Kuppan T. Heat Exchanger Design Handbook
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3
74
Chapter
8
While considering these benefits, the associated flow friction changes is also to be taken into
account
[2].
2
APPLICATION OF AUGMENTED SURFACES
For a two-fluid exchanger, augmentation should be considered for the fluid stream that has the
controlling thermal resistance. If both resistances are approximately equal, augmentation may
be considered for both sides of the exchanger. The majority of two-fluid exchanger applications
are served by four basic exchanger types
[5]:
(1) shell and tube,
(2)
tube-fin,
(3)
brazed plate-
fin, and
(4)
plate heat exchangers
(PHE).
If enhancement is employed, it must be applied to
three basic flow geometries
[5]:
1. Internal flow in circular tubes.
2.
External flow normal to tubes and tube banks.
3.
Channel flow
in
closely spaced parallel plate channels, e.g., brazed plate-fin heat
ex-
changers and plate heat exchangers.
3
PRINCIPLE
OF
SINGLE-PHASE HEAT-TRANSFER
ENHANCEMENT
Single-phase flow can be classified as internal flow or external
flow.
It can be further classified
as turbulent or laminar flow. The principal mechanisms of enhancement
in
turbulent of laminar
flow are the use of a secondary heat transfer surface and the disruption of the unenhanced
fluid velocity and temperature profile
[6].
The rate of heat transfer between a fluid and finned
surface can be expressed in a general form as
[7]:
where
h
is the heat-transfer coefficient,
qo
the overall surface efficiency,
A
the total heat
transfer area, and
A
T
the surface-fluid temperature difference.
In Eq.
1,
hq+4
represents the surface conductance. Obviously, high performance may be
achieved by using a heat-transfer surface with high value of
qJ,
h,
or both. Special surface
geometries provide enhancement by establishing a higher
hqJ
per
unit
surface area. Three
basic principles are employed to increase
hqJ
or
hA
[5,7,8]:
1.
Increase of
A
without appreciably changing
h;
e.g., finned tubes.
2.
Increase in
h
without an appreciable area
A
increase; e.g., surface roughness or turbulence
promoters. This principle is further explained in the next section.
3.
Increase
of
both
h
and
A;
for example, extended surfaces in compact heat exchangers have
increased heat transfer coefficient by means of surface promoters, such as perforations,
louvres, and corrugations or displaced promoters such as canted tubes.
3.1
Increase in Convection Coefficient Without an
Appreciable Area Increase
A
surface that results in performance enhancement due to increase in the heat transfer coeffi-
cient
h
without an appreciable transfer area is known as an enhanced heat-transfer
(EHT)
surface.
EHT
surfaces are vital to the development of heat-transfer augmentation devices. The
underlying principle of the enhanced heat-transfer surface is as follows.
As
a reasonable ap-
proximation, the heat-transfer coefficient is given by
[7]:
Heat- Transfer Augmentation
3
75
h
=
k/A
(2)
where
k
is the thermal conductivity of the fluid and
A
the thermal boundary layer thickness.
From
Eq.
2,
it can be seen that
h
can
be
increased by altering the flow pattern near the
surface in order to reduce the thermal boundary layer thickness.
3.2
Enhancement in Turbulent
Flow
Turbulent flow in a tube [after
61
exhibits a low-velocity flow region known as the laminar
sublayer immediately adjacent to the wall, with velocity approaching zero at the wall. Most of
the thermal resistance occurs in this low-velocity region. Any roughness or enhancement tech-
nique (e.g., swirl flow devices such as wire coil inserts, spiral inserts, and protrusions) that
disturbs the laminar sublayer will enhance the heat transfer.
For
fully developed flow in a
smooth tube, the dimensionless laminar sublayer thickness is given by:
Using the shear stress
2,
for a smooth tube, the ratio of laminar sublayer thickness
y
to tube
diameter
d
is given by:
For example, if Re
=
30,000
and
d
=
1
.O
in (25.4 mm), the laminar sublayer thickness
is
0.003
in
(0.0762
mm). Any roughness or enhancement technique that disturbs the laminar sublayer
will enhance the heat transfer.
