All the heat should be extracted if possible from the gases before they enter the smoke box. We should so arrange the flues, that without so much contracting the passage for the exit of the gases as to need too strong a blast, yet to confine the gases until their full value is extracted.
Several attempts have been made to apply the ideas of Clark and Overman, but as yet they have been very indirect and have met with only moderate success. (See Appendix, E.)
EVAPORATION, PRESSURE, TEMPERATURE, AND DENSITY.
324. The character of work to be done determines the nature of the steam to be used.
The quantity of work to be done shows the amount of steam to be produced.
The amount and character of the steam required, fixes the dimensions and proportions of the boiler.
A cubic foot of water, at a temperature of 62°, weighs 62.321 lbs.
A cubic foot of steam, generated at 212° Fahrenheit, under the atmospheric pressure (14.7 lbs. per square inch) weighs .03666 lbs.
Whence one cubic foot of water boiled at 212°, makes 1,700 cubic feet of steam.
The total heat of saturated steam (steam produced in contact with the water), consists of two parts at all temperatures; the latent and the sensible. The sensible heat is that shown by the thermometer, and varies with the pressure. The latent heat absorbed during the generation of steam, amounts to three fourths of the whole. As the temperature at which the steam is produced increases, the bulk produced from a given unit of water decreases, but the pressure and the total heat increase. (See C. R. M. p. 59, 61, Regnault’s experiments.)