Background of the Invention
1. Field of the Invention
The present invention relates to a heat transfer element,
more particularly to a heat transfer element for use in a power generating station
or a chemical processing plant. Such a heat transfer element can be in the form
of a sheet or tube, for example.
There are currently over six hundred power generating stations
in the European Union. An important feature of these stations is the provision of
heat exchangers consisting of a number of radiant panels which serve to transfer
heat within the station. There may be around 30,000 square metres of radiant panels
in a single heat exchanger. A power generating station may use up to twelve or more
The radiant panels should not only serve their primary
heat transfer function, they should also be robust to withstand the conditions in
which they operate. Thus, not only are physical conditions harsh, with hot air and
steam at up to about 150°C flowing at high speed past the panels, but also
corrosive chemicals, such as sulphurous and nitrous acids, are present in the air
stream. Furthermore, the panels may become clogged with soot or debris, which may
also impair their function. The panels are also subjected to rapid thermal cycling.
Conventionally, heat transfer elements used to make the
radiant panels have been manufactured from a metal with a vitreous enamel coating.
The metal base material, conveniently of mild steel, provides the necessary structural
strength to the element and also the required thermal conductivity. A coating of
vitreous enamel protects the metal base from the corrosive effects of the surrounding
Recently, attempts have been made to provide heat transfer
elements by spraying a metal base with a fluoropolymer. However, the resulting composite
element is not economical to manufacture.
United States Patent No. 4,461,347
there is proposed a heat exchanger assembly comprising coaxially arranged
inner and outer pipes. The inner pipe can be formed of high strength metal and ensheathed
by an extruded heat shrinkable plastics tube of non-reactive material, such as polytetrafluoroethylene
A plate heat exchanger comprising at least three plate
elements consisting of graphite and a fluoropolymer, such as polyvinylidene fluoride
is disclosed in
European Patent Specification No. 0 203 213 A1
British Patent Specification No. 2 255 148A
teaches a structurally composite metal and plastics tube in which the
metal forms a tubular core having openings throughout its length occupying at least
5% of its total surface area while the plastics material forms imperforate inner
and outer layers, each at least 0.1 mm thick, covering the inside and outside of
the metal core and integrally joined through the openings.
There is a need to improve upon the performance of heat
transfer elements in power generating stations. Thus, it would be desirable to provide
a heat transfer element with improved heat transfer properties, with improved anti-fouling
properties, with improved resistance to physical and chemical corrosion, and with
improved mechanical properties.
All of these desiderata are objects of the present invention.
A further object of the present invention is to provide
a heat transfer element with the improved properties referred to above but which
is relatively economical to manufacture.
Summary of the Invention
According to one aspect of the present invention there
is provided a heat transfer element comprising a polymer matrix having a fibrous
material interspersed therein, said heat transfer element comprising a fluoropolymer
at least on an outer surface thereof, the interspersion of the fibrous material
within the polymer matrix providing rigidity to the heat transfer element, a thermally
conductive material being distributed within the heat transfer element.
The fibrous material may comprise metal fibres, such as
iron, steel, or stainless steel fibres, in which case additional thermally conductive
material is not necessary. However, it is also possible, when using metal fibres,
to add a particulate metal such as particles of iron, steel, stainless steel or
The fibrous material may alternatively comprise glass fibres,
preferably glass fibres made from a chemically resistant glass, for example boron-free
glass fibres, or a mixture of glass fibres and fibres of a plastics material, such
as polypropylene or a fluoropolymer.
It is also contemplated that the fibrous material can comprise
glass fibres coated with a thermally conductive material.
The fibrous material can be incorporated in any convenient
form. Preferably the fibrous material comprises continuous fibres in one of the
forms conventionally used for making fibre reinforced articles. Examples include
randomly distributed or closely mingled fibres, or rovings braided to form continuous
tubes, formed into preimpregnated tapes, or woven into panels. The rovings may themselves
be precoated with, for example, a plastics material. One form of continuous tube
comprises loosely commingled or interwoven rovings, for example loosely interwoven
glass fibre rovings, wherein the individual rovings extend at a small angle, for
example about 10° to about 15°, to the tube axis. Such glass fibres may
be intermingled with polypropylene fibres or with fluoropolymer fibres or coated
with polypropylene powder or polyvinylidene powder. Another form of fibrous material
which can be used in the practice of the invention comprises a narrow band of parallel
fibres as warp interwoven with a similar narrow band of parallel fibres as weft,
with the warp and weft crossing each other substantially at right angles to one
another. Such narrow bands may be, for example, from about 0.2 cm to about 2 cm
It is also possible to use a mixture of metal and glass
fibres as the fibrous material.