3.3
Enhancement in Laminar
Flow
Laminar flow generally results in low heat-transfer coefficients. The fluid velocity and temper-
ature vary across the entire flow channel width
so
that the thermal resistance is not just in the
region near the wall as in turbulent flow. Hence, small-scale surface roughness is not effective
in enhancing heat transfer in laminar flow; the enhancement techniques employ some method
of swirling the flow or creating turbulence
[6].
Enhancement techniques may
be
(1)
passive methods, which require no direct application of
external power, and
(2)
active methods, which require external power. These methods are
discussed in Ref.
1.
Table
1
gives a list
of
passive and active methods. Two or more
of
the
methods may be used to provide a “compound enhancement” greater than that of the individual
method. Each method has its own merits and demerits. According to Bergles [4], the effective-
ness of these methods depends strongly on the mode of heat transfer, which might range from
single-phase free convection to dispersed-flow film boiling.
Although many enhancement techniques have been proposed and tested, only a small
number are currently used in heat exchangers
[l].
Of these, passively enhanced tubes are
relatively easy to manufacture, cost-effective for many applications, and can be used for ret-
rofitting existing units, whereas active methods, such as vibrating tubes, are costly and com-
plex
[9].
376
Chapter
8
Table
1
Enhancement Techniques
[4]
Passive techniques Active techniques
(No
external power needed) (External power required)
Extended surfaces Mechanical aids
Treated surfaces Surface vibration
Rough surfaces Fluid vibration
Displaced enhancement devices
Electric or magnetic fields
Swirl flow devices
Injection or suction
Surface tension devices
Additives for fluids
Additives for gases
5
HEAT-TRANSFER MODE
The heat-transfer augmentation techniques can be applied for various heat transfer modes
such as
1.
Single-phase natural convection
2.
Single-phase forced convection
3.
Pool boiling
4.
Forced convection boiling
5.
Condensation
6
PASSIVE TECHNIQUES
6.1
Extended
Surfaces
Extended surfaces are routinely employed in compact heat exchangers and shell and tube
exchangers either for liquids or for gases. The application of extended surfaces for liquid and
gas service are discussed next.
Extended Surfaces for Gases
In forced convection heat transfer between a gas and
a liquid, the heat transfer coefficient of
the gas may be of the order
of
1/50
to
1/10
that of the liquid, Ref.
(8).
The use of extended
surfaces
will
reduce the gas side thermal resistance. However, the resulting gas side resistance
may still exceed that of the liquid.
Enhanced Su$ace Geometries on Tube-Fin Exchangers.
Enhanced surface geometries that
have been used
on
circular tube-fin heat exchangers include
[8]:
(1)
plain circular fin,
(2)
slotted
fin,
(3) punched and
bent
triangular projections,
(4)
segmented
fin,
and
(5)
wire loop
extended surface. Some of the enhanced fin geometries
of
individual fins are shown
in
Fig.
1.
Figure
1
Enhanced circular
fin
geometries. (a) Plain circular fin;
(b)
slotted fin; (c) punched and
triangular projections; (d) segmented fin; and (e) wire-loop extended surface.
377
Heat- Transfer Augmentation
All of these geometries provide enhancement by the periodic development of thin boundary
layers on small-diameter wires or flat strips, followed by their dissipation in the wake region
between elements.
Enhanced Su@ace Geometries for Plate-Fin Exchangers.
Many enhanced surface geometries
are used on plate-fin exchangers.
To
be effective, the enhancement technique must be applica-
ble to low Reynolds number flows. Two basic concepts have been extensively employed to
provide enhancement [5,8]:
Special channel shapes, such as the wavy channel, which provide mixing due to secondary
flows or boundary layer separation within the channel.
Repeated growth and wake destruction of boundary layers; e.g., offset strip fin, louvred fin,
pin fin, and to some extent the perforated fin.
Extended Surfaces for Liquids
Extended surfaces used with liquids may be on the inner or outer surface of the tubes. Because
liquids have higher heat-transfer coefficients than gases, fin efficiency considerations require
shorter fins with liquids than with gases.
