Page 205 - Design and Operation of Heat Exchangers and their Networks

194 Design and operation of heat exchangers and their networks

One of the most important inputs for the heat transfer and pressure drop
evaluation is the magnitude of “scaled” j and f factors for compact heat
exchanger surfaces. As soon as one of the geometrical dimensions is changed
(such as fin pitch, height, or thickness), the surface is no longer geometrically
similar to the original surface for which experimental j and f data are avail-
able. In such cases, either theoretical or experimental correlation available to
designer should be incorporated in the computer program derive “scaled”
j and f factors to the new geometry. Care must be exercised to avoid exces-
sive extrapolation.
The optimization search for each variable may take many trials. For each
trial, a complete set of calculations may be performed to evaluate heat trans-
fer and pressure drop. If there are several variables for optimization, a total
number of heat exchanger rating calculations may be very large. However,
the computer time for such optimization is relatively low.
Finally, to express the objective function mathematically may be a very
difficult task. Input from many departments of industry, or from a number of
different industries may be needed to arrive at the mathematical objective
function. This function must be reviewed from time to time to reflect
changes, which may have taken place.

5.2 Optimal design of shell-and-tube heat exchangers
Shell-and-tube heat exchangers are the most common type of thermal
equipment employed in chemical process industries. The design of shell-
and-tube heat exchangers involves the determination of thermalhydraulic
performances in both tube and shell sides. The correlations for heat transfer
performance and pressure drop in the tube side are available in Chapter 2.
The calculation of the shell-side heat transfer performance and pressure drop
will be illustrated in the following sections.
The optimal design of shell-and-tube heat exchangers involves many
parameters. They can be divided into two sets: the user-defined parameters
and the parameters to be optimized. For example, in the work of Costa and
Queiroz (2008), the fluid allocation, materials, shell type, head type, baffle
type, tube pitch ratio, tube layout pattern, and tube wall thickness were clas-
sified as the user-defined parameters, and the shell inner diameter, total
number of tubes, number of tube passes, tube length, tube outside diameter,
baffle spacing, and baffle cut were taken as the optimization variables.