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T able o.N 168 Am. Soc. of Heat.-Vent. Engineers Guide, , 1922 .000090 .000103 1000118 .000026 .000029 .00027 .000019. .00027 .000021 .00026 .000023 .00029 .08654 .00019 .00029 .000016 .00028 .08468 .00018 .00028- .000016 .00028 .08286 .00018 .00028 :000017 .05471 .00023 .00022' .05689 .00024 .00022 .05906 .00026 .05216 .00026 .00021 .05430 .00026 .00021 .05644 .00029 .04931 .00029 .00021 .05141 .00031 .00021 .05352 .00033 .05653 .00018 .00023 .05882 .00018 ,00023 '.06111 .00018 .00023 .06339 .00019 .00023 1.06569 .00020 .00023 .000059 .05467 .00019 .00023 .05692 .00019 .00023 .05917 .00019 .00023 .06142 .00020 .00023 i.06367 .00022 .00023 .000068 .05262 .00021 .00022 .05483' .00021 .00022 .05704 .00021 .00022 .05925 .00022 .00022 .06147 .00024 .00022 ,000078 .00024 .06629 .00016 .00024 .06870 .00016 .00024 .07112 .00024 .06465 .00016 .00024 .06703 .00017 .00024 .06939 .00023 .06293 .00017 .00023 .06526 .00018 .00023 .06759 .06297 .00015 .00025 .06542 .00015 .00025 .06789 .00015 .00025 .07034 .00015 .00025 .07280 .00016 ,00025 .000034 uoiss3jd3(j q|ng 33J83(I J3d UJ 3SE3J3UI Ctf 8,J3av -xoJddv H Iti \reg ui s!iIrOJ3d C JAV S.J3U1 >r q|ng Aiq DUJ -330 J3d az < o 5 M s.J33a nv p3)Bjn)BS -Jj `n3 J3d -j/w t-t- OO oooo H UJ jcg ui S- 3SI*I 0 0 M S.-*3UI 01 < 09 t/5 N DUqJ|n-Zg3XQjgJ3d AV s"3a 0000 CO ooooooo .00026 .00025 .07609 .07440 .00016 .00016 .00026 .00025 .00024. ,000039 .00024 .000044 .00023 .000051 ocoooooo NOOCNOOMOO <OhN<OOhoN<OOoCN CCooCoOor}* .00016 .00016 .07353 .07193 .07569 .00016 .00027 .07839 .00017 .00027 .08110 .07409 .00016 .00027 .07675 .00016 .00027 .07942 .07252 .00016 .00026 .07512 .00016 .00026 .07773 .00026 .00025 .00029 .00028 .00028 2 (*" <U > xtQuf:Ji. 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Soc. of HeaT.-Vent. Engineers Cuide, 1922 f69 Example. As an example of the use of the table No. 3 we will assume a case where it is desired to find the weight in pounds per cubic foot of air at a temperature of 83 dry- and 68 wet-bulb (or a depression of 15) when the barometric pressure is 29.40 inches. From the table No. 3 we find that the weight of saturated air at 80 and 29.00 inch barometer is 0.07034 lb. per cu. ft. Also the decrement to be subtracted is 0.00015 lb. per degree of temperature above 80. That is, the weight at 83-and 29.00 inches would'be 0.07034 -- (3 X 0.00015) = 0.06989, lb. per cu, ft.. The increment is to be added per 0.1" increase in. barometer above 29.00 inches is 0.00025, so that the weight of the saturated mixture at 83" and 29:40 inches will be 0.06989 + (4 X 0.00025) = 007089 lb. per cu. ft. From the last column in the table we find the increase in weight for each degree wet-bulb depression for a temperature of 83 to be 0000034 + 0.3 (0.000039 -- 0.000034) = 0.0000355. Then the weight of moist air at 83, 15 wet-bulb depression, and 29.40 incli barometer will be' 0.07034 -- 0.00045 + 0.001 + 0.00053 = 0.07142 lb. per cu. ft. Specific Heat of Air The specific heat of air is the ratio of the heat required to. raise the tempera ture of a given weight of air through one degree as compared to the heat re quired to raise the temperature of the same weight of water from 62 to 63 degrees Fahr., i. e., it is the B. t. u. required to raise one pound of air one degree Fahr. The specific heat of air may be expressed as either of two factors, specific heat at constant pressure or at constant volume. It is the specific heat at con stant pressure that is ordinarily referred to. The factors commonly used here tofore have been those determined by Renault--specific heat at constant pressure = 0.2375, and at constant volume = 0.1689. But recent investigation tends to show that the value 0.2375 is too low. and that it should be Cp = 0.24112 + 0.00CKX)9t or for ordinary purposes approximately 0.2415.* For the specific heats of.various substances see the table on page I5C. Relation of Velocity to Pressure The laws governing the flow or air are perhaps less understood than almost any other branch of engineering data. The flow of air under high pressures must necessarily be investigated thermodynamically and the formulae are more or less complicated. For ordinary fan work, however, where air is at low pressure but slight error is introduced if. the same formulae are applied to the flow of air. as are commonly used for the flow of water. The basic formula for such calcu lations is ' :: V. = l/2^h (10) where V3 = velocity in ft. per second, or V = 60 J/' 2 gh where V = velocity in ft, per min. g = acceleration due to gravity in feet per second -h = head in ft. causing flow But we also have 'd h -- h J2W !; (11) (12) where h' = head expressed in in. of water d = density of water W = weight of air in lbs. per cu. ft. _____ * "Rational Psychrometric Formulae," by Willis H. Carrier, Am. Soc. Mech. Engrs., De cember, 1911, also W. F. G. Swan, Phil. Trans. Royal Soc., Series A, Vol. 210, pp. 199-238J