Internally Extended Sueaces.
Widely varied integral internally finned tubes (Fig.
2)
have
been developed for tube-side heat-transfer augmentation, and they are commercially available
in copper and aluminum. Most of them are for single-phase flows. In recent years, tubes with
special internal geometries for augmentation of boiling in refrigeration systems have been
developed. Typical, commercial names includes Thermofin tubes A,
B,
EX, and HEX. Other
possible methods for tube-side augmentation include internal roughness geometries, inserts
such as twisted tape inserts, and wire coil inserts.
The current trend for enhancement of intube evaporation of refrigerants is toward the use
of
internal microfin
tubes.
The reasons for their popularity are the fact that (1) they increase
heat transfer significantly while only slightly increasing pressure drop and
(2)
the amount of
extra material required for microfin tubing is much less than that required for other types of
internally finned tubes [lO]. Cross-sectional views of typical microfin tubes are shown in Fig.
3
[ll]. These are made from copper tubes and the popular ones are 9.525 mm
(3/8
in) outer
diameter and have
60
to
70
spiral findinch, fin height from 0.10 to 0.19 mm, and spiral angles
from 15 to
25"
[1,11]. These dimensions are in contrast to other types of internal finning that
seldom exceed
30
findinch and fin heights that range from several factors higher than the
microfin tube height. The heat-transfer coefficients, based on area of an equivalent smooth
a
b
C
d
e
f
8
h
Figure
2
Representative commercially available
tubes
with internal enhancement. (a) Twisted tape
insert in plain tube; (b) spiral internally finned tube;
(c)
quintuplex finned tube; (d) star shaped insert in
plain tube; (e) matron internally finned tube;
(f)
corrugated tube; (8) duplex internally finned tube; and
(h)
straight internally finned tube. (From Ref.
3.)
---
3
78
Chapter
8
w
-
--\.--
-
-
TUBE
2
TUBE
6
w
TUBE
3
TUBE
7
TUBE
4
TUBE
a
w
TUBE
5
TUBE
9
Figure
3
Fin profiles for micro-fin tubes used in refrigerant evaporators and condensers. (From
Ref.
11.)
tube, are increased
30
to
80%
above the smooth tube values, depending on the vapor quality
and fin profile
[I].
External
Low-Fin
Tubes.
Low-fin tubes are generally
0.75-1
in (19.2-25.4 mm)
OD
with
fins of 1/16 or 1/8 in (1.6 or 3.2 mm) height [12], and standard fin densities are 11,
19,
26,
30, and 40 findin (433-748 findm).
A
low-fin tube is ideally suited for a shell and
tube
exchanger. The outside surface of the low-fin tube is typically three times that of a plain tube.
Tube diameters are the same as plain tubes; which permits low-fin and plain tubes to be
interchangeable. External low-fin tube slii-faces are generally economical if the outside heat-
transfer coefficient is less than one-third of the inside heat-transfer coefficient [13]. The low-
fin tube is also effective with compressed gases, namely, in intercoolers and aftercoolers and
they are employed in condensing and evaporating duties, particularly with refrigerants and
hydrocarbons [7]. Low-fin tubes are used for applications such as
[13]:
1. Sensible heat applications like cooling of gases or viscous fluids.
2.
Condensing fluids with relatively low surface tension; low-fin tubes can enhance condens-
ing heat transfer typically by about
30%.
3.
Boiling-Low-fin tubes permit nucleate boiling at lower temperature differences than can
be achieved with plain tubes.
4.
Fouling fluids-Fouling is not accelerated with low-fin tubes; it is often reduced. Low-fin
tubes are more easily cleaned by chemical or mechanical means than are plain tubes.
6.2
Treated Surfaces
Treated surfaces involve fine-scale alternation of the surface finish or provision of either con-
tinuous or discontinuous coating. Treated surfaces such as hydrophobic coatings and porous
coatings (Fig. 4) are most effective for phase changes but are not applicable to single-phase
convection. The roughness height is below that which affects single-phase heat transfer
[
11.
Heat-Transfer Augmentation
3
79
Liquiv
':',
Vapor out
Figure
4
Treated surface.