Thus one preferred from of heat transfer element according
to the invention comprises:
- a polymer sheet having a fibrous material interspersed therein and comprising
a fluoropolymer at least on an outer surface of the sheet, the interspersion of
the fibrous material within the sheet providing rigidity to the element; and
- a thermally conductive material distributed within the heat transfer element.
Heat transfer elements according to the invention have
a number of significant advantages over conventional heat transfer elements, in
particular the conventional elements used to form the radiant panels of power generating
The provision of a fluoropolymer sheet significantly improves
the anti-fouling properties of the heat transfer elements of the invention. Fluoropolymers
have low surface energy and good lubricity and are therefore able to resist fouling
by soot and debris to a greater extent than has been the case with conventional
ceramic materials. Furthermore, fluoropolymers tend to be extremely resistant to
chemical attack and are well adapted to withstand the corrosive action of the sulphurous
and nitrous acids present in the air stream flowing past the elements when in use.
This resistance to chemical attack prevents surface solvation, which could otherwise
worsen the flow characteristics of the surface.
Detailed Description of the Invention
In one embodiment of the invention, the fibrous material
is itself a thermally conductive material, for example a metal such as iron, mild
steel, or stainless steel.
One advantage of using a thermally conductive material
as the fibrous material is that it may not then be necessary to provide any further
thermally conductive material in the element. In this case, the fibrous material
will itself serve as the sole thermally conductive material in the element. However,
it may in some cases be preferred to distribute a thermally conductive material
within the element by means other than the fibrous material. Thus, in one preferred
embodiment of the invention, the thermally conductive material comprises a particulate
or filamented material, for example, a particulate or filamented metal such as iron
or steel. This particulate or filamented material may be mixed with the fluoropolymer
prior to compression moulding or lamination of the fluoropolymer onto the fibrous
material. The resulting heat transfer element according to the invention will comprise
a fibrous material, which may if desired be of metal or some other thermally conductive
material but which may alternatively be or include a thermal insulator or a material
having a relatively low thermal conductivity, such as glass fibres, preferably made
from chemically resistnat glass such as boron-free glass, and a fluoropolymer sheet
having the thermally conductive particulate or filamented material distributed within
the fluoropolymer sheet or polymer matrix.
Although glass fibres exhibit relatively low thermal conductivity
properties, it has been found that adequate thermal conductivity can be imparted
to the heat transfer elements of the invention by utilising high volume proportions
of glass fibres, for example up to about 60% by volume of the heat transfer element.
The use of such levels of glass fibres is economically advantageous because the
polyvinylidene fluoride or other fluoropolymer is typically about 6 times more expensive
than glass fibres. Hence the invention enables the production of heat transfer elements
in a relatively economical manner, even though utilising a relatively expensive
fluoropolymer in its manufacture.
In general the desired heat conductivity properties can
be achieved by varying the loading of the fibrous material and/or by mixing a filler
with good thermal conductivity properties such as metal fibres or metal powder with
a material with lower thermal conductivity such as glass fibres. Typically the amount
of glass fibres can range from about 20% by volume to about 60% by volume of the
heat transfer element. The proportion of metal fibres or particles used can range
up to about 25% by volume but is usually not greater than about 20% by volume of
the heat transfer element.
The polymer sheet or matrix may consist entirely of a fluoropolymer
or admixtures of a fluoropolymer with compatible thermoplastic polymers, antioxidants
and other additives. In this case the fibrous material is interspersed within the
fluoropolymer. This can be achieved by laminating a pad of fibrous material, for
example a pad of chemically resistant glass fibres or metal fibres, between two
sheets or films of fluoropolymer. However, in an alternative embodiment of the invention,
the polymer sheet may comprise an underlayer of a plastics material, in which the
fibrous material is interspersed, and an overlayer of fluoropolymer. The plastics
material is preferably an acrylic polymer or alloy. This arrangement may be desirable
for economic reasons. When the plastics material, such as a relatively inexpensive
acrylic polymer, is laminated or compression moulded onto the fibrous material,
the thermoplastic acrylic polymer flows into and around the fibres and provides
a relatively cheap filler onto which the fluoropolymer may be coated. Of course,
the lamination or compression moulding of the fibrous material with the inexpensive
acrylic filler and the fluoropolymer may be done simultaneously by applying heat
and pressure to a sandwich having an outer film of fluoropolymer, an intermediate
layer of acrylic polymer and an inner layer of fibrous material. In this case, the
fibrous material may become interspersed in both the acrylic polymer and the fluoropolymer.
