Note: Descriptions are shown in the official language in which they were submitted.
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HEAT EXCHANGER WITH IMPROVED CORROSION RESISTANCE
Field of the Invention
The present invention relates to a plate package for a plate heat
exchanger and a plate heat exchanger with improved corrosion resistance.
Background
Plate heat exchangers may be used for different types of fluids.
However, some fluids are considered very corrosive. When heat exchanging
at least one corrosive fluid the demands on the heat exchanger increases.
Today the choice is often between materials which may corrode giving a short
life time of the plate heat exchanger with a risk of contaminating the fluid
or a
heat exchanger made of a more corrosion resistant material, the latter being
very expensive in comparison. Unfortunately, several materials that are
considered corrosion resistant are not able to be used for all parts of
permanently assembled plate heat exchangers since the materials used are
unable to give satisfying permanent joining. Brazed plate heat exchangers
may be made of a corrosion resistant plate material but the brazing material
is
a less corrosion resistant material thus constituting an obstacle for the heat
exchangers to be used in connection with certain liquids or gases. Then the
brazing technique itself may mix plate material and brazing material during
assembly of the heat exchanger giving rise to more easily corroding areas.
Also, corrosion resistant materials that can be applied to the plates of a
heat
exchanger before assembly can make it difficult or impossible for such a heat
exchanger to achieve satisfying permanent joining with good anti-corrosion
properties.
Coating materials like plastics are considered not enough fatigue and
corrosion resistant for highly corrosive fluids. The stress put on a plastic
coating on a plate of a plate heat exchanger e.g. in the form of high
pressures
and/or high temperatures also makes the coating degrade and/or lose its
adhesion to the plate. Also, high pressure differences and high temperature
differences during use of a plastic coated heat exchanger may cause the
coating to degrade and e.g. flake. Plastics also exhibit inferior thermal
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transmittance properties compared to metals which a plate heat exchanger is
made of.
Tantalum is a very corrosion resistant metal towards many fluids and it
is known to make heat exchangers of this metal. However, tantalum is an
expensive metal and is mechanically considerably weaker than other known
materials for use in heat exchangers such as stainless steel. Thus, thicker
plates must be used to withstand the mechanical stress put on a heat
exchanger made of tantalum.
WO 92/16310 discloses a method of surface protecting heat transfer
plates in a heat exchanger using plastics as a surface protecting material.
According to the method a gaseous medium containing the plastics is
introduced into the assembled plate heat exchanger which then forms a layer
on the surfaces of the heat exchanger plates.
GB 1,112,265 discloses tubular heat exchangers in contact with highly
corrosive media. In the document it is disclosed that mounting plates may be
coated or lined with tantalum and the tubes may be made of tantalum.
WO 96/06705 discloses fully brazed heat exchangers which are
resistant to corrosive media due to the brazing joints between the plates are
protected by a coating resisting the corrosive media. The plates are made of
stainless steel, the solder is copper solder and the protective coating
intended
to cover the brazing joints is a metal such as tin or silver.
US 2010/0051246 discloses a high-temperature and high-pressure
corrosion resistant process heat exchanger, wherein the third system coolant
channel surfaces of the heat transmission fin and heat transmission plate,
which come in contact with sulphuric acid and/or sulfite, are subjected to ion
beam coating and ion-beam mixing using a material having high corrosion
resistance such as SiC, A1203, silicon steel and tantalum.
JP 4,334,205 discloses a plate heat exchanger with plates made of
titanium, stainless steel, copper, nickel or alloys thereof. In order to
suppress
elution of electrode material from a plate a coating treatment is performed on
at least 30% of the heat transfer plate electrode areas by the side of a
cooling
water passage. The coating may comprise platinum metal oxide, manganese,
tantalum, tin etc.
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EP 110,311 discloses a flat heat-exchange plate comprising two plates
which may be surface coated with tantalum or a tantalum alloy and at least
one duct. The two plates are attached to each other to form the flat heat-
exchange plate by use of an adhesive coat.
It would be desirable to find new ways to ensure more corrosion
resistant heat exchangers in order to be able to process highly corrosive
media and increase the life time of the heat exchangers. It is also desirable
to
be able to produce corrosion resistant heat exchangers from cheaper base
materials that have good mechanical properties and are easily and effectively
permanently assembled. It would also be desirable that all parts of a heat
exchanger, e.g. both plates and joints, which are in contact with a highly
corrosive fluid are equally highly corrosion resistant. Further, it would be
desirable to achieve more fatigue and corrosion resistant internal parts of
heat exchangers in contact with highly corrosive fluids. It would also be
desirable to find corrosion and fatigue resistant materials applied on the
inside of a plate heat exchanger, which materials show good adhesion. Still
further, it would be desirable to achieve a good or improved heat transfer in
the plate heat exchanger.
