U.S. patent application number 12/599580 was filed with the patent office on 2010-10-14 for oxygen heat exchanger.
This patent application is currently assigned to AGC Flat Glass Europe SA. Invention is credited to Eric Baudelet, Gabriel Constantin, Olivier Douxchamps, Bertrand Leroux, Bruno Symoens, Remi Tsiava.
Application Number | 20100258263 12/599580 |
Document ID | / |
Family ID | 38577527 |
Filed Date | 2010-10-14 |
United States Patent
Application |
20100258263 |
Kind Code |
A1 |
Douxchamps; Olivier ; et
al. |
October 14, 2010 |
OXYGEN HEAT EXCHANGER
Abstract
The present invention relates to a heat exchanger for the supply
of oxygen or of a gas mixture containing at least 50% oxygen, the
temperature at the outlet of the exchanger not being below
300.degree. C., it preferably being above 400.degree. C., the
oxygen or the oxygen-rich gas feeding one or more burners of a
glass melting furnace, the heat of the combustion gases being used
directly or indirectly to heat the oxygen or the oxygen-rich gas in
the exchanger, in which the exchange power is between 20 and 300
kW, preferably between 40 and 250 kW and particularly preferably
between 80 and 170 kW.
Inventors: |
Douxchamps; Olivier; (Jumet,
BE) ; Baudelet; Eric; (Jumet, BE) ; Leroux;
Bertrand; (Paris Cedex 07, FR) ; Constantin;
Gabriel; (Paris Cedex 07, FR) ; Tsiava; Remi;
(Paris Cedex 07, FR) ; Symoens; Bruno; (Jumet,
BE) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND MAIER & NEUSTADT, L.L.P.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
AGC Flat Glass Europe SA
Bruxelles (Watermael-Boitsfort)
BE
L'AIR LIQUIDE SOC ANON POUR L'ETUDE ET L'EXPLOIT
Paris Cedex 07
FR
|
Family ID: |
38577527 |
Appl. No.: |
12/599580 |
Filed: |
May 7, 2008 |
PCT Filed: |
May 7, 2008 |
PCT NO: |
PCT/EP2008/055615 |
371 Date: |
June 25, 2010 |
Current U.S.
Class: |
165/11.1 ;
165/133 |
Current CPC
Class: |
F28F 19/00 20130101;
F23L 15/04 20130101; F28D 7/16 20130101; C03B 5/2353 20130101; F28F
21/08 20130101; Y02E 20/34 20130101; Y02P 40/50 20151101 |
Class at
Publication: |
165/11.1 ;
165/133 |
International
Class: |
F28F 27/00 20060101
F28F027/00; F28F 13/18 20060101 F28F013/18 |
Foreign Application Data
Date |
Code |
Application Number |
May 10, 2007 |
EP |
07107942.0 |
Claims
1. A heat exchanger for supplying oxygen or a gaseous mixture
composed of at least 50% oxygen, the temperature of which at the
outlet of the exchanger is not less than 300.degree. C., wherein
the oxygen or gas rich in oxygen supplies one or more burners of a
glass melting furnace, and the heat of the combustion gases is used
directly or indirectly to heat the oxygen or gas rich in oxygen in
the exchanger, in which the exchange power is in the range of
between 20 and 300 kW.
2. The exchanger according to claim 1, in which the power per unit
area of exchange in contact with the oxygen or gas rich in oxygen
is in the range of between 5 and 15 kW/m.sup.2.
3. The exchanger according to claim 1 supplying at most three
burners of the furnace.
4. The exchanger according to claim 1 of tubular configuration, in
which the oxygen or gas rich in oxygen circulates in the tubes, the
internal walls of which are in contact with a heat transfer
gas.
5. The exchanger according to claim 4, in which the total
cross-section of the tubes carrying the oxygen or gas rich in
oxygen provides the highest circulation rate in these tubes so long
as the rate does not exceed 120 m/s.
6. The exchanger according to claim 1, in which the pressure of the
oxygen or gas rich in oxygen is kept below 3 bar, preferably below
2 bar.
7. The exchanger according to claim 1, in which the surfaces in
contact with the oxygen or gas rich in oxygen are polished so that
their roughness does not exceed 6.mu..
8. The exchanger according to claim 7, in which the tubes in which
the oxygen or gas rich in oxygen circulates are substantially
straight and their walls have a thickness that is not more than 3
mm.
9. The exchanger according to claim 4, in which the chamber
enclosing the tubes is formed from several elements joined by
flanges, wherein tightness is assured at these flanges by composite
seals, the sealing element of which is made of material that is
inert with respect to oxygen.
10. The exchanger according to claim 9, in which the sealing
element is a ring composed of compressible mineral material.
11. The exchanger according to claim 1, in which the oxygen or gas
rich in oxygen is heated indirectly by the combustion gases,
wherein a first exchange between these is effected with an
intermediate heat transfer gas, which is then passed to the
exchanger to heat the oxygen or gas rich in oxygen, wherein the
heat transfer gas is formed by a gas that is inert with respect to
oxygen.