The use of compression moulding or lamination, for example
continuous belt lamination, to form the heat transfer element is preferred, particularly
when forming the heat transfer element as a sheet. However, it may sometimes be
appropriate, for example when an inexpensive acrylic polymer is used, to powder
coat the fluoropolymer onto a base portion formed after cooling of the acrylic base
sheet with interspersed fibrous material. However, the use of compression moulding
or lamination allows the manufacturer to minimise the thickness of the coating,
thus improving the thermal transfer properties of the element and allowing cost-effective
manufacture of the element by minimising the quantity of the expensive fluoropolymer
Typically a heat transfer element in the form of a sheet
has an overall thickness of from about 0.4 mm to about 1.2 mm.
The heat transfer element of the invention may also be
formed as a tube by extrusion of a fluoropolymer melt and interspersed fibrous material.
Other conventional methods of forming fibre reinforced plastics tubes may be used.
For example, a tube can be formed by spirally winding one or more layers of a fibre
reinforced plastics tape on to a mandrel and compressing or fusing the tape portions
one to another as appropriate. If more than one layer of tape is used then the fibre
directions of the two layers can be different. If the tape does not itself comprise
a fluoropolymer, then a fluoropolymer tape or film can simultaneously or thereafter
be applied to the fibre reinforced layer or layers and laminated thereto by application
of heat and/or pressure. If the fibrous reinforcement is a poor conductor, for example
glass fibres, then metal powder or metal fibres can be incorporated either in the
fibre reinforced layer or in the fluoropolymer coating layer.
Suitable equipment for manufacture of tubular heat transfer
elements in accordance with the invention can be achieved using, for example technology
developed by Automated Dynamics of 407 Front Street, Schenectady, New York 12305,
United States of America in order to effect fibre placement during tube formation,
or the discontinuous double pressing operation as provided by BST Beratung und System
Technik GmbH of Am Flughaven 7613, 88406 Friedrichshafen, Germany.
The tube or pipe can be of any convenient cross section
such as round, oval or square. It can have fins or other structural features integrally
formed therewith. Its diameter can vary within wide limits, for example from about
1 cm up to about 25 cm or more, e.g. about 38 mm. It can have couplings or other
fittings integrally moulded therein. The tube or pipe can vary in internal dimensions
or wall thickness along its length.
When the heat transfer element of the invention comprises
a sheet, it can be bent, corrugated or otherwise formed into a desired shape, using
appropriate conditions of heat and/or pressure.
The fluoropolymer used in the present invention is preferably
a fluorohydrocarbon polymer, such as polyvinylidene fluoride (PVDF) or a copolymer
with at least 80% by weight of vinylidene fluoride and up to 20% by weight of at
least one other fluorine based monomer. Suitable fluorine based monomers which may
be used with vinylidene fluoride are tetrafluoroethylene, hexafluoropropylene and
vinyl fluoride, having the characteristics listed in
United States Patent Nos 4,770,939
. The fluoropolymer is most preferably PVDF and is commercially available
from Atochem North America, Inc. under the trade designation KYNAR 500 PC, KYNAR
710, KYNAR 711 or KYNAR 2800.
The fluoropolymer may be mixed with another thermoplastic
polymer. The preferred thermoplastic polymers are acrylic polymers with units derived
from acrylates or methacrylates, such as copolymers derived from an alkyl acrylate
or alkyl methacrylate, preferably, methyl methacrylate or from at least one other
olefinically unsaturated monomer. Acrylic acid and methacrylic acid are also suitable
as the other olefinically unsaturated monomer. Advantageously, the copolymers comprise
at least 75% by weight of units derivable from an alkyl methacrylate and up to 25%
by weight of units derivable from one or more other olefinically unsaturated monomers.
The thermoplastic polymer is preferably poly(methyl acrylate) or poly (methyl methacrylate)
or an alkyl methacrylate/alkyl acrylate copolymer. These thermoplastic polymers
have the characteristics listed in
United States Pat. Nos. 4,770,939
and are commercially available from Rohm & Haas Company under the trade
description Acryloid/Paraloid B-44®. These materials are described
United States Patent No. 5,229,460
. Another preferred acrylic polymer is available from Atohaas under the
trade designation OROGLAS HFI-10.
The use of an acrylic polymer in admixture with the fluoropolymer
can improve the wetting properties of the material and thus help to ensure even
coating of the fibrous material in the heat exchange element of the invention.
