| 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,
| F | = | | H − h + s ( tsup − tsat ) | | –––––––––––––––––––––––––––––––––– | | 970.4 |
| ( 3 ) |
| 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, |
| tsup | = | final temperature of steam, |
| tsat | = | temperature of saturated steam, corresponding to pressure existing, |
| ( tsup − tsat ) | = | 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.
Steam gauges indicate the pressure above the atmosphere. As has been pointed out, the atmospheric pressure changes according to the altitude and the variation in the barometer. Hence, calculations involving the properties of steam are based on absolute pressures, which are equal to the gauge pressure plus the atmospheric pressure in pounds to the square inch. This latter is generally assumed to be 14.7 pounds per square inch at sea level, but for other levels it must be determined from the barometric reading at that place.