isw.T 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



273 



EXPERIMENTS ON COALS. 



Report on the Coals Suited to the Steam Navy. By Sir Henky 

 DE LA Beche and Dr. Lyon Playfair. 



Experiments necessary to ascertain the true practical value of 

 coal involve a very liirgre series of observations, extended over a 

 considerable period, and directed to special objects of inquiry. 

 The qualities for which particular kinds of fuel are pre-eminent 

 being so varied, it is impossible to deduce general results from a 

 limited series of observations. Even in the economical appli- 

 cation of coals, their evaporative value, or their power of forming 

 steam, one variety of coal which may be admirably adapted from its 

 quick action for raising steam in a short period, may be far ex- 

 ceeded by another variety, inferior in this respect, but capable of 

 converting a much larger quantity of water into steam, and there- 

 fore more valuable in the production of force. A coal uniting 

 these two qualities in a high degree might still be useless for naval 

 purposes, on account of its mechanical structure. If the cohesion 

 of its particles be small, the effect of transport or the attrition of 

 one coal against another by tlie motion of a vessel might so far 

 pulverise it as materially to reduce its value. Even supposing the 

 three qualities united, rapidity and duration of action with consi- 

 derable resistance to breakage, there are many other properties 

 which should receive attention in the selection of a fuel without 

 the combination of which it might be valueless for our steam navy. 



Thei-e is an important difference existing between varieties of 

 coals in the bulk or space occupied by a certain weight. For the 

 purposes of stowage-room this cannot be ascertained by specific 

 gravity alone, because the mechanical formation of the fragments 

 of coal may enable one of less density to take up a smaller space 

 than that occupied by another of a higher gravity. This is far 

 from an imaginary difference, being sometimes as great as 60 per 

 cent., and not unfrequently 40 per cent. The mere theoretical de- 

 termination of the density of coals would, therefore, give results 

 useless for practice. The space occupied between two varieties of 

 coals, often equally good as regards their evaporative value, differs 

 occasionally 20 per cent. — that is, where 80 tons of one coal could be 

 stowed, 100 tons of another of equal evaporative value might be 

 placed, by selecting it with attention to its mechanical structure. 



These facts are mentioned merely to show that a hasty gene- 

 ralization should not be made, and to account for our drawing at- 

 tention to these various points as a means of preventing the 

 selection of a fuel from any one quality. AV'e do not, in the 

 present stage of this inquiry, consider it proper to offer any recom- 

 mendation of our own as to particular kinds of fuel, leaving the 

 experimental facts to decide for themselves. 



After preliminary experiments had proved that no practical 

 result could be attained by mere laboratory research, it was 

 determined to test each variety of coal on a scale of sufficient 

 magnitude to check the theoretical views by the practical results. 

 As it was impossible for either of us to devote our whole time to 

 this inquiry, our services being required by other official duties, we 

 appointed assistants* to superintend its special parts, under our 

 general direction. 



It will be obvious that there are several circumstances which 

 must receive attention before the true evaporative value of a fuel 

 can be obtained. Thus, the water in the tanks has a varying tem- 

 perature during the day, dependent on atmospheric changes, and 

 is always different from that in the boiler. The temperature of 

 water in the boiler also varies with the external temperature, and 

 tlie circumstances under which the experiments are made. The 

 shape of a Cornish boiler favours an inequality in the tempe- 

 rature of the water in its various parts, the colder and denser 

 water sinking to the bottom, and having a tendency to remain 

 there ; so that the temperature of water at the surface is far from 

 being the mean temperature of water in the boiler, the difference 

 between the surface and bottom water being, on an average, 70^. 

