Selection and Use of Printed Circuit Heat Exchangers 0 INTRODUCTION/PURPOSE 1 SCOPE 2 FIELD OF APPLICATION 3 DEFINITIONS 4 CONSTRUCTION 5 HEAT TRANSFER AND PRESSURE DROP 6 FOULING 7 MECHANICAL AND MATERIALS ASPECTS 8 COMPACTNESS 9 FLEXIBILITY 10 COST 11 GBHE EXPERIENCE 5 12 BIBLIOGRAPHY APPENDICES A HEAT TRANSFER AND PRESSURE DROP IN WAVY PASSAGES

1. GBH Enterprises, Ltd.
Process Engineering Guide:
GBHE-PEG-HEA-510
Selection and Use of Printed Circuit
Heat Exchangers
Information contained in this publication or as otherwise supplied to Users is
believed to be accurate and correct at time of going to press, and is given in
good faith, but it is for the User to satisfy itself of the suitability of the information
for its own particular purpose. GBHE gives no warranty as to the fitness of this
information for any particular purpose and any implied warranty or condition
(statutory or otherwise) is excluded except to the extent that exclusion is
prevented by law. GBHE accepts no liability resulting from reliance on this
information. Freedom under Patent, Copyright and Designs cannot be assumed.
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2. Process Engineering Guide:
Selection and Use of Printed
Circuit Heat Exchangers
CONTENTS
SECTION
0
INTRODUCTION/PURPOSE
2
1
SCOPE
2
2
FIELD OF APPLICATION
2
3
DEFINITIONS
2
4
CONSTRUCTION
2
5
HEAT TRANSFER AND PRESSURE DROP
3
6
FOULING
3
7
MECHANICAL AND MATERIALS ASPECTS
4
8
COMPACTNESS
4
9
FLEXIBILITY
4
10
COST
4
11
GBHE EXPERIENCE
5
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3. 12
BIBLIOGRAPHY
5
APPENDICES
A
HEAT TRANSFER AND PRESSURE DROP IN
WAVY PASSAGES
DOCUMENTS REFERRED TO IN THIS PROCESS
ENGINEERING GUIDE
6
7
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4. 0
INTRODUCTION/PURPOSE
This Guide is one of a series on heat transfer prepared for GBH Enterprises.
The Printed Circuit Heat Exchanger (PCHE) is typically a proprietary piece of
heat transfer equipment made by select manufacturers..
It has very small passages, making it very compact and opening up possibilities
for innovative design. It also has the flexibility of arrangement of the process
fluids of a plate fin heat exchanger. (For information about plate fin heat
exchangers see [Ref. 1]).
1
SCOPE
The main purpose of this guide is to indicate the situations where a PCHE might
be appropriate and should receive serious consideration in the choice of a heat
exchanger. It considers problems that might arise. For a general discussion on
the selection of heat exchanger type see GBHE-PEG-HEA-506.
2
FIELD OF APPLICATION
This Guide applies to engineers in GBH Enterprises world-wide who may be
involved in the selection and specification of heat exchangers.
3
DEFINITIONS
For the purposes of this Guide, the following definition applies:
PCHE
Printed Circuit Heat Exchanger
With the exception of terms used as proper nouns or titles, those terms with initial
capital letters which appear in this document and are not defined above are
defined in the Glossary of Engineering Terms.
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5. 4
CONSTRUCTION
The passages carrying the fluids in a PCHE are chemically milled into flat metal
plates which are then diffusion bonded together to make up solid blocks or
'cores'. (Diffusion bonding occurs when two clean flat plates are brought into
close contact with each other at high temperature (under) in an inert atmosphere.
The diffusion process leads to a new crystal structure in the metal which, under
the right conditions, gives a join indistinguishable from the parent metal.)
Typical dimensions for a passage are 1 to 2.5 mm wide by 0.5 to 1 mm deep.
Fluid headers are welded to the cores to control the flow of fluids to the
appropriate passages. A typical core is 600 mm by 225 mm by 250 mm. Cores
can be welded together to build up larger units. Typical surface/volume ratios are
500-1000 m2/m3 compared with 50-100m2/m3 for shell and tube exchangers.
