14-06-2012, 05:35 PM
Increasing Heat Exchanger Performance
ABSTRACT
Engineers are continually being asked to improve processes and increase efficiency.
These requests may arise as a result of the need to increase process throughput,
increase profitability, or accommodate capital limitations. Processes which use heat
transfer equipment must frequently be improved for these reasons. This paper
provides some methods for increasing shell-and-tube exchanger performance.
INTRODUCTION
Increasing heat exchanger performance usually means transferring more duty or operating the exchanger at a
closer temperature approach. This can be accomplished without a dramatic increase in surface area. This
constraint directly translates to increasing the overall heat transfer coefficient, U. The overall heat transfer
coefficient is related to the surface area, A, duty, Q, and driving force, ΔT. This equation is found in nearly all
heat exchanger design references1-3.
Enhanced surfaces
Since there are so many different types of heat exchanger enhancements, it is highly unlikely that a commercial
simulator could support them all. Furthermore, some propriety data from the manufacturers of the heat transfer
enhancement might never be released. However, that does not mean that process and project engineers can not
perform some of the preliminary calculations for new technologies.
The following provides background information on many different types of heat exchanger enhancements. Heat
exchanger enhancement must always satisfy the primary goal of providing a cost advantage relative to the use of
a conventional heat exchanger6. Other factors that should be addressed include fouling potential, reliability and
safety.
Finning
Tubes can be finned on both the interior and exterior. This is probably the oldest form of heat transfer
enhancement. Finning is usually desirable when the fluid has a relatively low heat transfer film coefficient as does
a gas. The fin not only increases the film coefficient with added turbulence but also increases the heat transfer
surface area. This added performance results in higher pressure drop. However, as with any additional surface
area, the fin area must be adjusted by an efficiency. This fin efficiency leads to an optimum fin height with respect
to heat transfer. Most of the heat transfer and film coefficients for finned tubes are available in the open literature
and supported in most commercial heat exchanger rating packages. Recent papers also describe predicting
finned tube performance10. Data for the performance of low finned tubes as compared to generalized correlations
are also available in the literature11.
Tube Inserts
Inserts, turbulators, or static mixers are inserted into the tube to promote turbulence. These devices are most
effective with high viscosity fluids in a laminar flow regime9,12-15. Increases in the heat transfer film coefficients
can be as high as five times. Inserts are used most often with liquid heat transfer and to promote boiling. Inserts
are not usually effective for condensing in the tube and almost always increase pressure drop. Because of the
complex relationships between the geometry of the insert and the resulting increase in heat transfer and pressure
drop, there are no general correlations to predict enhancements. However, through the modification of the
number of passes, a resulting heat transfer coefficient gain can be achieved at lower pressure drop in some
situations9.
CONCLUSION
Engineers can evaluate increasing heat exchanger performance through a logical series of steps. The first step
considers if the exchanger is initially operating correctly. The second step considers increasing pressure drop if
available in exchangers with single-phase heat transfer. Increased velocity results in higher heat transfer
coefficients, which may be sufficient to improve performance. Next, a critical evaluation of the estimated fouling
factors should be considered. Heat exchanger performance can be increased with periodic cleaning and less
conservative fouling factors.