Summary of the invention
It is an object of the present invention to solve the above mentioned
problems. Thus, it is an object of the present invention to provide good
mechanical properties and high corrosion resistance of all parts of a heat
exchanger in contact with highly corrosive fluids. It is also an object of the
present invention that good heat transfer is obtained.
This object is achieved by a permanently joined plate package for a
plate heat exchanger being coated with a tantalum containing coating
everywhere on the inside, such as both plates and joints, in at least one flow
side of the plate package. By applying a coating comprising tantalum highly
corrosive media such as hydrochloric acid can be used in a plate heat
exchanger without a rapid degradation of the heat exchanger.
The present invention relates to a permanently joined plate package for
a plate heat exchanger made of stainless steel or carbon steel wherein at
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least all surfaces in contact with media of at least one of the flow sides of
the
plate package have an alloy bonded coating of a tantalum containing
compound. The present invention also relates to a plate heat exchanger
comprising said plate package.
One embodiment of a plate heat exchanger according to the present
invention include the heat exchanger having frames and/or mounting plates
that are a part of at least one of the flow sides of the heat exchanger and
said
frames and/or mounting plates are made of tantalum, or stainless steel or
carbon steel having an alloy bonded coating of a tantalum containing
compound, preferably stainless steel or carbon steel having an alloy bonded
coating of a tantalum containing compound, more preferably stainless steel
having an alloy bonded coating of a tantalum containing compound.
Another embodiment of a plate heat exchanger according to present
invention is when the plate heat exchanger is permanently joined and is made
of stainless steel or carbon steel and all surfaces of at least one of the
flow
sides of the plate heat exchanger have an alloy bonded coating of a tantalum
containing compound.
Detailed description of the invention
A conventional permanently joined plate package or plate heat
exchanger may be made more corrosion resistant than it was from the
beginning with the present invention.
A plate heat exchanger is composed of multiple, thin metal plates that
have very large surface areas and fluid flow passages which may enable heat
transfer. A heat exchanger is provided with at least two inlets and two
outlets
for the fluids to be heat exchanged. Additional fluids may be used then
requiring additional inlets and out lets of the heat exchanger. Plate heat
exchangers comprise a series of heat transfer plates. These heat transfer
plates form what is called a plate package in the heat exchanger. The heat
transfer plates are made of thin sheets of metal and are often provided with
corrugations or other protuberances in their heat transferring portions, which
in a heat exchanger abut against each other by a large force at a great
number of contact places distributed across the heat transferring portions.
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Then the heat transfer plates are assembled interspaces are formed between
the plates. These plate interspaces are intended for at least one heat
exchanging fluid flowing through. In a plate heat exchanger the at least two
fluids are flowing through the interspaces next to each other allowing the
heat
5 transfer to take place. These interspaces between the plates intended for
flow
of one of the fluids is in the present application considered as being part of
a
flow side. In the present application the wording flow side is connected to
the
construction of a heat exchanger or plate package for the fluids, i.e. the
fluid
flow passages. Since at least two fluids are used in a plate heat exchanger,
it
has has at least two flow sides, one flow side for a warm fluid and one flow
side for a cold medium. For each flow side, all parts of a plate package or a
heat exchanger being in contact with either the warm or cold flowing fluid are
considered belonging to that flow side, e.g. plates, plate interspaces,
joints,
connections, inlet and/or outlet ports in frames or mounting plates. In a
plate
package or plate heat exchanger according to the present invention at least
one of the flow sides is designed for highly corrosive fluids when in use.
With the present invention simple rigid base materials for heat
exchangers such as stainless steel, copper and carbon steel can be used and
with a tantalum containing coating be made corrosion resistant to highly
corrosive fluids. With the present invention also other parts of the plate
package or heat exchanger like the joints which may be more sensitive parts
of the plate heat exchanger due to e.g. welding during the assembly of the
heat exchanger are coated with a corrosion resistant material. The joints may
also be sensitive parts of the heat exchanger due to soldering, fusion bonding
or brazing during assembly of the heat exchanger. The term fusion bonding
relates to the use of an iron based brazing material in accordance with the
disclosures of e.g. EP 1 347 859 131 and WO 02/098600. Assembly of a heat
exchanger using soldering, fusion bonding or brazing the joints may be made
of a different material than the plates. During the assembly process the
soldering or brazing material is applied to the plates, fully or partially
covering
the plates, and the soldering or brazing material may during the assembly be
mixed with additional coatings on the plate material or in some cases even
the plate material itself creating more corrosion sensitive parts of the heat
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exchanger. Since at least both plates and joints of a plate package or plate
heat exchanger according to the present invention are coated the heat
exchanger is made more corrosion resistant. Thus, in this way the joints or
areas on the plates close to the joints can no more be a weak link for the
heat
exchanger.
