1839.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



265 



waggon and the locomotive, tend to show that they rank, according to 

 their respective merits, in the order in which they stand, the Cornish 

 beinpf greatly superior to the two others. 



With respect to the actual economy of the Cornish boiler, we cannot 

 but concur in Mr. Parkes' opinion, though we should explain it dif- 

 ferently. Instead of saying that the slow rate of combustion " invoices 

 the necessity of employing a very e.xtensive surface, or proportion of 

 boiler to evaporation," we should say, that a more extensive evapo- 

 rating surface absorbs more caloric from the gases and vapours before 

 they arrive at the chimney, and thus admits of a slower rate of com- 

 bustion. We are not informed of the thickness of coal on the grate 

 in any of the experiments referred to ; but if we suppose it to have 

 been the same in all, the rate of combustion will be inversely as the 

 area of the grate, and directly as the quantity of fuel burned per hour ; 

 or directly as the quantity burned per hour on each square foot of 

 grate. But if the thickness of the layer of coal is not the same, the 

 rate of combustion is directly as the quantity of coal burned per hour 

 on each square foot of grate, and inversely as the thickness of the 

 stratum of coal on the grate. Mr. Parkes states in the note at the 

 foot of page 22, that he found thickness of fuel far more economical 

 than an excessive extent of grate surface, thus pointing out the eco- 

 nomical effect of a circumstance, which he has nevertheless not 

 included in what he calls the exponents. 



In that part of his paper in which he treats of the waggon boiler, the 

 author professes, on his own behalf and that of the profession generally, 

 absolute ignorance of the rate at which heat is transmissible through 

 metal of varying thickness, as well as the rate at which it is absorb, 

 able by water at different temperatures (see page 13). If this admis- 

 sion is well founded, it is much to be regretted that so important a 

 part of the investigation should have been omitted in his experiments, 

 and that he should not have delayed the publication of his paper until 

 he could have resolved the difficulty. We are inclined to think that 

 he will find, should he be disposed to pursue the inquiry, that the 

 rate at which heat is transmissible is very nearly directly as the differ- 

 ence between the temperatures of the gases in the flue, and of the 

 water in the boiler, and inversely as the thickness of the plate. Un. 

 fortunately Mr. Parkes has left us entirely in the dark as to the tem- 

 perature of the gases in the flues, so that we cannot with any degree 

 of certainty institute a comparison of the evaporative results with 

 regard to that circumstance ; but if we suppose the mean temperature 

 in the flues to be 800'~', while that of the water is in the one case 300*^, 

 and in the other only 220", the thickness of the plate in the former 

 case being double that in the latter, we shall find the transmission of 

 heat through equal areas of plate to be in the ratio — - to 580, or as 

 25 is to 58 ; or, the transmission would be equal if the surfaces were 

 as 58 to 25. Now it appears that the Cornish engineers allow 7 times 

 as much surface as the general waggon boiler practice for the vapori- 

 zation of equal weights of water in equal times, or in the proportion 

 of 175 to 25, which is 3 times as much as would be necessary, under 

 the conditions assumed above, to evaporate the same quantity of 

 water. The mean difference of temperature should therefore be 

 reduced to one-third, or 467°. The consequence must be, that the 

 heated air will reach the chimney, with an excess of temperature over 

 that of the water in the boiler less by at least two-thirds than in the 

 waggon boiler ; and, the same quantity of heated air having thus a 

 greater effect, less fuel will be burned to produce only the same effect, 

 as we have already explained. It must be remembered that the eva- 

 poration will not be increased in the ratio of the heated surface when 

 an equal quantity of coal is burned, the increasing evaporation dimi- 

 nishing the temperature of the hot air, for which reason the plate with 

 which it comes afterwards in contact will not be heated to so high a 

 temperature, and will therefore not evaporate so much water as an 

 equal area of the other parts of the plate, whose temperature is more 

 elevated. 



Compared to the Warwick boiler, the Cornish has only 275 times as 

 much heated surface as would evaporate the same quantity of water, 

 the temperature of the plate being the same; and if we take into con- 

 sideration that its temperature in the Cornish must have been on an 

 average nmch lower than in the Warwick, in consequence of the 

 greater extent of surface in proportion to the fuel burned, the saving 

 will not appear so disproportioned to the increased area as Mr. 

 Parkes seems to think by the following expression in page 13 : " We 

 must not conclude that it requires 7 times as great a surface exposed 

 to heat U7ider Hie circumstances, to realise an additional product of 

 only 22 per cent, from fuel." It should also be remembered that the 

 surface is here considered with reference to the fuel consumed instead 

 of the water evaporated, which would have been the fairest comparison. 

