Page 323 - Mechanical Engineers' Handbook (Volume 4)

312 Heat Exchangers, Vaporizers, Condensers

Correction for Mixture Effects
The above heat-transfer coefficients apply only to the condensate film. For mixtures with a
significant difference between the dew-point and bubble-point temperatures (condensing
range), the vapor-phase heat-transfer coefficient must also be considered as follows:
1
h (44)
c
(1/h cƒ 1/h )
v
The vapor-phase heat-transfer rate depends on mass diffusion rates in the vapor. The well-
known Colburn–Hougen method and other more recent approaches are summarized by
Butterworth. 19 Methods for mixtures forming immiscible condensates are discussed in
Ref. 20.
Diffusion-type methods require physical properties not usually available to the designer
except for simple systems. Therefore, the vapor-phase heat-transfer coefficient is often esti-
mated in practice by a ‘‘resistance-proration’’-type method such as the Bell–Ghaly method. 21
In these methods the vapor-phase resistance is prorated with respect to the relative amount
of duty required for sensible cooling of the vapor, resulting in the following expression:
h n
q
t
v
q sv h sv (44a)
The exponent n can range from about 0.7 to 1.0 depending on the amount of mixing of the
light and heavy components. Use n 1.0 for a well-mixed (high-velocity) vapor and decrease
n for low-velocity systems with large molecular weight range.
For more detail in application of the resistance proration method for mixtures, see Ref.
14 or 21.

Pressure Drop
For the condensing vapor, pressure drop is composed of three components—friction, mo-
mentum, and static head—as covered in Ref. 14. An approximate estimate on the conser-
vative side can be obtained in terms of the friction component, using the Martinelli separated
flow approach:
P P 2 l (45)
ƒ
l
where P two-phase friction pressure drop
ƒ
P friction loss for liquid phase alone
l
The Martinelli factor 2 l may be calculated as shown in Eq. (32). Alternative methods for
23
shellside pressure drop are presented by Diehl and by Grant and Chisholm. These methods
22
24
were reviewed by Ishihara and found reasonably representative of the available data. How-
ever, Eq. (32), also evaluated in Ref. 24 for shellside flow, should give about equivalent
results.
3.3 Shell and Tube Reboilers and Vaporizers
Heat exchangers are used to boil liquids in both the process and power industries. In the
process industry they are often used to supply vapors to distillation columns and are called
reboilers. The same types of exchangers are used in many applications in the power industry,
for example, to generate vapors for turbines. For simplicity these exchangers will all be
called ‘‘reboilers’’ in this section. Often the heating medium is steam, but it can also be any