Page 188 - Design and Operation of Heat Exchangers and their Networks

176 Design and operation of heat exchangers and their networks

4.2.3 Condensation in tubes
A dimensionless correlation for predicting condensation heat transfer coef-
ficients inside tubes was presented by Shah (1979) as
αd 0:8
_ x
Nu ¼ ¼ 0:023Re 0:8 Pr 0:4 ð 1 _xÞ +3:81 _xÞ 0:04 0:76 =p 0:38
ð
lo l r
λ l
(4.166)
where Re lo ¼Gd/μ l , p r ¼p/p cr .
The validity range of Eq. (4.166) is 7 d 40mm, 0.002 p r 0.44,
2
21 t s 310°C, 3 u g 300m/s, 0 _x <1, 10.8 G<210.6kg/m s,
6 2
158 q<1.893 10 W/m , 100 Re lo 63,000, and 1 Pr l 13.
Later, Shah (2009) improved his correlation by identifying three heat
transfer regimes. For downward flow in vertical tubes, Regime I occurs
when
1
J g , (4.167)
2:4Z +0:73
where the dimensionless superficial gas velocity is defined as

_ xG
J g ¼ p ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi (4.168)
gdρ ρ ρð l v Þ
v
and Shah’s correcting parameter Z is defined as

0:8 0:4
Z ¼ 1=_x 1Þ p (4.169)
ð
r
Regime III occurs when
J g 0:89 0:93e 0:087=Z 1:17 (4.170)

For horizontal channels (2016a), Regime I occurs when
2
G d h
We l ¼ > 100 (4.171)
ρ σ
l
0:62
and J g 0:98 Z +0:263ð Þ (4.172)
Regime III occurs if
0:95
J g (4.173)
1:254 + 2:27Z 1:249
For both cases, Regime II lies between Regime I and III.