Document B9jkNwoeraen52X601oqvJeL

American Society of Heating and Ventilating Engineers Guide, 1930 temperature, dew-point temperature, and dew-point differentials for 50 per cent relative humidity illustrates this relationship most clearly: Dry-bulb temperature.............. ................ 65.0 Dew-point temperature................................... 45.8 Difference between dew-point and drybulb temperature........................................ 19.2 70.0 50.5 19.5 75.0 80.0 85.0 90.0 55.25 59.75 64.25 68.75 \ 19.75 20.25 20.75 21.25 It will be seen from an inspection of this table that the difference between the dew-point temperature and the room temperature is approxi mately 20 deg. throughout this range of dry-bulb temperatures or, to be more exact, the differential increases only 10 per cent for a range of practically 25 deg. This principle holds true for other humidities and is due to the fact that the pressure of the water vapor practically doubles for every 20 deg. through this range. The approximate relative humidity for any difference between dew point and dry-bulb temperature may be expressed as: 100 ini1 2 20 (5) where I, = dew-point temperature. This principle is very useful in determining the available cooling effect obtainable by the use of ventilation with saturated air when a desired relative humidity is to be maintained in a room, even though there may be a wide variation in room temperature. This problem is one which applies to certain industrial conditions, such as those in cotton mills, tobacco factories, etc., where relatively high humidities are carried and' where one of the principal problems is to remove the heat generated by the machinery. It also permits the use of a differential thermostat,, responsive to both the room temperature and to the dew-point tempera ture, to control the relative humidity in the room. , Rate of Evaporation In problems of air conditioning and drying, as well as in other industrial applications of evaporation, such as cooling towers, for example, it is desirable to determine the rate of evaporation. There are two distinct cases of evaporation. The first 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 for evaporation 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 wetbulb 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- 166 Chapter 7--Air Conditioning Fig. 2. Heat Transmitted by Evaporation 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: ^ = (a + to) (' - e) (6) where ^7 = rate of evaporation. at a = the rate of evaporation in still air. b = the rate of increase with velocity. ' = the vapor pressure of the liquid. e = the vapor pressure in the atmosphere. v = 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, e1 is the vapor pressure corresponding to the wet-bulb tem perature of the air. This wet-bulb 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 w = 0.093 - ) (approximately) 167 '(7)