Page 175 - Design and Operation of Heat Exchangers and their Networks

Thermal design of evaporators and condensers 163

The validity range of Eqs. (4.62), (4.63) is: 3.5 ρ l /ρ g 1500, 0.005
p/p cr 0.8.

4.1.2.7 Nucleate flow boiling for upward flow in vertical tubes
The nucleate flow boiling heat transfer coefficient for upward flow in a ver-
tical tube can be evaluated from the following equations:

α b,up
F
¼ C F F q F p r d F w (4.64)
α 0
where α 0 is the heat transfer coefficient for a specific fluid at q 0 ¼20,000W/m 2
and p r0 ¼0.1. The values of α 0 for many fluids 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 1 of Kind and Saito (2013) or
evaluated with
0:27
ð Þ ,2:5 (4.65)
C F ¼ min 0:435 M=M H 2
n
ð
F q ¼ q=q 0 Þ (4.66)
For inorganic fluids, hydrocarbons and halocarbons
n ¼ 0:8 0:1 10 0:76p r (4.67)

For cryogenic fluids
n ¼ 0:7 0:13 10 0:48p r (4.68)

The relationship of the heat transfer coefficient to reduced pressure
p r ¼p/p cr is expressed by
1:7
¼ 2:816p 0:45 +3:4+ p 3:7 (4.69)
r 7 r
F p r
1 p
r
The effect of the surface roughness is modified by
0:133
ð
F w ¼ R a =R a0 Þ (4.70)
with R a0 ¼1μm.
The relationship to tube diameter is
0:4
F d ¼ d 0 =dð Þ (4.71)
with d 0 ¼0.01m.
The range of validity of the earlier equations is 1mm d 32mm,
0.001 p r 0.985, 0.005μm R a 5μm.