Page 193 - Design and Operation of Heat Exchangers and their Networks

Thermal design of evaporators and condensers 181

Jokar et al. (2004) carried out the experiments on condensation heat
transfer of R134a in two brazed plate heat exchangers with the same plates
of 311mm by 112mm, chevron angle of β¼60degrees but different num-
bers of plates (i.e., 40 and 54). The refrigerant vapor was completely con-
densed in the condenser. The single-phase flow correlations for Nusselt
number and friction factor are given in Eqs. (4.139), (4.140). They used
the model of Wang and Zhao (1993), Eq. (4.186), for the condensation heat
transfer correlation, which yielded

αd h,b 0:622 1 0:33 0:248
Nu ¼ ¼ 0:0336Re H Pr ð ρ =ρ Þ (4.211)
lo l l v
λ l
where Re lo and H are defined by Eqs. (4.187), (4.188), respectively. Later,
Jokar et al. (2006) provided another correlation

αd h,b
Nu ¼
λ l
1:3
2 2 1:05 2 0:05 2
G ρ Δh v ρ σ ρ l
¼ 3:371Re 0:55 Pr 0:3 l l l
lo l 2 2
ρ c p,l ΔT G μ G ρ ρ
l l l v
(4.212)
The condensation friction factor is correlated as ( Jokar et al., 2004, 2006)
Δp f ρ d h,b 7 1:6
m
f ¼ ¼ 2:139 10 Re m ð 960 Re m 4160Þ (4.213)
2
2G L
in which
Gd h,b
Re m ¼ (4.214)
μ m
The mean density is calculated with Eq. (4.192), and the average viscosity is
defined by

h i 1
μ ¼ _x=μ v,sup +1 _xÞ=μ l,sub (4.215)
ð
m
with the mean value of _x ¼ 0:5 because in their experiments, the vapor was
completely condensed.
Jeong et al. (2007) reported the condensation heat transfer characteristics
of the brazed plate heat exchangers using refrigerant R22. The number of
plates of the two exchangers is 10 and 20, respectively. The chevron angle
β is 30degrees (how it is defined is not mentioned). The measured conden-
sation heat transfer coefficients are presented in terms of their variations with