Kuppan T. Heat Exchanger Design Handbook
Подождите немного. Документ загружается.
164
Chapter
4
Figure
4
Tube-fin layout:
(a)
inline; and (b) staggered.
aluminum and copper; for heat recovery applications they are made from carbon steel. Headers
and tube plates are made of carbon steel.
Tube Layout.
The two basic arrangements of the tubes in a tube bank are staggered and in-
line. These arrangements are shown in Fig.
4.
The only construction difference between these
two is that in the staggered arrangement each alternate row
is
shifted half a transverse pitch,
and the two arrangements differ in flow dynamics. Due to compactness and higher heat trans-
fer, the staggered layouts are mostly used. However, if the air stream is laden with dust and
abrasive particles etc., in-line layout is preferred, being less affected and having ease of
cleaning.
1.4
Continuous Fins on
a
Tube
Array
This type of tube-fin geometry (Fig. lc) is most commonly used in
(1)
air conditioning and
refrigeration exchangers in which high-pressure refrigerant is contained on the tube side,
(2)
as radiators for internal combustion engines, and
(3)
for charge air coolers and intercoolers for
cooling supercharged engine intake air of diesel locomotives. The tube layout pattern is mostly
staggered.
Tube Details
Round or flat tubes (rectangular tubes with rounded corners) usually with a staggered tube
arrangement are used. Elliptical tubes are also being used. Round tubes are used for high-
pressure applications and also when considerable fouling is anticipated. The use of flat tubes
is limited to low-pressure applications, such
as
vehicular radiators. Flat or elliptical tubes,
instead of round tubes, are used for increased heat transfer in the tube and reduced pressure
drop outside the tubes
[2,9].
High-parasitic-form drag is associated with flow normal to the
round tubes. In contrast, the flat tubes yield lower pressure loss for flow normal to the tubes
due to lower form drag and avoid the low-performance wake region behind the tubes.
Fin Details
Continuous fin exchangers use flat, continuous, plain, wavy, louvered fins. The plain continu-
ous fins are used in those applications in which the pressure drop is critical, although a larger
amount of surface area is required for the specified heat transfer compared to the wavy or
interrupted fins.
Exchangers for Air Conditioning and Refrigeration
Evaporators and condensers for air conditioning and refrigeration are usually the tube-fin type
when air is one of the fluids. These exchangers are referred to as coils when air is one of the
165
Compact Heat Exchangers
fluids. Tube-fin geometry is becoming widespread in the current energy conservation era,
be-
cause the bond between the fin and tube is made by mechanically or hydraulically expanding
the tube against the fin, instead of employing welding, brazing, or soldering [2]. Formation of
a mechanical bond requires very little energy compared to the energy required to solder, braze,
or weld the fin to the tube. Because of the mechanical bond, the operating temperature is
limited.
Tube-fin details for air conditioning and refrigeration applications are furnished by Shah
[2]: Evaporator coils for small-capacity systems (less than 20 tons) use 7.9, 9.5, and 12.7
mm
(6,
i,
and
i
in) tubes and fin densities in the range of 315-551 finslm (8-14 finslin). For
refrigeration applications, lower fin densities (157-315 finslm or
4-8
finslin) are used due to
the possibility of frosting and blockage. For condenser coils, tube diameters used are 7.9, 9.5,
and 12.7
mm
(6,
i,
and
i
in), and
fin
densities range from 394 to 787 fins/m (10-20 finslin)
and fin thickness from 0.09 to 0.25 mm (0.0035-0.10 in).
Radiators
Radiators are compact liquid-to-gas heat exchangers widely used in automotive vehicles. Major
construction types used for radiators include (1) soldered design with flat brass tubes with flat
plain or louvered copper fins-tubes and header sheets are made from brass and the water tank
from brass or plastics-and (2) brazed aluminurn design with flat tube and corrugated fins,
usually louvered fins or flat tube continuous fins. These two construction types are shown in
Fig. 5. Louvered cores are preferred
as
they allow
a
lower fin density than a nonlouvered core
of the same area for the same thermal performance. The soldered brass tubes and copper fins
(Fig. 6) were primarily used before the 1980s. Because of light weight and significantly better
durability and operating life, brazed aluminum tubes and aluminum corrugated multilouver fin
radiators have almost replaced the copper-brass radiators [2]. Typical fin density ranges from
400 to 1000 finslm (10-25 findin), and the surface area density
(p)
ranges from
900
to 1650
m2/m3 (275-500 ft’/ft’).
