VARIATION IN PROPERTIES OF
SATURATED STEAM WITH PRESSURE
___________________________________________________
| | | | | |
| Pressure |Temperature | Heat of | Latent | Total |
| Pounds | Degrees | Liquid | Heat | Heat |
| Absolute | Fahrenheit |B. t. u. |B. t. u.|B. t. u.|
|__________|____________|_________|________|________|
| | | | | |
| 14.7 | 212.0 | 180.0 | 970.4 | 1150.4 |
| 20.0 | 228.0 | 196.1 | 960.0 | 1156.2 |
| 100.0 | 327.8 | 298.3 | 888.0 | 1186.3 |
| 300.0 | 417.5 | 392.7 | 811.3 | 1204.1 |
|__________|____________|_________|________|________|
As the pressure and temperature increase, the latent heat decreases.
This decrease, however, is less rapid than the corresponding increase in
the heat of the liquid and hence the total heat increases with an
increase in the pressure and temperature. The percentage increase in the
total heat is small, being 0.5, 3.1, and 4.7 per cent for 20, 100, and
300 pounds absolute pressure respectively above the total heat in one
pound of steam at 14.7 pounds absolute. The temperatures, on the other
hand, increase at the rates of 7.5, 54.6, and 96.9 per cent. The
efficiency of a perfect steam engine is proportional to the expression
(t - t_{1})/t in which t and t_{1} are the absolute temperatures of the
saturated steam at admission and exhaust respectively. While actual
engines only approximate the ideal engine in efficiency, yet they follow
the same general law. Since the exhaust temperature cannot be lowered
beyond present practice, it follows that the only available method of
increasing the efficiency is by an increase in the temperature of the
steam at admission. How this may be accomplished by an increase of
pressure is clearly shown, for the increase of fuel necessary to
increase the pressure is negligible, as shown by the total heat, while
the increase in economy, due to the higher pressure, will result
directly from the rapid increase of the corresponding temperature.