This section is from the book "The Tinman's Manual And Builder's And Mechanic's Handbook", by Isaac Ridler Butt. Also available from Amazon: The Tinman's Manual And Builder's And Mechanic's Handbook.
When water in a vessel is subjected to the action of fire, it readily imbibes the heat or fluid principle of which the fire is the immediate cause, and sooner or later, according to the intensity of the heat, attains a temperature of 212° Fahrenheit. If at this point of temperature the water be not enclosed, but exposed to atmospheric pressure, ebullition will take place, and steam or vapor will ascend through the water, carrying with it the superabundant heat, or that which the water cannot under such circumstances of pressure absorb, to be retained and to indicate a higher temperature.
Water., in attaining the aeriform state, is thus uniformly confined to the same laws under every degree of pressure; but as the pressure is augmented, so is the indicated temperature proportionately elevated: hence the various densities of steam, and corresponding degrees of elastic force.
The preceding Table is peculiarly adapted for estimating the power of steam engines on the condensing principle, because in such the effective force of the steam is the difference between the total force and the resisting vapour retained in the condenser. The following Table is more adapted for estimating the effects of non-condensing engines, as, in such, the atmospheric pressure is not generally taken into account, engines of this principle being supposed to work in a medium; or, the atmospheric pressure on the boiler, to cause a greater density of steam, is equal to the resisting atmosphere which the effluent steam has to contend with on leaving the cylinder.
Table of the Elastic Force of Steam, the Pressure of the Atmosphere not being included.
Elastic Force in
Temperature in decrees of
Steam Water being 1.
Cubic in. of
Water in a cubic foot of
inch, of Mer.
Steam, independent of the heat indicated by an immersed thermometer, also contains heat that cannot be measured by any instrument at present known, and, in consequence of which, is termed latent or concealed heat; the only positive proof we have of its existence being that of incontestable results or effects produced on various bodies. Thus, if one part by weight of steam at 212° be mixed with nine parts of water at 628, the result is water at 178 6°; therefore, each of the nine parts of water has received from the steam 116.6° of heat, and consequently the steam has diffused or given out 116.6 X 9 = 10494 - 33.4 = 1016° of heat which it must have contained. Again, it is ascertained by experiment, that if one gallon of water be transformed into steam at 212°, and that steam allowed to mix with water at 52°, the whole will be raised to the boiling point, or 212". From these and other experiments, it is ascertained that the latent heat in steam varies from 940° to 1044°, the ratio of accumulation advancing from 212°,as the steam becomes more dense and of greater elastic force; hence the severity of a scald by steam to that by boiling water.
The rules formed by experimenters as corresponding with the results of their experiments on the elastic force of steam at given temperatures vary, but approximate so closely that the following rule, because of being simple, may in practice be taken in preference to any other.
Rule.-To the temperature of the steam in degrees of Fahrenheit, add 100. divide the sum by 177, and the 6th power of the quotient equals the force in inches of mercury.
Ex. Required the force of steam corresponding to a temperature of 312°. 312 + 100 ÷ 177 = 2.3276 = 159 inches of mercury.
But the Table is much better adapted to practical purposes, as the various results or effects are obtained simply by inspection.