Effect
of
Fin Density on Fouling
A
major question in the choice of detailed dimensions for a core is the appropriate fin spacing.
As
the number of fins per inch (or per meter) is increased, the unit can be made progressively
more compact. At the same time, it becomes more sensitive to clogging by dirt, soot, airborne
dirts, insects, and the like, and more sensitive to nonuniformity in the fin spacing. In practice
it has been found that for most automotive applications, radiators can have 10-12 finslin.
Fourteen fins per inch can be fabricated with little increase in cost per unit surface area, and
Figure
5
Forms
of
radiators: (a) soldered flat fin-tube; and (b) flat tube louvred fin design: (i) com-
gated fin,
(ii)
flat continuous fin.
166
Chapter
4
Figure
6
Soldered radiator. (Courtesy
of
Banco Products, Vadodara, India.)
this closer fin spacing may be justified for installations where space is a constraint and fouling
of the surfaces is not a problem
[9].
One-Row Radiators
[
101
In conventional multirow radiators, the fin array is not in contact with the tubes over the full
fin depth. The region of no contact occurs between the tube rows and is illustrated in Fig.
7.
This hinders heat conduction from the tubes to the fin region between the tube rows. If the
fins are lowered in this region, the conduction path is further impaired. Hence the thermal
Figure 7
Region
of
no contact between tube rows
of
a radiator
167
Compact Heat Exchangers
Brass tube with cross-ribbed turbulator
Thickness=O.l52
mm
Figure
8
Single tube row radiator design.
performance per unit fin depth is low compared to the fin region in contact with the tubes.
Remedies for this problem follow:
1.
Use a one-row core
in
place of the multirow design (Fig.
8).
This design eliminates
the low-performance fin area between tube rows, and offers either material savings and lower
air pressure drop, or increased thermal performance. In the United States, manufacturers usu-
ally make 19.1 mm
(i
in) tubes in
0.152
mm
(0.006
in) thick brass.
To
prevent the tube wall
from becoming concave during core assembly, heavier wall thickness is usually adopted. An-
other method followed to prevent the tube wall from becoming concave during assembly is to
use cross-ribbed turbulator tubes. The indented cross ribs increase the cross-sectional moment
of
inertia of the tube wall, increasing its stiffness. The indentations in the wall of turbulator
tubes may be beneficial for low-flow-rate applications, where the flow may be lamina. For
turbulent flow, the increase in coolant-side heat-transfer coefficient will be marginal, because
the dominant thermal resistance is typically on the air side, and the ribs or indentations produce
large increase in pressure drop.
2,
An alternative method of eliminating the ineffective fin area is to eliminate the space
between the tube rows. In other words, have the tubes touching in the axial direction (Fig.
8).
This is referred
to
as the “tubes-touching” design. To achieve tubes-touching design, the ends
of the tubes must be shaped to provide the requirement of minimum ligament between the
tubes in the header plate. For the same fin depth in the air flow direction, the tubes-touching
design should provide the same thermal performance as a one-row design.
Manufacture of Continuous Finned Tube Heat Exchangers
The manufacturing procedure for continuous finned tube heat exchanger is discussed next.
Detailed manufacturing practices are further described in the chapter on Heat Exchanger Fabri-
cation: Brazing and Soldering.
Mechanically Bonded Core.
In a rounded hole design, the flat fins are formed with holes
larger than the tubes to be inserted. Tubes are assembled in a fixture and fins are inserted. The
fin
gap (i.e., the
fin
density) is controlled by the ferrule height (Fig.
9).
After assembly, the
tubes are expanded
so
that metal-to-metal contact is being achieved. The tube ends are gener-
Figure
9
Ferrule height
of
a tube-fin
heat
exchanger.
I68
Chapter
4
ally expanded into the header plates. Sometimes, on thinner tube plates, the tubes are brazed.
The tubes, fins, and headers together form the core, which is then attached to the side sheet
assembly and the header for the tube-side fluid or water tank, as the case may be, and tightly
sealed with gaskets.
Soldered Core.
In a soldered design, which is mostly followed for flat tube-fin exchangers,
the radiator cores are built from tinned brass tubes and copper fin sheets. The fins are punched
with holes slightly larger than the tube dimension, and tubes are inserted through the holes.
