Steam:
Its Generation and Use

heat must, therefore, be an efficient source of energy as well as sensible heat. 1 hat it is just as much so when working between the same limits of temperature, was demonstrated by Ran- kine in a series of articles published in the Engin- eer in 1857. And, in fact, it may be said there would be no available energy if there was no latent or specific heat. We may, perhaps, understand this point a little better by means of an illustration suggested by Carnot, which, though based upon the theory of the materiality of heat, is still just as true under the correct theory. In fact, the second law of thermo-dynamics is equally applicable to a pon- derable body as to heat, and may be summed up in the well-known adage, “ Water will not run up hill.” The figure represents a sec- tion of a building in which is situated a tank of water, or any other fluid, which is used to drive a water - motor upon a floor below, after which the fluid is discharged, whence it may or may not find its way to the sea-level — the line of absolute zero. Now it is evident the greatest possible effect obtainable in the motor-engine is repre- sented by the weight of fluid, Q, multiplied by its fall to > the point of discharge. The height of the surface of the tank above sea-level is and the height of its discharge from same datum-line is ~2, while its fall is r, — rs> and the greatest efficiency of the motor is expressed by U = Q ( ri Ts)- But the total energy of the fluid is represented by Q , and the efficiency of the motor expressed in terms of total energy is : It is evident that the same law holds good what- ever be the character of the fluid in the tank. Now, the quantity Q, — which may represent the latent heat, while the height, rb represents temperature —may be greater or less with the same height. If Q = 0, then there would be no available energy, for there would have been none expended. It will also be seen that if in the sup- posed steam-engine above calculated, 0 be sub- stituted for .475, the specific heat of the steam, there would be no energy in the engine. From the mere inspection of the above form- ula, in view of this illustration, it is readily seen : ist. That the useful effect can only equal the total heat expended when the temperature at which it is rejected is absolute zero, in which case it matters not at what temperature the heat may be received. 2d . That with a given minimum temperature, the higher the maximum temperature the greater will be the proportion of total heat converted into useful work. 3. That it is of greater importance to lower the temperature at which heat is rejected than to raise that at which it is received. There are, however, practical limits to these several values: ist. The temperature of rejection cannot be carried below that of the substance into which it is rejected — in practice it must be several de- grees above it — and is independent of the fluid employed. As there is, in practice, nothing available colder than air or water, r3 cannot easily be less than ioo° Fahr., 560° absolute. 2d. The temperature of reception cannot be greater than the highest temperature of combus- tion, nor greater than the surfaces' of the piston and cylinder will stand; nor greater than will produce in the given fluid the highest allowable pressure. 3d . The highest pressure is limited by the strength of the mechanism and safety of its oper- ation, and is also independent of the fluid. As all fluids, except mercury and turpentine, attain this limit of pressure before the limit of tempera- ture, the pressure is the practical limiting condi- tion in this direction. Obviously, then, as the limits of lowest avail- able temperature and of highest practical pres- sure are the same for all vapors, it becomes evi- dent that the fluid having the highest tempera- ture at the limit of pressure, other things being equal, has the advantage, theoretically, in possi- ble economy. Of all available liquids, water fulfils this condition best, and therefore it is use- less to search for another vapor as a substitute for steam, unless it can be shown that the losses incidental to the use of the latter are necessarily enough greater than those incidental to some other fluid, to more than counterbalance this ad- vantage. That there are such compensating ad- vantages is not probable, and they would, indeed, need to be very great to offset the cost of fluid, water being free of cost in nearly all situations.