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 | Specific
| gravity | 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
---------+----------+----------
=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 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 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. 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.
[Illustration: FIG. 151.--Ice crystals.]
[Illustration: 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 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
[Illustration: 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 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._ 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.
[Illustration: 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 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.
[Illustration: 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°.
[Illustration: FIG. 156.--A vacuum pan.]
=187. Distillation of Water.=--Usually when solids are dissolved in
liquids the vapor coming from the liquid contains 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 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 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.)
[Illustration: 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 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 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.
[Illustration: FIG. 158.--Cross-section view of cylinder and steam chest
of a steam engine.]
[Illustration: FIG. 159.--The steam drives the piston to the left.]
[Illustration: 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 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 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.
[Illustration: FIG. 161.--Length-section of modern, fast-passenger
locomotive. _A_, cylinder valve--piston type valve; _B_,
cylinder--piston at out end of stroke; _C_, boiler tubes--flues from
fire-box; _D_, fire-tube type superheater; _E_, draught screen; _F-A_,
fire-brick arch to protect tubes from direct heat; _F-B_, firebox; _G_,
grate; _H_, exhaust nozzle; _I_, safety valve nest; _T_, throttle lever;
_R_, throttle rod; _Y_, throttle valve.]
=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 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.
[Illustration: FIG. 162.--Apparatus for determining the mechanical
equivalent of heat.]
[Illustration: 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)]
[Illustration: 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 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.
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 { concrete }
8 in. thick = 0.46 { cement } 50 per cent. more than brick.
12 in. thick = 0.33 {
stone 33-1/3 per cent. more than brick.
Window = 1.090 {
Wood as wall = 0.220 { concrete } 20 per cent. more than
Double window = 0.560 { reinforced } 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?
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?
[Illustration: 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
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.
[Illustration: FIG. 164.--Cut away view of a modern automobile engine,
with parts requiring attention most frequently, indicated. (Courtesy of
the "Automobile Journal")]
The common gasoline or gas engine is called a four-cycle (better
four-part cycle) engine (see Fig. 164), since it requires 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.
[Illustration: 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, 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.
[Illustration: 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 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.
[Illustration: FIG. 167.--The principle of the steam turbine.]
[Illustration: 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 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.
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_ _B_²)/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.
[J] American League of Automobile Manufacturers.
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 (V_{1}/V_{2} = T_{1}/T_{2}), 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.