The tube outer surfaces and
fin
surfaces are solder coated for achieving bonding. After the
header plates are assembled, the core is baked in an oven. During baking the solder metal
on
the tube outer surface and on the fin surface melts and give perfect metal-to-metal contact.
Alternately, a fluxed core assembly is dip soldered.
1.5
Surface Selection
Various surfaces are being used in compact heat exchanger applications.
A
proper selection of
a surface is one of the most important considerations in compact heat exchanger design. The
selection criteria for these surfaces are dependent upon
(1)
the qualitative and
(2)
the quantita-
tive considerations
[
1
11.
Qualitative Considerations
The qualitative considerations include the following:
1.
Heat-transfer requirements
2.
Operating temperature and pressure
3.
Flow resistance
4.
Size or compactness and weight
5.
Mechanical integrity
6.
Fouling characteristics
7.
Availability of surfaces, manufacturing considerations
8.
Low capital cost, and low operating cost, especially low pumping cost
9.
Designer’s experience and judgment
10.
Maintenance requirements, reliability, and safety
Heat-transfer requirements with low flow resistance to minimize the pumping cost and a
compact, low-weight unit are the important criteria for selecting a surface for a compact heat
exchanger. These aspects are common for all types of heat exchangers and have been covered
earlier. Surface selection influenced by fouling, manufacturing considerations, and cost are
discussed next with additional details.
Fouling
of
Compact Exchangers.
Fouling is one of the major problems in compact heat
exchangers, particularly with various fin geometries and fine flow passages that cannot
be
cleaned mechanically. Hence, compact heat exchangers may not be applicable in highly fouling
applications. There is a belief and some evidence that integral and low-finned tubes foul either
at essentially the same rate or in some cases at a lower rate than plain tubes and also they are
easier to clean the most types of fouling
[7].
Other than low-finned tubes,
if
fouling is a
problem, with the understanding of the problem and applying innovative means to prevent/
minimize fouling, compact heat exchangers may be used in at least low to moderate fouling
applications. Methods to reduce fouling are discussed in Chapter
9.
Manufacturing Considerations.
Most of the heat exchanger manufacturing industries make
only a limited number of surfaces due to limited availability of tools and
to
market potential.
Compact Heat Exchangers
I69
Select a surface that a company can manufacture, even though theory and analysis can be used
to arrive at some high-performance surfaces that may pose problems to manufacture.
Cost.
Cost
is one of the most important factors in the selection of surfaces, such that the
overall heat exchanger is least expensive either in initial cost or in both initial and operating
costs. If a plain fin surface can do the job for an application, a plain surface is preferrable
to
expensive complicated surface geometries.
Quantitative Considerations
The quantitative considerations include performance comparison of surfaces with some simple
“yardsticks.” Surface selection by a quantitative method is made by comparing performance
of various heat exchanger surfaces and choosing the best under some specified criteria for a
given heat exchanger application. Various methods have been proposed in the literature for
such comparisons for compact heat exchanger surfaces and for other heat-transfer devices.
Some
of them have been reviewed by Bergles et al. [12] from the viewpoint of performance
enhancement. Shah
[
1 I] categorizes the surface selection method as follows:
1. Comparisons based on
j
and
f
factors
2. Comparison of heat transfer as a function of fluid pumping power
3. Miscellaneous direct comparison methods
4.
Performance comparisons with a reference surface
These methods are discussed in Chapter 8.
2
PLATE-FIN HEAT EXCHANGERS
The definition given in the Plate-Fin Heat Exchangers Guide to Their Specification and Use
[13] is as follows: Plate-fin heat exchangers (PFHE) are a form of compact heat exchanger
consisting of a stack of alternate flat plates called “parting sheets” and fin corrugations, brazed
together as a block. The basic elements of PFHE and the brazed unit are shown in Fig.
10.
Fluid streams
flow
along the passages made by the corrugations between the parting sheets.
The corrugations serve
as
both secondary heat-transfer surfaces and mechanical supports for
the internal pressures between layers. In liquid or phase-change (to gas) applications, the part-
ing sheets may be replaced by flat tubes on the liquid or phase-change side. Plate-fin heat
exchangers are widely used in various industrial applications because of their compactness.
