CHAMBERS'S INFORMATION FOR THE PEOPLE. 



latter, and combines with the former, which it 

 takes from the air. This combination generates 

 intense heat, which is freely communicated to the 

 carbon particles just disengaged from the hydro- 

 gen, and at this temperature the carbon is able to 

 combine with the oxygen to form carbonic acid. 

 The efficiency of the combustion depends mainly 

 on the completeness of this operation, for should 

 the carbon particles not come in contact with the 

 oxygen until they have lost temperature, they 

 cannot combine, and will consequently pass away 

 as black smoke ; or if the supply of air is insuffi- 

 cient, carbonic oxide will be formed, and this 

 combination yields only one-third the heat pro- 

 duced by the production of carbonic acid. 



The standard used to compare the evaporative 

 power of different fuels is the number of pounds 

 of water one pound of each of them will evaporate. 

 The water is supposed to have been previously 

 heated to 212, and to remain at that temperature 

 during evaporation. The theoretical evaporative 

 power of i Ib. of average coal (according to this 

 standard) is 14^ Ibs. of water, and of i Ib. of 

 average coke, 13^ Ibs. The actual evaporation 

 obtained varies very much, being dependent on 

 the form and arrangement of the boiler and flues, 

 the quality of fuel, and the care and attention of 

 the stoker. In ordinary factory boilers, it is often 

 as low as 6 or 7 Ibs. ; in locomotive and marine 

 engines, from 9 to 10 Ibs. We have seen, how- 

 ever, from 12 to 13 Ibs. water evaporated by first- 

 rate fuel in very good examples both of stationary 

 and marine boilers. When small coal, or dross, is 

 used in Cornish boilers of ordinary construction, 

 the evaporation we have sometimes found to be 

 as low as 3 Ibs. of water per pound of fuel. 



We have already said that steam is water (in a 

 gaseous condition), plus a large amount of latent 

 heat The whole of this heat has to be communi- 

 cated to it by the fuel, just as much as the sensible 

 heat. The latent heat of steam at 212 is equiv- 

 alent to about 996 F. The sensible heat, how- 

 ever, between 32 and 212 F. is only 180, so 

 that its latent heat is 5^ times its sensible heat 

 above 32. In other words, if any given weight 

 of fuel be required to raise a certain quantity of 

 water from 32 to 212, then it will require 5^ 

 times that quantity to convert the water into steam 

 at that temperature. 



We have above referred to the oxygen of the 

 air as being an essential to combustion. This 

 gas constitutes about one-fifth by bulk of the air ; 

 and as i Ib. of fuel requires (on the average) T.\ 

 Ibs. (or 30 cubic feet) of oxygen for its perfect 

 combustion, about 150 cubic feet of air must be 

 admitted into the furnace for each pound of fuel 

 burned. This is the amount chemically required, 

 but as much air passes through the furnace with- 

 out assisting the combustion, it is found necessary 

 in practice to admit 50 or 100 per cent. more. 



The proportion which the area of the fire-grate 

 bears to the total heating surface, as well as to the 

 amount of fuel consumed, is of the utmost im- 

 portance to the efficiency of the boiler. The 

 boilers of Cornish engines have a large grate 

 surface, and burn the fuel very slowly, the lowest 

 consumption reached being about 4 Ibs.* coal per 

 square foot of grate per hour. Ordinary factory 

 boilers burn about 15, and marine boilers about 



* Some authorities state a to 2$ Ibs., but this seems doubtful 

 420 



20 Ibs. coal per foot of grate per hour. The 

 draught induced by a chimney does not admit of 

 a larger consumption than about 30 Ibs.; but in 

 locomotive boilers, where there is an artificial 

 draught, small grate surface, and very quick com- 

 bustion, 70 or 80 Ibs. of fuel are commonly 

 burned, and the consumption is sometimes far 

 greater. With grates not burning more than. 

 20 Ibs. of coal, the whole of the air is admitted 

 through the spaces between the fire-bars ; but 

 always in locomotives, and generally in marine 

 boilers, a certain amount of air has to be admitted 

 above the bars. As long as the boilers are 

 properly designed and proportioned, there seems 

 to be no difference in economy between fast and 

 slow combustion ; but one is more convenient in 

 some cases, and the other in others, and the 

 particular circumstances of each case decide the 

 kind of boiler to be used. 



Many experiments have been made to deter- 

 mine the relative economic value of the different 

 kinds of fuel, and most valuable tables giving the 

 detailed results of these are published. Welsh 

 coals and anthracites stand at the head of British 

 fuels, then Lancashire and Newcastle coals, and 

 lastly Scotch coal ; the last being deficient in 

 carbon, and too rich in combined oxygen. The 

 quality of coal used at a factory must be regu- 

 lated by a consideration of its relative price and 

 economic value. In steam-ships, superior qualities 

 are generally used, because in them the space 

 taken up by the coal is important, as every cubic 

 foot saved from the coal-space can be made use 

 of for cargo. Coke makes less smoke than coal, 

 and is found more efficient where great local 

 intensity of heat is required ; coal, however, is 

 better where the combustion has actually to be 

 developed and carried on through flues, &c. 

 at some considerable distance from the furnace. 



We must now proceed to the consideration of 

 the apparatus used in the generation of steam 

 namely, boilers and furnaces. 



The oldest form of boiler used was probably the 

 sphere ; this was superseded by a cylinder set on 

 end, and this further improved by making its 

 bottom concave, and its top hemispherical. Watt's 

 ' wagon ' boiler superseded these, and continued 

 in use for many years. 



Smeaton seems to have been the first to make 

 a boiler with an internal furnace, although his 

 arrangement has long been superseded. As the 

 working steam pressures came to be increased, it 

 became evident that that form of boiler which 

 required the fewest internal stays to prevent its 

 bursting had many advantages over any other. 

 The spherical form is the strongest of all, but has 

 the disadvantage that the heating surface is very 

 small indeed compared to the volume of water 

 which it holds. Next to the sphere (as to strength 

 against bursting) comes the cylinder, and this 

 form is now universally used for boilers that have 

 to withstand any considerable pressure. Fig. 2 

 shews a section of a ' Cornish ' boiler, an invention 

 of Trevithick's which holds its ground to the 

 present day, and seems likely to do so for many 

 years to come. This boiler is simplicity itself, though 

 heavy and bulky. With care, it can be made to- 

 give very good evaporative results ; and having 

 thus fair economy with maximum strength and 

 simplicity, and minimum liability of getting out 

 of order, as well as small first cost, it is preferred 



