Page 173 - Design and Operation of Heat Exchangers and their Networks

Thermal design of evaporators and condensers 161

2 3 1=2
2
ρ j j l
g g
K ¼ 4 5 (4.56)
g 1 ρ =ρ μ cosθ
g
l
l
where (dp/dz) l designates the pressure drop of the liquid flowing alone in the
tube and (dp/dz) g is the pressure drop of the gas flowing alone in the tube. θ
is the angle between the tube axis and the horizontal, positive for downward
flow. The transition curves are correlated according to the data taken from
Fig. 4 of Taitel and Dukler (1976) as

2 3
ð
lg f T ¼ 0:095456 0:08997lgX 0:03097 lgXÞ 0:0034326 lgXÞ
ð
(4.57)

2 3
ð
lg f F ¼ 0:67728 + 0:83232lgX +0:25945 lgXð Þ +0:027107 lgXÞ
(4.58)
2
lg f K ¼ 0:77997 0:11641lgX 0:2378 lgXð Þ
3 4
ð
ð
+0:0064732 lgXÞ +0:010108 lgXÞ (4.59)
in which X is the Lockhart-Martinelli parameter
s ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ð dp=dzÞ l
X ¼ (4.60)
ð dp=dzÞ
g
According to Fig. 4.3, the flow regime for two-phase flow in a horizontal
tube can be determined as follows:

Bubble flow : X 1:6 andT f T
Intermittent flow : X 1:6,T < f T and F f F
Stratified flow : F < f F andK < f K
Wavy flow : F < f F andK f K
Annular flow : X < 1:6and F f F

4.1.2.4 Flow boiling heat transfer
The calculation method for saturated flow boiling heat transfer coefficient
has been developed by Kind and Saito (2013) and will be introduced as
follows:

q ffiffiffiffiffiffiffiffiffiffiffiffiffiffi
3 3 3
α ¼ α + α (4.61)
c b
where α c is convective flow boiling heat transfer coefficient and α b is nucle-
ate flow boiling heat transfer coefficient.