For many years, PFHEs have been widely used for gas separation
in
cryogenic applications
and for aircraft cooling duties. In aerospace applications weight saving is of paramount impor-
tance. An important guidebook on PFHE specification and use
is
Ref. 13.
This
guidebook
covers construction, thermal design, mechanical design, manufacture by brazing, inspection
and quality control, packing, and despatch.
2.1
Essential Features
Salient features of PFHE are discussed by Shah and include the following
[
1,2]:
1.
Plate-fin surfaces are commonly used in gas-to-gas exchanger applications. They offer
high area densities (up to about
6000
m2/m3 or 1800 ft2/ft3)).
2.
The passage height
on
each side could be easily
varied.
Different fins (such as the plain
tri-
angular, louver, perforated, or wavy fin) can
be
used between plates for different applications.
3.
Plate-fin exchangers are generally designed for low-pressure applications, which operating
pressures limited
to
about
1000
Wag (150 psig).
4.
The maximum operating temperatures are limited by the type of fin-to-plate bonding and
I70
Chapter
4
Of
Figure
10
Plate-fin heat exchanger: (a) basic elements; and (b)
Two
designs
of
PFHE.
the materials employed. Plate-fin exchangers have been designed from
low
cryogenic
op-
erating temperatures (all-aluminum
PFHE)
to about
800°C
(
1500°F)
(made
of
heat-resis-
tant alloys).
5.
Fluid contamination (mixing) is generally not a problem since there is practically zero
fluid leakage from one side to the other of the exchanger.
2.2
Application
for
Fouling Service
The
PFHE
is limited in application to relatively clean streams because of its small
flow
pas-
sages. It
is
generally not designed for applications involving heavy fouling since there
is
no
Compact
Heat
Exchangers
I71
easy method of cleaning the exchanger. If there is a risk of fouling, use wavy fins and avoid
serrated fins. Upstream strainers should be employed where there is any doubt about solids in
the feed. Measures to overcome fouling problems with PFHE are described by Shah [2].
2.3
Size
Maximum size is limited by the brazing furnace dimensions, and by the furnace lifting capac-
ity. The heating characteristics of denser blocks also impose limitations on the maximum size
which can be brazed. Typical maximum dimensions for low-pressure aluminum PFHEs are 1.2
m
x
1.2 m in cross section
x
6.2
m along the direction of flow
[
1,2,13], and
1
.O
m
x
1
.O
m
x
1.5
m for nonaluminum PFHEs [13]. Higher pressure units, being heavier per volume, make both
handling and brazing more difficult,
thus
reducing the economic maximum size. Duties that call
for larger units are met by welding together several blocks, or by manifolding pipework.
2.4
Advantages
of
PFHEs
The principal advantages of PFHEs over other forms of heat exchangers are summarized in
Ref 13 and include the following:
1.
Plate fin heat exchangers, in general, are superior in thermal performance to those of the
other type of heat exchangers employing extended surfaces.
2.
PFHE can achieve temperature approaches as low as 1°C between single-phase streams
and 3°C between multiphase streams. Typically, overall mean temperature differences of
3-6°C
are employed
in
aluminum PFHE applications
[
131.
3.
With their high surface compactness, ability to handle multiple streams, and with alumi-
num’s highly desirable low temperature properties, brazed aluminum plate-fins are an
obvious choice for cryogenic applications.
4.
Very high thermal effectiveness can be achieved; for cryogenic applications, effectiveness
of the order
of
95%
and above is common.
5.
Provided the streams are reasonably clean, PFHEs can be used to exchange heat in most
processes, for the wide range of stream compositions and pressurehemperatwe envelopes
~31.
6.
Large heat-transfer surface per unit volume is possible.
7.
Low weight per unit heat transfer.
8.
Possibility of heat exchange between many process streams.
9.
Provided correct materials are selected, the PFHE can be specified for temperatures rang-
ing from near absolute zero to more than
800°C,
and for pressures up to at least
140
bar
[
11. According to Shah
[
11, usually PFHEs do not involve both high temperature and
high pressure together.
10.
PFHE offers about 25 times more surface area per equipment weight than the shell and
tube heat exchanger
[
131.
2.5
Li
m
i
t at ion
s
of
P
F
H
E
s
Limitations other than fouling and size include the following
[
11:
1.
With a high-effectiveness heat exchanger and/or large frontal area, flow maldistribution
becomes important.
