Kuppan T. Heat Exchanger Design Handbook
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354
Chapter
7
Table
3
Comparison Between PHE and Shell and Tube Heat Exchanger
[6]
Features Traditional PHE STHE
Multiple duty
Possible
Impossible
Piping connections
From one direction (on frame
From several directions
plate)
Heat-transfer ratio
3-5
1
Operating weight ratio
1
3-10
Hold-up volume
Low High
Space ratio
1
2-5
Welds
None
Welded
Sensitivity to vibrations
Not sensitive
Sensitive
Gaskets
On every plate
On each flanged joint
Detection of leakage
Easy
to
detect on exterior Difficult
to
detect
Access for inspection
On each side
of
plate
Limited
Time required for opening
15 min with pneumatic tight-
60-90
min
ener
Repair
Easy to replace plate andor
Requires tube plugging (de-
gasket
creased capacity)
Modification
Easy by adding
or
removing
Impossible
plates
Fouling
10-25%
that of STHE
bourd
and the
chevron
or
herringbone
pattern. Figure
7
shows both types. The construction
features of the these two patterns are discussed next.
Intermating Troughs Pattern
In the intermating troughs or washboard pattern, the corrugations are usually pressed deeper
than the plate spacing. The plates nestle into one another when the plate pack
is
assembled
Figure
7
Two most widely used corrugation types. (a) Intermating troughs or washboard pattern; and
(b) chevron or herringbone pattern.
355
Plate Heat
Exchangers
Figure
8
Cross-section
of
two neighboring plates (contact). (a) Intermating troughs; and (b) and (c)
chevron troughs. (From
Ref.
5.)
(Fig. 8a). The plate spacing is maintained by dimples, which are pressed into the crests and
troughs and contact one another on adjacent plates. Since the washboard type has fewer contact
points, and due to its greater corrugation depth than the herringbone type, it operates at lower
pressures. The maximum channel gap,
b
(shown in Fig. 8a), varies from
3
to
5
mm and the
minimum channel gap varies from
1.5
to
3
mm.
Typical liquid velocity range
in
turbulent flow
is from
0.2
to
3
m/s,
depending upon the required pressure drop
[5].
Chevron or Herringbone Trough Pattern
In the herringbone pattern, the corrugations are pressed to the same depth as the plate spacing.
The
chevron angle is reversed on adjacent plates
so
that when the plates are clamped together
the corrugations cross one another to provide numerous contract points. The herringbone type
therefore has greater strength than the washboard type, which enables it to withstand higher
pressures with smaller plate thickness
[9].
It is the most common type in use today. The contact
arrangements of the plates are shown in Fig. 8b.
The
corrugation depth generally varies from
3
to
5
mm. Typical liquid velocities (in turbulent
flow)
range from
0.1
to
1
m/s
[5].
Plate Materials
Materials that are suitable for cold pressing and corrosion resistant are the standard materials
of construction. Table
4
gives a list
of
most common materials used for fabrication of plates.
Carbon steel
is
rarely used due to its poor corrosion resistance.
Table
4
Common PHE Materials
Stainless steel AISI
304
Incoloy 825
Stainless steel AISI
316
Monel400
Avesta SMO 254
Hastelloy
B
Titanium, titanium-0.2%
Hastelloy C-276
palladium stabilized
Aluminum brass 7612212
Tantalum
Cupronickel
(70/30)
Inconel 600
Cupronickel
(901
10)
Inconel 625
Diabon
F
100
356
Chapter
7
4.2 Gasket Selection
When selecting the gasket material, the important requirements to be met are chemical and
temperature resistance coupled with good sealing properties and shape over an acceptable
period
of life
[3].
Much work has been done to develop elastomer formulations that increase
the temperature range and chemical resistance of gaskets. Typical gasket materials and their
maximum operating temperature are given in Table
5.