6.3
Rough Surfaces
Rough surfaces refer to tubes and channels having roughness elements in the form of regularly
repeated ridge-like protrusions perpendicular to the stream direction. The configuration is gen-
erally chosen to promote turbulence rather than to increase the heat-transfer surface area. The
surface roughness may be applied to any
of
the usual prime or extended heat exchange surfaces
WI:
1. Flat plates
2.
Circular tubes
3.
Annuli having roughness on the outer surface of the inner tube
4.
Fins
Rough surfaces are produced in many configurations, ranging from random sand-grain
type roughness to discrete protuberances. Helical repeated-rib roughness is readily manufac-
tured and results in good heat-transfer performance in single-phase turbulent flow without
severe pressure drop. The main effect is that the ribs cause flow separation and reattachment
[l]. The principal parameters are ridge height
e
and pitch
p.
Examples of repeated-rib rough-
ness are shown in Fig. 5
[
141.
Both two-and three-dimensional integral roughness elements are
also possible. Within the category of surface roughness, a three-dimensional roughness of the
sand-grain type appears to offer the highest performance
[
151. Roughness elements have been
produced by the traditional processes of machining, forming, casting, or welding
[7].
Various
inserts or wrap-around structures can also provide surface proturbances
[
11.
Applications.
Surface roughness is attractive for turbulent-flow applications with high Prandtl
number fluids for which the increase in heat-transfer coefficient is greater than the friction
factor increase
[
151. In laminar-flow applications, small-scale roughness is not effective, but
twisted tape inserts and internally finned tubes appear to be the favored augmentation tech-
niques
[5].
6.4
Tube Inserts and Displaced Enhancement Devices
This class refers to devices that are inserted inside a smooth tube. The tube inserts are relatively
low in cost, relatively easy to insert and to take out of the tubes for cleaning operations.
Enhancement Mechanism
Tube inserts can create one of or some combinations of the following conditions, which are
favorable for enhancing the heat transfer with a consequent increase in the flow friction [16]:
1. Interrupting the development of boundary layer of the fluid flow and increasing the degree
of flow turbulence.
380
Chapter
8
.-.
h
bjffl;J-.
A
HELICAL
RIBS
YIAE
COILS
-.nn
PROFILE
SHAPES
Figure
5
Examples
of
repeated
rib
roughness. (Ravigururajan and Bergles,
1985.)
2.
Increasing the effective heat-transfer area if the contact between the tube inserts and the
tube wall is excellent.
3.
Generating rotating and/or secondary flow.
Forms
of
Insert Device
Various
forms
of insert devices are
[8]:
1.
Devices that cause the flow to swirl along the flow length, e.g., swirl strips and twisted
tapes.
2.
Extended surface insert devices that provide thermal contact with the tube wall (this may
also swirl the flow), e.g., star inserts.
3.
Wall-attached insert devices that mix the fluid at the tube wall.
4.
Displaced insert devices that are displaced from the tube wall and cause periodic mixing
of the gross flow.
Displaced Enhancement Devices
Displaced enhancement devices are inserted into the flow channel
so
as to improve the fluid
flow near the heat-transfer surface
[
173.
These devices include
[83
streamline shape, disks,
static mixer, and meshes or brushes, and wire coil inserts.
Coil Inserts.
Coil inserts provide an efficient and inexpensive way to enhance the heat trans-
fer inside heat-exchanger tubes
[
181.
In applications that require frequent cleaning, their easy-
Heat-Transfer Augmentation
381
to-retrofit characteristic offers a significant advantage over integrally enhanced tube types;
however, the coil inserts can have
an
inherent disadvantage if imperfect contact exists between
the coil and the tube wall. For inserts, the parameters that affect the enhancement are the
disruption height, the disruption spacing or pitch, the helix angle, and the disruption shape
[9].
Spiral spring inserts are shown in Fig.
6.
Inserts
for
HeatingKooling Mode Operation.
