Page 183 - Design and Operation of Heat Exchangers and their Networks

Thermal design of evaporators and condensers 171

Han et al. (2003a) performed experiments on evaporative heat transfer and
pressure drop in brazed plate heat exchangers with R410A and R22. Plate
heat exchangers with chevron angles of 45, 55, and 70degrees were used.
The single-phase heat transfer correlation was given as

α sp d h,ψ 0:64 0:32 0:09
Nu sp ¼ ¼ 0:295Re ψ Pr β ð β in radiantÞ (4.131)
λ
The evaporative heat transfer correlations are expressed as

αd h,ψ n 0:3 0:4
Nu ¼ ¼ CRe eq,ψ Bo eq Pr l (4.132)
λ l
where

0:041 2:83
C ¼ 2:81 l w =d h,ψ β (4.133)

0:082 0:61
n ¼ 0:746 l w =d h,ψ β (4.134)
G eq d h,ψ
Re eq,ψ ¼ (4.135)
μ l
with Eqs. (4.104), (4.107) for G eq and Bo eq , respectively.
The friction factor was correlated as
Δp f ρ d h,ψ n 0
m
0
f ¼ ¼ C Re eq,ψ (4.136)
2
2G L

β
0 5:27 3:03
C ¼ 64,710 l w =d h,ψ (4.137)

β
0 0:62 0:47
n ¼ 1:314 l w =d h,ψ (4.138)
(Note: In Eq. (12) of Han et al. (2003a),“N cp ”(N cp ¼2) should be an
error, and might be replaced with “2.”)
Jokar et al. (2006) reviewed the two-phase heat transfer and pressure
drop correlations in the literature and carried out their own experiments
on evaporation and condensation heat transfer of R134a in three plate heat
exchangers of different sizes (i.e., 34, 40, and 54 plates), with the chevron
angle of β¼60degrees. For single-phase flow, the resulting correlations
are given as

α sp d h,b 0:79 n
Nu sp ¼ ¼ 0:089Re Pr (4.139)
λ
in which n¼0.3 for cooling and n¼0.4 for heating. The hydraulic diam-
eter is taken as the characteristic length and is defined as twice the mean