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162 Chapter 4

Vapor Cooling
Condensation of a Mixture

s \
Condensation Begins

Heating

Heat Removed or Added

Figure 4.6 Heating and cooling curves for a heat exchanger.

tially linear, then the logarithmic-mean temperature difference can be used for
each segment as shown below.
The total surface area,

. (4.3)
Ao = A,+A 2 + ..+A n
Substituting Equation 4.1 into Equation 4.3 for each segment we find that

Ql Q2 Qn
A 0 = (4.4)
U 0(At) LMi U 0(At) LM2

If the segments are chosen so that

(4.5)

then

Q/n f 1 1 1 1
A n =- (4.6)
U 0 L (At) LMi (At) L (At) LMn J

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