Page 190 - Design and Operation of Heat Exchangers and their Networks

178 Design and operation of heat exchangers and their networks

4.2.4 Condensation in plate heat exchangers
Condensation heat transfer in plate heat exchangers can be affected by geo-
metrical parameters, plate surface characteristics, fluid characteristics, and
operation conditions. Till now, there is no general correlation even for com-
monly used plates with herringbone corrugation. Some correlations from
different sources of the literature are listed as follows.
Wang and Zhao (1993) presented the experimental results of water steam
condensation in a plate condenser. The chevron angle β seems to be
45degrees. They used the equivalent diameter as the characteristic length,
but the definition was not declared. It might be the hydraulic diameter (from
Eq. 4.103)

d h,b ¼ 2b (4.184)
where b is the plate spacing. The heat transfer coefficient for cooling water is
given as

0:4
0:319Re 0:6425 Pr , Re 1000
α sp d h,b
Nu sp ¼ ¼ (4.185)
0:4
λ 0:3489Re 0:6418 Pr ,Re > 1000
The correlation equation for condensation is given as
αd h,b 0:983 0:33 0:248
Nu ¼ ¼ 0:00115 Re l =HÞ Pr ð ρ =ρ Þ (4.186)
ð
l l v
λ l
where
Re l ¼ G 1 _x out Þd h,b =μ (4.187)
ð
l
_ x out is the vapor mass fraction at the outlet of the plate condenser and H is a
dimensionless parameter expressing the influence of subcooling of the
condensate

ð
c p,l T s T w Þ
H ¼ (4.188)
ð
Δh v +0:68c p,l T s T w Þ
In their experiments, _x out is ranged from 0 to 0.64.
Yan et al. (1999) investigated the condensation heat transfer and pressure
drop for R134a in a plate heat exchanger with a chevron angle of
β¼60degrees. The exchanger consisted of three plates that formed two
counterflow channels, downflow of the condensing R134a in one channel
releasing heat to the cold upflow of water in the other channel. The char-
acteristic length is d h,b defined by Eq. (4.184). The vapor mass fraction
changes over the exchanger were not given. The correlation of the