CHAPTER 5
HEAT TRANSFER THEORY (cont'd)

HEAT TRANSFER FROM CONDENSING VAPOURS

The rate of heat transfer obtained when a vapour is condensing to a liquid is very often important. In particular, it occurs in the food industry in steam-heated vessels where the steam condenses and gives up its heat; and in distillation and evaporation where the vapours produced must be condensed. In condensation, the latent heat of vaporization is given up at constant temperature, the boiling temperature of the liquid.

Two generalized equations have been obtained:

(1) For condensation on vertical tubes or plane surfaces

h_v = 0.94[(k³r²g/m) x (l/LDT)]^0.25                            (5.34)

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.

h_h = 0.72[(k³r²g/m) x (l/DDT)]^0.25                                                                            (5.35)

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 heat-transfer 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 non-condensing vapour are air in the vapours from an evaporator and in the jacket of a steam pan. The adverse effect of non-condensable vapours on overall heat transfer coefficients is due to the difference between the normal range of condensing heat transfer coefficients, 1200-12,000 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:

for condensing steam           12,000 J m^-2 s^-1 °C^-1
for condensing ammonia         6,000 J m^-2 s^-1 °C^-1
for condensing organic liquids 1,200 J m^-2 s^-1 °C^-1

1/U = 1/h₁ + x/k + 1/h₂
      = 1/1750 + 0.001/45 + 1/6000
      = 7.6 x 10^-4

Heat-Transfer Theory > HEAT TRANSFER TO BOILING LIQUIDS

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