264 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



[July, 



the latter. On the first appearance of the work iinJer review, we 

 entertaineil great hopes that it woukl supply this deficiency, hut were 

 much disappointed ; for on perusing it carefully, we found it so full 

 of theoretical errors and fallacious arguments, as to render it unfit to 

 serve as a guide, either in the construction of boilers, or in the 

 management of the fires under them, except in a general way, which 

 is now little needed, as the principles advocated by Mr. Parkes are 

 already e.\tensively acknowledged and put in practice, especially by 

 the Cornish engineers. 



Tire author proposes, in this paper, " to investigate and compare the 

 peculiar properties of various kinds of steam-boilers, as exemplified 

 in their practice; to show their points of agreement and disagree- 

 ment ; to exhibit their respective merits and demerits as evaporative 

 .vessels; to point out some general laws which may contribute to give 

 greater uniformity to the results of evaporation from any assigned 

 heated surface of boiler, and enable the employer of a boiler not only 

 to ascertain if he is using his fuel economically or wastefully, but to 

 apply a remedy, should he find his practice imperfect." 



In our opinion, the proposed end has not been attained in the work 

 before us ; nevertheless, the table of experiments, which we presume 

 to be correct, cannot but be very instructive, and may, with the 

 addition of other experiments, form a basis from which a more suc- 

 cessful analyst may deduce the general laws which Mr. Parkes endea- 

 voured to discover. We have now to show the grounds on which 

 we base the opinion we have just expressed ; for which purpose we 

 shall follow the reasoning contained in the paper as concisely as pos- 

 sible, and, therefore, confine ourselves principally to the leading 

 points of the argument, supporting our remarks by quotations from 

 the work itself. 



Part I. 

 " On the qualities of steam boilers, and on the influence exercised over 

 evaporation, by their piopoitions and practical management." 



This part commences with a kind of introduction, in which the 

 author enumerates the subjects treated in the sequel, and mentions, 

 among others, the influence of time in producing the relative degrees 

 of economy, which he reserves for a separate examination at the end 

 of the paper, giving the following vague definition of time, as he in- 

 tends it to be here understood : — 



" By time, I mean the relative ]ieriods of the duration of a given amount of 

 heat about the boilers, and about equal areas of their surface." 



The rate of combustion, one of the elements of time, as defined by 

 our author, is considered by him as one of the circumstances which 

 have the greatest influence on the evaporative economy of a boiler; 

 hut, if we perfectly comprehend, as we believe we do, the meaning 

 attached by the author to that expression, viz , the quantity of fuel 

 burned under one boiler in a given time, this is not a matter of choice, 

 but must depend on the required rate of evaporation ; and slow com- 

 bustion, in the same sense, must be, not s cause, but a sign of 

 economy effected by some other means. It should be distinctly 

 understood that we have made use of the term slow combustion, in the 

 sense in \vhich we conceive it to be employed by the author, namely, 

 as signifying a comparatively small quantity of fuel burned under one 

 boiler in a given time : but we would rather have that expression 

 convey the idea, that the quantity of fuel burned in a given time is 

 small in proportion to the quantity contained in the furnace. The 

 rapidity of combustion, in this sense, must obviously be regulated, in 

 some measure, by the nature of the fuel ; for the more bituminous 

 varieties of coal, if submitted to a comparatively moderate heat, suffer 

 distillation, and a great proportion passes unburned through the 

 boiler in the form of smoke. In one instance, in Lancashire, Mr. 

 Parkes tells us, the coal he attempted to burn, on his plan of thick 

 fires, on extensive grates, with slow combustion, contained so much 

 tar as to run in streams through the bars, and catch fire in the ash-pit. 

 Thus the limit of alow combustion, properly so called, or rather, the 

 most advantageous rate of combustion, is determined by the nature of 

 the fuel used. 



At the head of the observations on each class of boilers are placed 

 certain quantities, considered by the author as forming the principal 

 points of contrast and comparison between them. These are :— the 

 tinae in which one pound of coal is burned under one boder ; the 

 weight of coal burned on each square foot of grate per hour ; the 

 weight of water evaporated by one square foot of heated surface per 

 hour from 212° ; and the weight of water evaporated by lib of coal 

 from 212°. These quantities are called by the author exponents, as 

 he considers them to be "indicative or e^fpoHew^ia^of thequahtyof the 

 boder, and of the effects of the practice upon it." Mr. Parkes is pro- 

 bably not aware that these terms are already appropriated in mathe- 

 matics to a very dift'erent signification. 



The first of thtffje circumstances can evidently have no influence on 



the economy of a boiler ; for, supposing two boilers to be equally 

 economical, the weight of coal burned imder each in a given time is 

 necessarily proportional to the quantity of water evaporated in that 

 time. 



