Kuppan T. Heat Exchanger Design Handbook
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214
Chapter
4
Assume an overall exchanger size and
configuration
-
Frontal area, Cover legnth,
number
of
flow
-passages and surface
geometry.
t
I
Calculate
Ifiwnlo-phyeical
fluid
propertiee.
I
Calculate
the
dme
geometrical
properties:
&Ao,D,oC,
j3
and
6
t
Select
(jlf),,
I
n=
1,2
I
I
haluate
these
variables:
Re
,
j
,
f,
b
,
T~
,
ulo
,
k,,
k,
,
on
both
aides
Estimate variable
property
corrections
:
compute
(UA)
n
I
Compute
%oatat
area
and
hence detennine
flow
length
on
both
sides.
Calculate (Ap)n;
f
(Ap)n
I
specified
complete the design:
Else assume new G1 and G2,
and itereate
till
convergence
n
=
1,2
Figure
39
Sizing
of
a
compact
heat exchanger.
215
Compact Heat Exchangers
Table
3
Geometric Characteristics
[
171
Hot side (fluid
1)
Cold side (fluid
2)
Dh,h
oh
ah
[
171
illustrate the procedure for sizing a crossflow heat exchanger.
To
illustrate this,
a
simple
crossflow heat exchanger is considered
as
shown in Fig.
20.
It is assumed that each fluid is
unmixed throughout. The two fluids are designated by h and c. The problem
is
to determine
the three dimensions
L,,
&,
and
L3.
The first step after chosen the two surfaces is to assemble
the geometric characteristics
of
the surface as given in Table
3.
A first estimate of
Gh
and
G,
can be made using the following approximation relation
After the determination of mass velocity, carry out the calculations similar to
a
rating problem.
The sequence of parameters to be calculated after determination
of
mass velocity is given in
Table
4.
The pressure drop results are then compared with those specified for the design, and the
procedure is repeated until the two pressure drops are within the specified value.
Table
4
Sequence of Parameters to
Be
Calculated for
Sizing
Problem
After Calculating Mass Velocity
[
171
216
Chapter
4
8.3
Optimization
of
Plate Fin Exchangers and Constraints
on Weight Minimization
[SS]
Minimizing the material volume, or weight, of the plate-fin heat exchanger core does not
necessarily represents the optimum solution, even for weight-sensitive aerospace applications;
size and shape can be important design considerations
[a].
Minimum weight core could be
longer than is desirable because
of
the low fin efficiency implied. If the fin is too thin,
its
efficiency will be low, and to compensate both core length and flow area will be high, giving
an excessively high weight. On the other hand, a very thick fin giving high fin efficiency will
yield a low core length but with low porosity (free flow area) and weight. Minimizing the
components’ thickness can also contribute for low weight. But there are limitations in reducing
components beyond certain limits as follows:
The thicknesses of fin material and separating plates have lower limits set by pressure-retaining
capability.
Plate fin cores are usually made from sheet stock of a fixed range of thickness; rolling finstock
to a special “optimum” thickness could be uneconomic.
It may not be possible to form fins of sufficient dimensional accuracy if they are too thin;
if
they can
be
made, they might deform unsatisfactorily on brazing.
There may also be lower thickness limits set by erosion and corrosion problems.
9
EFFECT
OF
LONGITUDINAL HEAT CONDUCTION
ON THERMAL EFFECTIVENESS
The E-NTU and LMTD methods discussed are based on the idealization
of
zero longitudinal
heat conduction in the wall in the fluid flow direction. If a temperature gradient is established
in the wall in the fluid flow direction, heat transfer by conduction takes place from the hotter
to the colder region
of
the wall in the longitudinal direction. This longitudinal conduction in
the wall flattens the temperature distributions, reduces the mean outlet temperature
of
the cold
fluid, increases the mean outlet temperature of the hot fluid, and thus reduces the thermal
effectiveness
[63,65].
The reduction in the effectiveness at a specified
NTU
may be quite
significant for a compact exchanger designed for high thermal effectiveness (above
-80%)
and a short flow length,
L.
