric units, it was found that the expenditure of 42,700 gram centimeters (427 gram meters) would result in producing enough heat to warm one gram of water one centigrade degree. The facts just given may be summarized as follows: 778 foot-pounds of energy are equivalent to 1 British thermal unit and 42,700 gram centimeters, or 427 gram meters, of energy are equivalent to 1 calorie. This relation of work units to heat units is called the mechanical equivalent of heat.
195. The Heat Equivalent of Fuels and Efficiency Tests of Engines.?To determine the efficiency of a steam engine it is necessary to know not only the mechanical equivalent of heat but also the heat produced by burning coal or gas; 1 lb. of average soft coal should produce about 12,600 B.t.u. Now since 778 ft.-lbs. are equivalent to one B.t.u. the energy produced when 2 lbs. of average soft coal is burned is 778 ? 12,600 ? 2 = 19,605,600 ft.-lbs. In actual practice 2 lbs. of average soft coal burned will develop about 1 horse-power for 1 hour. 1 horse-power-hour = 33,000 ft.-lbs. ? 60 = 1,980,000 ft.-lbs. Now efficiency equals (work out)/(work in) 1,980,000/19,605,600 = 1/10 or 10 per cent.. This is the efficiency of a good steam engine. Ordinary ones require 3 lbs. of coal burned to each horse-power-hour produced or they are but 2/3 as efficient or have but about 7 per cent. efficiency.
[Pg 220]
Heat of Combustion of Various Fuels
Data in this table are taken from U. S. Geological Survey, Bulletin No. 332, and U. S. Bureau of Mines, Bulletin No. 23.

B.T.U.  Calories

per lb. per gram
Alcohol, denatured 11,600 6,450
Coal, anthracite, average 12,600 7,500
Coal, bituminous, average 19,000 7,000
Gasoline 19,000 10,550
Illuminating gas 18,000 10,000
Kerosene 19,990 11,050
Constants for Heat Transmission
Data from "Ideal Fitter," American Radiator Co.
B.t.u. transmitted per square foot per hour per degree (Fahrenheit) difference in temperature between inside and outside air.
Brick work

4 in. thick = 0.68
8 in. thick = 0.46
12 in. thick = 0.33
concrete cement 50 per cent. more than brick.
stone 33-1/3 per cent. more than brick.

Window = 1.090
Wood as wall = 0.220
Double window = 0.560
concrete reinforced 20 per cent. more than brick.

Important Topics

1. Heat a manifestation of energy.
2. Steam-engine and its action.
3. Mechanical equivalent of heat and heat equivalent of fuels and efficiency of engines.

Exercises

1. Construct a working model of the cylinder and steam chest of a steam engine and be prepared to explain its action.
2. At $5.00 per ton how many B.T.U.'s should be produced from 1 cent's worth of bituminous coal?
[Pg 221]
3. Try the following experiment: Place a quart of water in a teakettle and place it over the fire for 5 minutes, and note the rise in temperature and compute the number of B.T.U.'s entering the water. Place another quart of water at the same temperature in an aluminum or tin dish and heat for 5 minutes, note the rise in temperature and compute the heat used before. Which of the dishes shows the greater efficiency? How do the efficiencies of the two dishes compare? How do you account for any differences in the efficiencies found?
4. How high would 8 cu. ft. of water be lifted if all of the energy produced by burning 1 lb. of coal were used in raising it?
5. What is the mechanical equivalent of a pound of coal expressed in horse-power hours?
6. If a furnace burns 100 lbs. of coal a day and its efficiency is 50 per cent. how many B.T.U.'s are used in warming the house?
7. How many B.T.U.'s can be obtained by burning 1/2 ton of bituminous coal?
8. when a pound of water is heated from 40?F. to 212?F., how many foot-pounds of energy are absorbed by the water?
9. How many loads of coal each weighing 2 tons, could be lifted 12 ft. by the energy put into the water in problem 8?
Fig. 163.?An eccentric.
10. When 3 cu. ft. of water are used for a hot bath and the water has been heated from 50?F. to 112?F., how many B.T.U.'s have been absorbed by the water?
11. If the average temperature of water at the surface of Lake Michigan is 50?F., how many B.T.U.'s would be given off by each cubic foot of water at the surface, if the temperature of the water should drop 5?F.?
12. In a cold storage plant carbon dioxide gas is used. The pipe[Pg 222] leading from the compression pump to the expansion valve passes through a condensing tank of cold water. Why?
13. When the gas is compressed in a cold storage plant, what becomes of the energy used by the compression pump?
14. An eccentric (Fig. 163), is a round disc mounted a little to one side of its center, A, on the engine shaft B. A band, C, on the circumference of the disc is connected by a rod, D, with the slide valve in the steam chest. How is the rotary motion of the shaft changed into a backward and forward motion of the slide valve?