In one embodiment of the present invention permanently assembled
plate heat exchangers or plate packages for plate heat exchangers made of
stainless steel or carbon steel are coated with a tantalum containing
compound. The plate packages or heat exchangers may e.g. be permanently
assembled by welding, soldering, fusion bonding or brazing. A tantalum
containing compound is introduced into the heat exchanger in at least those
plate interspaces being intended for through flow of one of the two heat
exchanger fluids, i.e. at least one of the flow sides designated for being
used
for highly corrosive fluids when in use. Inside the heat exchanger or plate
package, the tantalum containing compound is deposited on all surfaces of at
least one of the flow sides of the heat exchanger or plate package, e.g.
plates, joints and other parts intended to be in contact with heat exchanger
fluids.
The use of a tantalum containing compound according to the present
invention provides a plate package or plate heat exchanger with very good
properties. Tantalum shows better heat transfer properties then plastics which
are not considered thermally conductive materials. According to the present
invention it is important to be able to present a coating or layer which does
not impair the heat transfer. Tantalum shows good heat transfer properties.
Further, the tantalum containing coating according to the present invention is
chemically bonded to the materials of the plate package and plate heat
exchanger. The tantalum containing compound is bonded by alloying to said
materials. In this context an alloy bonding is a metallic solid solution
composed of two or more elements from two or more different metal bodies
composed of different materials, in the present invention tantalum and the
plate material, e g stainless steel, copper or carbon steel, in an interface
layer
between the bodies. Such an alloying bonding give rise to more fatigue
resistant plate packages and heat exchangers compared to e.g. heat
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exchangers coated with plastic materials. Since the tantalum is partially
alloyed to the material the adhesion is superior. This makes it easy for the
tantalum containing coating to follow the plate and joint materials movements
due to thermal and pressure changes within the plate heat exchanger when
going form out of use to use and also during use. The tantalum containing
coating has a gradual transition of compounds within itself. When looking at
the tantalum containing coating in a cross cut view, the intermediate phase
closest to the heat exchanger material, e.g. a plate, show an alloy of
tantalum
containing compound and the plate material, a gradual transition is thereafter
made to only the tantalum containing compound, which thereafter is gradually
transferred into tantalum oxide since the outer surface of the tantalum
containing compound is oxidized. Thus, since not all of the tantalum
containing coating applied to parts of a heat exchanger is an alloy with said
parts it is considered that the tantalum containing compound is partially
alloyed to the heat exchanger parts.
The film thickness of the tantalum coating must not be too high
because that would influence the heat transfer properties in a negative way
since an enlarged barrier, an increased plate thickness, between the heat
transferring fluids decreases the heat transfer. If the film thickness is to
low
the effect of the coating may not last as long as suspected when in contact
with a highly corrosive fluid.
According to the present invention a tantalum containing compound is
coated on the inside of a plate package or heat exchanger using a deposition
process with chemical reactants in fluid form. The method of coating a
permanently joined plate package or heat exchanger in accordance with the
present invention, comprises the steps: 1) introducing gas or vapor phase
chemical reactants into said plate package or heat exchanger in at least one
of the flow sides of the heat exchanger, wherein at least one of the reactants
is a reactant comprising tantalum, 2) formation of a solid film comprising a
tantalum containing compound on the surfaces of said plate package or heat
exchanger from the reaction of the gas or vapor phase chemical reactants.
The application process relates to formation of a non-volatile solid film
on a substrate, in the present case parts of a plate package or heat
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exchanger, from the reaction of gas or vapor phase chemical reactants,
wherein at least one reactant is a reactant comprising tantalum. A reaction
chamber is used for the process, into which the reactant gases or vapors are
introduced to decompose and react with the substrate or in the case of
multiple applications the previously applied layer to form the film. Inside
the
reaction chamber the reactants are forced into the plate package or heat
exchanger. In one embodiment the reactant comprising tantalum in fluid form
is tantalum pentachloride.
The application process disclosed above could also be used for parts
of a heat exchanger such as frames or mounting plates not part of a
permanently joined heat exchanger. Such coated frames or plates may then
be used together with a permanently joined plate package coated in
accordance with the present invention.
