Physics Part 35

[Ill.u.s.tration: FIG. 159.--The steam drives the piston to the left.]

[Ill.u.s.tration: 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, pa.s.ses 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 s.h.i.+fted to its first position and the process is repeated. It will a.s.sist 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 s.h.i.+fts 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.

[Ill.u.s.tration: FIG. 161.--Length-section of modern, fast-pa.s.senger 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 ill.u.s.trate 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.

[Ill.u.s.tration: FIG. 162.--Apparatus for determining the mechanical equivalent of heat.]

[Ill.u.s.tration: 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)]

[Ill.u.s.tration: 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 ident.i.ty 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 40F. to 212F., 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?

[Ill.u.s.tration: FIG. 163.--An eccentric.]

10. When 3 cu. ft. of water are used for a hot bath and the water has been heated from 50F. to 112F., 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 50F., 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 5F.?

12. In a cold storage plant carbon dioxide gas is used. The pipe leading from the compression pump to the expansion valve pa.s.ses 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 circ.u.mference 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.

[Ill.u.s.tration: FIG. 164.--Cut away view of a modern automobile engine, with parts requiring attention most frequently, indicated. (Courtesy of the "Automobile Journal")]