Page 177 - Design and Operation of Heat Exchangers and their Networks

Thermal design of evaporators and condensers 165

with d 0 ¼0.01m. The relationship to surface roughness F w is given by
Eq. (4.70).
The relationship between the perimeter-averaged heat transfer coeffi-
cient and the flow parameters G and _x can be expressed as
h i
0:25 0:1 0:3
F G ¼ G=G 0 Þ 1 p q=q cr,pb _ x (4.80)
ð
r
2
where G 0 ¼100kg/m s. For p r 0.1
q cr,pb =q cr,0:1 ¼ 3:2p 0:45 ð 1 p r Þ 1:2 (4.81)
r
For p r <0.1
0:17 0:8
q cr,pb =q cr,0:1 ¼ 1:2 p + p (4.82)
r r
h i 0:5
ρ ρ ρ 0:25 Pr 0:245 at p r ¼ 0:1 (4.83)
q cr,0:1 ¼ 0:144Δh v l g g ð gσ=ρ Þ l
l
The range of validity of the earlier equations is 4mm d 25mm,
0.005μm R a 5μm, and 0.03 p r 0.93 for cryogenic fluids and
0.005 p r 0.85 for other fluids, λ w δ t 0.7W/K.

4.1.2.10 Nucleate flow boiling in horizontal tubes with thin
tube wall, λ w δ t <0.7W/K
For flow boiling in horizontal tubes, the effects of nonuniform circumfer-
ential wall temperature and partial wetted wall surface can be taken into
account by modifying the exponent n h and coefficient C F,h (Kind and Saito,
2013)
n h ¼ κn h,λ w δ t 0:7 (4.84)

C F,h ¼ ψC F,h,λ w δ t 0:7 (4.85)
where the factor κ can be written by

κ ¼ 0:675 + 0:325tanh 3:711 λ w δ t 0:0324ð½ ފ (4.86)
The value of ψ depends on the flow pattern (see Fig. 4.3).
For stratified and wavy flow

ψ ¼ 0:46 + 0:4tanh 3:387 λ w δ t 0:00862ð½ ފ (4.87)
For slug flow
ψ ¼ 0:671 + 0:329tanh 3:691 λ w δ t 0:00842ð½ ފ (4.88)
For annular flow

ψ ¼ 0:755 + 0:245tanh 3:702 λ w δ t 0:0125ð½ ފ (4.89)