The weight ratio of the fluoropolymer to the thermoplastic
acrylic polymer, if used, is preferably in the range of from about 90:10 to 40:60,
preferably from about 75:25 to 65:35, for example about 70:30.
A low melting point fluorine-based terpolymer may also
be added to the fluoropolymer/thermoplastic acrylic polymer mixture. A terpolymer
is a polymer made from three monomers. Such a low melting point terpolymer would
have, for example, a melting point of not higher than 150°C. A suitable terpolymer
is vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene, having a melting
temperature of about 87° to 93°C and a melt viscosity of about 11,000
to 13,000 Poise at 125°C. The preferred terpolymer is commercially available
from Atochem, North America, Inc. under the trade designation KYNAR ADS®.
The weight ratio of the fluoropolymer to the terpolymer, if used, is in the range
of from about 50:50 to 99:1.
The mixture may also contain other additives, such as corrosion
inhibiting pigments, dry flow promoting agents, antioxidants, adhesion promoters
and ultra-violet-absorbing materials, although not required. One preferred additive
is an antioxidant, such as 2,2-bis[3-[3,5-bis(1,1-dimethylethyl)-4-hydroxyphenyl]-1-oxopropoxy]methyl]-1,3-propanediyl
3,5-bis(1,1-dimethylethyl)-4-hydroxybenzene-propanoate, available from Ciba-Geigy
under the trade designation Irganox 1010.
The fluoropolymer composition can be formed into a thin
film for lamination to the outside of a heat transfer element in accordance with
In order that the invention may be properly understood
and fully carried into effect, a number of preferred embodiments thereof will now
be more particularly described in the following Examples:
A fluoropolymer composition comprising the following ingredients
% by weight
Paraloid™ B-44 Beads
The materials were mixed in a high speed MIXACO™
mixer and fed into a twin screw extruder and extruded at about 200°C. The extrudate
was quenched in a water bath and then pelletised.
The pelleted composition was extruded through a single
screw extruder with a single slot die to form a continuous film with a thickness
of around 120 µm.
The resulting film was used to coat a fibrous pad of mild
steel by placing a sheet of film on each side of the pad and subjecting the covered
pad to a temperature of 200°C and a pressure of 0.625 tonnes per square inch
(95 bar) in a heating press.
The resulting heat transfer element has a thickness of
about 1 mm and has excellent heat transfer, anti-fouling, structural and flow characteristics.
A film of fluoropolymer coating composition was prepared
as described above in Example 1 and was used to coat a fibrous mild steel pad by
covering both sides of the pad with film and passing the covered pad through a twin
belt laminator. Acetate release sheets were placed over the fluoropolymer film to
prevent adherence of the fluoropolymer to the belts of the laminator.
The resulting heat transfer element is approximately 1
mm thick and has excellent heat transfer, anti-fouling, structural and flow characteristics.
A fluoropolymer coating composition as specified in Example
1 was prepared and mixed with stainless steel filings in a ratio of three parts
by weight of the coating composition to one part by weight of stainless steel filings.
The resulting composite material was laminated onto a fibre glass pad using the
method described in Example 2 to form a heat transfer element having a thickness
of about 1 mm with excellent heat transfer, anti-fouling, structural and flow characteristics.
Examples 1 to 3 were repeated using a fluoropolymer composition
of the following ingredients:
% by weight
In each case, a heat transfer element with excellent heat
transfer, anti-fouling, structural and flow characteristics was produced.
A laminate comprising two pre-manufactured Solex 8008 100%
fluoropolymer films, each 0.150 mm thick, and two 110 g/m2 Advantex™
pre-manufactured fibrous chemically resistant glass mats were combined together
with a fibrous pad of steel approximately 0.6 mm thick by laminating them together
in a twin belt laminator using a pressure of less than 5 bar and a temperature of
230°C. The resulting laminate has a thickness of 0.91 mm and has excellent
economic performance, and heat transfer, anti-fouling, structural and flow characteristics.
A pipe is prepared by tape winding preprepared tapes comprising
60% by volume chemically resistant glass.fibre together with 40% by volume of Kynar
711. This was obtained in the form of a very fine powder and was coated using a
fluidised bed on to the glass fibres and then consolidated using a heated die. The
resultant tape was 0.4 mm thick and 20 mm wide and was wound on to a mandril with
60% of the tape in the length of the pipe and 40% in the inner and outer surfaces
of the pipe at an angle of +/- 20°. The resultant pipe performed well under