(4) Heat Engines

196. The Gas Engine.?One of the heat engines in common use to-day is the gasoline engine. It is used to propel automobiles and motor boats, to drive machinery, etc. The construction and action of a gasoline engine may be understood by studying a working model, or by proper diagrams.
The common gasoline or gas engine is called a four-cycle (better four-part cycle) engine (see Fig. 164), since it requires[Pg 223] four movements of the piston to complete one cycle or series of changes. This is illustrated in Fig. 165 1, which represents a cross-section of the cylinder of the gasoline engine with the piston moving downward. At the upper end of the cylinder are two ports or openings. One, the exhaust port, is closed, the inlet port is open and a mixture of gas and air is entering. Fig. 165 2 shows the piston returning; both ports are closed and the "charge" of air and gas is being compressed. As the piston reaches the end of its stroke in compressing the charge, an electric spark explodes or "fires" the charge of gas and air. The hot burning gas expands suddenly driving the piston downward with great force (Fig. 165 3). The piston rod is attached to the crank of a heavy fly-wheel and this is given sufficient energy or momentum to keep it going through the next three strokes. Fig. 165 4 represents the returning piston pushing out the burnt "charge" through the open exhaust valve e. On the next downward motion of the piston the valve e closes. It opens, and new charges of gas and air enter and the "cycle" is repeated.
Fig. 165.?The four strokes of a gas engine cycle.
In order to make the motion more even and continuous and also to secure more power, more than one cylinder is attached to the same shaft and fly-wheel. Two, three,[Pg 224] four, six, eight and even more cylinders have been attached to one shaft. Four or six cylinders are commonly used in automobile gasoline motors. To lessen the sound of the "exhaust," the latter is sent through a "muffler" which often reduces the noise to a low throbbing. (See Fig. 166.) The gasoline engine is more efficient than the steam-engine, since the fuel, gas, is burned in the cylinder and not in a separate furnace. The combustion of the fuel in the cylinders makes some special cooling device necessary to prevent their overheating. This usually consists of a casing about the cylinders. Between the cylinder and this casing is water which on being heated passes to a tank or radiator. In the radiator the water cools and then returns to the space between the cylinders and casing thus keeping up the circulation.
Fig. 166.?An efficient automobile muffler. (Courtesy Popular Science Monthly.)
197. Efficiency of Gas Engines.?One may test the efficiency of a gas engine by determining the amount of power developed and comparing it with the mechanical equivalent of the fuel burned. Illuminating gas is sometimes employed to drive gas engines. One cubic foot of illuminating gas should produce 600 B.T.U. when burned. The efficiency of the gas or gasoline engines is sometimes as high as 25 per cent. This engine is free from smoke and is also compact and quickly started. While the fuel, gas or gasoline, is somewhat expensive it is light and easily[Pg 225] carried. Suppose a gas engine produces 1 horse-power and uses 20 cu. ft. of gas an hour, what is its efficiency? 1 horse-power-hour = 550 ? 60 ? 60 = 1,980,000 ft.-lbs. 20 cu. ft. of gas = 20 ? 600 ? 778 = 9,336,000 ft.-lbs.
Efficiency = work out/work in = 1,980,000/9,336,000 = 21.2 per cent.
Fig. 167.?The principle of the steam turbine.
Fig. 168.?Path of steam in DeLaval steam turbine. (a) and (c) movable blades, (b) stationary.
198. The Steam Turbine. One form of the steam-engine that is coming into general use is the turbine. (See Fig. 167.) This consists of a shaft to which are attached blades, the shaft and blades being contained in a closed case. Steam is admitted by nozzles and strikes the blades so as to set them and the shaft in motion. There are also stationary blades (see Fig. 168), which assist in directing the steam effectively against the rotating parts. The[Pg 226] steam turbine is used for large power plants. (See Fig. 293.) It is very efficient, makes very little vibration, and occupies about one-tenth the floor space that a reciprocating engine of equal power uses. Some large ocean steamers are now driven by steam turbines.

