Document zzEr0Ln8KM8dk7O1z3onjEYNn

American Society of Heating and Ventilating Engineers Guide, 1934 100 per cent,-,the dry-bulb exceeds the wet-bulb, and. the wet-bulb exceeds the dew-point temperature. RELATION OF DEW-POINT TO RELATIVE HUMIDITY A peculiar relationship exists between the dew-point and the relative humidity and this is found most useful in air conditioning work. This is, that for a fixed relative humidity there is substantially a constant difference between the dew-point and the dry-bulb temperature oveiya considerable temperature range. Table 1 giving the dry-bulb and dew point temperatures, and dew-point differentials for 50 per cent relative humidity, illustrates this relationship clearly. Table 1. Dry-Bulb and Dew-Point Temperatures for 50 Per Cent Relative Humidity Dry-bulb temperature.............. ................. 1 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 in per cent as: TOO where t, = dew-point temperature. This principle is very useful in determining the available cooling effect obtainable 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 con ditions, 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 temperature, to control the relative humidity in the room. Table 2 gives, for different temperatures, the density of saturated vapor (St), the weight of saturated vapor mixed with 1 lb of dry air (Wt), (for a relative humidity of 100 per cent and a barometric pressure (D) of 4 Chapter 1--Thermodynamics of Air Conditioning Since the difference in vapor pressures is substantially proportional to the difference between the wet- and dry-bulb temperatures (i.e., the wetbulb depression) the rate of evaporation is also for case two substantially proportionate to the wet-bulb depression. In case two, the rate of sensible heat transfer from the air to the liquid to produce evaporation is substantially the same as the rate of heat transfer with the same type of surface, without moisture being present, but with the same temperature differences. In other words, the rate of heat transfer depends upon the temperature difference only, whether the surface is wet or not. For example, it has been shown that the rate of heat transfer with air flowing across staggered coils (transverse flow) may be represented by the formula: _____ 1_ Ut = 0.0447 + 50.66 v (18) where Ut = heat transfer expressed in Btu per hour per square foot per degree difference in temperature between steam and air, for transverse flow. At a velocity of 400 fpm, Ut = 5.8; at a velocity of 800 fpm, Ut = 9.3. Referring to Fig. 4, showing the rate of heat transmission by evapo ration for different air velocities, it will be noted that for transverse flow there are 560 Btu per hour per square foot transferred per inch difference of vapor pressure at a velocity of 400 fpm and 910 Btu per hour per square foot per inch difference in vapor pressure at a velocity of 800 fpm. One inch of vapor pressure difference corresponds approximately to 95 deg difference between the wet- and dry-bulb temperature. Dividing by 95, 17