 But we certainly cannot, under any circumstances, expect the saving 



of fuel to be proportional, or nearly so, to the increased surface for 

 transmitting the heat. 



We cannot agree with the author, that the gain of 41i percent, in 

 the effect of fuel is " miserably small, compared with the strides made 

 in the economy of steam," nor is it at all necessary to conclude, from 

 this inconsiderable economy, " that our methods of generating heat 

 and steam, and of constructing evaporative vessels, have attained the 

 utmost perfection which the strict laws of nature and the limited 

 ingenuity of man forbid us from passing." 



We now come to the most important part of the paper, where the 

 author discusses the various circumstances which affect the results 

 obtained in the different boilers, and compares them numerically. 

 It is intituled 



** An investigation of the relative time during; which the products of com- 

 bustion, from equal weights of fuel, continue in operation on equal areas 

 of the surface of the boilers j with an estimato of the quantity and intensity 

 of heat applied to thera." 



We shall discuss the several propositions of this investigation 

 seriatim, and endeavour to show whether they are based on sound 

 principles or not. The chapter commences with the following 

 words : — 



" The structure of the parts, and the mode of setting a boiler, occasion the 

 heat applied to it to travel greater or less distances, and to pass over very 

 unequal extents of surface, in equal or unequal* times. The distances tra- 

 velled I shall consider as determined by the length of the circuit which the 

 heat is compelled to traverse from the grate till it quits tlie boiler. The time 

 in which it performs the circuit is tlie period of the duration of a particle 

 of heat about the boiler, and is the first question to be considered. 



" The rate of combustion, or the time in which a pound of fuel is burned, 

 seems to me to be the best practical measure of the velocity of the products of 

 that combustion about a boiler. The mind readily apprehends, that if a 

 pound of coal be consumed under one boiler, in half the time that it is con- 

 sumed under another, the velocity of the current must be twice as rapid in 

 the one case as in the other ; but if the velocity be expressed in feet per 

 minute, or miles per hour, no information is conveyed of an appreciable or 

 practical nature ; nor does that expression reach the source or origin of tlie 

 current, viz , the rate of combustion.'' 



We must protest against this mode of estimating the velocity of the 

 products of combustion, as affording no measure whatever of that 

 velocity in boilers of various dimensions, or in which different systems 

 of firing are followed. Suppose, for example, two boilers, with grates 

 in the ratio of 1 : 2, the section of the flue, in the same ratio, but its 

 length the same in both, and let all the arrangements bo such that the 

 same quantity of air shall pass through, and the same weight of fuel 

 be burned on one square foot of grate in each; then, according to the 

 above rule, the velocity of the current of the products of combustion 

 should be as 1 : 2, while, in reality, it would evidently be equal in 

 both boilers. As another illustration, let equal weights of fuel be 

 burned in the same time, under two boilers, similar in every respect; 

 but let the stratum of fuel be twice as thick on one grate as on the 

 other ; we have already shown that, with the same draught, more fuel 

 would be burned in a given time on the former than on the latter, and 

 that therefore the draught must be checked by means of the damper, 

 in order to confine the consumption to an equality with the other ; it 

 is then clear that in this case the velocity of the current in the boiler 

 where the thick fire is used must be considerably less than in the 

 other. But by the above rule it would be found to be equal in both ; 

 therefore, the rate of combustion, or time in which a pound of fuel is 

 burnt, cannot be adopted as the measure even of the relative " velocity 

 of the products of combustion about a boiler." 



The author considers the value of time, as an element influencing 

 evaporative results, to be referable to, — 



" 1st. To the rate of combustion. 



" 2nd. To the distance passed over by the products of combustion before 

 they quit the boiler. 



" 3rd. To the time in which the heat traverses the boilers. 



" 4th. To the period of the duration of the heat about equal areas of 

 surface, . 



" It is necessrry to state that the rate of combustion now spoken of, is not 

 the rate reckoned on the square foot of grate, but the consumption of fuel in 

 an unit of time under one boiler of each class." 



We have already proved that this rate of combustion has not the 

 remotest relation to evaporative economy. 



" Proposition 1 . The velocities of the current of heated matter through 

 each boiler, will be to one another directly as the rates of combustion, an 

 inversely as the time in which equal weights of fuel are burned." 



This propositition, being in substance the same as what precedes, 

 calls for no further notice, 



» The words or unequal must have crept in ttirough isadveitence, for they cauno 

 possibly have any sigaiflcalloa In the way ihey are used. 



C 2 



