APPLIED MECHANICS. 



[THE FEED-PI-MF. 



1 . (>i von the length of stroke to find the speed. 

 Kult. Divide 100 by the stroke (in feet), the quotient 



is the number of revolution* per minute. 



JSfomfJt. What is the speed of an engine having 2 ft. 

 6 ins. stroke t 



- 40 revolutions per minute. 



2. Given the speed to find the stroke. 



JfoJe. Divide 100 by the number of revolutions per 

 minute, the quotient is the length of stroke in feet. 



/<lr. What must be the stroke of an engine 

 making 33 revolutions per minute 1 



* - 2 80 feet, or about 2 ft. 10 ins. 



:',. . 



STEAM PASSAGES. The dimensions of the steam 

 passages should be proportioned to the area of the 

 cylinder; for while the piston travels at its quickest 

 peed, increasing rapidly the space to be filled with 

 team, the passages should admit the steam with suffi- 

 cient velocity to sustain the pressure on the retreating 

 piston. It is found, practically, that the area of the 

 steam-pipe may be advantageously jj^th of that of the 

 cylinder, or the diameter of the one ,', ; !i of that of the 

 other. Thus, for an engine having a cylinder 30 ins. 

 diameter, the steam-pipe should be 5 ins. diameter. 

 When it is intended that the piston should travel more 

 rapidly than the average rate of 200 feet per minute, 

 the steam-pipe should be proportionally large. In such 

 oases, its diameter may be advantageously reckoned at 

 Jth of the diameter of the cylinder. The ports which 

 admit the steam into the cylinder should always present 

 an area of passage considerably greater than that of the 

 steam-pipe, for during a great part of every stroke they 

 are partially closed by the slide. The exhaust-pipe, 

 wliich conveys the steam from the cylinder, should be 

 larger than the steam-pipe ; for the waste steam should 

 be permitted to become rapidly expanded in volume, in 

 order that its back pressure ou the piston may be di- 

 minished as much as possible. 



FEED-PUMP. The size of the feed-pump should al- 

 ways be greatly in excess of that which is absolutely re- 

 quired for supplying the amount of water boiled off in steam 

 for the engine . Occasionally, the valves of the pump leak ; 

 there may be leaks in the boiler ; some of the water may 

 pass over in priming ; and a good deal may be wasted in 

 blowing off. The pump should, therefore, be capable of 

 supplying at least 3 times as much water as is actually 

 due to the steam supplied to the cylinder. The pump is 



generally arranged so as to make one stroke for each 

 revolution of the engine, or each double stroke of the 

 piston. If, then, wo take a case whore the diameter of 

 the rvliii'l.T in 30 inches, the stroke 4 feet, and the 

 average pressure 30 Ibs. above that of the atmosphere 

 that is to say, the steam at 3 atmospheres we find that, 

 during each revolution, the cylinder, twice filled with 

 steam, uses a volume of about 6S,0<>0 cubic im-ln-i. 

 Steam at 1 atmosphere of pressure being about 1,000 

 times the volume of water, its volume at 3 atmospheres 

 is Jrd of that, or about 533 times that of water. The 

 water necessary to generate 68,000 cubic inches of steam 

 at 30 Ibs. pressure, therefore, amounts to 128 cubic 

 inches ; and as the feed-pump should be capable of 

 supplying thrice this quantity, it must deliver at each 

 stroke 384 cubic inches. Should we make the stroke of 

 the pump 2 feet, half that of the piston, its area must be 



OQJ 



10 square inches, or its diameter 4 inches. 



As a general rule, for the dimensions of the feed- 

 pump in non-condensing engines, we may offer the fol- 

 lowing : 



Multiply the square of the diameter of the cylinder 

 (in inches) by the length of stroke (in foot), divide by 90, 

 and the quotient is the product of the square of the 

 diameter (in inches) of the pump, by its length of stroke 

 (in feet). 



Example. Required the size of the feed-pump for an 

 engine having a cylinder 30 inches diameter, and a stroke 

 of 4 feet. 



22L= = 40, the product of diameter squared 

 by stroke. If we take the stroke of the pump 2 feet, 



then = 20 is the square of the diameter, or the dia- 

 2 



meter is about 4J inches, because 4J X 4J X 2 = 40 

 nearly. 



Many treatises on the steam-engine give rules for the 

 dimensions of all the principal parts of an engine. We 

 think, however, that these rules are not, in many cases, 

 practically available, because a difference in general 

 design and arrangement, or in average pressure and 

 speed, occasions very considerable variations in the pro- 

 portions of the parts. Careful study of well-made 

 engines, and actual experience in their construction and 

 working, form the true sources of information as to 

 their due proportions. 



CHAPTER VI. 



THE STEAM-ENGINE (Continued). 



Contents. CONDENSING F.NOINBS SAVERY'S NEWCOMBN'S WATT'S IMPROVEMENTS CONDENSER AND AIR-PUMP 



INJECTION SURFACE CONURNSBR PARALLEL MOTION BKAM ENGINE MAKINB ENGINE, PADDLE, 8CRKW 



COMBINED BNOINtH ROTARY ENGINES SUPER-HEATING STEAM APPLICATIONS OF STEAM POWER PUMPING 



DRIVING MACHINERY LOCOMOTION PROPULSION OF VESSELS. 



THE Condensing Engine., in all main points, resembles 

 the non-condensing engine ; but it requires some ad- 

 ditional parts, in order that the vacuum produced by 

 condensing the steam may be employed as a source of 

 additional power. If we suppose that the steam, on 

 leaving the bottom of the cylinder, instead of flowing out 

 into the atmosphere, which resists its egress with a 

 pressure of 15 llw. per square inch, wore conducted into 

 a vesiwl totally void of air or steam, this resisting force 

 would be entirely removed, and the effect of the Hiram 

 prenning on the upper side of the piston, would be in- 

 creased by that quantity. If the vacuum in the vessel 

 were not perfect that is to say, if there were contained 

 in it some rare fluid, such as air or steam, or a mixture 

 nf tioth, greatly sMHNttitd, and capable of pressing with 

 a forco of only '2 or 3 Ibs. on the square inch the pres- 

 sure of the iiteam on the piston would be increased by a 



quantity 2 or 3 Ibs. less than 15 Ibs. per square inch. 

 Generally, if we reckon the pressure of steam in the 

 boiler as its absolute pressure, not its excess over the 

 atmosphere, and deduct the pressure of fluid in the 

 vacuum vessel, the difference will be the effective pressure 

 on the piston. Thus, with steam in the cylinder ex- 

 erting a pressure of 10 Ibs. above that of the atmosphere, 

 or having an absolute pressure of 25 Ibs. per square inch, 

 wliile the vacuum vessel contains a fluid pressing with a 

 force of 2 Ibs. per square inch, the eflective pressure on 

 the piston is 25 2 = 23 Ibs. per Hqtiare incli. The 

 condition of the fluid in the vacuum vessel, as to pressure, 

 is generally measured by a barometer. 



Tlio upper part of tho vessel A (Fig. 202) is connected 

 with a glass tube C, about :>0 inch.-; long, dipping into a 

 cup of mercury B. Were tho space in A an :ili.<ohit.ii 

 vacuum, tho atmospheric pressure on the nurfaco 