The second circumstance affects the results in two wnys.—Firstli/, a 

 larger grate is generally accompanied by a larger surface to receive 

 the radiated heat, which moderates the action of the latter on the sur- 

 face, and thus adds to its durability. — Secondly, the thickness of the 

 stratum of fuel, and the weight burned in a given time, being the 

 same, the combustion must proceed more slowly in that furnace which 

 has a larger area of grate, and therefore, contains a greater quantity 

 of fuel at one time. The economy effected by this means may be 

 explained thus : — 



It is a matter of every-day experience in common life that, below 

 a certain limit, the more atmosjiheric air is admitted in a given time 

 to a given quantity of fuel in a state of incandescence, the more 

 rapidly the latter will be consumed ; there is also, no doubt, that the 

 air which has passed through the fuel into the flue of a boiler, of the 

 ordinary construction, contains still a large proportion of uncombined 

 o.\ygen, some part of which might still be employed in effecting the 

 combustion of an additional quantity of fuel, if properly apphed. 

 Suppose, for example, a square foot of grate covered with a stratum 

 of coal 6 inches in thickness, and supplied with such a quantity of 

 atmospheric air, that 5lbs of coal shall be burned in an hour ; it is 

 clear, that if a second stratum of coal of the same thickness as the 

 first be added, it will in a short time be heated to incandescence, and 

 as all the air which has passed through the lower stratum comes in 

 contact with the upper one, a portion of the latter will burn by com- 

 bining with some of the free oxygen remaining in it, and thus a 

 greater quantity of fuel will be burned per hour on the square foot of 

 grate, when the stratum is 12 inches, than when it is only 6 inches 

 thick, the supply of air being the same. To reduce the consumption 

 of coal, in the second case to tlie same as in the first, it will therefore 

 be necessary to diminish the supply of air, the consequence of which 

 will be that a smaller quantity of heated air will pass up the chimney; 

 and since the principal loss of eftect is to be attributed to the abstrac- 

 tion of heat by the air which passes up the chimney, the gain, or 

 rather saving of heat effected by means of thick fires will be propor- 

 tional to the diminution of draught, the air being supposed to arrive 

 at the foot of the chimney, at the same temperature, under all circum- 

 stances, which is probably the case, when the heated or evaporating 

 surface is the same. 



The experiments made by Mr. Parkes on the summit of the chimney, 

 and mentioned by him in a former paper (Trans. Inst. C. E., Vol. II., 

 page 167), are in accordance with these views as far as we are 

 more acquainted with the particulars ; but, unfortunately, we are 

 neither informed of the temperature of the air in the chimney, nor of 

 the volume of air passing through the furnace, nor of the temperature 

 of the flues, these points having probably not been ascertained. We 

 only know that the air arrived at the top of the chimney on the old 

 plan at such a temperature, that the water in an open copper vessel 

 exposed to the current was constantly in ebullition, while on the new 

 plan its temperature rarely exceeded 180°. Whatever the actual 

 difference of temperature of the hot air may have been, it must be 

 referred to two distinct causes ; Jirst, to the difference in the rate of 

 combustion, less air being required to burn the same quantity of 

 coal ; so that if all other circumstances had been the same, and the 

 air had therefore arrived at the foot of the chimney at the same tem- 

 perature as on the old plan, nevertheless, the quantity of heated air 

 impinging against the vessel of water being diminished, the same 

 effect could not be produced upon the water, as when a greater quan- 

 tity of hot air impinged against the vessel. Besides which, the air 

 ascending the chimney with less velocity, had more time to cool, and 

 therefore, lost more of its temperature before arriving at the top of 

 the chimney, where the vessel of water was placed. Secondly, to the 

 addition of another boiler ; for the evaporating surface was thus so 

 much augmented as to abstract a much greater amount of heat from 

 the air during its passage over it, which therefore arrived at the foot 

 of the chimney with a lower temperature. In the last mentioned 

 work, page l6g, Mr. Parkes states that he found that 75lbs of coke, 

 l)roduced from lOOlbs of coal, evaporated as much water as lOOlbs of 

 the self-same coal. This observation, if correct, corroborates our views 

 explained above ; for the combustion of the gases contained in the 

 coal could not have taken place without evolving some heat, the 

 whole of which must consequently have been employed in raising the 

 temperature of an extra quantity of air over and above that which 

 was necessary to burn the gases distilled from the coal. 



The third circumstance is certainly an important cause, and the 

 fourth is the evidence and measure of the evaporative economy. 

 The observations on the three kinds of boiler, the Cornish, the 