2.
Due to short transient times, a careful design
of
controls is required for startup compared
with shell and tube exchangers.
3.
Flow oscillations could be a problem.
I
72
Chapter
4
2.6
Applications
Current areas of application include
[
131:
1.
Cryogenicdair separation
2. Petrochemical production
3. Syngas production
4.
Aerospace
5.
Land transport (automotive, locomotive)
6.
Oil and natural gas processing
PFHEs are used in all modes of heat duty, including
[
131:
1.
Heat exchange between gases, liquids, or both
2. Condensing
3. Boiling
4.
Sublimation (“reversing” heat exchangers)
5.
Heat or cold storage
2.7
Economics
The PFHE is not necessarily cheaper for a given heat duty than other forms of heat exchangers,
because the method used for constructing PFHEs is complex and energy-intensive
[
131.
2.8
Flow Arrangements
The fins on each side can be easily arranged such that overall flow arrangement of the
two
fluids can result in crossflow, counterflow, cross-counterflow, or parallel flow, though parallel
flow is mostly not used. Properly designed, the PFHE can be made to exchange heat in perfect
counterflow, which permits PFHEs to satisfy duties requiring a high thermal effectiveness.
Crossflow, counterflow, and cross-counterflow arrangements are shown in Fig.
11.
The
con-
struction of a multifluid plate-fin exchanger is relatively straightforward except for the inlet
and outlet headers for each fluid.
2.9
Fin Geometry Selection and Performance Factors
Plate-fin surfaces have plain triangular, plain rectangular, wavy, offset-strip, louver, perforated,
or pin fin geometries. These fin geometries are shown in Fig. 12 [8]. Plate-fin heat exchangers
are superior to tube-fin heat exchangers from heat-transfer and pressure-drop points of view
for the given total packaging space. Fin density varies from 120 to
700
fins/m (3-18 findin),
thickness from
0.05
to 0.25 mm
(0.002-0.01
in),
and fin heights may range from 2 to
25
mm
(0.08-1.0
in)
[2].
Plain Fin
Plain fin corrugation is the simplest type of finning. These surfaces are straight fins that are
uninterrupted (uncut) in the fluid flow direction. Triangular and rectangular passages are more
common. The triangular fin is generally not structurally as strong as the rectangular fin for the
same passage size and fin thickness. When the fins are straight along the flow length, the
boundary layers tend to
be
thick resulting in lower values of the heat transfer coefficient. When
they are wavy or off-set strip fin along the flow length, the boundary layers are thinner
or
the
growth of boundary layers is disrupted periodically, respectively, resulting in a higher heat
transfer coefficient.
Plain fin surfaces have pressure-drop and heat-transfer characteristics similar to flow
Compact Heat Exchangers
173
-L
t
7
I 1
Figure
11
PFHE
flow arrangement:
(a)
crossflow;
(b)
counterflow;
and
(c) cross-counter flow, Tayl~r'~.
through small-bore tubes, i.e., relatively low pressure drop and heat transfer, but a high ratio
of heat transfer to pressure drop
[13].
For
a
flow channel with rectangular
or
triangular cross
section, under turbulent flow conditions, standard equations for turbulent flow in circular tubes
may be used to calculate
j
andf, provided the Reynolds number (Re) is based on the hydraulic
diameter,
Dh.
If the Re based on hydraulic diameter is less than
2000,
one may use theoretical
laminar flow solutions for
j
and
f.
Plain fins are preferred for very low Reynolds number applications and in applications
where the pressure drop is very critical. Condensing duties require minimal pressure drop or
else the heat release curve can significantly alter and the overall duty may not be met. There-
fore for condensation, plain fins are normally specified.
Plain-Perforated Fin
Perforated fins have either round or rectangular perforations, having the size and longitudinal
and transverse spacings as major perforation variables. The flow passages are either triangular
or rectangular.
A
metal strip is first perforated, then corrugated. Perforated areas vary from
about
5
to
25%
of
the sheet area
[13].
When the corrugations are laid across the
flow,
the
stream is forced
to
pass through these small holes. Salient performance features include:
Qec
tsogular
Triangular
Pin
fin
Offset
strip
Wavy
Lduvewd
Qerfomted
fin
fih
Fin
sin
e
io
.Fir,
Figure
12
PFHE
fin geometries.