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Condensation 315

stream, the coolant, and the specific shell-and-tube system to be used. Because the
calculation of individual heat transfer coefficients lies beyond the scope of this man-
ual, a conservative estimate is made for the overall heat transfer coefficient. This
yields a conservatively large surface area estimate. (For the calculation of individual
heat transfer coefficients, consult refs. 1–3.) The calculation procedure here assumes
countercurrent flow, commonly found in industrial applications. However, some
applications employ cocurrent flow or use fixed heat exchangers. The following pro-
cedure is valid for cocurrent flow, but requires an adjustment to the logarithmic mean
temperature difference (1–3).
To size countercurrent condensers, use the following equation to determine the
required heat transfer area:
∆
A = H U T (8)
con load LM
2
where A is the condenser (heat exchanger) surface area (ft ), U is the overall heat trans-
con
2
fer coefficient (Btu/h-ft ºF), ∆T is the logarithmic mean temperature difference (ºF);
LM
e (
T − T o) − T ( − T )
i
cool,
∆T LM = e ( [ cool, T ( con T )] (9a)
ln T − T cool, o) con − cool, i
where T is the emission stream temperature (ºF), T is the coolant outlet temper-
e cool,o
ature (ºF), T is the condensation temperature (ºF), and T is the coolant inlet
con cool,i
temperature (ºF). For cocurrent flow, this equation becomes
e (
T − T ) − ( T con − T cool, o)
cool,
i
e ( [
∆T LM = (9b)
ln T − T ) ( T con − T cool, o)]
cool,
i
Assume that the approach temperature at the condenser exit is 15ºF. In other words,
T = (T − 15) (9c)
cool,i con
Also, the temperature rise of the coolant fluid is specified as 25ºF; that is,
T = (T + 25) (9d)
cool,o cool,i
where T is the coolant exit temperature.
cool,o
In estimating A , the overall heat transfer coefficient can be conservatively assumed
con
2
as 20 Btu/h-ft ºF. The actual value will depend on the specific system under consider-
ation. This calculation is based on refs. 2 and 6, which report guidelines on typical
overall heat transfer coefficients for condensing vapor–liquid media.
3.5. Coolant Selection and Coolant Flow Rate
The next step is to select the coolant based on the condensation temperature required.
Use Table 1 to specify the type of coolant. For additional information on coolants and
other properties, see refs. 3 and 7.
The heat extracted from the emission stream is transferred to the coolant. From the
energy balance, the flow rate of the coolant can be calculated as follows:

Q = H load [ C T ( − T )] (10)
coolant p coolant cool, o cool, i