The application process of the tantalum containing composition is
preferably done by Chemical Vapor Deposition (CVD) or Atomic Layer
Deposition (ALD), preferably by CVD.
A basic CVD process consists of the following steps: 1) a predefined
mix of reactant gases and diluent inert gases are introduced at a specified
flow rate into the reaction chamber; 2) the gas species move to the substrate;
3) the reactants get adsorbed on the surface of the substrate; 4) the
reactants
undergo chemical reactions with the substrate to form the film; and 5) the
gaseous by-products of the reactions are desorbed and evacuated from the
reaction chamber.
The growth of material layers by ALD consists of repeating the
following characteristic four steps: 1) Exposure of the first precursor. 2)
Purge
or evacuation of the reaction chamber to remove the non-reacted precursors
and the gaseous reaction by-products. 3) Exposure of the second precursor -
or another treatment to activate the surface again for the reaction of the
first
precursor. 4) Purge or evacuation of the reaction chamber. Each reaction
cycle adds a given amount of material to the surface, referred to as the
growth per cycle. To grow a material layer, reaction cycles are repeated as
many as required for the desired film thickness.
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In one embodiment the method of coating a permanently joined heat
exchanger made of stainless steel, copper or carbon steel comprises the
steps: 1) introducing gas or vapor phase chemical reactants into said heat
exchanger in at least one of the flow sides of the heat exchanger, wherein at
least one of the reactants is a compound comprising tantalum, 2) formation of
a solid film comprising tantalum on the surfaces of said heat exchanger from
the reaction of the gas or vapor phase chemical reactants, is for the steps 1)
and 2) preferably carried out at a temperature of 600-1000 C, more preferably
700-9000C.
In another embodiment of the present invention steps 1) and 2) are
carried out at atmospheric pressure, subatmospheric pressure or at very low
pressure.
According to the present invention it is important that the heat
exchange plates of a plate package or plate heat exchanger not only are
permanently joined to each other along their peripheral portions, it is also
important that at a variety of areas of contact in their heat exchange
portions
are permanently joined. If plates are only joined along their peripheral
portions other areas of contact may move/be dislocated during use. If only
contact surfaces along their peripheral portions are permanently joined the
plates may separate at some areas of contact which are not permanently
joined during use when the plate heat exchanger is e.g. pressurized on one of
the fluid flow sides. In the case of areas of contact shifting due to e.g.
pressurizing, a coated heat exchanger which is not joined at all areas of
contact within the fluid flow would then have areas not coated exposed to the
fluid in the heat exchanger and thus resulting in corroding areas if the fluid
used is corrosive. Thus, it is important that all areas of contact between
plates, where the areas of contact are in contact with or surrounded by
corrosive fluid, are permanently joined by welding, soldering, fusion bonding
or brazing.
A permanently joined plate package for a plate heat exchanger as
disclosed herein is to be interpreted as a non-accessible plate package
wherein at least all areas of contact between plates in contact with corrosive
fluid are permanently joined. Thus, since the plate package is non-accessible
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it is to be interpreted that the complete plate package may not be
disassembled in any way.
Such a plate package according to the present invention can be used in a
plate heat exchanger having e.g. frames and/or mounting plates of any
5 material, as long as they are not in contact with the corrosive fluid in at
least
one of the flow sides. If e.g. frames or mounting plates are a part of at
least
one of the flow sides of the heat exchanger and is in contact with a highly
corrosive fluid said frames and/or mounting plates preferably are made of
tantalum, or stainless steel or carbon steel having an alloy bonded coating of
10 a tantalum containing compound on at least the parts of the at least one of
the flow sides of the heat exchanger. For such frames and/or mounting plates
preferably stainless steel or carbon steel having an alloy bonded coating of a
tantalum containing compound are used, more preferably stainless steel
having an alloy bonded coating of a tantalum containing compound.
A permanently joined plate heat exchanger as disclosed herein is to be
interpreted as a non-accessible heat exchanger comprising a permanently
joined plate package wherein at least all areas of contact between plates in
contact with corrosive fluid are permanently joined. Thus, since the plate
heat
exchanger is non-accessible it is to be interpreted that the plate heat
exchanger may not be disassembled. For a plate heat exchanger this means
that not even any frames or mounting plates that are located around a plate
package and are to be in contact with at least one corrosive heat exchange
fluid can be removed. The permanently joined plate heat exchanger
according to the present invention is for the parts in contact with at least
one
fluid, e.g. a corrosive fluid, impossible to disassemble in any way. The
wordings permanently joined and permanently assembled in view of plate
packages and plate heat exchangers are regarded as being interchangeable
in the present application.
