Page 194 - Design and Operation of Heat Exchangers and their Networks

182 Design and operation of heat exchangers and their networks

the mean vapor mass fraction (between inlet and outlet) in the test section.
The correlation equation for R22 is given as
αd h,b _ x 0:01 0:19 0:3
ð
Nu ¼ ¼ 24:50:9+ _xÞ Re Pr (4.216)
eq
λ l
Longo (2010a) presented the heat transfer coefficients and the pressure
drop measured during saturated vapor condensation of refrigerants
R236fa, R134a (early reported by Longo and Gasparella, 2007 and Longo,
2008) and R410A (early reported by Longo, 2009) inside a brazed plate heat
exchanger with chevron angle of β¼65degrees. The experimental heat
transfer coefficients have been compared against the classical Nusselt analysis
for laminar film condensation on vertical surface (gravity controlled)

2 3 1=4
αL gρ Δh v L
l
Nu ¼ ¼ ψNu Nusselt ¼ ψ0:943 Re eq 1600 (4.217)
λ l λ l μ ΔT
l
and the equation of Akers et al. (see Longo, 2010a) for forced convection
condensation inside tube
ψ ð L ψ ð L
αd h,b 1=3 1=3
Nu ¼ ¼ Nu x,Akers dx ¼ 5:03Re eq Pr l dx Re eq 1700
λ l L 0 L 0
(4.218)

where the equivalent Reynolds number is defined by Eq. (4.190). It is found
that the deviations from the experimental data are acceptable. The deviation
for gravity-controlled condensation and for forced convection condensation
is about 10% and 20%, respectively. The condensation frictional pressure
drop of R236fa, R134a, and R410A is expressed against the kinetic energy
per unit volume of the refrigerant flow computed by the homogeneous
model (see Eq. 4.192 for ρ m ):

Δp f KE=V
¼ 2:00 (4.219)
1 kPa 1J=m 3
in which the kinetic energy per unit volume is defined as

G 2
KE=V ¼ (4.220)
2ρ m
This implies that the condensation friction factor is a constant, f¼500d h,b /L.
However, it seems to be suitable only for a special brazed plate heat
exchanger. Longo (2010b) further presents the experimental results for