Page 182 - Design and Operation of Heat Exchangers and their Networks

170 Design and operation of heat exchangers and their networks

mean value of vapor quality encountered in the PHE was used for the cor-
relation equations. The vapor quality changes Δx over the exchanger are
reported to range from 0.126 to 0.337.
Ayub (2003) made a literature search on single-phase correlations avail-
able for plate heat exchangers and introduced the correlations for the evap-
oration heat transfer coefficient and the friction factor, which are applicable
to various system pressure conditions and plate chevron angles. The corre-
lations based on their actual field data were presented as

αd h,ψ 2 0:4124 0:12 0:35
Nu ¼ ¼ C Re Δh v =L ð p=p cr Þ ð 65degrees=βÞ (4.125)
ψ
λ l
where C¼0.259 for flooded and thermosiphon operation, C¼0.156 for
direct expansion, △h v in kJ/kg, and L in m.
The Reynolds number is defined as
Re ψ ¼ Gd h,ψ =μ l (4.126)

and the hydraulic diameter is defined as
d h,ψ ¼ 2b=ψ (4.127)

in which ψ is the area enlargement factor and b is the plate spacing. For the
herringbone-type plate, b¼2a and ψ can be calculated according to Martin
(1996) as

1 p ffiffiffiffiffiffiffiffiffiffiffiffiffi p ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
2
ψ ¼ 1+ 1+ X +4 1+ X =2 (4.128)
6
with the corrugation parameter
X ¼ 2πa=l w (4.129)
The Fanning friction factor is given as follows:

2
m
f ¼ nRe ψ 1:89 + 6:56R 3:69R ð 30degrees β 65degreesÞ
(4.130)
where
R¼β/30degrees
m¼0.137, n¼2.99 for Re ψ 4000
m¼0.172, n¼2.99 for 4000<Re ψ 8000
m¼0.161, n¼3.15 for 8000< Re ψ 16,000
m¼0.195, n¼2.99 for Re ψ >16,000