an 



APPLIED MEC1IANICS. 



[THE CONDENSER. 



pi*Um, 



of escaping into the atmosphere as in 



non-condensing engines, flows by this pipe into the con- 

 denser, which is placed in a cistern of cold water, and 

 has a pipe and cock, I, for the admission of a jut of cold 

 water to condense the steam. This jet is called the in- 

 jection, and its quantity is regulated by means of the 

 cook, worked by a rod passing upwards, with a handle 

 in some place convenient for the attendant. The bottom 

 of the condenser communicates by a passage, having a 

 valve G, with the air-pump A. The bucket or piston P 

 of the air-pump is fitted with valves opening upwards, 

 ami is moved upwards and downwards by a rod connected 

 with some reciprocating part of the engine, and passing 

 through a stuffing-box in the air-pump cover. Near 

 the top of the air-pump there is a passage Q, fitted with 

 a discharge valve opening into the hot-well K, from which 

 the feed-water is pumped to the boiler, the overplus or 

 waste Ixsing discharged by a waste-pipe. The cistern in 

 which tho BOBtMOMt .'iinl air-pump are placed, is kept 

 'antly filled with cold water by a pump called the 

 oil I n;it,'i- jnim/i, supplying it by a pipe N at tho bottom, 

 while the heated water overflows by a suitable waste- 

 pipe. While the steam from the cylinder flows into the 

 cold condenser, and meets the cold water diffused through 

 it by the injection jet, it becomes condensed into water, 

 and falls with the injection-water to the bottom, occupy- 

 ing very little volume compared with that which it 

 occupied while in the state of steam, and leaving the 

 pace of the condenser as a partial vacuum. But water 

 always contains a quantity of air mingled with it, which 

 passes over with the steam from the boiler to the 

 cylinder, and thence to the condenser, and the injection- 

 water also j>arts with a portion of the air it contains, so 

 that after a time the condenser would become filled with 

 the air so liberated, and with the water of injection and 

 condensation, unless means were taken to remove 

 them. 



As the air-pump bucket descends, the dischar<.e-valve 

 Q being closed, no air or water am enter from above to 

 fill the space left void by tho descent of the bucket, but 

 tho air and water from tho condenser pass through tho 

 valves G and H, and enter this space from In-low. On 

 the ascent of the bucket, the bucket- valves P being 

 closed by the pressure of air ami water alxive them, the 

 roiit.-nts of tin; pump are discharged through tho valvi-s 

 Q into tho hot well. Thus by the alternate descent and 

 ascent of the air-pump bucket, when its capacity and 

 thi! amount of injection are properly proportioned, 

 almost a iwrfoct vacuum is maintained in the eoiid. 

 anil the effective pressure of the steam on every sr 

 inch of tliu piston is increaseil |.\ 

 that which it would be, were tho stojitn permit! 



rutii thu atmosphere. 



ridrnwr is g.'iu-rally littrd with a lloic-valve H, 



which comes into play on starting the engine thus. Tho 



MI- having UH.-II stopped, ti. ,| air-pump 



limy haw become quite filled with water through the 



injection-cock ; and on starting the engine again, no 

 vacuum could be produced while they are thus ir 



'. But by opening a small valve called the blow- 

 Viruuyh talrr, a communication is made betwcvu the 

 team-pipe from the boiler, and the eduction-pipe leading 

 from the cylinder to tho condenser. The pressure of 

 the steam in the boiler is thus brought to bear on tho 

 water in the condenser, and forces it out by the blow- 

 valve H, the steam occupying its place. On shutting the 

 blow-through valve and opening the injection-cock, the 

 steam in the condenser is condensed, and a vacuum 

 formed, so that the engine may be started. As long as 

 there is even a partial vacuum in tho condenser, 

 the atmospheric pressure on the blow-valve keeps it 

 closed. 



The bottom-valve, bucket-valves, and discharge-valves 

 of the air-pump, are frequently made of vulcanised 

 caoutchouc sheet, cut into discs and laid over gratings. 

 Water or air forced through the perforations in the 

 gratings, raises the flexible discs of caoutchouc, and 

 passes round their edges ; but neither air nor water can 

 return, for tho atmospheric pressure forces the caoutchouc 

 discs firmly down on the gratings, and thereby effectually 

 closes their openings. 



INJECTION -It is a peculiar property of all vapours, 

 that, besides their sensible heat, or tho temperature to 

 which they raise the thermometer, they contain a great 

 amount of latent heat, not measured by tho thermometer, 

 but by its effect, when tho condition of the vapour is 

 changed. The latent heat of steam, when its tempera! uro 

 or sensible heat is 212, is estimated to be about 1,000. 

 This does not mean that the latent heat could raise a ther- 

 mometer 1,000, but simply that a pound of steam at 212 

 being condensed by its mixture with 1,000 Ibs. of water 

 at any temperature, such as G0, could raise the tem- 

 perature of the whole mass of water 1. In other words, 

 if it were found that the combustion of a certain weight 

 of fuel could raise the temperature of a given mass of 

 water from 211 to 212, it would require 1,000 times 

 that quantity of fuel to convert the water into steam, 

 having still the sensible temperature of 212. This great 

 latent heat is something essential to the condition of 

 water in a state of vapour ; for as soon as any portion of 

 it is removed, by bringing the steam into contact with a 

 cold substance, a part of the steam is immediately con- 

 densed into water ; and the remainder, expanding to fill 

 the space thus left void, loses density and pressure as it 

 gains volume. In estimating the quantity of injection- 

 water necessary for condensing the steam of an engine, 

 we must therefore bear in mind that it is not alone the 

 sensible temperature, but also the latent heat of tho 

 steam which we have to absorb by the cold water in- 

 jected. Let us assume that 1 cubic foot of water, having 

 been converted into steam in the boiler, and having acted 

 on the piston in the cylinder, flows into tho condenser 

 at a temperature of 212, and containing 1,000 of latent 

 heat, and that it there mingles with a quantity of water 

 at r<J ', suuiciciit to condense it and produce an ultimate 

 temperature of 112 throughout the mixture. The total 

 heat of tho steam being 1,212, has to be reduced to 

 112 that is, the steam has to lose 1,100 of temperature; 

 the injection-water entering at 02, and being raised to 

 112, has to gain 50. The quantity of injection-water 

 must therefore bo 22 cubic foot ; for 22 cubic feet raised 

 >ro equivalent to 1 cubic foot reduced 1,100, because 

 22 X 50=1100. 



The temperatures wo have assumed are such as would 

 frequently occur in practice ; and generally it will be 

 found that the quantity of injection-water required for 

 an engine is from 15 to 25 times the quantity required 

 for feed. 



Tho capacity of the air-pump is generally Jth of that 



of the cylinder, tho stroke being usually J, and the dia- 



C i that of the cylinder. The power necessary to 



work tho air-pump of a condensing engine is about j'jjth 



of tho total po,-. 



SURFACE-CONDENSER. Condensation by injec- 

 tion, such as has been d. >ove, is the system 

 that lias been almost universally adopted since Newcuiueu 





