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Multiple shell and tube heat exchangers in the series are employed to handle the temperature cross in the chemical process industries. Depending on the degree of temperature cross, certain number of heat exchangers (either E or F shell type)
Multiple shell and tube heat exchangers in the series are employed to handle the temperature cross in the chemical process industries. Depending on the degree of temperature cross, certain number of heat exchangers (either E or F shell type) need to be connected in series such that the temperature cross in each exchanger is within allow-able limit. Determination of the number of exchangers for the given terminal temperatures is essential during heat exchanger design phase. In this paper, using finite difference calculus, modeling has been done to calculate the number of shells required for both E and F shell cases. In addition, equations are developed to determine hot and cold fluid temperature profiles across all heat exchangers. Design procedure is illustrated with the help of a case study and the capital cost of both cases is compared. Issues related to E shell and F shells are also discussed.17091
© 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Keywords: Multiple shells in series; Heat exchanger design; Temperature cross; E shell; F shell; Finite difference
1. Introduction
There are several shell configurations designated as E, F, G, H,
J, K and X by the Tubular Exchanger Manufactures’ Associa-
tion Inc. These are described in detail in literature (Perry and
Green, 1997). E shell is a single-pass shell, and the number
of tube passes may be one or multiples of two (two is most
common). The shell side fluid enters at one end and leaves
the other end of the opposite side. F shell is a two-pass shell
that has a longitudinal baffle piding the shell into two com-
partments, shell fluid enters at one compartment, travels the
entire length of the shell through that compartment, turns
around and flows through the another compartment of the
shell and finally leaves at the same end of the other side.
The number of tube passes for F shell may be two or mul- 源自六"维%论:文*网!加7位QQ324'9114 www.lwfree.cn
tiples of four (four is most common). Considering first the
1–2 heat exchanger in Fig. 1(a), the tube fluid in the first tube
pass is in parallel with the shell fluid, and in the second tube
pass the tube fluid is in the counter flow with the shell fluid.
Hence, the log mean temperature difference (LMTD), which
applies to either parallel or counter flow but not to a mix-
ture of both types, cannot be used to calculate the true oreffective mean temperature difference (EMTD) without cor-
rection. Similarly for 2–4 heat exchanger, as may be seen in
Fig. 1(b), contact between shell fluid and tube fluid is a mix-
ture of both parallel and counter flows and hence correction
factor is necessary to get the EMTD. This EMTD is generally
obtained by multiplying LMTD of true counter current flow
by a flow correction factor (FT). This factor is correlated in
terms of two dimensionless ratios, R and P by Nagle (1933)
and Underwood (1934). For instance, derivations can be found
in Kern (1997) and following assumptions were made dur-
ing the derivation: stream flows are at steady state, overall
heat transfer coefficient and specific heat remain constant
throughout the exchanger, there is no phase change and heat
losses are negligible. Eqs. (1) and (2), Kern (1997) are used
to calculate FT for 1–2 exchanger and FT for 2–4 exchanger,
respectively.
For 1–2 heat exchangerNomenclature
a, b, c cost law coefficients
bc cost of base line exchanger
A heat transfer area (m2)
AT heat transfer area of total exchangers (m2)
B arbitrary constant
C capital cost of total heat exchangers
cp specific heat of cold fluid (kJ/kgK)
Cp specific heat of hot fluid (kJ/kgK)
Cpn average specific heat of hot fluid for nth
exchanger (kJ/kgK)
Eb f.o.b. price of total heat exchangers on January 管壳换热器英文文献和中文翻译:http://www.lwfree.cn/fanyi/20180107/18633.html
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