Document gbk8m92Jd67j2xjnVrmYg5LB3

American Society .of Heating and Ventilating'Engineers Guide, 1934 eqiiatiOn is.'jy = or, for the same temperature range at constant pres- Pi ti ,, - , ..... . Vi 7 2 sure, Pc, the relation is ^ = jr. "' In general, for any weight of gas, W, since volume is proportional to weight, the relation between P, V and T is ' PV = WRT (!) where ., , . .. P = the absolute pressure of the gas in pounds per square foot. F.= the volume of the weight W in cubic, feet................................. ................... W = the weight of the gas in pounds. R = a constant depending on the nature-of the gas. | T = the absolute temperature in 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 1 will apply to most engineering computations.: : j '-`f Additional information on the properties of air will be found in Chapiters 1 and 2. i PROPERTIES OF WATER Composition of Water. Water is a chemical compound (H20) formed by the union of two volumes of hydrogen and one volume of oxygen, or two parts by weight of hydrogen and 16 parts by weight of oxygen. ; ;;; Density of Water: Water is at its greatest density at 39.2 F, and it expands when heated or cooled from, this temperature. At 62 F a U. b. gallon of 23i Cu ini of water weighs approximately lb, and a .cubic foot of water is equal to 7.48 gal. The specific volume of water depend? on the temperature arid it is always the reciprocal of its specific density. (See Table 4). . .. . i WriterPressures. Pressures are often stated in feet or inches of Water column.: At 62 F, with h equal to the head in feet, the pressure ol a column of water is 62.383/i lb per square foot, or 0.433fe lb per square inch. A columri of water 2.309 ft (27.71 in.) high exerts a pressure of one pound per square inch at 62 F. Boiling Point,of Water. Tle boiling point of water varies with the pressure; it is lower at higher altitudes. A change in pressure will always be accompanied-by a change in the boiling point, and there will be a .cor- resppnding change in the latent heat of evaporation. Specific Heat. The specific heat of water, or the amount of heat (Btu) required to raise the/ temperature of one pound of water one degree Fahren heit, varies with the temperature, but it is commonly assumed to be unity at all temperatures. ;Steam tables are -based :on exact -values, however. The specific heat of .ice at 32 K is 0.492 Btu per pound. The amount of heat required to raise one pound of .water at 32 F through a knpwn temperature interval depends on theaverage specific heat tor the temperature range. '. \ - - Sensible and, Latent Heat. The heat necessary to raise the temperature of one pound of:water from 32 F to the boiling point is known as the heat 580 ' T able 3. W eights of Saturated and Partly Saturated A ir for Various Barometric and H ygrometric Conditions31.? - Pounds per Cubic Popt i Chapter 41 -Properties of Air, Water and Steam :S81