4.3 Frames
Frames are classified
[7]
as
(1)
B
frame, suspension-type frame used with larger PHEs,
(2)
C
frame, compact cantilever-type frame used with smaller PHEs, and
(3)
F
frame, an intermediate
size suspension-type frame. The frame is usually constructed in carbon steel and painted for
corrosion resistance. Where stringent cleanliness requirements apply, such as in pharmaceuti-
cals and in dairy, food, and soft drinks industries, the frame may be supplied in stainless
steel. Stainless-steel-clad frames are available
for
highly corrosive environments. The units are
normally floor mounted, but small units may be wall mounted.
4.4 Nozzles
Nozzles are located in one or both end plates. In single-pass arrangement, both the inlet and
outlet ports for both fluids are located in the fixed head end, and hence, the unit may be opened
up without disturbing the external piping. But with multipass arrangements the ports must
always be located on both heads. This means that the unit cannot be opened up without disturb-
ing the external piping at the movable head end.
To
be corrosion resistant, nozzles are usually
same as plate material. Typical nozzle materials include stainless steel type
316,
rubber clad,
titanium, Incoloy
825,
and Hastelloy
C-276.
4.5 Tie Bolts
The tie bolts are usually made
of
1%
Cr-0.5%
MO
low-alloy steel. The packing of large units
may be compressed by hydraulic, pneumatic, or electric tightening devices.
4.6 Connector Plates
It is possible to process three
or
more fluids in a single gasketed PHE. This is being achieved
by means of connector plates. This practice is widely used in food processing and permits
heating, cooling, and heat recovery of the fluids in a single unit
[9].
Table
5
Gasket Materials and Maximum
Operating Temperature
Styrene butadiene rubber
(80°C)
Nitrile rubber
(14OOC)
Ethylene propylene rubber, EPDM
(
150°C)
Resin-cured butyl rubber
(140°C)
Fluorocarbon rubbers
(
1
SOOC)
Ruoroelastomer (Viton)
(100°C)
Compressed asbestos fiber (CAF)
(260°C)
Silicon elastomers (low-temperature applications)
Plate Heat Exchangers
357
4.7
Connections
Connections are usually made of the same material as the plates to avoid the galvanic corrosion
damage. Rubber-lined connections can be used for some duties.
5
OTHER FORMS OF PLATE HEAT EXCHANGERS
Continued developments and product innovations are taking place among various manufactur-
ers to overcome limitations of pressure and temperatures, and to handle products of high vis-
cosity, high fibrous contents, and high corrosivity. As a result, various forms of
PHEs
have
come into the market. Some new forms
of
PHEs
are discussed next. Text for items
5.1
to
5.7
has been taken from product literature of Alfa Laval, Pune, India
[6].
5.1
Brazed Plate Heat Exchanger, Based
on
Reference
6
The brazed plate heat exchanger evolved from the conventional plate heat exchanger in answer
to the need for a compact plate heat exchanger for high-pressure and high-temperature duties.
Like the gasketed plate heat exchanger, the brazed plate heat exchanger is constructed of a
series of corrugated metal plates but without the gaskets, tightening bolts, frame, or carrying
and guide bars. It consists simply of stainless steel plates and two end plates. The plates are
brazed together in
a
vacuum oven to form a complete pressure-resistant unit. The two fluids
flow in separate channels. This compact design can easily be mounted directly on piping
without brackets and foundations. Brazed plate heat exchangers accommodate a wide range of
temperatures, from cryogenic to more than
200°C.
Because of the brazed construction, the
units are not expandable, but get their reputation from their relatively tiny size.
5.2
Wide-Gap Plate Heat Exchanger, Based
on
Reference
6
The wide-gap-about
16
mm
(5/8
in)-plate heat exchanger provides a free-flow channel for
liquids and products containing fibers or coarse particles or high-viscosity liquids that normally
clog or cannot be satisfactorily treated in shell and tube heat exchangers.
5.3
Flow-Flex Tubular Plate Heat Exchanger,
Based on Reference
6
In Flow-Flex, the tubular plate heat exchanger, the plate pattern builds the tube channels with
a free cross section on one side and conventional plate channels on the other. This allows
Flow-Flex to handle asymmetrical duties, that is, dissimilar flow rates at a ratio of at least two
to one (for the same fluid properties and pressure drop). This vibration-free construction is
also suitable for low-pressure condensing and vaporizing duties and for fibrous and particle-
laden fluids.