 Other circumstances naturally affect the evaporative powers of 

 the coal, as for example the fact that all the water exposed to the 

 action of the fire in the boiler is not converted into steam, and 

 that wood is used to light the fire. Another circumstance of con- 

 siderable importance, is the expansion or contraction of the boiler 

 from an increase or diminution of the temperature. In the early 

 stage of the experiments, those conducted by Messrs. Wilson and 

 Kingsbury, it was thought unnecessary to make a correction for 

 this variation in conditions ; but on ascertaining experimentally 



• The assistants employed were Mr. Wilson, Principal of the Poyal Agricultural Col- 

 lege, Mr. Kingsbury, Mr. J. Arthur Phillips, and Mr. UntchinsOD. Mr. Wrightson, a 

 pupil of Llebig, was entrusted with the analyses of the coal, Mr. Galloway analysed the 

 gases, and Mr. Howe also assi&tifd in the analyses. 



that the difference was as much as 69*625 lb. of water in the con- 

 tents of the boiler, between the temperature 150' and 212', it 

 became desirable to make an allowance for it, even when the dif- 

 ference between the initial and final temperature was not greater 

 than 10°. Other circumstances of less importance, but influencing 

 the results, have been neglected, because the application of such 

 corrections would have only complicated the results, and would 

 have had little practical value when the errors of observation in 

 such approximative experiments remain so large. Among these 

 may be mentioned the quantity of gases evolved during combus- 

 tion, the elevation in temperature of the air entering the fire-place, 

 the barometrical and hygrometric conditions of the atmosphere, 

 the radiation from the boiler (very small in amount, owing to its 

 brick covering), the hygrometric state of the fuel, or the heat ne- 

 cessary for obtaining mechanical draught in the chimney. In most 

 of these cases the necessary observations have been made, to en- 

 able the corrections to be applied, should it afterwards appear 

 desirable. 



In making the calculation for the evaporative value of a fuel, 

 the quantity consumed was divided into two portions, the first 

 being that necessary to raise the whole mass of water exposed to 

 the fire from tlie mean temperature to 212', the second portion 

 being that required to evaporate the water taken from the tanks 

 from a temperature of 212". To enable this to be done, the mean 

 temperature of the whole mass of the water is ascertained — that is, 

 the temperature of the water in the boiler at its initial tempera- 

 ture after being mixed with the tank-water at its average tempe- 

 rature. The average of the latter was the mean of several obser- 

 vations taken during the day, and is designated by t'. 



Let w be the weight of water from the tanks at temperature t' ; 

 W the weight of water in the boiler at temperature t'\ this 

 being obtained from surface temperature corrected by experiment ; 

 t, temperature after mixture. 



Then t = — -..y-j . 



The correction for the wood was made from data procured 

 by Messrs. Wilson and Kingsbury, but it can only be employed 

 for the particular wood used, as in subsequent experiments the 

 evaporative value was found very different from anotlier quality 

 obtained. The co-efficient of the evaporative power of the wood 

 may be deduced from experiment, in which a certain weight of 

 Mater was raised from a known temperature to the boiling point, 

 and then a certain portion of it evaporated. The following for- 

 mulse have been used by Mr. Kingsbury for the calculation : — 



N is the total weight of wood used in raising (^V -j- w) (the 

 weight of water in the boiler, and of that let down from the tanks 

 during the experiment) from the mean temperature / to 212^ ; 

 then it is necessary to find the weight N' necessary to evaporate 

 f from 212^ 



w 

 Then -j; = e, the evaporating power. 



Let TO be the weight of wood required to raise W -}- to from t to 



212', the number 1000 being assumed as the latent heat of steam. 



N to evaporate W -\- w from 212' 



N' to evaporate W -\- w from 212° 



/ n 



Then m -j- N' =: N. Now 



_ . re W -f u) 



N w 



212 



.W = n. 



W + w' 

 I (N - N') = (212 - t)n z=. (212 - t) W / ^ + "" j . 



N / = N' 



r^^ +Jf (212-0 + /} 

 = ^'{^^^2-')^^^ + "') + ^}' 



w (212 - t) {M V +w) + Iw _ 



• ■ • Sr — N I ~ ^ 



or, introducing the value of t as given by the first formula, 

 (/ + 21 2 - t) w + ( 212 - f) W _ 



N/ " *• 



If q be the quantity of wood used in lighting the fire, e q will be 

 the weight of water evaporated from 212' by the wood, and must 

 be deducted from the weight of water evaporated in calculating 

 the work done by the coal. 



The co-efficient of the evaporating power of the coals, or the 



36 



