When water at the boiling point is turned into steam at the same temperature, there are required 965.7 B.t.u. for each pound of water changed into steam. In the table, this is the latent heat of the vapor of water at 0, gage pressure. As the pressure and corresponding temperature rise, the latent heat becomes less. At 10 pounds gage pressure, the temperature of the steam is practically 240°F., but the heat of vaporization is 946 thermal units. When the steam is changed back into water, as it is when condensed in the radiators, this latent heat becomes sensible and is that which heats the rooms. The steam enters the radiators and, coming into contact with the relatively colder walls, is condensed. As condensation takes place, the latent heat of the steam becomes sensible heat and is absorbed by the radiators and then transferred to the air of the rooms. PROPERTIES OF STEAM ---------+----------+-------------+--------- Absolute | Gage | Temperature | Latent pressure | pressure | | heat ---------+----------+-------------+--------- 0 | 14.7 | 212.00 | 965.70 1 | 15.0 | 213.04 | 964.96 2 | 16.0 | 216.33 | 962.63 3 | 17.0 | 219.45 | 960.49 4 | 18.0 | 220.40 | 958.32 5 | 19.0 | 225.25 | 958.30 6 | 20.0 | 227.95 | 954.38 7 | 21.0 | 230.60 | 952.50 8 | 22.0 | 233.10 | 950.62 9 | 23.0 | 235.49 | 949.03 10 | 24.0 | 237.81 | 947.37 11 | 25.0 | 240.07 | 945.76 12 | 26.0 | 242.24 | 944.25 13 | 27.0 | 244.32 | 942.74 14 | 28.0 | 246.35 | 941.29 15 | 29.0 | 248.33 | 939.88 16 | 30.0 | 250.26 | 938.50 17 | 31.0 | 252.13 | 937.17 18 | 32.0 | 253.98 | 935.45 19 | 33.0 | 255.77 | 934.57 20 | 34.0 | 257.52 | 933.32 21 | 35.0 | 259.22 | 932.10 22 | 36.0 | 260.88 | 930.92 23 | 37.0 | 262.50 | 929.76 24 | 38.0 | 264.09 | 928.62 25 | 39.0 | 265.65 | 927.51 ---------+----------+-------------+--------- Whenever water is evaporated, heat is used up at a rate that in amount depends on its temperature and the quantity of water vaporized. This heat of vaporization is important, not only in problems which relate to steam heating but in all others where vapor of water exerts an influence--ventilation of buildings, atmospheric humidity, the formation of frost, refrigeration, and many other applications in practice; this factor is one of the important items in quantitative determinations of heat. It will appear repeatedly in considering ventilation and humidity.