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T able 3. W eights of Saturated and Partly Saturated A ir for Various Barometric and H ygrometric CoNDmoNsa.b Pounds per Cubic' Foot American Society of Heating and Ventilating Engineers Guide, 1936 From Fan Engineering. o f MboAisctoAn ivre ininenPtoaunnddsacPceruraCtuebicchaFrotoft,o rq by uickly John determining E. Younger. the weight of air under any condition of dry-bulb, wet-bulb, Published In Mechanical Engineering, June, 1925. and pressure is A Chari for Determining the Weight Chapter 1^--Fundamentals of Heating and Air Conditioning 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 or, for the same temperature range at constant pres- i ii V T% sure, Pc, the relation is ^ = jy In general, for any weight of gas, W, since volume is proportional to weight, the relation among P, V, and T is PV = WRT (3) where P = the absolute pressure of the gas, pounds per square foot. V = the volume of the weight W, cubic feet. W = the weight of the gas, pounds. R -- a constant depending on the nature of the gas. The^average value of R for air is 53.34. T = the absolute temperature, degrees Fahrenheit. This is the characteristic equation for a perfect gas, and while no gases are perfect in this sense, they conform so nearly that Equation 3 will apply to most engineering computations. HUMIDITY Humidity is the water vapor mixed with dry air in the atmosphere. Absolute humidity has a multiplicity of meanings, but usually the term refers to the weight of water vapor per unit volume of space occupied, expressed in grains or pounds per cubic foot. With this meaning, absolute humidity is nothing but the actual density of the water vapor in the mixture and might better be. so called. A study of Keenan's Steam Tables2 indicates that water vapor, either saturated or super-heated, at partial pressures lower than 4 in.'of mercury may be treated as a gas with a gas constant R of 1.21 in the characteristic equation of the gas pV = wR (t + 460). Within such limits, the density (d) of water vapor is <* = -fr = i.2i (< + 460) (Punds cubic foot) . (4) where '= t *+- 4DU (grains per cubic foot) = actual partial pressure of vapor, inches of mercury. ./ = dry-bulb temperature, degrees Fahrenheit. 'Published by American Society of Mechanical Engineers, see abstract in Table 6. (4b)