In the heating mode operation, the rotating
flow is noted to have favorable centrifugal convection effect, which can increase the heat-
transfer coefficient, whereas in the cooling mode operation, the rotating flow may have an
adverse centrifugal convection effect, which even may decrease the convection heat-transfer
coefficient
[
161. Thus tube inserts that can generate rotating flow, such as swirl strips or twisted
tapes, are generally not used in oil coolers; instead, coil or spiral springs are used, since spiral
springs usually do not generate rotating flow.
6.5
Swirl
Flow
Devices
Swirl flow devices include a number of geometric arrangements
or
tube inserts for forced flow
that create rotating and/or secondary flow inside tube. Such devices include inlet vortex genera-
tors, twisted tape inserts, and axial core inserts with screw type windings [1,16]. The augmen-
tation is attributable to several effects [l]: increased path length
of
flow, secondary flow ef-
fects, and, in the case of tapes, fin effects.
Twisted Tape Insert
The twisted tap insert shown in Fig.
7
has been extensively investigated. Variants of the twisted
tape that have been evaluated include mechanically rotated twisted tapes, short sections of
twisted tape at the tube inlet or periodically spaced along the tube length, and periodically
spaced “propeller” inserts that are mechanically rotated by the flowing fluid [8].
Corrugated Surfaces
Corrugated tubes are associated with circumferential or spiral indentations (spirally fluted) as
shown in Fig.
8.
Flows in corrugated channels are associated with secondary flows, suppression
of the secondary flow by counteracting centrifugal forces, and destruction of the secondary
flow by the onset of turbulence
[7].
In condensing services, corrugated tubes increase the heat-
transfer coefficient because only a thin film of the liquid condensate is left on the crests, while
most of it drains into the troughs due to the surface tension and gravity effects. The thin film
offers very little resistance, and hence the condensing coefficient is very high.
Among the corrugated tubes, spirally corrugated tubes provide several advantages over
other rough surfaces. They have easier fabrication, limited fouling, and higher enhancement in
the heat-transfer rate compared to increase in the friction factor
[19].
The tube parameters that
affect enhancement of corrugated surfaces are the pitch, the depth and type of the grooves, and
number of groove starts.
Doubly Enhanced Surfaces
If enhancement is applied to the inner and outer tube surfaces, a doubly enhanced tube results.
Applications for such doubly enhanced tubes include condenser and evaporator tubes. Typical
forms
of
doubly enhanced tubes include
[8]:
(1) helical rib roughness on inner surface and
integral fins on outer surface,
(2)
internal fins on inner surface and porous boiling coating on
outer surface,
(3)
twisted tape insert on inner surface and integral fins on outer surface,
(4)
corrugated roughness on inner and outer surfaces, and
(5)
corrugated tubes. The aluminum,
spirally fluted tube of GA Technologies is an example. Figure
9
illustrates five basic ap-
proaches that may be employed to provide doubly enhanced tubes.
382
Chapter
8
Type of roughness of
Three dimensional
Ridge type
two-
Groove type
two
roughness ("uniform
dimensional roughness
dimensional roughness
internal surface
roughness)
("repeated ribs")
Bask geometry
Variations of basic
geometry by change
of
ple (shows probable
range of p/e of interest)
Possible "element
shapes" for basic
geometry
(a)
I
1
Figure
6
(a) Catalog
of
roughness geometries and
(b)
internal roughness.
383
Heat- Transfer Augmentation
Figure
7
Twisted tape insert. (a) Insert; and
(b)
insert in a plain tube.
Turbulators for Fire Tube Boilers
A
popular technique for enhancement of heat transfer in fire tube boilers is to introduce turbula-
tors into the boiler tubes to increase the gas-side heat-transfer coefficients, thereby increasing
heat recovery from flue gases
[20].
This will reduce fuel consumption and improve boiler
efficiency. Turbulators are made from a narrow, thin metal strip bent and twisted in zig-zag
fashion to allow periodic contact with the tube wall.
6.6
Surface Tension Devices
Surface tension devices consist of relatively thick wicking material
or
grooved surfaces on
heat-transfer devices to direct the flow of liquid in boiling and condensing.
J?
p
x
starts
-1
Figure
8
Spirally fluted tubes.