Page 181 - Design and Operation of Heat Exchangers and their Networks

Thermal design of evaporators and condensers 169

mentioned earlier and proposed a correlation for heat transfer coefficient in
the subcooled flow boiling of R134a in the PHE as
Nu sub =Nu lo ¼ 1:2Fr 0:75 +13:5Bo 1=3 Ja 1=4 (4.116)

where
G 2
Fr ¼ (4.117)
2
ρ gd
l h,b
c p Δt sat ρ l
Ja ¼ (4.118)
Δh v ρ g
with △t s ¼t w,m t s and t w,m ¼(t w,i t w,onb )/2. The position of onset of
nucleate boiling (onb) is estimated from the flow visualization.
Later, Hsieh and Lin (2003) proposed a new form of the correlations for
R410A evaporation in plate heat exchangers as

α ¼ Eα lo + Sα pool 2000 < Re lo < 12;000, 0:0002 < Bo < 0:002ð Þ (4.119)
in which the Dittus-Boelter correlation (Dittus and Boelter, 1930) is used for
α l (from Eq. 2.32)
λ l 0:8 0:4
α lo ¼ 0:023Re Pr (4.120)
lo l
d h,b
and the Cooper correlation (Cooper, 1984) is used for α pool
α pool ¼ 55q 0:67 M 0:5 p r 0:12 0:2lgR a ½ lg p r 0:55 ð 4:11Þ, (4.121)
ðފ
2
2
with α pool in W/m K, q in W/m , molecular mass M in kg/kmol, roughness
R a in μm, and p r ¼p/p cr . The correlations of the enhancement factor E and
the suppression factor S are given as
4
E ¼ 1+ 2:4 10 Bo 1:16 +1:37X 0:86 (4.122)
tt
6 2 1:17 1

S ¼ 1+1:15 10 E Re (4.123)
lo
The correlation for the fraction factor differs from their earlier work and
is given as
Δp f ρ d h,b 1:12
m
f ¼ ¼ 23,820Re eq (4.124)
2
2G L
The expression of the Lockhart-Martinelli parameter X tt is not given. It
can be calculated according to its definition, Eq. (4.60). In their work, a