Page 335 - Mechanical Engineers' Handbook (Volume 4)

324 Heat Exchangers, Vaporizers, Condensers

removal of liquid from the reboiler (blowdown) should be provided to ensure good operation.
Even for thermosiphon reboilers, if designed for low heat fluxes (below about 2000 BTU/
hr/ft , 6300 W/m ), the circulation through the reboiler may not be high enough to prevent
2
2
heavy components from building up, and some provision for blowdown may be advisable
in the bottom header.

5 USE OF COMPUTERS IN THERMAL DESIGN OF PROCESS HEAT EXCHANGERS
5.1 Introduction
The approximate methods for heat transfer coefficient and pressure drop given in the pre-
ceding sections will be used mostly for orientation. For an actual heat exchanger design, it
only makes sense to use a computer. Standard programs can be obtained for most geometries
in practical use. These allow reiterations and incrementation to an extent impossible by hand
and also supply physical properties for a wide range of industrial fluids. However, computer
programs by no means solve the whole problem of producing a workable efficient heat
exchanger. Many experience-guided decisions must be made both in selection of the input
data and in interpreting the output data before even the thermal design can be considered
final. We will first review why a computer program is effective. This has to do with (1)
incrementation and (2) convergence loops.

5.2 Incrementation

The method described in Section 2.1 for calculation of required surface can only be applied
accurately to the entire exchanger if the overall heat transfer coefficient is constant and the
temperature profiles for both streams are linear. This often is not a good approximation for
typical process heat exchangers because of variation in physical properties and/or vapor
fraction along the exchanger length. The rigorous expression for Eq. (1) is as follows:
A dQ
o
U MTD
o
Practical solution of this integral equation requires dividing the heat transfer process into
finite increments of Q that are small enough so that U may be considered constant and
o
the temperature profiles may be considered linear. The incremental area, A , is then cal-
o
culated for each increment and summed to obtain the total required area. An analogous
procedure is followed for the pressure drop. This procedure requires determining a full set
of fluid physical properties for all phases of both fluids in each increment and the tedious
calculations can be performed much more efficiently by computer. Furthermore, in each
increment several trial and error convergence loops may be required, as discussed next.

5.3 Main Convergence Loops
Within each of the increments discussed above, a number of implicit equations must be
solved, requiring convergence loops. The two main types of loops found in any heat ex-
changer calculation are as follows.

Intermediate Temperature Loops
These convergence loops normally are used to determine either wall temperature or, less
commonly, interface temperature. The discussion here will be limited to the simpler case of