Document gDLwkrb9Je0p3y8V4xO17X2wq

American Society of Heating and Ventilating Engineers Guide, 1936 be accompanied by a change in the boiling point, and there will be a cor responding change in the latent heat of evaporation. These values are given in Table 6. Specific Heat. The specific heat of water, or the amount of heat (Btu) required to raise the temperature of one pound of water one degree Fahren heit, varies with the temperature, but it is commonly assumed to be unity at all temperatures. Steam tables are based on exact values, however. The specific heat of ice at 32 F is 0.492 Btu per pound. The amount of heat required to raise one pound of water at 32 F through a known temperature interval depends on the average specific heat for the temperature range. Sensible and Latent Heat. The heat necessary to raise the temperature of one pound of water from 32 F to the boiling point is known as the heat of the liquid or sensible heat. When more heat is added, the water begins to evaporate and expand at constant temperature until the water is entirely changed into steam. The heat thus added is known as the latent heat of evaporation. Table 7. Thermal Properties of Water Temperature Deo F 32 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 212 220 240 260 280 300 350 400 450 500 550 600 700 Sat. Press. Lb per Sq In. 0.0887 0.1217 0.1780 0.2561 0.3628 0.5067 0.6980 0.9487 1.274 1.692 2.221 2.887 3.716 4.739 5.990 7.510 9.336 11.525 14.123 14.696 .17.188 .24.97 35.43 49.20 67,01 134.62 247.25 422.61 681,09 1045.4 1544.6 3096.4 Volume Cu Ft per Lb 0.01602 0.01602 0.01602 0.01603 0.01605 0.01607 0.01610 0.01613 0.01616 0.01620 0.01625 0.01629 0.01634 0.01639 0.01645 0:01650 0.01656 0.01663 O.O16601 0.01670 0.01676 0.01690 0.01706 0.01723 0.01742 0.01797 0.01865 0:0195 0.0205 0.0219 .0.0241 0.0394 Weight Lb per Cu Ft 62.42 62.42 62.42 62.38 62.31 62.23 62.11 62.00 61.88 61,73 61.54 61.39 61.20 61.01 60.79 60.61 60.39 60,13 59.92 59.88 59.66 .59,17 . 58.62 58:04.57.41 55.65 53.62 51.3 ' 48.8 . 45.7 41.5 25.4 - Specific Heat 1.0093 1.0048 1.0015 0.9995 0.9982 0.9975 0.9971 0.9970 . 0.9971 0.9974 0.9978 0.9984 0.9990 0.9998 1.0007 1.0017 1.0028 1.0039 1.0052 1.0055 1.0068 1.0104 1.0148 1.020 1.026 1.044 1.067 1.095 1.130 1.200 1.362 30 Chapter 1--Fundamentals of Heating and Air Conditioning RATE OF EVAPORATION In problems of air conditioning and drying, as well as in other industrial applications of evaporation, such as cooling towers, it is desirable to determine the rate of evaporation. There are two distinct cases of evaporation. Thefirst case is that in which the source of heat is primarily from the water itself and in which the air temperature may even be raised. The second is that in which the heat forevaporation is obtained entirely from the air itself, in which case the air is cooled and the temperature of the water remains substantially constant at the wet-bulb temperature. Both cases, however, may be reduced to a common basis of calculation. It has been found that the increase in the rate of evaporation is nearly in direct proportion to the increase in the air velocity, and that it is in direct proportion to the difference in vapor pressure between the vapor pressure of the water and the pressure of the vapor in the air. . The general formula covering the experimental data may be expressed as follows: ^ = (o + bv) (e1 -- e) (17) where rate of evaporation, the rate of evaporation in still air. the rate of increase with velocity, the vapor pressure of the liquid. the vapor pressure in the atmosphere. velocity. . ' , The only difference between case one and case two is that in case one the vapor pressure of the liquid is one of the known or assumed factors, being dependent upon the known temperature of the liquid, while in case two, e' is the vapor pressure corresponding to the wet-bulb tem perature of the air. This wetrbulb or evaporation temperature is dependent upon the drybulb temperature and the moisture content, or upon the total heat of the air. as indicated in the previous paragraph. The effect of air velocity depends upon whether the flow of air is parallel to the surface or perpendicular to the surface elements. For a flow of air parallel to a horizontal surface 1 + ggo ) (' - ) (approximately) (18) where / w = pounds evaporated per square foot per hour. v = velocity of atmosphere over surfaces, feet per minute. e1 = vapor pressure of the water corresponding to its temperature, e = vapor pressure in the surrounding atmosphere.1 For transverse flow, as across a tubular surface, the rate of evaporation is nearly doubled. These relationships are indicated graphically on the chart, Fig. 3.