The shell and tube exchangers and plate exchangers
are
usually
designed for effectiveness of
60%
or less per pass. Therefore, the influence of heat conduction
in the wall in the flow direction is negligible for these effectiveness levels. The presence of
longitudinal heat conduction in the wall is incorporated into the thermal effectiveness formula
by an additional parameter,
h,
referred to as the longitudinal conduction parameter. Another
parameter that influences the longitudinal wall heat conduction is the convection conduction
ratio,
q@,
Thus, in the presence of longitudinal heat conduction in the wall, the exchanger
thermal effectiveness
is
expressed in a functional form
[65]:
9.1
Longitudinal Conduction Influence on Various
Flow
Arrangements
The influence
of
longitudinal conduction in the case of parallel flow is not discussed because
high-performance exchangers are not designed as for parallel flow arrangements. For high-
performance exchangers, the effect is discussed in Chapter
7,
Regenerators. In a crossflow
arrangement, the wall temperature distributions are different in the two perpendicular flow
Compact Heat Exchangers
21
7
directions. This results in a two dimensional longitudinal
conduction effect, and is accounted
for by the cold-side and hot-side
Z
parameters defined as
[65]:
Note that the flow lengths
L,
and
Lh
are independent in a crossflow exchanger. If the separating
wall thickness is
a,
and the total number of parting sheets is
Np,
then the transfer area for
longitudinal conduction is given by
[65]:
Ak,c
=
2N&,a and
Ak,h
=
2N&,a
(133)
Chiou
[66]
analyzed the problem for a single-pass crossflow heat exchanger with both
fluid unmixed and tabulated values for the reduction in thermal effectiveness due to longitudi-
nal wall heat conduction.
10
AIR COOLERS
Air-cooled heat exchangers (ACHE), or “fin-fans,” (Fig.
40)
are an alternate heat rejection
method that is used frequently in place of the conventional water-cooled shell and tube heat
exchanger to cool a process fluid. They can
be
used in all climates. Generally they are not
compact heat exchangers
[
11.
ACHE are increasingly found in a wide spectrum of applications
including chemical, process, petroleum refining, and other industries. Recently very large air-
cooled condensers and dry-cooling towers have been constructed at power plants located in
areas where cooling water is unavailable or costly
[67].
A fan located below the tube bundle
forces air up through the bundle, or a fan above draws the air through the bundle. The fans
are axial-flow fans varying from
4
to 12 ft diameter and having four to six blades. The fan
blades may be of aluminum, plastic, or, in the case of corrosive atmospheres, stainless steel
[68].
The drive can be an electric motor with gears or V-belts. Before discussing the construc-
tion details and design aspects, the factors that favor air cooling over water cooling are dis-
cussed.
10.1
Air Versus Water Cooling
Two primary methods of process cooling are
(1)
water cooling and (2) air cooling. The choice
between air or water as coolant depends on many factors, like (1) cooler location, (2) space
for cooling system,
(3)
effect of weather,
(4)
design pressure and temperature,
(5)
danger of
contamination,
(6)
fouling, cleaning, and maintenance, and
(7)
capital costs. Environmental
concerns such as shortage of makeup water, blowdown disposal, and thermal pollution have
become an additional factor in cooling system selection.
Air Cooling
Air cooling is increasing in use, particularly where water is in short supply. Factors that favor
air cooling include the following:
1. Air is available free in abundant quantity with no preparation costs.
2. Air cooler design is well established, and can perform well with a reasonable degree of
reliability.
3.
Water is corrosive and requires treatment to control both scaling and deposition of dirt,
whereas air is mostly noncorrosive. Therefore, material selection is governed by process
fluids routed through the tube side.
218
Chapter
4
Section
Bundle
)+--U
c
~~~~
,
..
..
..
View
I
I
s
.,
N
'.
.'
..
8
,
L-,Unit
A
-4
Unit
B
-*I
L
Bank
J
Figure
40
Details of aircooled heat exchanger (ACHE). (a) ACHES consists of bundles combined
into bays combined into units and units combined into banks; (b) details of fin tube; and (c) typical
headers for
air coolers offer flexibility.
4.
Mechanical design problems are eased with air coolers since the process fluid is always
on the tube side.
5.
Danger of process fluid contamination is much greater with water cooled system.
6.
Air-side fouling can be periodically cleaned by air blowing, and chemical cleaning can be
carried out either during half-yearly or yearly attention. Water-cooled systems may require
frequent cleaning.
7.
Maintenance costs for air coolers are about
20-30%
of those for water cooled system
[68].
Compact Heat Exchangers
21
9
Figure
40
Continued.