The manufacturer's brochure [Ref. 2] shows typical plates and complete heat
exchangers and give more detailed technical information: it is not intended to
repeat that material here.
5
HEAT TRANSFER AND PRESSURE DROP
Clearly with such small passages the Reynolds Number is low, though for such
fluids as air and water in the larger passages it is not normally quite in the
laminar region. The method of creating the flow passages does not constrain
them to be straight and the normal pattern is a zig-zag, with a typical section
length of 5 to 10 mm. The resultant frequent tripping of the flow disrupts the
boundary layer, giving better heat transfer for the same expenditure of pressure
drop, especially at low values of Reynold's Number.
As with other heat exchanger types, the design can be optimized or adapted to
meet the pressure drop allowed as required. This applies even for reboilers or
condensers inserted into the bottom or top of a distillation column, where natural
circulation reigns.
Data for passages of roughly the same form as those in a PCHE (single phase
only), are to be found in Kays and London [Ref. 3], though select manufacturers
have their own data and adapt the pattern to suit the application. The Kays and
London data (based on experiments with plate fin geometries) are given in
Appendix A so that designers can carry out their own crude sizing without
reference to proprietary designs.
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6. The complete flexibility in the layout of the channels in the plates makes almost
any arrangement, including counter-current, possible. This is explored below.
6
FOULING
It is well known that plate heat exchangers suffer less from fouling than shell and
tube units, no doubt due to the constant change of flow direction imparted by
conventional plate design. The PCHE can be expected to have the same
characteristic. However, unless the duty is completely clean or the fouling known
to be self limiting, some form of chemical cleaning should be available to cope
with any fouling that may develop.
What will rightly be of concern to designers is the risk of complete blockage of
passages, since mechanical cleaning is not possible and chemical cleaning of
such tiny passages when blocked is unlikely to be effective. It is necessary to be
very clear about what the risks really are in any particular potential application. It
is suggested that the following need to be considered:
(a)
Thick fouling layer. If the thickness of the layer is a significant proportion of
2 mm then pressure drop will be badly affected. Such duties are
unsuitable for a PCHE.
(b)
Unstable, velocity sensitive fouling. Should the dirt tend to grow faster in a
low velocity region, then any exchanger with parallel passages presents
the possibility of unstable behavior. If one passage has a slightly lower
velocity, the dirt will accumulate faster in it, thereby slightly reducing the
velocity. Chemical cleaning is needed before the situation deteriorates too
far.
(c)
Solids larger than the holes. These can be filtered out if present in small
amounts.
(d)
Solids a little smaller than the holes. These may not be a problem if
present in small numbers but might cause blockages if they are sufficiently
numerous to interfere with each other. They also can be filtered out if not
too many.
.It is seen that a PCHE should not be used in badly fouling duties but might be
acceptable for many that are moderately fouling, subject to confirmation case-bycase. For cooling water or other situations where the suspended solids are not of
simple shape the passages can be widened to slots so that solids will readily
pass through.
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7. 7
MECHANICAL AND MATERIALS ASPECTS
PCHE's are normally made of type 316L stainless steel. They are also available
in a range of more exotic corrosion resistant materials, those containing ever
greater proportions of nickel, including nickel itself. Titanium construction has not
yet reached the market place. The very thin ligaments between channels make
the PCHE especially sensitive to corrosion, without the possibility of the
equivalent of a quick retube. More care should be taken than is usual with
stainless steel to ensure that the corrosion rate really is next to zero.
The integrity of the diffusion bonding between plates at the edge is an important
criterion for success. Quality is maintained by adherence to procedures and can
be checked by surface crack detection on the outside and by leak and pressure
testing. In the author's opinion the integrity of a PCHE is greater than that of a
carefully built shell and tube exchanger.
Conventional welding on a PCHE is mainly concerned with the attachment of
manifolds. For most duties the stress in such a weld is nominal because the
manifold is of small diameter. No notable technical problems arise. Other welds
are needed when a number of cores have to be joined together to make a larger
unit, but again these do not normally see any high stresses.
