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HEATING VENTILATING AIR CONDITIONING GUIDE 1940 Table 4. Weight of Saturated and Partly Saturated Air3 Dbt-Bulb Temp FDeg 28.5 or rosWeight Saturated Air Various Barometric and Htgrometric Conditions--Pounds per Cubic Foot Barometric Pressure laches of Mercury 29.0 29.5 30.0 . 30.5 31.0 Increase In Weight Per 0.1 in. Rise in Barometer Approx. Average Increase in Weight Per Deg Wet-Bulb Depression 30 0.07703 0.07839 0.07974 0.08110 0.08245 0.08381 0.00027 0.000017 32 0.07671 0.07806 0.07940 0.08075 0.08210 0.08345 0.00027 0.000017 34 0.07638 0.07772 0.07907 0.08041 0.08175 0.08310 0.00027 0.000018 36 0.07605 0.07739 0.07873 0.08007 0.08141 0.08274 0.00027 0.000018 38 0.07573 0.07706 0.07840 0.07973 0.08106 0.08239 0.00027 0.000019 40 0.07541 0.07674 0.07806 0.07939 0.08072 0.08205 0.00027 o.ooooisf 42 0.07509 0.07641 0.07773 0.07905 0.08038 0.08170 0.00026 0.000020 44 0.07477 0.07609 0.07740 0.07872 0.08004 0.08135 0.00026 0.000020 46 0.07445 0.07576 0.07707 0.07838 0.07970 0.08101 0.00026 0.000021 48 0.07413 0.07544 0.07674 0.07805 0.07936 0.08066 0.00026 0.000021 50 0.07381 0.07512 0.07642 0.07772 0.07902 0.08032 0.00026 0.000022 52 0.07350 0.07479 0.07609 0.07739 0.07868 0.07998 0.00026 0.000023 54 0.07318 0.07447 0.07576 0.07706 0.07835 0.07964 0.00026 0.000023 56 0.07287 .0.07415 0.07544 0.07673 0.07801 0.07930 0.00026 0.000024 58 0.07255 0.07383 0.07512 0.07640 0.07768 0.07896 0.00026 0.000025 60 0.07224 0.07352 0.07479 0.07607 0.07734 0.07862 0.00026 0.000026 62 0.07193 0.07320 0.07447 0.07574 0.07701 0.07828 0.00026 0.000027 64 0.07161 0.07288 0.07414 0.07541 0.07668 0.07794 0.00026 0.000028 66 0.07130 0.07256 0.07382 0.07508 0.07634 0.07760 0.00026 0.000029 68 0.07098 0.07224 0.07350 0.07475 0.07601 0.07727 0.00026 0.000030 70 0.07067 0.07192 0.07317 0.07442 0.07568 0.07693 0.00026 0.000031 72 0.07035 0.07160 0.07285 0.07410 0.07534 0.07659 0.00025 0.000032 74 0.07004 0.07128 0.07252 0.07377 0.07501 0.07625 0.00025 0.000033 76 0.06972 0.07096 0.07220 0.07343 0.07467 0.07591 0.00025 0.000034 78 0.06940 0.07064 0.07187 0.07310 0.07434 0.07557 0.00025 0.000036 80 0.06909 0.07032 0.07155 0.07277 0.07400 0.07523 0.00025 0.000037 82 0.06877 0.07000 0.07122 0.07244 0.07366 0.07489 0.00024 0.000039 84 0.06845 0.06967 0.07089 0.07211 0.07333 0.07454 0.00024 0.000040 86 0.06812 0.06934 0.07056 0.07177 0.07299 0.07420 0.00024 0.000042 88 0.06780 0.06901 0.07022 0.07143 0.07264 0.07385 0.00024 0.000043 90 0.06748 0.06868 0.06989 0.07109 0.07230 0.07351 0.00024 0.000045 92 0.06715 0.06835 0.06955 0.07075 0.07195 0.07316 0.00024 0.000047 94 0.06682 0.06801 0.06921 0.07041 0.07161 0.07280 0.00024 0.000049 96 0.06648 0.06768 0.06887 0.07006 0.07126 0.07245 0.00024 0.000051 98 0.06615 0.06734 0.06853 0.06972 0.07091 0.07209 0.00024 0.000053 100 0.06581 0.06700 0.06818 0.06937 0.07055 0.07174 0.00024 0.000055 Approximate average decrease in weight per 0.1 F rise in dry-bulb temperature equals. 0.000017 lb per cubic foot. 6 CHAPTER 1. AIR, WATER AND STEAM The specific heal of air is the number of Btu required to raise the tem perature of 1 lb of air 1 F. Distinction should always be made between the instantaneous specific heat at any existent temperature and the mean specific heat, which is the average specific heat through a given tempera ture range. The mean specific heat is the value required in most calcu lations. The specific heats at constant pressure, Cp, and the specific heats, Cv, at constant volume are different. The specific heat at constant pressure is commonly used and it varies, under a pressure of one atmos phere, from a minimum at 32 F from which it increases with either increase or decrease of temperature. The value of 0.24, as the mean specific heat at constant pressure, is sufficiently accurate for use at ordinary tem peratures. Values for instantaneous and mean specific heats are given in Table 3. The mean specific heat of water vapor at constant pressure is taken as 0.45 for all general engineering computations. 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 82 F and 29.00 in. barometer is found to be 0.07000 lb per cubic foot. There is a decrease of 0.00017 lb per degree drybulb temperature above 82 F. There is an increase of 0.00024 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 0.000039 lb per degree wet-bulb depression when the dry-bulb is 83 F. Tabu lating the items: 0.07000 = weight of saturated air at 82 F and 29.00 in. Hg. bar. -- 0.00017 = decrement for 1 deg dry-bulb, 1 X 0.00017. + 0.00096 = increment for 0.4 in. bar., 4 X 0.00024. + 0.00059 = increment for 15 deg wet-bulb depression, 15 X 0.000039. 0.07138 = 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 F, 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 Pi and Pi represent the initial and final absolute pressures, and Vt and Fj represent corresponding volumes of the same mass, say one pound of Vi Pi gas, then yy = -5 , or Pj Vi = Pi Fj, but since Pi Vi for any given case is V i Pi a definite constant quantity, it follows that the product of the absolute pressure and volume of a gas is a constant, orPF = 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