Page 90 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

4.4 Design e F T method 87

While the above expressions are more accurate, it can be shown that F T value for a 1e2 and a 1e8
exchanger for the same service differ by a maximum of 2% and often any exchanger having one
shell pass and two or more even-numbered tube passes can be treated as a 1e2 exchanger with F T
as

(4.3)
F T ¼ðX=YÞ
where
q ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
X ¼ ðR þ 1Þ lnfð1 SÞ=ð1 RSÞg (4.4)
and

8 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 9
p
2
ðR þ 1Þ =
<2 S R þ 1
Y ¼ðR 1Þ ln (4.5)
p ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
: ;
2 S R þ 1 þ ðR þ 1Þ
A low value of F T means higher surface requirement for the exchanger being designed. The
design value of F T is always kept above 0.8. For F T < 0:8, there is a ‘temperature cross’ with heat
transfer from the cold stream back to the hot fluid at some point within the exchanger. In addition,
the slope of F T versus S curve becomes very steep under this condition, making the exchanger
performance sensitive to any changes in design temperatures. However, the value of 0.8 is only
approximate and higher values of F T are required at very large and very small values of R. F T can
be increased by using several shells in series or by increasing the numbers of passes in the same
shell (up to a practical limit of 6). Nevertheless, the curves become steeper with increasing
number of shell passes and require higher F T values. In extreme cases, counterflow exchangers
with F T ¼ 1 may be the only practical configuration. F T is also equal to 1 if either of the fluid is
isothermal as in case of condensation of saturated steam. Usually the single shell arrangement is
more economical, even with more complex internals.
It is important to note that Eqs. (4.3)e(4.5) has been derived from Eqs. (4.1) and (4.2) for n ¼ 1.
In case a single shell pass does not serve the design, this is reflected not only by a value of F T
lower than 0.8 but also an imaginary value of Y from Eq. (4.5). Higher numbers of shell passes
have to be opted in such cases and F T calculated from Eq. (4.1) or (4.2).
6. Based on estimated A o , decide the exchanger layout and geometry e tube details (pitch,
dimension and passes). Table 4.5 may be referred for this. As a first guess, 19.05 mm or 25.4 mm
may be adopted with P T ¼ 1.25 D o .
Tubes of different material can be chosen from Appendix Table F1.A, which presents the
specifications as per TEMA code. Tube diameters and thickness as per IS 4503:1967 is shown in
Appendix Table F1.B.
7. Decide on exchanger type. This may be based on information summarised in Table 4.6.
8. Estimate the shell inner diameter D s from the expression

s ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
CL A o P 2 T
D s ¼ 0:637 (4.6)
CTP D o L e