Page 129 - Design and Operation of Heat Exchangers and their Networks

Steady-state characteristics of heat exchangers 117

With this trick, the correction factor for the logarithmic mean temper-
ature difference was obtained as

2
2
ln 1 + CN = 1+ ν CN 2 1
1
F ¼ 2 (3.232)
2
ð 1 ν ÞCN
1
where ν is the ratio of the channel numbers in small and big spiral heat
exchangers, respectively:
p ffiffiffiffiffiffiffiffiffiffiffiffiffi
ν ¼ A 0 =A 1 ¼ n 0 =n 1 (3.233)
2πn 1 khs
(3.234)
CN 1 ¼ p ffiffiffiffiffiffiffiffiffiffiffiffi
_ _
C h C c
3.4.4 Parallel-flow spiral heat exchanger
Due to its poor thermal performance, the parallel-flow arrangement in spiral
heat exchanger is rarely used, except for some special technical reasons. With
a similar method, Bes (2001) obtained the effectiveness of the parallel-flow
spiral heat exchanger as
ð
1 e f NTU 1 + RÞ
ε ¼ (3.235)
f 1+ RÞ
ð
where
1 1
f ¼ + p ffiffiffiffi p ffiffiffiffi (3.236)
2 1= R + R

3.5 Plate-fin heat exchangers

The plate-fin heat exchangers have been very often used in the aircraft
industry since 1940s and cryogenic industry for more than 40years. Thou-
sands of these exchangers have been installed in chemical process plants over
the past 50years. Their range of application includes air separation; separa-
tion and purification of light hydrocarbons; liquefaction of natural gas;
separation, purification, and liquefaction of helium; production of olefins;
and purification and liquefaction of hydrogen. They are also used in ammo-
nia production, offshore oil and gas processing, nuclear engineering, and
syngas production. Some of the special features of plate-fin heat exchangers
2 3
are their high density of heat transfer surface on the order of 1000m /m and
their ability to accommodate up to 10 or 12 streams within a single heat
exchanger unit (Bell, 1990).