In a boiler trial the temperature of the feed water is 60 degrees Fahrenheit and the pressure under which steam is delivered is 160.3 pounds gauge pressure or 175 pounds absolute pressure. The total heat of one pound of steam at 175 pounds pressure is 1195.9 B. t. u. measured above the standard temperature of 32 degrees Fahrenheit. But the water fed to the boiler contained 28.08 B. t. u. as the heat of the liquid measured above 32 degrees Fahrenheit. Therefore, to each pound of steam there has been added 1167.82 B. t. u. To evaporate one pound of water under standard conditions would, on the other hand, have required but 970.4 B. t. u., which, as described, is the latent heat of evaporation at 212 degrees Fahrenheit. Expressed differently, the total heat of one pound of steam at the pressure corresponding to a temperature of 212 degrees is 1150.4 B. t. u. One pound of water at 212 degrees contains 180 B. t. u. of sensible heat above 32 degrees Fahrenheit. Hence, under standard conditions, 1150.4 - 180 = 970.4 B. t. u. is added in the changing of one pound of water into steam at atmospheric pressure and a temperature of 212 degrees. This is in effect the definition of the latent heat of evaporation. Hence, if conditions of the trial had been standard, only 970.4 B. t. u. would be required and the ratio of 1167.82 to 970.4 B. t. u. is the ratio determining the factor of evaporation. The factor in the assumed case is 1167.82 ÷ 970.4 = 1.2034 and if the same amount of heat had been absorbed under standard conditions as was absorbed in the trial condition, 1.2034 times the amount of steam would have been generated. Expressed as a formula for use with any set of conditions, the factor is, H - h F = ----- (2) 970.4 Where H = the total heat of steam above 32 degrees Fahrenheit from steam tables, h = sensible heat of feed water above 32 degrees Fahrenheit from Table 22. In the form above, the factor may be determined with either saturated or superheated steam, provided that in the latter case values of H are available for varying degrees of superheat and pressures. Where such values are not available, the form becomes, H - h + s(t_{sup} - t_{sat}) F = ---------------------------- (3) 970.4 Where s = mean specific heat of superheated steam at the pressure existing in the trial from saturated steam to the temperature existing in the trial, t_{sup} = final temperature of steam, t_{sat} = temperature of saturated steam, corresponding to pressure existing, (t_{sup} - t_{sat}) = degrees of superheat. The specific heat of superheated steam will be taken up later. Table 19 gives factors of evaporation for saturated steam boiler trials to cover a large range of conditions. Except for the most refined work, intermediate values may be determined by interpolation.