Page 64 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

60 Chapter 3 Double pipe heat exchanger

for the inner fluid
G o D e
(3.15b)
m o;average
and Re o ¼
based on D e for the annulus
Turbulent flow
0:32
(3.16a)
Flow in tubes; with 5 % tolerance: f i ¼ 0:0014 þ 0:125=ðReÞ
0:42
Flow in clean; iron and steel pipes; with 10 % tolerance: f i ¼ 0:0035 þ 0:246=ðReÞ (3.16b)
Laminar flow
Flow in tubes: f i ¼ 16=Re i (3.16c)
and for the outer fluid

2 3
2
1 ðD o =D io Þ
4 5 (3.16d)
f o ¼ð16=Re o Þ
2 2
1 þðD o =D io Þ þ 1 ðD o =D io Þ =lnðD o =D io Þ
For longitudinal finned tubes, the friction factor for the annular region is
h i
2
f of ¼ exp 0:08172ðlnRe of Þ 1:7434ðlnRe of Þ 0:6806 for ðRe > 400Þ (3.16e)
and
f of ¼ 16=Re of for ðRe 400Þ (3.16f)

Since fins tend to destabilise laminar flow, the critical Reynolds number is 400 in the finned
annulus.
A minor modification is often made to Eq. 3.14 by incorporating a viscosity correction
factor (f) to account for the effect of variable fluid property on friction factor in non-isothermal
flow, viz

0:14

m
for laminar flow (3.17a)
f ¼
m w
0:25
m
for turbulent flow (3.17b)
f ¼
m w
This modifies the pressure drop equation (Eq. 3.14a,b) for the outer and inner fluid as
2
4f o G L o 1
o
DH f;o ¼ 2 (3.18a)
2gr De 0 f
o