5.4
Twin-Plate Heat Exchanger, Based on Reference
6
The twin-plate heat exchanger is a welded plate heat exchanger designed especially for han-
dling aggressive media. The twin-plate pack consists of welded channels that alternate with
traditional gasketed channels. The aggressive medium
flows
in the welded channels,
and
the
only gaskets in contact with the aggressive medium are two circular porthole gaskets between
the welded plate pair. The porthole gaskets are available in highly resistant elastomers and
nonelastomer materials. The channels for the nonaggressive medium are sealed by traditional
elastomer gaskets. Use of semiwelded units is increasing in the chemical, petroleum refining,
and refrigeration industries
[
101.
358
Chapter
7
5.5
Double-Wall Plate Heat Exchanger, Based on Reference
6
The double-wall plate heat exchanger is designed for use with media between which contami-
nation or a hostile reaction can occur if the two media should mix. Double plates replace the
single plates normally between the two media. The channels formed by assembling the double
plates are sealed by conventional gaskets. In the unlikely event of leakage, the media will be
easily visible on the outside of heat exchanger. A hole in one of the double plates results in
leakage between them. A gasket defect causes an external leak, either directly from the periph-
eral gasket or from between the plates at the site of the failure.
5.6
Diabon
F
Graphite Plate Heat Exchanger,
Based on Reference
6
The Diabon
F
graphite plate heat exchanger is a heat exchanger with graphite plates developed
for use with media normally too corrosive for exotic metals and alloys. The Diabon
F
100
graphite plates are made
of
a composite material composed
of
graphite and fluoroplastics. The
material is compressed into corrugated plates fitted with thin, flat, corrosion-resistant gaskets.
The graphite plates offer excellent corrosion resistance and good heat-transfer properties
in
combination with low thermal expansion and high pressure.
5.7
Glue-Free Gaskets (Clip-On Snap-On Gaskets), Based on
Reference
6
The clip-on snap-on gaskets are glue-free gaskets designed to perform in heavy industrial
applications in the same manner as traditional glued gaskets. The glue-free gaskets are attached
to
the plate by fasteners situated at regular intervals around the plate periphery. The gaskets
simply slip into place during fitting and slip out for regasketing procedures, eliminating gluing
procedures. Both operations can be performed on site without removing the plate from the
frame. All these contribute to reduced service costs and downtime.
5.8
All Welded Plate Exchangers, After Larry Trom
[lO]
This design entirely eliminates the gaskets and by developing a fully welded plate exchanger
further enhances the reliability as well as the temperature and pressure limits of the gasketed
plate exchanger. Welded plate exchangers, unlike gasketed and semiwelded models, can neither
be expanded in surface area nor be cleaned readily by mechanical methods. Only chemical
cleaning is possible.
6
WHERETOUSEPLATEHEATEXCHANGERS
The characteristics
of
plate heat exchangers are such that they are particularly well suited to
liquid-liquid duties in turbulent flow
[8].
Incidentally, a fluid sufficiently viscous
to
produce
laminar flow in a shell and tube heat exchanger may be in turbulent flow
in
a
PHE.
In the
1930s,
PHEs
were introduced to meet the hygienic demands of the dairy industry. Today the
plate heat exchanger is universally used in many fields: heating and ventilating, breweries,
dairy, food processing, pharmaceuticals and fine chemicals, petroleum and chemical industries,
power generation, offshore oil and gas production, onboard ships, pulp and paper production,
etc. Plate heat exchangers also find applications in water-to-water closed-circuit cooling-water
systems using a potentially corrosive primary cooling water drawn from sea, river, lake, or
Plate Heat Exchangers
359
cooling tower, to cool a clean, noncorrosive secondary liquid flowing in a closed circuit
[8].
In this application, titanium (99.8%)-palladium
(0.2%)
stabilized titanium is used because of
its outstanding corrosion resistance.
6.1
Applications
for
Which Plate Heat Exchangers
Are
Not
Recommended
PHEs
are not recommended for the following services
[3]:
1.
Gas-to-gas applications.
2.
