Document ZnL054n0nX3qrk3QXnGvg1mMJ

T able 4. Weights of Saturated and Partly Saturated Air for Various Barometric and H ygrometric Conditions,b Pounds per Cubic Foot From Fatt Engineering. bA convenient and accurate chart for quickly determining the weight of air under any condition of dry-bulb, wet-bulb, and pressure is A Chart fo r Determining the Weight ttf M oist A ir in Pounds per Cubic Foot, by John E. Younger. Published in Mechanical Engineering, June, 1925. Heating Ventilating Air Conditioning. Guide 1938 6 Chapter 1. Air. Water and Steam Table 4 is intended to aid in determining the density of moist air, taking into account its temperature, pressure, and moisture content. Example 1. To show the use of Table 4: Given air at 83 F dry-bulb and 68 F wetbulb (or a depression of 15 deg) with a barometric pressure of 29.40 in. of mercury. What will be the weight of this air in pounds per cubic foot? Solution. From Table 4 the weight of saturated air at 80 F and 29.00 in. barometer is found to be 0.07034 lb per cubic foot. There is a decrease of 0.00015 lb per degree drybulb temperature above 80 F. There is an increase of 0.00025 lb for each 0.1 in. above 29.00 in. From the last column of Table 4 it is found that there is an increase of approxi mately 0.000035 lb per degree wet-bulb depression when the dry-bulb is 83 F. Tabu lating the items: 0.07034 = weight of saturated air at 80 F and 29.00 bar,. -- 0.00045 = decrement for 3 deg dry-bulb, 3 X 0.00015. + 0.00100 -- increment for 0.4 in. bar., 4 X 0.00025. + 0.00053 = increment for 15 deg wet-bulb depression, 15 X 0.000035. 0.07142 = weight in pounds per cubic foot of air at 83 F dry-bulb, 68 F wet-bulb, ' 29.40 in. bar. It is usual to assume that dry air, moist air, and the water vapor in the air follow the laws of perfect gases. This assumption while not absolutely true, especially with saturated vapor at temperatures much above 140'JY is sufficiently accurate for practical purposes and it greatly simplifies computations. Boyle's Law refers to the relation between the pressure and volume of a gas, and may be stated as follows: With temperature constant, the volume of a given weight of gas varies inversely as its absolute pressure. Hence, if P, and Pt represent the initial and final absolute pressures, and V\ and Vt represent corresponding volumes of the same mass, say one pound of yp gas, then ~ -- --or Pj Vi = P2 V2, but since Pi Vi for any given case is Kj Pi a definite constant quantity, it follows that the product of the absolute pressure and volume of a gas is a constant, or PV = C, when T is kept constant. Any change in the pressure and volume of a gas at constant temperature is called an isothermal change. Charles' Law refers to the relation among pressure, volume, and tem perature of a gas and may be stated as follows: The volume of a given weight of gas varies directly as the absolute temperature at constant pressure, and the pressure varies directly as the absolute temperature at constant volume. Hence, when heat is added at constant volume; Vc, the resulting equation is P = -T=r,' or, for the same temperature range at. constant pres- P-i 11 sure, Pc, the relation is Vi II XV In general; .for any weight of gas, W, since volume is proportional to weight, the relation among P, V, and T is where PV = WRT (3) P = the absolute pressure of the gas, pounds per square foot. V = the volume of the weight W, cubic feet. . 7 P