NO. 4. How much remains to be done in connection with economizing the fuel consumed by our engines in the production of work, will be comprehended, if we fully realize the fact, that not more than ten per cent of the real power of the coal burnt under our most perfect modern steam boilers, is turned to useful practical account. In the year 1702, Savary constructed a steam engine by means of which, a weight of 1,000 tuns could be raised one foot high by the combustion of one bushel of coal. In 173 0, Otto Guericke Newcomen made an atmospheric engine which lifted 3,500 tuns by the consumption of the same amount of fuel. Watts original engine raised 6,000 tuns, the modernized Watts engine raises 15,000 tuns by the same weight of coal. The average duty of the ordinary improved Cornish engines of our day is equivalent to 56,000 tuns raised one foot high by the combustion of the same quantity of coal as above. Large as this last amount may seem to a superficial observer, it is yet infinitely below the probable realizations of the future, as the following computation conclusively demonstrates. If we consider the calorific value of the combustion of one pound of average coal, as equal to 6,000 or 7,000 centigrade units of heat, andif each-of these units, as has been proved by recent elaborate researches, is equivalent to 420 kilogramme-tres or nearly 2,700 foot pounds, we find, that one pound of coal produces a force equal to 16,200,000 or 18,900,000 of footpounds. The most economical engines in the world, do not on an average, reach a higher figure than 1,398,094 foot-pounds per pound of coal consumed, this being only 0074 to 0086 of the whole theoretical amount, or only from 7yt. to 81 per cent of the real power concentrated by nature in one pound of coal. We must be careful not to confound the amount of foot pounds which are equivalent to the combustion of one pound of coal with the quantity of heat needed to vaporize a certain amount of water, this being a quite different thing. If it takes 6855 centigrade units of heat to evaporate one lb. of water and if one lb. of coal evolves on an average 6,500 centigrade units, then theoretically 1022 lbs. of water should have to be evaporated from a lb. of such coal. We find in practice, that 8 lbs. of water from 1 lb. of coal may be taken as a fair average, so that in this case 2-20 lbs. of water or 22 per cent only have really been lost. By greater care and attention, this amount of waste may be further reduced, so as to assimilate still more closely the practical with the theoretical results. What really becomes of the 90 per cent, more or less, of foot-pounds, lost, during the combustion of one lb. of coal, as mentioned above ? This is a question which we expect to be asked and which we will here attempt to briefly elucidate. The equivalent of the combustion of one lb. of coal being as above stated 18,900,000 foot lbs; we may suppose 10 per cent of this quantity to be converted into useful work by the engine and another 10 per cent to be entirely wasted (by remediable causes), during the production of steam. This gives a total consumption of 20 per cent or of 3,780,000 foot-pounds, leaving an apparent loss of 15,120,000 foot-pounds which have vanisied during the vaporization of 10 lbs. of water. In such a case we might affirm that 15,120,000 foot-pounds have really been absorbed or rendered latent in the work of converting one lb. of water into steam, for which purpose every centigrade unit of heat evolved by the coal must have had to furnish no less than 2,520 foot-pounds of hidden work. A much larger quantity of fuel than 10 per cent, is however, in most cases, wasted by remediable causes. We have shown in previous articles, in the SCIENTIFIC AMERICAN, how bad stoking causes a waste of fuel, which may reach 25 per cent; how, the necessary blowing off in cases of salt or impure water produces a loss of S3 per cent, and how priming and scale may add another 30 per cent to the above. This however is but a fraction of what often takes place, as waste by radiation of heat and consequent condensation of steam in the boilers, steam-pipes, and cylinders are another source of very considerable loss. This radiation may, to a considerable extent, be obviated by the use of external coverings of felt or canvas, by superheating the steam, by steam jackets, or better yet, by the combined effect of these various remedies. Leaks are another frequent cause of logs of fuel, the amount of which can only be determined by the calculation of the units of heat in every lost pound of steam or water, remembering that the waste by leakage of one lb. of steam exceeds by 5J times at least, that which would originate from the leakage of one lb. of hot water from the boiler. As is known to every tyro, repacking of the slides, pistons, blow-off nozzles etc., are the preventives of lose by leakage. An imperfect vacuum leads to a waste of fuel, as the required power will in Such a case, have to be obtained from a lower step of expansion and with a. corresponding increase in the consumption of fuel. The neglect to ease and stop in time, the urging of the fires, the unnecessary friction of any of the rubbing surfaces of the engine,.the excess or the deficiency of draft in the furnaces, the use of bad coals, and many other causes too numerous to be here enumerated, all concur to increase to an almost indefinite extent, the waste of fuel. The sum total of remediable waste in our ordftiary carelessly managed engines, frequently reaches formidable figures, and this before the very eyes of the proprietors of the same who seem totally blind to the fact, that wasting fuel if iaju. rioas to their pockets. 167 It is not of the highest importance, as is sometimes be- i lieved, that an engine should run beautifully smooth and easy; it is however, most essential, that every bushel of coals burned under its boilers should be made to furnish their maximum of usefulness, a result, which can be attained only by constant care and vigilance, two words which in themselves comprise the whole duty of the engineer, and ought to be his motto.
This article was originally published with the title "“Waste” and “Economy” of Fuel" in Scientific American 20, 11, 166-167 (March 1869)