The present invention relates to application of a solid film of a tantalum
containing coating onto surfaces within a permanently joined plate package or
plate heat exchanger. The tantalum containing compound used as coating,
preferably metal tantalum, tantalum oxide and/or tantalum nitride, applied on
the surfaces of the heat exchangers to be in contact with highly corrosive
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fluid. In a preferred embodiment the tantalum containing compound is metal
tantalum and/or tantalum oxide, preferably metal tantalum. If the tantalum
coating is made of metal tantalum naturally the uppermost part of the coating
is oxidized and thus is tantalum oxide, and the nethermost part of the coating
is then alloyed with the materials of a permanently joined plate package or
plate heat exchanger.
The permanently joined plate package and permanently joined heat
exchanger coated in accordance with the present invention is made of
stainless steel or carbon steel. Stainless steel and carbon steel are
considered materials with good mechanical properties. The permanently
joined plate package or permanently joined heat exchanger is assembled
using welding, soldering, fusion bonding or brazing, preferably using welding
fusion bonding or brazing. Brazing is preferably done by use of copper as
brazing material. Preferably the heat exchanger is made of stainless steel and
was assembled using welding, fusion bonding or brazing, preferably fusion
bonding or copper brazing.
According to the present invention the coating comprising tantalum
applied onto the surfaces in at least one of the flow sides designated for
being
used for highly corrosive fluids has preferably a film thickness of about 1-
300
pm, preferably 1-125 pm, more preferably 1-50 pm, even more preferably 10-
40 pm and most preferably 15-25 pm.
Examples
Two copper brazed stainless steel units, CB14, and two Alfa Fusion
stainless steel units, AN14, from Alfa Laval have been processed with the
CVD process to coat with tantalum. Conventional Alfa Fusion units, AN14,
were used as reference. All units contained plates of stainless steel but in
CB14 they were copper brazed and in AN14 they were fusion bonded
together.
Process:
Tantalum reacts with chlorine gas to form TaCI5. The gas is led into an
vacuum oven at 850 C were the TaCI5 will react with available surfaces
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(stainless steel, copper, carbon steel, graphite etc) to form a CVD coating of
tantalum. The pressure of the gas is about 25 mB, and the process is running
for about 8 hours.
The chorine released during the process will react with hydrogen to
form hydrochloric gas which is led out of the process and neutralized with
sodium hydroxide.
TaCI5 gas is led from the centre pipe to the units. The small, hanging
spacers attached to the inlet and outlet are used for evaluation of the
thickness of the tantalum layer. According to weight measuring of the spacers
before and after process the average thickness of the tantalum layer is about
45 pm in the inlet and 38 pm in the outlet.
Analysis:
The tantalum CVD processed units (CB 14 and AN 14 units) were
corrosion tested with 75 C hydrochloric acid during 48 h. The hydrochloric
acid used for the test showed almost no change in color after recirculation in
the tantalum treated units. The tantalum coated CB 14 and AN 14 units
showed no without leaking or other signs of corrosion damages during or after
the corrosion test. After the corrosion test the units were pressure tested
with
compressed air at 8 bar. No external or internal leaks were found in the
units.
A conventional AN14 unit was corrosion tested in hydrochloric acid as
well. For the conventional AN14 unit the hydrochloric acid reacted strongly
with the stainless steel surfaces under emission of hydrogen gas, the acid
had to be replaced a couple of times because of depletion. A strong green
colorization from iron chloride was found in the acid from the standard unit.
The conventional AN14 unit showed no leakage after 90 minutes, but after 6
hours numerous large leaks were detected.
After the corrosion tests the units were cut up and cross cuts of the
surfaces were metallograhipcally prepared and examined with microscope.
The tantalum treated units were cut up and four cross cuts were examined
from each unit. The CB 14 unit showed very good adhesion between the
copper and tantalum in all investigated locations. The CB 14 unit showed
slightly better adhesion between the copper and tantalum than the stainless
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steel and tantalum in the AN 14 unit. A reason for this might be that the
surface of the AN 14 unit may have been contaminated or, to a lower extent,
be because of the higher surface roughness in the AN 14 unit.
The thickness of the tantalum layer varies from about 105-125 pm in
the areas around the inlet to just over 10 pm at the diagonal maximum
distance from inlet on the AN 14 unit.
The thickness of the tantalum layer varies from about 150 pm in the
inlet to thin, most probably less than 5 pm at the diagonal maximum distance
from inlet on the CB 14 unit.
