Page 176 - Design and Operation of Heat Exchangers and their Networks

164 Design and operation of heat exchangers and their networks

4.1.2.8 Nucleate flow boiling for downward flow in vertical tubes
The nucleate flow boiling heat transfer coefficient for downward flow in a
vertical tube is less than that in upward flow in accordance with the relation-
ship as

α b,down ¼ 0:75α b,up (4.72)
4.1.2.9 Nucleate flow boiling in horizontal tubes with thick
tube wall, λ w δ t ≥0.7W/K
For flow boiling in horizontal tubes, the circumferential wall temperature
will not be uniform, and the wall surface might not be wetted completely.
Therefore, the heat transfer correlation differs from that in vertical tubes.
The nucleate flow boiling heat transfer coefficient in a horizontal tube with
λ w δ t 0.7W/K can be expressed as

α b,h α b,up
¼ min C F,h F q,h F p r ,h F d,h F w F G , (4.73)
α 0 α 0
where α 0 can be found in Table 2 of Kind and Saito (2013). In the absence of
data, α 0 can be evaluated by Eq. (4.14). C F can be obtained from Table 3 of
Kind and Saito (2013) or evaluated with

0:11
ð Þ ,2:5 (4.74)
C F,h ¼ min 0:789 M=M H 2
n h
ð
F q,h ¼ q=q 0 Þ (4.75)
For inorganic fluids, hydrocarbons and halocarbons
n h ¼ 0:9 0:36p 0:13 (4.76)
r
For cryogenic fluids

n h ¼ 0:9 0:44p 0:085 (4.77)
r
where the reduced pressure p r ¼p/p cr .
The relationship of the heat transfer coefficient to reduced pressure is
expressed by

1:6p 6:5
¼ 2:692p 0:43 + r (4.78)
r 4:4
F p r
1 p
r
The relationship to tube diameter is
0:5
F d ¼ d 0 =dð Þ (4.79)