Increase heat and
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Decrese heat,
increase pressus
Page No.
Answers
Answer:
Explanation:
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. The
methods consider whether the exchanger is performing correctly to begin with, excess
pressure drop capacity in existing exchangers, the re-evaluation of fouling factors and
their effect on exchanger calculations, and the use of augmented surfaces and
enhanced heat transfer. Three examples are provided to show how commercial
process simulation programs and shell-and-tube exchanger rating programs may be
used to evaluate these exchanger performance issues. The last example shows how
novel heat transfer enhancement can be evaluated using basic shell-and-tube
exchanger rating calculations along with vendor supplied enhancement factors.
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, ∆T. This equation is found in nearly all
heat exchanger design references1-3.
As stated in this form, U can be calculated from thermodynamic considerations alone. This calculation results in
the required U such that the heat is transferred at the stated driving force and area. Independent of this required
U based on thermodynamics, an available U can be determined from transport considerations. For this
calculation, U is a function of the heat transfer film coefficients, h, the metal thermal conductivity, k, and any
fouling considerations, f. An exchanger usually operates correctly if the value of U available exceeds the U
required.
For basic shell-and-tube exchangers, there are a number of literature sources that describe how to estimate heat
transfer film coefficients based on the flow regime and the type of phase change: boiling or condensing1-4. As a
point of reference, Table 1 shows some typical values for the different film coefficients.
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. The
methods consider whether the exchanger is performing correctly to begin with, excess
pressure drop capacity in existing exchangers, the re-evaluation of fouling factors and
their effect on exchanger calculations, and the use of augmented surfaces and
enhanced heat transfer. Three examples are provided to show how commercial
process simulation programs and shell-and-tube exchanger rating programs may be
used to evaluate these exchanger performance issues. The last example shows how
novel heat transfer enhancement can be evaluated using basic shell-and-tube
exchanger rating calculations along with vendor supplied enhancement factors.
Hydrocarbon Engineering, March 1998
Bryan Research & Engineering, Inc.
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Table 1
Examples of Heat Transfer Film Coefficients