Document ypQX7OzgkZJ8aNZBnrKB92992

Heating Ventilating Air Conditioning Guide 1939 relationship is, that for a fixed relative humidity there is substantially a constant difference between the dew-point and the dry-bulb temperature over a considerable temperature range. Table 5, giving the dry-bulb and dew-point temperatures and the dew-point differentials for 50 per cent relative humidity, illustrates, this relationship clearly. Table 5. Temperatures for 50 Per Cent Relative Humidity Dry-bulb temperature.___________________ 65.0 Dew-point temperature.--________________ 45.8 Difference between dew-point and dry19.2 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: 100 where ~ 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 and tobacco factories, where re latively high humidities are carried and where one of the principal prob lems is to remove the heat generated "by the machinery. It also permits the use of a differential thermostat, responsive to both the room tempera ture and the dew-point temperature, to control the relative, humidity in the room. Table 6 gives, for different temperatures, the density of saturated vapor, dt, the weight of saturated vapor mixed with 1 lb of dry air, Wt, (at a relative humidity of 100 per cent and a-barometric pressure, B, of 29.92-in. of mercury), the specific volume of dry air, and the volume of an air-vapor mixture containing 1 lb of dry air (at a relative humidity of 100 per cent and a pressure of 29.92 in. of mercury). The preceding equations or the data from Table 6 may be conveniently used in solving the following typical problems: Example S. Humidifying and Heating. Air is to be maintained at 70 F with a relative humidity of 40 per cent (!> = 0.4) when the outside air is at 0 F and 70 per cent Chapter 1. Air. Water and Steam relative humidity (<1* = 0.7) and a barometric pressure, B, of 29.92 in. of mercury. Find the weight of water vapor added to each pound of dry air and the dew-point temperature of the humidified air. Solution. From Equation 5a and Table 6, Wt = 0.622 ( " 0.000548 lb per pound of dry air. Wt = 0.622 " 0.00618 lb per pound ot dry air. The water vapor added per pound of dry air must be (Wt -- Wi) or 0.005632 lb: By inspection of Table 6, Wt -- 0.00618 at 44.5 F, so this is the dew-point temperature of the humidified air. An approximation of the same result from Table 6 is W, = 0.7 X 0.0007852 = 0,00054964 lb per pound of dry air. Wt = 0-4 X 0.01574 = 0.006296 lb per pound of dry air. The water vapor added per pound of dry air is approximately 0.00574636 lb and the dew-point temperature is approximately 45 r. The degree of approximation is evident. Example 8. Dehumidifying and Cooling. Air with a dry-bulb temperature of 84 F, a wet-bulb of 70 F, or a relative humidity of 50 per cent (<b = 0.5), and a barometric pressure, B, of 29.92 in. of mercury is to be cooled to 54 F. Find the dew-point tem perature of the entering air and the weight of vapor condensed per pound of. dry air. Solution. From Equation 5a and Table 6, Wt = 0.622 = 0.01248 lb per pound of dry air. Wt = 0.622 (~29 92 -042003 ) = a00887 lb per pound o! dry a!f- Since Wi = Wt when t -- 63.4 F, this is the dew-point temperature of the entering air. The weight of vapor condensed is (IV) -- Wt) or 0.00361 lb per pound of dry air. An approximate result is Wi = 0.5 X 0.02543 = 0.012715 lb per pound of dry air. 1 Wt = 1 X 0.008856 = 0.008856 lb per pound of dry air, since the exit air issaturated. Since Wi = Wt at t = 64 F, this is the dew-point temperature of the entering air. The weight of vapor condensed is 0.003859 lb per pound of dry air. The degreeof approxi mation is again evident. Since Table 6 was prepared, the new steam tables, Thermodynamic Properties of Steam, by J. H. Keenan and F. G. Keyes, have been pub lished. For the last two years an A.S.H.V.E. Research Technical Advisory Committee on Psychrometry has been formulating standard psychrometric data, on a tentative basis. Included in this Committee's work is a revision of Table 6 to bring it into conformity with the Keenan and Keyes tables. Pending acceptance by the Society of the Committee's report, the tabular data and psychrometric chart published in earlier editions of The Guide have been retained. ADIABATIC SATURATION OF AIR . The process of adiabatic saturation, or evaporative.cooling, is.of con siderable importance in air conditioning. Suppose that unsaturated air be made to pass at a steady rate through a tun 11