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American Society of Heating and Ventilating Engineers Guide, 1935 where Ut -- heat transfer expressed in Btu per hour per square foot per degree difference in temperature between steam and air, for transverse flow. At a velocity of 400 fpm, Ut = 5.8; at a velocity of 800 fpm, Ut -- 9.3. Referring to Fig. 3, showing the rate of heat transmission by evapo ration for different air velocities, it will be noted that for transverse flo.w there are 560 Btu per hour per square foot transferred per inch difference of vapor pressure at a velocity of 400 fpm, and 910 Btu per hour per square foot per inch difference in vapor pressure at a velocity of 800 fpm. One inch of vapor pressure difference corresponds approximately to 95 deg difference between the wet- and dry-bulb temperature. Dividing by 95, the value of 5.9 Btu per square foot per degree difference in temperature is obtained for a velocity of 400 fpm, and 9.55 Btu per square foot for a velocity of 800 fpm. It will be noted that for these two cases the heat transfer by evapo ration per degree difference in temperature corresponds almost exactly with the heat transfer by convection coils. The similarity may be noted by comparing the formula for heat transfer in parallel flow, where - 0.026 + "! V (20) with the heat transfer by evaporation with parallel flow. The relationship will be seen to be very close in both cases and would indicate that the heat transfer by evaporation is actually brought about by a process of con vection. 28 Chapter .-i--Fundamentals of Heating: and Air Conditionings The difference in form of the two formulae may be due in part to errors in observation at the higher and lower velocities. In cooling air and condensing out the moisture therefrom the heat transfer is considerably more rapid than when the air is dry arid no moisture is condensed. In general the rate of heat transmission on the air side is increased an amount which is proportionate to the latent heat removed as compared with the sensible heat removed. That is, if the latent heat removed was 50 per cent of the sensible heat removed, then the conductivity of the surface in contact with the air would be increased approximately 50 per cent. REFERENCES A Review of Psychrometric Charts, by C. O. Mackey (Heating and Ventilating, June, July, 1931). A New Psychrometric Chart, by C. A. Bulkeley (A.S.H.V.E. Transactions, Vol. 32, 1926). Air Conditioning Applied to Cold Storage and a New Psychrometric Chart, by C. A. Bulkeley (Refrigerating Engineering, February, 1932). Air Conditioning Theory, by John A. Goff (Refrigerating Engineering, January, 1933). Rational Psychrometric Formulae, by W. H. Carrier (A.S.M.E. Transactions, Vol. 33, Temperature of Evaporation, by W. H. Carrier (A.S.H.V.E. Transactions, Vol. 24, 1918). Principles of Engineering Thermodynamics, by Kiefer and Stuart. Basic Theory of Air Conditioning, by Lawrence Washington (Western Conference on Air Conditioning, San Francisco, Calif., February 9-10, 1933). Mixtures of Air and Water Vapor, by C. A. Bulkeley (Refrigerating Engineering, January, 1933). Temperature of Evaporation of Water into Air, by W. H. Carrier and D. C. Lindsay (A.S.M.E. Transactions, 1924). Chemical Engineering, by Lewis, Walker and McAdams. v.V Fan Engineering, Buffalo Forge Co. The Psychrometric Chart, by E. V. Hill (Aerologist, April, May,-June, 1932). PROBLEMS IX PRACTICE 1 Given air at 70 F dry-bulb and 50 per cent relative huimdity with a.baro metric pressure of 29.00 in. Hg, find the weight of vapor per pound of dry air. Weight of saturated vapor per pound of dry air -- Wt = 0.01578 lb (Table 5). Satura tion pressure of the vapor at 70 F = t = 0.7386 in. Hg. From Equation 7, 0.01578 X 0.5 (29.00 - 0.7.386) --T 29.00 - (0.5) (0.7386) ` '^ W = 0.00779 lb of vapor per pound of dry air at 70 F dry-bulb and 50 per cent relative humidity. A pproximate Method: 0.01578 X 0.5 = 0.00789 lb of vapor per pound of dry air at 70 F dry-bulb and 50 per cent relative humidity. : 2 f Given air with a dry-bulb temperature of 80 F, relative-humidity of 55 per cent, and a barometric pressure of 29.92 in. Hg, calculate the. weight of a cubic foot of the mixture. .: . . : Weight of saturated vapor per cubic foot = 0.001580 lb (Table 5);i-' :n tt * 29