Fluids with very high viscosity may pose flow distribution problems, particularly when
cooling is taking place; flow velocities less than
0.1
m/s
are not used because they give
low heat-transfer coefficients and low heat exchanger efficiency.
3.
Less suitable for vapors condensing under vacuum.
7
THERMOHYDRAULIC FUNDAMENTALS
OF
PLATE HEAT EXCHANGERS
The design of plate heat exchangers is highly specialized in nature, considering the variety of
plate corrugations (there are more than
60
different plate) available to suit varied duties. The
plate design varies from manufacturer to manufacturer, and hence the thermohydraulic perfor-
mance. Unlike tubular heat exchangers, for which design data and methods are easily available,
plate heat exchanger design continues to be proprietary in nature. Manufacturers have devel-
oped their own empirical correlations for the prediction of thermal performance applicable to
the exchangers marketed by them. Therefore, specific and accurate characteristics of specific
plate patterns are not available in the open literature. In recent years, some correlations have
been reported in many references. These correlations are mostly approximate in nature, to suit
preliminary sizing of the units for a given duty.
No
published information is available on
accurate heat-transfer correlations. Hence, it is better to leave the thermal and
also
mechanical
design aspects to a plate manufacturer who offers
an
optimum design for the process parame-
ters submitted by the consignee. In the following paragraphs, the salient features of the thermal
design methods for approximate sizing are presented.
7.1
High- and Low-Theta Plates
A
plate having a low chevron angle,
p
(Fig. 9) (about
25-30"),
provides high heat transfer
combined with high pressure drop, whereas a plate having a high chevron angle (about
60"-
65")
provides a low heat transfer combined with low pressure drop,
[8].
Manufacturers specify
the plates having low values
of
chevron angle as
high-theta plates
and plates having high
values of chevron angle as
low-theta plates.
Theta is used by manufacturers to denote the
number of heat transfer units
(NTU).
The expression for theta is given by
360
Chapter
7
Figure
9
Chevron angle.
The values of
6
achieved per pass for various plate types
vary
enormously
(1.15
to about
4),
and NTU for high-theta plates is of the order of
3.0
and for low-theta plates
0.50.
7.2
Thermal Mixing
One of the thermal design problems associated with PHE is the exact matching of the thermal
duties; it is very difficult to achieve the required thermal duty and at the same time utilize the
available pressure drop fully
[3].
This problem is overcome through a procedure known as
thermal mixing. Thermal mixing provides the designers with a better opportunity to utilize the
available pressure drop without excessive surface, and with fewer standard plate patterns
[9].
Thermal mixing
is
achieved
by
two methods:
I.
Using high- and low-theta plates.
2.
Using horizontal and vertical plates.
Thermal Mixing Using High- and Low-Theta Plates
In
this method, the pack of plates may be composed
of
all high-theta plates (say,
p
=
30"),
or
all low-theta plates (say,
p
=
60"),
or a combination
of
high- and low-theta plates arranged
alternatively in the pack to provide
an
intermediate level
of
performance.
Thus
two plate
configurations provide three levels of performance plates, as shown in Fig.
10.
H
channel(high+high)
M
channel
(high+low)
L
channel
(low+low)
Figure
10
Thermal mixing using a combination
of
high theta and low theta plates.
361
Plate Heat Exchangers
Thermal Mixing Using Horizontal and Vertical Plates
In this method, two combinations of geometric patterns are selected to provide three levels of
performance plates (Fig.
11)
[7]:
1.
Horizontal-style plates: Accu-ThermTM
“H’
style plates have a horizontal herringbone em-
bossing. These plates have higher heat-transfer coefficients and slightly larger pressure
drop.
2.
Vertical-style plates: Accu-ThermTM
“V”
style plates have a vertical herringbone emboss-
ing. These plates have lower pressure drop and slightly lower heat transfer coefficients.
3.
Combination-style plates: Accu-ThermTM
“H”
and “V” style plates have been combined to
obtain an intermediate thermal and pressure drop performance.