Operating costs for water coolers are higher, because of higher cooling water circulation
pump horsepower and water treatment costs.
8.
Air cooling eliminates the environmental problems like heating up
of
lakes, rivers, etc.,
blowdown, and washout.
Air cooling has the following disadvantages:
1.
Air coolers require large surfaces because of their low heat-transfer coefficient on the air
side and the low specific heat of air. Water coolers require much less heat-transfer surface.
2.
Air coolers cannot be located next to large obstructions if air recirculation is to be avoided.
3.
Because
of
air’s low specific heat, and dependence on the dry-bulb temperature, air cannot
usually cool a process fluid to low temperatures. Water can usually cool a process fluid
from
10°F
to
5°F
lower than air, and recycled water can be cooled to near the wet-bulb
temperature of the site in a cooling tower
[68].
4.
The seasonal variation in air temperatures can affect performance and make temperature
control more difficult. Low winter temperatures may cause process fluids to freeze.
5.
Air coolers are affected by hailstorms and may be affected by cyclonic winds.
6.
Noise is a factor with air coolers.
10.1
Tube Bundle Construction
An air cooler tube bundle consists of a series of finned tubes set between side frames, passing
between header boxes at either end. The number of passes on the tube side (the process fluid
side) is made possible by internal baffling of the header boxes.
For
horizontal bundles, vertical
baffles result in a side-by-side crossflow arrangement, whereas horizontal baffles result in a
220
Chapter
4
counter-crossflow arrangement relative to airflow. Inlet and outlet nozzles may or may not be
on the same header box. Tubes are attached to the tube sheet by expansion or welding.
Tube Bundle Fin Geometry
The finned tube (Fig. 40b) specifications include extruded, embedded, welded single footed,
and double footed. These fins can be solid or serrated, tension wound
or
welded, and of
a
variety of metals. The fin most commonly used for air coolers is aluminum, tension wound
and footed. Embedded fins have less contact resistance to heat transfer than do tension-wound
fins, but corrosion is likely to
be
induced at the groove between fin and tube [68].
The tubes are usually 1 in (25.4 mm) diameter; fin density varies from
7
to 16 findin
(276-630 fins/m), fin height from to
2
in (9.53-15.88 mm), and fin thickness from 0.012 to
0.02
in (0.3-0.51
mm).
The tubes are arranged in standard bundles ranging from
4
to 40 ft
(1.22-12.20 m) long and from
4
to 20
ft
(1.22-6.10 m) wide, but usually limited to 9.90-10.8
ft (3.2-3.5 m).
Fin Materials
Aluminum fins are used because of high thermal conductivity, light weight, and formability,
but their use is limited to low-temperature applications. Either SO1-containing or marine atmo-
Figure
41
Orientation
of
ACHE.
Compact Heat Exchangers
22
I
spheres may corrode alurninum fins,
so
carbon steel might be preferred for such conditions.
As the base tube is usually carbon or low-alloy steel, galvanic corrosion at the fidtube junction
is expected. Hence, finned tubes of carbon steel can be hot-dip galvanized to protect against
galvanic corrosion and
also
to provide a metallic bond between the fin and the bare tube [68].
Carbon steel tubes with carbon steel or alloy steel fins are used in boiler and fired heater
applications.
Header Boxes
Many types of header boxes are offered by industry. They can be grouped into three general
classifications [69]:
1. Plug type boxes
2.
Removable cover plate boxes for high-fouling services
3.
All welded construction
See
North
[69] for header boxes design.
Orientation of Tube Bundle
Air-cooled exchanger bundles can be installed either vertically or horizontally. The most com-
mon orientation is in the horizontal plane. A considerable reduction in ground area can be
made if bundles are vertically mounted, but the performance of the unit is greatly influenced
by the prevailing wind direction. In general, the use of vertically mounted bundles
is
confined
to small, packaged units.
A
compromise, which requires about half the ground area of the
horizontal unit, is the A- or V-frame unit. In this type, two bundles sloped at 45-60” from the
horizontal are joined by their headers at top or bottom to form the sloping side of the
A
(i.e.,
roof type) or
V,
respectively. The A-frame type with forced draft fans below (Fig.
42)
is the
more common and is used in steam condensing applications.
