Document ymk15QBKkJvO3QvnN842ROKk3

Heating Ventilating Air Conditioning Guide 1939 Table 4. Weight of Saturated and Partly Saturated Air3 Dry-Bulb Temp Dm P 28.S Weight o? Saturated Air fob Various Barometric and Htgrometric Conditions--Pounds per Cubic Foot Barometric Pressure Inches of Mercury 29.0 29.5 30.0 - 30.S 31.0 Increase In Weight Per 0.1 in. d;~ in Barometer Approx. Average Increase in Weight Per Deg Wet-Bulb Depression 30 32 34 36 0.07703 0.07839 0.07671 0.07806 0.07638 0.07772 0.07605 0.07739 0.07974 0.07940 0.07907 0.07873 0.08110 0.08075 0.08041 0.08007 0.08245 0.08210 0.08175 0.08141 0.08381 0.08345 0.08310 0.08274 0.00027 0.00027 0.00027 0.00027 0.000017 0.000017 0.000018 0.000018 38 40 42 44 0.07573 0.07706 0.07541 0.07674 0.07509 0.07641 0.07477 0.07609 0.07840 0.07806 0.07773 0.07740 0.07973 0.07939 0.07905 0.07872 0.08106 0.08072 0.08038 0.08004 0.08239 0.08205 0.08170 0.08135 0.00027 0.00027 0.00026 0.00026 0.000019 0.000019 0.000020 0.000020 46 48 50 52 0.07445 0.07576 0.07413 0.07544 0.07381 0.07512 0.07350 0.07479 0.07707 0.07674 0.07642 0.07609 0.07838 0.07805 0.07772 0.07739 0.07970 0.07936 0.07902 0.07868 0.08101 0.08066 0.08032 0.07998 0.00026 0.00026 0.00026 0.00026 0.000021 0.000021 0.000022 0.000023 54 56 58 60 0.07318 0.07447 0.07287 0.07415 0.07255 0.07383 0.07224 0.07352 0.07576 0.07544 0.07512 0.07479 0.07706 0.07673 0.07640 0.07607 0.07835 | 0.07964 0.07801 0.07930 0.07768 ! 0.07896 0.07734 0.07862 0.00026 0.00026 0.00026 0.00026 0.000023 0.000024 0.000025 0.000026 62 64 66 68 0.07193 0.07320 0.07161 0.07288 0.07130 0.07256 0.07098 0.07224 0.07447 0.07574 0.07414 0.07541 0.07382 I 0.07508 0.07350 0.07475 0.07701 0.07668 0.07634 0.07601 0.07828 0.00026 0.07794 I 0.00026 0.07760 0.00026 0.07727 0.00026 0.000027 0.000028 0.000029 0.000030 70 72 74 76 0.07067 0.07192 0.07035 0.07160 0.07004 0.07128 0.06972 0.07096 0.07317 0.07285 0.07252 0.07220 0.07442 0.07410 0.07377 0.07343 0.07568 0.07534 0.07501 0.07467 0.07693 0.07659 0.07625 0.07591 0.00026 ' 0.000031 0.00025 0.000032 0.0002S 0.000033 0.00025 0.000034 78 80 82 84 0.06940 0.07064 0.06909 0.07032 0.06877 0.07000 0.06845 0.06967 0.07187 0.07155 0.07122 0.07089 0.07310 0.07434 0.07557 0.07277 i 0.07400 0.07523 0.07244 0.07366 0.07489 0.07211 | 0.07333 I 0.07454 0.00025 0.00025 0.00024 0.00024 0.000036 0.000037. 0.000039 0.000040 86 88 90 92 0.06812 0.06934 0.06780 0.06901 0.06748 03)6868 0.06715 0.06835 0.07056 0.07022 0.06989 0.06955 0.071770.07143 0.07109 0.07075 0.07299 0.07264 0.07230 0.07195 0.07420 0.07385 0.07351 0.07316 0.00024 0.00024 0.00024 0.00024 0.000042 0.000043 0.000045 0.000047 94 96 98 100 0.06682 0.06801 0.06648 0.06768 0.06615 0.06734 0.06581 0.06700 0.06921 0.06887 0.06853 0.06818 0.07041 0.07006 0.06972 0.06937 0.07161 0.07126 0.07091 0.07055 0.07280 0.07245 0.07209 0.07174 0.00024 0.00024 0.00024 0.00024 0.000049 0.000051' 0.000053 0.000055 dry-bulb temperature equals 0.000017 lb Approximate average decrease In weight per 0.1 F rise in per cubic foot. 6 Chapter 1. Air, Water and Steam The specific heat 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 1 peratures. Values for instantaneous and mean specific heats are given H 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 ib 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 Ib 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 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 Fj and Vi represent corresponding volumes of the same mass, say one pound of Vt Pi gas, then pr = -- or Pi Vi = Pi Vi, but since Pi Vi for any given case is 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