Document b5Q2zwEYVv5jnDL6gGXLXE0JO

American Society of Heating and Ventilating Engineers Guide, 1936 Since the difference in vapor pressures is substantially proportional to the difference between the wet- and dry-bulb temperatures the wet- bulb depression) the rate of evaporation is also, for case two, substantially proportionate to the wet-bulb depression. In case two, the rate of sensible heat transfer from the air to the liquid to produce evaporation is substantially the same as the rate of heat transfer with the same type of surface, without moisture being present, but with the same temperature differences. In other words, the rate of heat transfer depends upon the temperature difference only, whether the surface is wet or not. For example, it has been shown that the rate of Woo 1uco 4- /Mn i two t tooo 900 .5 600 -1- 700 600 _Sl s V t4' 600 ? <00 300 a XX * too too. 01-- $ 1 700 600 r # \ w ijfg*, vr rotonst /CoCm rfin. ne (>i ftx>-Cat Her Exp erit %s rOO 'Gkt) c. Air Veto vty inf eet for. Him tie i 11 I 1 ri Fig. 3. Heat Transmitted by Evaporation heat transfer with air flowing across staggered coils (transverse flow) may be represented by the formula: 1 where 0.0447 + -^ \V <i9) Ut = heat transfer, expressed in Btu peg hour per square foot per degree difference in temperature between steam and air, for transverse flow. At a velocity of 400 fpm, /t = 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 flow there are 560 Btu per hour per square foot transferred per iiicl? difference of vapor pressure at a velocity of 400fpm, and 910 Btu per hour per square foot per inch difference in vapor pressure at a velocity of 800 fpm. 0ne inch of vapor pressure difference corresponds approximately to 95 deg difference between the wet- and dry-bulb temperature. Dividing by. 95, 32 . - Chapter'!^--Fundamentals of Heating and Air Conditioning .: 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. Y'. ' 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, UD = 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. ......... The difference in form of the two formulae may be due in part tri 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 and 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, 1911). /. 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). - ,^,r, Chemical Engineering, by Lewis, Walker and McAdams. Fan Engineering,.Buffalo Forge Co. . . The Psychrometric Chart, by E. V. Hill {Aerologist, April, May, June, 1932). i