Page 61 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

3.2 Design 57

Since double-pipe exchangers employ longitudinal fins, E f in Eq. 3.4 is given by Eq. 2.18 repro-
duced below
L f;eq ¼ h f þ t f =2 (2.18)

If both fluids are in turbulent flow, the heat transfer coefficients (h i ) and (h o ) for plain tubes may be
computed from the same correlation using a suitably defined
equivalent diameter (D e ); otherwise, special attention must
be given to the annular region. Referring to Table 2.6 and
Individual Heat Transfer Coefficients
using the nomenclatures defined therein, for turbulent flow,
Re > 10,000.

0:8 0:33 0:14
hD e =k ¼ 0:027 (3.6a)
w
ðD e G=mÞ ðC p m=kÞ ðm=m Þ
[Some prefer replacing 0.027 with 0.023 for double pipe]
Intermediate flow range (10,000 > Re > 2100)

h i h i
2=3 1=3 0:14 2=3
w (3.6b)
hD e =k ¼ 0:116 ðD e G=mÞ 125 ðC p m=kÞ ðm=m Þ 1 þðD i =LÞ
Laminar flow, Reð¼ D e G=mÞ<2100

1=3 0:14
w
hD e =k ¼ 1:86½ðk=D e ÞðC p m=kÞD e =LŠ ðm=m Þ (3.6c)
The heat transfer coefficients (h o ) for finned tube in the annulus has been expressed in terms of j H
!
1=3 0:14

h 0 D e C p m m r V o D e
o
factor j H ¼ as a function of Reynolds number Re o ¼
k k m m
w
by Kern and Krauss (1972).
1=3
0:9145 7 2:618
j H ¼ 0:0263Re þ 4:9 10 Re for N f ¼ 24 (3.7a)
o o
1=3
1:032 7 2:618
j H ¼ 0:0116Re o þ 4:9 10 Re o for N f ¼ 36 (3.7b)
Eq. 3.7a and 3.7b predict nearly the same values of j H for Re o > 1000.
Fluid flow properties usually are functions of the flow temperature and may be evaluated at the
caloric temperature. If the temperature difference of flow is moderate or the fluids have a viscosity less
than 1 cP at cold terminal temperature, T f,avg (arithmetic average temperature) is used instead of
caloric temperature.
m w in the Sieder-Tate correction factor ðm=m Þ of Eq. 3.6 is estimated at the average wall tem-
w
perature of the inner pipe given by