Page 98 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

4.4 Design e F T method 95

Nu De calculated from Eq. (4.11) gives h o as
Nu De k s
(4.13)
h o ¼
De
h o does not change much from initial estimate unless the number of tubes changes by a large
2
amount. h o z5000 W m K is often used as an initial guess.
In case of single-phase heat transfer, the following nature of variation with respect to shell-side
and tube-side fluid mass velocities is considered
Turbulent flow:
h o a G 0:55 and h i a G 0:8 (4.14a)
s t
Laminar flow:
h o a G 0:85 and h i a G 0:33 (4.14b)
s t
Heat transfer coefficients in case of condensation and boiling are only slightly affected by mass
velocity and depend primarily on the physical properties of the fluid. Therefore, these
coefficients are often considered to be almost constant.
12. Calculate the overall heat transfer coefficient U from Eq. (3.3) and compare it with the design
U D value
1 1 1 R di A total A total
(3.3)
¼ R do þ
þ R w þ
þ
U h o E f;effective A i h i A i
where R w , the tube wall resistance, is calculated for bare tubes as

D o D o
ln (4.15a)
R w ¼
2k w D o 2t
and for integral circumferentially finned tube as

t D o þ 2N f h f ðD o þ h f Þ
ln (4.15b)
R w ¼
k w D o t
D o is bare tube outside or root diameter (mm), h f is fin height (mm), t is tube wall thickness (mm),
2
N f is the number of fins per mm and k w is the tube material thermal conductivity (W/m K). The R d
needs to be in the range suggested for similar service (see Table 2.4).
13. Check if U > U D.
This ensures that the exchanger will have a margin in the heat transfer load Q when design inlet
temperatures, fluid flow rates and properties are set. If the calculated value differs significantly,
return to Step 5 with the calculated U.
If the estimated duty falls below the required duty, then as next trial, the exchanger physical
details are altered e.g. increase in the number of passes, increased tube length, etc. In some cases,
interchanging the shell-side and the tube-side fluids can also be an alternative.
14. Optimise the design: repeat Steps 5 to 10, as necessary, to determine the cheapest exchanger that
will satisfy the duty. Usually that corresponds to the one with the smallest area.