Steam:
Its Generation and Use

the fire, the higher the temperature of the fire and the lower that of the escaping gases the bet- ter the economy, for the losses by the chimney gases will bear the same proportion to the heat generated by the combustion as the temperature of those gases bears to the temperature of the the fire. That is to say, if the temperature of the fire is 2,500° and that of the chimney gases 500° above that of the atmosphere, the loss by the chim- ney will be — 20 per cent. Therefore, as the escaping gases cannot be brought below the temperature of the absorbing surface, which is practically a fixed quantity, the temperature of the fire must be high in order to secure good economy. 'I he losses by radiation being practically pro- portioned to the time occupied, the more coal burned in a given furnace in a given time, the less will be the proportionate loss from that cause. It therefore follows that we should burn our coal rapidly and at a high temperature, to secure the best available economy. THEORY OF HEAT ENGINES.* In any heat engine it is essential that there should be, ist, a working fluid ; 2d, a source of heat; and 3d, a receptacle for unexpended heat, both of which latter must be external to the working fluid. In its operation there must be a reception of heat by the working fluid, at a cer- tain temperature, a conversion of heat into work, «and a discharge of unconverted heat at a lower temperature than that at which it was received. 1 lie difference between such higher and lower temperatures is called the “range of tempera- tures,” and the engine is called a “perfect en- gine ’ ’ when the whole heat corresponding to its range of temperature is converted into work. Sadi Carnot, in 1824, seems to have been the first to enunciate the principle, now universally recognized, that the ratio of the maximum me- chanical effect in a perfect heat engine to the total heat expended upon it, is a function solely of the two constant temperatures, at which re- spectively heat is received and rejected, and is independent of the nature of the intermediate agent or working fluid, though at that day the dynamic theory of heat was not known,and Carnot supposed that all the heat received in the boiler, or its equivalent, was transferred to the conden- ser. Subsequent researches of Joule, Rankine and others, have established the following prop- ositions : ist. In any heat engine the maximum useful ejfect (expressed in foot pounds or in percentage) * From “ Substitutes for Steam,” by Geo. H. Babcock, read before the American Society of Mechanical Engineers, May, 1886. Transactions, Vol. VII., p. 710. bears the same relation to the total heat expended (expressed in foot pounds or as unity) that the range of temperature bears to the absolute tem- perature at which heat is received. 2d. In any heat engine the minimum loss of heat bears the same relation to the total heat ex- pended as the temperature at which the heat is rejected bears to the temperature at which it is received, both being reckoned from absolute zero, 460° j- below the zero of Fahrenheit’s scale. These two propositions, expressed in algebraic formulae, are: (1) U which, if H j, becomes 4 1 the well-known equation LT - - ; and, ri (2) L H in which also, if H— 1 Z = — '1 But as L 4“ U— i, U— i — -f-, which is identical with (1) differently written. At this point we need to divest ourselves of an idea which is common, and which naturally comes from the terms used, that “latent” heat is necessarily wasted heat —or, in other words, that if all the heat received was expended in ele- vating the temperature, instead of a large share of it going into the ‘ ‘ latent ’ ’ condition, we should be able to turn a larger percentage of it into power. It has been upon this erroneous supposi- tion that most of the searches for substitutes for steam have been based. To show its fallacy, practically, it is only necessary to consider the action of an engine using steam as a gas without expenditure of latent heat, and compare it with the results attained in engines in which the latent heat is expended in the boiler and discharged in the condenser. Wc will assume th<it steam be supplied at ioo° temperature — 1 pound pressure, or 28 inches vacuum nearly — that it be worked through Carnot’s cycle between that temperature and 3200— the temperature of saturated steam at 75 pounds gauge pressure. The efficiency of this cycle would be, by above formula, — 780 = .28. The heat expended per pound of steam would be 220 X -475 X 772 = 80,674 foot pounds of energy, of which the engine would utilize 28 per cent., or 22,588 foot pounds. There would, .u r I • , 1,980,000 therefore, be required ——------- = 87 6 pounds 22,588 steam per hourly horse-power, and that in a per- fect engine ; but, working within the same limits, in a very imperfect engine, using water with its large latent heat, in actual practice, a horse- power is obtained for from 16 to 18 pounds, or about one-fifth the quantity of fluid. Latent + See note, p. 13. 19