25
weak function of mass velocity (0.33 power), so the
self-correction of turbulent flow is absent.
The result is that many of the tubes become
virtually plugged, and a few tubes carry most of the
flow. Stability is ultimately achieved in the high
flow tubes as a result of high mass velocity and
increased turbulence, but because so many tubes
carry little flow and contribute little cooling, a
concurrent result is high pressure drop and low
performance. The point at which stability is
reached depends on the steepness of the viscosity
versus temperature curve. Fluids with high pour
points may completely plug most of an exchanger.
This problem can sometimes be avoided by designing
deep bundles to improve air flow distribution.
Bundles should have no more than one row per pass
and should preferably have at least two passes per
row, so that the fluid will be mixed between passes.
When fluid has both a high viscosity and a high pour
point, long cooling ranges should be separated into
stages. The first exchanger should be designed for
turbulent flow, with the outlet temperature high enough
to ensure an outlet Reynolds number above 2,000 even
with reduced flow. The lower cooling range can be
accomplished in a serpentine coil (a coil consisting of
tubes or pipes connected by 180° return bends, with a
single tube per pass). The low temperature serpentine
coil should, of course, be protected from freezing by
external warm air recirculation ducts.
Closed loop tempered water systems are often more
economical, and are just as effective as a serpentine
coil. A shell and tube heat exchanger cools the
viscous liquid over its low temperature range on the
shell side. Inhibited water is recirculated between the
tube side of the shell and tube and an ACHE, where
the heat is exhausted to the atmosphere.
For viscous fluids which are responsibly clean, such as
lube oil, it is possible to increase the tube side
coefficient between four- and tenfold, with no
increase in pressure drop, by inserting turbulence
promoters and designing for a lower velocity. It is
then advantageous to use external fins to increase the
a
ir-side coefficient also. In addition to the increase in
heat transfer coefficient, turbulence promoters have
the great advantage that the pressure drop is
proportional to the 1.3 power of mass velocity, and
only to the 0.5 power of viscosity, so that non-
isothermal flow are much more stable. The simplest
and probably the most cost-effective promoters are the
swirl strips, a flat strip twisted into a helix.
VI. COST
The approximate purchase price may be determined
from Figure 18, which gives the price per square foot
of bare tube surface as a function of the total bare
tube surface and the number of tube rows. The prices
indicated are FOB factory, and do not include freight
or export crating charges. The prices are based on 1-
inch OD X 12 BWG X 32 foot long steel tubes with
extruded aluminum fins, fabricated steel headers with
steel shoulder plugs, 100 psig design pressure, TEFC
motors, and HTD drives. Price multiplication factors
are included for different tube materials.
It can be seen from these curves that the price per
square foot varies little for installations in excess of
7,000 square feet of bare tube surface. It is also
evident that the reduction in unit price as a function of
the number of tube rows becomes progressively less
as the number of rows increases.
REFERENCES
1. Steven, R.A., J. Fernandez, and J. R. Wolf: “Mean Temperature
Difference in One, Two and Three-pass Crossflow Heat
Exchangers,” Trans, ASME, Paper Nos. 55-A-89 and 55-A99,
1955.
2. Kays, William M., and Al. L. London, Compact Heat Exchangers,
Third Edition, McGraw-Hill Book Company, New York, 1984.
3. Seider, E. N. and G. E. Tate, “Heat Transfer and Pressure Drop of
Liquids in Tubes.” Ind. Eng. Chem.,Vol. 28, 1429-1435, 1936.
4. Kern Donalds Q., Process Heat Transfer, McGraw-Hill Book
Company, New York, 1950.
5. Rohsenow, Warren M., and James P. Hartnett, Handbook of
Heat Transfer, McGraw-Hill Book Company, New York, 1973.