THE EXPANSION OF STEAM.

H3

PROPERTIES OF STEAM AND OTHER GASES.

Pressure, density, and temperature are the important character-istics of steam, as they are the properties which regulate the econo-mical production and application of steam power. Steam as a gasis amenable to the common laws of gaseous fluids; and, accordingto those laws, the pressure, the density, and the temperature bearfixed relations to one another. The influence of temperature onthe expansion of gases under constant pressures is nearly uniformfor equal increases of temperature, and is nearly the same fordifferent gases. The expansion of air may be assumed to representthat of other gases, and it is found by experiment that air expandsT i 7j -th of its volume at 32 0 for each degree of temperature communi-cated.

The relation betwixt pressure and volume under constant tem-peratures is also sensibly uniform within ordinary limits. For anexpansion of four times the initial volume, experiments on variousgases show a corresponding diminution of pressure in the ratio of1 to 3'99, ör sensibly 1 to 4.

The total or constituent heat of saturated steam is at all temper-atures separable into two parts—latent and sensible heat. Thesensible heat is that indicated by the thermometer, and it varies asthe pressure. The latent heat absorbed during the conversion ofwater into steam constitutes by far the greater proportion of thetotal heat. Thus for saturated steam we have the followingvalues:—

Pressure. Temperature. Latent Heat . Total Heat.

147 Ibs. ... 212° ... 966 ü, 6 ... U78 0, 6

90 lbs. ... 3 20° *2 ... 89i°'4 ... i2ii°-6

The difference of total heat is, in this case, 33 0 in favour of thehigher pressure. It appears, then, that by expansion perfectly drysteam becomes slightly surcharged, in virtue of the excess of totalheat due to higher pressures; and should it contain a portion ofwater in a state of Suspension, a small part of this water must beevaporated during expansion.

For steam, and for gases generally, the following ratios may beadopted:—

With a constant temperature, the pressure varies simply as thedensity, and inversely as the volume.

With a constant pressure, expansion is uniform under a uniform