Page 317 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Encyclopedia of Physical Science and Technology En007c-310 June 30, 2001 17:30

Heat Exchangers 263

to effect the necessary thermal change if the duty has been in the fundamental understanding of the details of the heat
fixed. In either case the rating program will also calculate transfer and fluid mechanics processes leads to modifica-
the pressure drops for both streams in the exchanger. tions in the structure of the heat exchanger. Particularly,
If the calculation shows that the required amount of heat there is now a concerted effort to understand and catego-
cannot be transferred or if one or both allowable pressure rize the fundamental fluid mechanical processes operating
drops are exceeded, it is necessary to select a different, in various kinds of “enhanced” surfaces. For example, it
usually larger, heat exchanger and rerate. Alternatively, has been found that a spirally fluted tube swirls the fluid
if one or both pressure drops are much smaller than al- as it flows through the tube, giving rise to secondary flows
lowable, a better selection of parameters may result in in the immediate vicinity of the surface which increase
a smaller and less costly heat exchanger, while utilizing the heat transfer coefficient with little increase in pres-
more of the available pressure drop. sure drop. Similar efforts are underway with flows outside
The design modification program takes the output from tubes and for two-phase (vaporizing or condensing) flows.
the rating program and modifies the configuration in such There is continuing development of new manufactur-
a way that the new configuration will do a “better” job of ing techniques, including explosive bonding of metal parts
solving the heat transfer problem. (e.g., tubes to tubesheets), oven brazing for compact heat
A computer-based configuration modification program exchangers (including stainless steel and titanium), and
is a complex one logically because it must determine what ceramic heat exchangers for very high temperature appli-
limits the performance of the heat exchanger and what cations.
can be done to remove that limitation without adversely In the software area, computer programs for the anal-
affecting either the cost of the exchanger or the operational ysis design of heat exchangers are moving constantly to-
characteristics of the exchanger which are satisfactory. If, ward using more fundamental and detailed calculations of
for example, it finds that the heat exchanger is limited by the actual flow field and the thermal transport within the
the amount of heat that it can transfer, the program will try flow field. The improvement in computer capabilities has
either to increase the heat transfer coefficient or to increase meantthatthedesignengineerhasdirectaccesstothemost
the area of the heat exchanger, depending on whether or highly advanced design methods at his desk rather than
not pressure drop is available. To increase the tube-side having to access a mainframe computer in batch mode.
coefficient, one can increase the number of tube passes, Meanwhile, the growing supercomputer availability is al-
thereby increasing the tube-side velocity. If shell-side heat lowing the research engineer to study the fundamental
transfer is limiting, one can try decreasing baffle spacing fluid mechanical and corresponding thermal transport pro-
or decreasing the baffle cut. To increase area, one can cesses in the complex geometries characteristic of actual
increase the length of the exchanger, or increase the shell heat exchangers.
diameter, or go to multiple shells in series or in parallel. Heat exchangers are a prime means of conserving en-
Clearly the possibilities are enormous, and the configu- ergy in process plants by exchanging heat between process
ration modification program must be very tightly written streams that need to be cooled and those that need to be
to avoid wandering off into impossible designs or loops heated. Much attention is directed toward optimization of
without an exit. A designer using a hand method makes heat conservation and recovery by selecting the proper
many decisions intuitively, based on the experience the heat exchanger network.
designer has built up. In any case, once a final design has With all the advances that have been made, there is still
been arrived at by the computer, the basic rationality and room for much more. Our knowledge of the mechanisms
approximate correctness of the solution should be verified of fouling is very limited, and meaningful predictive meth-
by an experienced designer. ods are almost nonexistent. Many important heat, mass,
and momentum transfer processes are still poorly under-
stood. Finally, some cases where the basic equations are
known still must be crudely approximated in design be-
VI. FURTHER DEVELOPMENTS IN cause the computational requirements for complete design
DESIGN AND APPLICATION still exceed those of the most powerful computers.

There is continuing rapid development in both the hard-
ware and the software of the process heat exchanger in-
dustry. New types of heat exchangers appear from time SEE ALSO THE FOLLOWING ARTICLES
to time, usually arrived at for solving a particular prob-
lem that is not quite properly or economically satisfied DISTILLATION • FLUID MIXING • HEAT TRANSFER •
by existing equipment. Even in well-known types, growth STEAM TABLES • THERMODYNAMICS