7.3
Flow
Area
Close spacing
of
the plates with nominal gaps ranging from
2
to
5
mm
(0.08-0.02
in), gives
hydraulic mean diameters in the range of
4-10
mm
(0.15-0.4
in)
[9].
The plates are embossed
so
that very high degrees of turbulence are achieved. Critical Reynolds numbers are in the
range
of
10-400,
depending on the geometry. These factors contribute to produce very high
Figure
11
Thermal mixing using combination
of
plate geometries. (a) Horizontal-horizontal; (b) hori-
zontal-vertical; and (c) vertical-vertical. (Courtesy of
M/S
Paul Muller Company, Springfield, Missouri.)
--
362
Chapter
7
heat-transfer coefficients. Nominal velocities for "waterlike" liquids in turbulent flow are usu-
ally in the range
0.3-1.0
m/s
(1-3.1 ft/s), but true velocities may be higher by a factor three
or four due to the effect of the corrugations
[8].
All
heat-transfer and pressure drop relation-
ships are, normally, based on channel velocity. The channel velocity is calculated by dividing
the flow per channel by the channel cross sectional area
[
111. The channel cross-sectional area,
A,
is
given by
A,=
Wb
(2)
where
b
is the mean plate gap and
W
is the effective plate width (gasket to gasket). Dimension
W
is shown schematically in Fig. 12.
7.4
Heat-Transfer and Pressure-Drop Correlations
Heat-Transfer Correlations
A
general correlation for turbulent
flow
is given by
[8]:
Nu
=
C
Re"Pr"(~&,)'
(3)
Typical reported values are
C
=
0.15-0.40
n
=
0.65-0.85
m
=
0.30.45
(usually
0.333)
x
=
0.05-0.20
!-----
NOMINAL
WIDTH
X1
I
x
to
,-I
EFFECTIVE
PLATE
WIDTH
EFFECTIVE LENGTH
OF
FLOW
w
=
NOMINAL
WIDTH
-
=d
x;+x;
GASKET
THICKNESS
(DIAGONAL ARRANGEMENT)
CORRUGATED
PLATE
PROFlLE
Figure
12
Major plate dimensions, W-Plate width and L-Effective flow length. (From Ref.
1
1
.)
363
Plate Heat
Exchangers
One of the most widely used plates has the following relationship [8]:
where the Reynolds number is based on equivalent diameter,
D,,
defined by
D,
=
4
Wb
2(W+
b)
=
2b
since
b
is very small compared to
W
The expression for hydraulic diameter
Dh
for
PHE
is given by:
Dh
=
4
x
minimum free flow area
wetted perimeter
2b
z-
4*
where
$*
is the ratio of actual (developed) surface area to projected surface area.
But to calculate Reynolds number, the hydraulic diameter expressed by
Eq.
6
is not used
because it is difficult to measure the parameter
$*
[5].
Buonopane and Troupe
[
121 present generalized relationships for a number of geometries
for laminar flow. It seems that the following Sieder-Tate type relationship applies [8]:
where
C,
=
1.86-4.50 depending on geometry, replace
Dh
by
D,,
and
L
is the effective plate
length (as shown in Fig.
12).
More correlations are summarised in references 4,
5,
7, and
11.
Pressure Drop
Friction Factor,
f.
The fanning friction factor,
f,
is of the form
[
131:
f=-
C?
Re
'
where
C,
is a constant characterizing each type of plate, and
y
covers different ranges. For a
typical plate, the
f
factor is given by
[
131:
(9)
Usher
[
141 reported that the friction factor for plate exchangers is
10-60
times that developed
inside tubular exchangers at similar Reynolds numbers for turbulent flow. Marriott [8] ob-
served that this value can be up to 400 times. Though the friction factors are high compared
to that of the tubular units, the pressure drop will be less due to the following reasons
[8]:
1.
The nominal velocities are low, and nominal plate lengths do not exceed about
6
ft (1829
mm),
so
that the term (G2/2g,)L in the pressure-drop equation is very much smaller than
would be the case in a tubular unit,
2.
Only a few passes will achieve the required NTU value,
so
that the pressure drop is
effectively utilized for heat transfer and losses due to unfruitful flow reversals are mini-
mized.