CHAPTER
5 Two generalized equations have been obtained: (1) For condensation on vertical tubes or plane surfaces
where l(lambda) is the latent heat of the condensing liquid in J kg^{1}, L is the height of the plate or tube and the other symbols have their usual meanings. 
(2) For condensation on a horizontal tube
where D is the diameter of the tube. These equations apply to condensation in which the condensed liquid forms a film on the condenser surface. This is called film condensation: it is the most usual form and is assumed to occur in the absence of evidence to the contrary. However, in some cases the condensation occurs in drops that remain on the surface and then fall off without spreading a condensate film over the whole surface. Since the condensate film itself offers heat transfer resistance, film condensation heat transfer rates would be expected to be lower than drop condensation heat transfer rates and this has been found to be true. Surface heattransfer rates for drop condensation may be as much as ten times as high as the rates for film condensation. The contamination of the condensing vapour by other vapours, which do not condense under the condenser conditions, can have a profound effect on overall coefficients. Examples of a noncondensing vapour are air in the vapours from an evaporator and in the jacket of a steam pan. The adverse effect of noncondensable vapours on overall heat transfer coefficients is due to the difference between the normal range of condensing heat transfer coefficients, 120012,000 J m^{2} s^{1 }°C^{1}, and the normal range of gas heat transfer coefficients with natural convection or low velocities, of about 6 J m^{2} s^{1 }°C^{1}. Uncertainties make calculation of condensation coefficients difficult, and for many purposes it is near enough to assume the following coefficients:
The heattransfer coefficient for steam with 3% air falls to about 3500 J m^{2} s^{1 }°C^{1}, and with 6% air to about 1200 J m^{2} s^{1 }°C^{1}.
Assuming the ammonia condensing coefficient, 6000 J m^{2} s^{1 }°C^{1} U = 1300 J m^{2} s^{1} °C^{1}. HeatTransfer Theory > HEAT TRANSFER TO BOILING LIQUIDS Back to the top 
