Читать книгу Mechanics of the Household - E. S. Keene - Страница 5

—The temperature of the steam is comparatively an unimportant factor in the amount of heat given up by the radiator. It is the heat liberated at the time the steam changes from vapor to water that produces the greatest effect in changing the temperature of the house. This evolution of heat by condensation is sometimes called the latent heat of vaporization. It is the heat that was used up in changing the water to vapor. The following table of the properties of steam shows the temperatures and exact amounts of latent heat that correspond to various pressures.

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 pressure	Gage pressure	Temperature	Latent 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.

At temperatures below the boiling point of water, the heat of vaporization gradually increases until, at the freezing point, it is 1092 B.t.u. Water vaporizes at all temperatures—even ice evaporates—and the cooling effect produced by evaporation from sprinkled streets in summer, or the chilling sensation brought about by the winds of winter are caused largely because of its effect. The evaporation of perspiration from the body is one of the means of keeping it cool. At the temperature of the body 98.6 the heat of vaporization is 1046 B.t.u.