Page 189 - Design and Operation of Heat Exchangers and their Networks

Thermal design of evaporators and condensers 177

The condensation heat transfer coefficient for downward flow in vertical
and horizontal tubes can be expressed as
8
< α I for RegimeI
α ¼ α I + α Nu for RegimeII (4.174)
:
α Nu for RegimeIII
where
0:0058 + 0:557p r
α I 0:8 0:04 0:76 0:38 μ l
ð
¼ 1 _xÞ +3:81 _xÞ _ x =p (4.175)
ð
r 14μ
α lo
v

3 1=3
gρ ρ ρ Þλ
ð
v
l
l
α Nu ¼ 1:32 l (4.176)
2
μ Re l
l
λ l 0:8 0:4
α lo ¼ 0:023Re lo Pr l (4.177)
d
Re lo ¼ Gd=μ (4.178)
l
Re l ¼ G 1 _xð Þd=μ l (4.179)
However, if We l <20, some data were underpredicted.
For horizontal tubes, the heat transfer coefficient can also be determined
according to the flow pattern (Shah, 2016b):
8
for intermittent,annular, and mist flow
α I
<
α ¼ α I + α Nu for wavy flow (4.180)
:
α Nu for stratified flow
For condensation heat transfer in mini-/microchannels (Shah, 2016a),
the heat transfer coefficient in Regime I (Eq. 4.175) is replaced by the cor-
relation of Cavallini et al. (2006)
0:3685 0:2363 2:144
α I 0:817 ρ l μ l 1 μ v 0:1
¼ 1+1:128_x Pr l (4.181)
α lo ρ v μ v μ l
For inclined tubes, the heat transfer coefficient of an inclined tube with
inclination of 30 to +30degrees is equal to that of the horizontal tubes. In
the range of the inclination of +30 to +90degrees (downward flow) a linear
interpolation between the heat transfer coefficient of the horizontal tube and
that of the vertical tube can be applied
α θ ¼ α θ¼0 degrees for 30degrees θ 30degrees (4.182)

α θ ¼ α θ¼0 degrees + α θ¼90 degrees α θ¼0 degrees θ 30ð Þ=60for 30degrees θ
90degrees
(4.183)