Page 134 - Design and Operation of Heat Exchangers and their Networks

122 Design and operation of heat exchangers and their networks

where A is the total heat transfer area, A¼A p +A f , A p is the heat transfer area
of the plates called as primary surface area, and A f is the extended surface area
called as secondary surface area. This area will not be equal to the heat trans-
fer area of fins if some parts of the fin area do not belong to the extended
surface.
As an example, for rectangular plain fins,
A f h fs
¼ (3.253)
A s fs
For the rectangular offset strip fins shown in Fig. 3.14,

ð
A f h fs l s + δ f Þ
¼ (3.254)
A ð h fs + s fs Þl s + h fs + s ofs δ f Þδ f
ð
where l s is the strip length and s ofs is the strip offset. For symmetrical strips,
s ofs ¼s f /2.

3.5.4 Heat transfer and pressure drop correlations
For rating and designing plate-fin heat exchangers, we are used to use cor-
relations of the Colburn j factor to evaluate the heat transfer coefficient of
fins. The Colburn j factor is a dimensionless factor for heat transfer defined
by Eq. (3.255):
Nu α
j ¼ ¼ Pr 2=3 (3.255)
RePr 1=3 c p G
The total pressure drop of a plate-fin heat exchanger core can be
obtained from Eq. (3.256) (Kays and London, 1984):

G 2 1 2 4fL 1 1 1 2
Δp ¼ 1 σ + K c + +2 1 σ K e
2 ρ in ρ d h ρ out ρ in ρ out
m
(3.256)
in which σ is the ratio of free flow area to frontal area. K c and K e are the loss
coefficients for abrupt contraction and abrupt expansion, respectively. Kays
and London (1984) provided the curves for K c and K e for tube bundles, rect-
angular fins, and triangular fins. However, as has been pointed by Kays and
London, for offset strip fins and louver fins, because of the interruption of the
fin surface, the curve for Re!∞ should be chosen, which can be computed
from the classic expressions (White, 2011) as follows:

K c 0:42 1 σ 2 (3.257)