Two potential problems should be brought to the mechanical designer's attention,
namely the tendency of some large PCHE's with flow across the width of the
plates to bend (like a banana) more than we are accustomed to (due to the high
temperature gradient) and the possibility of high internal stresses if cross flow
gives a high local temperature difference between channels.
8
COMPACTNESS
A PCHE requires less volume than any other commercial exchanger to fulfill a
given duty. The saving is mainly in passage length - as a rough guide the
approach area (that is, the gross area of the surface at which the heat transfer
channels terminate, the equivalent to the tube sheet) of a PCHE is the same as
for a conventional heat exchanger but passage length (equivalent to
tube length times number of passes) is about 10% as much.
Compactness opens up possibilities of economic arrangements impossible with
conventional geometry, such as putting the exchanger inside the end of a
process vessel or connecting directly to it. Alternatively the possibility of making
savings by letting the exchanger move with or be supported by the piping is
increased (see Clause 11).
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8. 9
FLEXIBILITY
As with the plate fin type, the method of manufacture of a PCHE permits great
flexibility in the mutual arrangement of the heat transfer surfaces. The equivalent
of two different fluids on the shell side is readily accomplished. An example of
where this can be a great advantage occurs when a multi-component fluid is to
be condensed by two or more coolants in series without condensate separation.
Another example is the incorporation of a small number of steam passages to
cope with a high freezing/melting point fluid.
It is possible to modify passages along their length to match fluid properties and
optimize design.
10
COST
A PCHE will normally compete well with other plate exchangers, particularly for
high temperature and pressure applications. The benefits depend on too many
factors for any helpful guide to be given here. There is one example, in the USA,
of a duty requiring a nickel construction where the PCHE was an easy winner on
price over the conventional plate, as well as having other advantages.
It must be remembered that the main plant items in a chemical plant only account
for about 25% of the total cost. The PCHE gives opportunities for significant
savings on piping and structures which must be taken into account in any cost
comparison when considering a particular case.
11
GBH ENTERPRISES EXPERIENCE
Our experience has served to confirm the benefits of compactness and to
emphasize the difficulty of handling a corrosion problem. Not only is there not
much material thickness to act as corrosion allowance, it is impossible to
examine the heat transfer surface visually or accurately to seal off particular
leaking passages. Apart from corrosion, our experience confirms the general
robustness of the PCHE and the ability of manufacturers to carry out the thermal
rating accurately.
The above observations are based on applications on nitric acid plants in
Europe, as detailed below.
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9. One very large unit in Western Europe - nitric acid plant replaced the coils in an
economizer within the existing shell. The alternatives were either a completely
new economizer or major surgery on the old one to get the replacement coils in.
It is in fact possible to enter the vessel alongside the four PCHE blocks, giving
better access than when it was full of tube banks.
Unfortunately there was a corrosion problem. This is thought to have been due to
continued reaction increasing the dew point in blocked or partially blocked
passages. It only requires one hole for boiler feed water to join the process and
lower the dew point everywhere. The problems of inspection repair and low
corrosion tolerance were thrown into sharp relief by this experience.
The other significant duty was heating a gas stream with condensing steam as
part of the NOx abatement project at Western European plant. Here the PCHE
could simply be welded into the line without supports of its own, thereby greatly
simplifying structure, piping and layout.
A PCHE has also been used successfully on a boiling CO2 duty in the Large
Scale Laboratory in Europe (see [Ref. 4]).
12
BIBLIOGRAPHY
[1]
'Plate fin heat exchangers - guide to their specification and use.' MA
Taylor (Ed). HTFS 1987
[2]
'Compact Heat Exchangers' – manufacturer sales brochure.
[3]
Compact heat transfer - a summary of basic heat transfer and flow friction
design data. W M Kays & A L London. McGraw-Hill 1958.
[4]
'Alternatives to Small Scale refrigeration plant'. P D Hills, M Ahmed & K
Connolly. Heat Exchanger Engineering, Vol,III, Advances in Design &
Operation, E A Foumeny & P J Heggs (eds), Ellis Horwood, 1993.
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10. APPENDIX A
HEAT TRANSFER AND PRESSURE DROP
IN WAVY PASSAGES (based on Kays and London data their figure 86)
Define symbols as follows:
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11. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown
Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass
Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance
Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals
Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com