10.2 American Petroleum Institute Standard 661
American Petroleum Institute Standard 661 [70] is meant for air-cooled heat exchangers for
general refinery service. The scope of the standard includes minimum requirements for design,
materials selection, fabrication, inspection, testing, and preparation for shipment of refinery
process air-cooled heat exchangers. The standard specifies that the pressure parts shall be
designed in accordance with ASME Code, Section VIII, Division 1 [71]. API Standard 661
should be specified and used as a guide in preparing the
requisition/specification.
When the
customer’s requirements deviate from API 661, the exceptions or special requirements should
be listed as “Specific Requirements” [72].
10.3 Problems with Heat Exchangers in
Low-Temperature Environments
Air-cooled exchangers are designed to perform at ambient temperatures ranging from 130°F
to -60°F. In extremely cold environments, overcooling of the process fluid may cause freezing.
This may lead to tube burst, and hence freeze protection is required to prevent plugging or
damage to the tubes. The minimum temperature also affects material selection for construction
of the cooler.
Temperature Control
Several methods are used to control the performance of air-cooled heat exchangers to meet
variations in weather and process requirements. The current practice for air cooler design in
cold climates for problem services include
(1)
Recirculation of hot air through the tube bundle;
222
Chapter
4
HtGH-
FIN
TUBE
YCTION
SUPPCN
TueEs
SUPPO(ITS
CHAN
WlS
FLOATING
1
HEADER
SUPPORT
/-
PLENUM
VE
NTURl
Ar
/
SECTI~SUPPORI
/CHANNELS
HOT
FLUID
IN
I__C
FlYEO
F
LOATt
NG
HEAOER
SUPPORT
Figure
42
Air
flow
through
ACHE:
(a) forced draft, and
(b)
induced draft.
(2)
steam coils provided to preheat air for start-up conditions-these may be mounted at the
cooler base to warm up the inlet air, and according to the recommendation of
API
661,
this
steam coil should be separate from the main cooler bundle;
(3)
control devices that enable part
of the process fluid to bypass the unit;
(4)
for some units, particularly viscous oil coolers,
cocurrent design is appropriate, where the hot fluid enters nearest the cold ambient wall, and
the outlet where wall temperature is critical is in the warm air stream
[69];
and
(5)
in certain
cases, simply controlling the process outlet temperature with fan switching, two-speed fan
drives, variable-speed motor automatic louvers, variable-pitch fans, or autovariable-pitch fans
is sufficient. Each case must be considered
on
its merits
to
decide on the best method of
control.
10.4
Forced Draft Versus Induced Draft
One of the design criteria affecting the performance of air coolers is the type of draft: forced
draft or induced draft. Both these arrangements are shown in Fig.
43.
Forced Draft
Forced draft fans have the advantage of handling cold air entering the exchanger, requiring
smaller volumes of air and less horsepower. They generally offer better arrangements for rnain-
tenance and they are easily accessible. They are best adapted for cold-climate operation with
warm-air recirculation. Also, forced draft fans afford a higher heat-transfer coefficient relative
to induced draft, because forced draft fans cause turbulence across the tube bundle
[68].
223
Compact Heat Exchangers
r
Recirculation
plume
Figure
43
Air recirculation through an ACHE. H, height from ground level;
w,
half
width;
md,
fresh
air; and m,, recirculated air.
Induced Draft
Compared with the forced draft design, induced draft design has the following advantages:
1.
Easier to shop assemble, ship, and install.
2.
The
hoods
offer protection from weather and hailstone protection.
3.
Easier to clean the underside when covered with lint, bugs, and debris.
4.
Better air distribution over the tube bundle.
5.
Less likely to be affected
by
hot air recirculation.
The disadvantages
of
induced draft design are:
1.
More difficult to remove bundles for maintenance.
2.
High-temperature service limited due to effect of hot air
on
the fans.
3.
More difficult to work on the fan assembly, due to heat from the bundle and due to their
location.
10.5
Recirculation
Recirculation
of
hot plume air (Fig.
44)
occurs in most forced draft air-cooled heat exchangers.
The resultant increase in inlet cooling air reduces the effectiveness of the heat exchanger. To
1,
L
.o
N
9
w
&
0.
.8
Qo
ln
9)
0.
>
3
.-
g
0.
5
w
0.
VI
0
0.02
0.06
0.10
0.14
Figure
44
Effectiveness
of
ACHE with air circulation.