Page 179 - Design and Operation of Heat Exchangers and their Networks

Thermal design of evaporators and condensers 167

ð
_ x cr,low ¼ min _x cr + Δ_x cr =2, 1Þ (4.100)
in which _x cr can be calculated with Eq. (4.96).

4.1.3 Flow boiling in plate heat exchangers
Since the advent of brazed plate heat exchangers in the 1990s, a lot of exper-
imental studies have focused on the flow boiling heat transfer in such
exchangers. In plate evaporators, the flow direction is usually upward,
and the parallel-flow arrangement is often adopted. Because of a large tem-
perature difference near the evaporator inlet, the onset of nucleate boiling
can be approached faster than the counterflow arrangement. Furthermore,
in an upward flow, the fluid pressure decreases along the flow direction,
which yields a decrease in the saturated temperature of the fluid; meanwhile,
the temperature of the hot fluid also decreases in the upward direction in the
parallel-flow arrangement. Therefore, the required mean temperature dif-
ference will be somewhat decreased.
A brazed plate heat exchanger with the chevron angle of β¼60degrees
was used by Yan and Lin (1999) for the investigation of the evaporation heat
transfer and pressure drop for R134a. The correlation of the single-phase
water-to-water test results yields
Nu sp ¼ 0:2121Re 0:78 Pr 1=3 ð μ=μ Þ 0:14 (4.101)
w
The evaporation heat transfer coefficients are correlated for
2000<Re eq <10,000 as

αd h,b 0:5 0:3 1=3
Nu ¼ ¼ 1:926Re Re eq Bo Pr (4.102)
l eq l
λ l
where
d h,b ¼ 2b (4.103)
" #
0:5
ρ l
G eq ¼ G 1 _x + _x (4.104)
ρ
v
Gd h,b
Re l ¼ (4.105)
μ l
G eq d h,b
Re eq ¼ (4.106)
μ
l
q
Bo eq ¼ (4.107)
G eq Δh v