Important Topics

1. The gas engine, its construction, action and efficiency.
2. The steam turbine.

Exercises

1. If coal costs $4.00 a ton, and gas, $0.80 per 1000 cu. ft. what amounts of heat can be secured from 1 cent's worth of each?
2. What will it cost to heat 30 gallons of water (1 gal. of water weighs about 8-1/3 lbs.) from 40?F. to 190?F. with coal costing $4.00 per ton and yielding 12,000 B.T U. per lb. if the heater has an efficiency of 50 per cent.
3. What will it cost to heat 30 gallons of water from 40?F. to 190?F. with gas at $0.80 per 1000 cu. ft. if the heating device has an efficiency of 75 per cent.
4. Construct a cardboard working-model showing the action of the gas engine and be prepared to explain the action of the various parts.
5. If 500 lbs. of iron should fall 2000 ft. and all of the resulting mechanical kinetic energy should be transformed into heat, what would be the amount of heat produced?
6. What are the special advantages of (a) the gasoline engine? (b) the turbine? (c) the reciprocating steam engine?
7. Do you burn coal or gas in your kitchen stove at home? Which is for you the more economical? Why?
8. What are the advantages of using a fireless cooker?
9. What is the efficiency of a locomotive that burns 3.2 lbs. of coal per horse-power-hour?
10. A gas engine developed in a test 0.34 horse-power for 1 minute. and 50 seconds, 0.5 cu. ft. of gas being used. The heat of combustion of the gas was 600 B.T.U. per cu. ft. Find the efficiency of the engine.
11. Find the horse-power of an engine, the diameter of the piston being 19 in., stroke 26 in.; it uses steam at an average pressure of 200 lbs. per square. inch and makes 100 strokes a minute.
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12. What is the efficiency of an engine and boiler that develops 200 horse-power, while burning 390 lbs. of soft coal per hour?
13. If a locomotive has an efficiency of 6 per cent. and develops 1700 horse-power how much coal is burned in an hour?
14. If an automobile engine burns 1 gallon of gasoline in an hour and develops 10 horse-power, what is its efficiency?
15. The A.L.A.M.[J] formula for horse-power is (N B2)/2.5 when the piston speed is 1000 ft. per minute, N being the number of cylinders and B, their diameter. Find the horse-power of a 4-cylinder engine, the cylinders having a diameter of 4 in.
16. Find the horse-power of a 6-cylinder automobile engine, if the cylinder diameter is 4.5 in.
17. A 4-cylinder automobile having 4-in. cylinders, uses 1 gallon of gasoline in 1 hour. Find its efficiency, if its average horse-power developed is 6.
18. The motor boat Disturber III, has 24 cylinders each with diameter 3.5 in. If the piston speed is 1000 ft. per minute, what is the horse-power? (See problem 15.)

Review Outline: Heat

Heat; sources (4), effects (5), units (2).
Temperature; thermometer scales (3), absolute temperature, 9C?/5 + 32? = F?.
Expansion; gases, Law of Charles (V1/V2 = T1/T2), liquids, peculiarity of water, solids, coefficient of expansion, uses, results.
Heat Transference; conduction, uses of good and poor conductors, convection, in nature, heating and ventilating systems, radiation, 3 peculiarities, value of sun's radiation.
Heat and Moisture; relative humidity, dew point, formation of dew, fog, rain, snow, etc., evaporation, effects, conditions.
Heat Measurement; specific heat, heat of fusion, of vaporization, combustion.
Vaporization; Boiling point, laws of boiling, distillation, artificial cooling.
Heat Engines; steam, gas.?construction, action, efficiency, mechanical equivalent of heat. Heat equivalent of fuels.

 

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