12. The exchanger according to claim 11, in which the heat transfer
gas is air, nitrogen, CO.sub.2 or steam.
13. The exchanger according to claim 11, in which the heat transfer
gas is formed from combustion gases diluted by means of at least
one of the gases: air, nitrogen, CO.sub.2 and steam.
14. The exchanger according to claim 11, in which the heat transfer
gas is firstly heated in a recuperator, which has been heated by
the combustion gases beforehand.
15. The exchanger according to claim 1, in which the material of
the surfaces in contact with the oxygen or gas rich in oxygen is
made from a metal alloy of which a sample exposed to the hot gas
does not exhibit a weight gain of more than 0.1 mg/cm.sup.2 after
1000 cycles of exposure, wherein each cycle includes increasing the
temperature to a value equal to or higher than 400.degree. C.,
maintaining this phase temperature for one hour and returning to
ambient temperature.
16. The exchanger according to claim 15, in which the alloy
complies with the condition of a weight gain of less than 0.1
mg/cm.sup.2 of exposed surface when the phase temperature is at
least 500.degree. C. in the test in oxidising atmosphere.
17. The exchanger according to claim 15, in which the alloy in
contact with the oxygen or oxygen-based gaseous mixture resists the
spontaneous combustion test according to standard ASTM G 124 at
least up to a pressure of 3 bar.
18. The exchanger according to claim 15, in which the alloy in
contact with the oxygen or oxygen-based gaseous mixture is a
ferritic steel alloy containing a percentage by weight of Cr of 12
to 30% and an Al content of 1 to 8%.
19. The exchanger according to claim 15, in which the alloy in
contact with the oxygen or oxygen-based gaseous mixture, for an
oxygen temperature not exceeding 500.degree. C., is an alloy
containing a percentage by weight of chromium in the range of
between 10 and 20% weight.
20. The exchanger according to claim 15, in which the alloy has an
Ni content higher than 25% and a Cr content from 10 to 30%.
21. The exchanger according to claim 20, in which the alloy is one
of those commercially available under the names "Inconel 600 H",
"600L", "601", "617", "625", "Incoloy 800H" or "800HT".
22. The exchanger according to claim 1, in which the elements are
in contact with the oxygen or the oxygen-based mixture, are brought
to a temperature in the range of between 300.degree. and
900.degree. C.
23. The exchanger according to claim 1, in which an oxygen detector
is placed in contact with the heat transfer gas, which is connected
to an alarm when the oxygen content is more than 1% higher than
that of the heat transfer gas.
Description
[0001] The present invention relates to heat exchangers intended to
heat oxygen or a gas rich in oxygen for the purpose of supplying
the burners of glass melting furnaces.
[0002] Glass melting furnaces, including those with the highest
production capacity, i.e. the furnaces supplying the "floats"
producing flat glass, are mostly equipped with burners that operate
with fossil fuels and air. The choice of this type of energy is
driven by economic reasons, considering the importance of energy
consumption. As an indication, usual melting furnaces producing
between 600 and 900 tonnes of glass per day require an available
power in the order of 50 to 80 megawatts.
[0003] The use of these furnaces not only results in a significant
energy consumption but also in the discharge of large volumes of
combustion gas. For these reasons, efforts are constantly being
made to reduce energy costs and the costs for processing these
combustion gases as far as possible.
[0004] The currently most widespread practice using air as
combustive does not lead to the lowest possible consumption of
energy, since nitrogen in the air is also brought to high
temperature and consequently absorbs a significant portion of the
energy released by combustion, and part of this energy is lost even
if the fumes undergo a recovery operation. Moreover, as is well
known, the presence of nitrogen leads to the formation of the
oxides responsible for so-called acid rain.
[0005] For these reasons solutions that use not only air but oxygen
or gases rich in oxygen have been developed and are already being
used in some applications.
[0006] However, the systematic use of oxygen encounters practical
and economic difficulties. The costs of the oxygen required is the
first of the economic difficulties. The energy balance gain must
more than compensate the additional cost of the oxygen. The
investments in specific material are also of importance in the
economics of the system, whether it concerns the burners, supplies
or, as studied below, the exchangers. In practice, the continuous
supply of oxygen to these large-scale assemblies requires
production installations on site or supply via gas pipeline, either
of which incurring substantial investments.
[0007] Considering the burdens imposed by the use of oxygen or gas
rich in oxygen in these glass melting furnaces to reach a
satisfactory economic balance, it is necessary to optimise the
choices made.
[0008] It appears that the best energy balance requires that
burners are run using oxygen and preheated fuels, with at least
part of the combustion gases being used for this preheating
operation.
[0009] While the use of hot oxygen is highly recommendable, putting
this into practice requires new solutions both for the
configuration of these installations and for their operation. Thus,
traditionally, the air used for the burners is preheated in
regenerators. These are towers lined with refractory materials,
into which the combustion gases are passed to heat the refractory
materials in a first phase and into which the air used in the
combustion is passed for reheating in a second phase. The
alternation of these phases results in a highly specific furnace
structure. The burners are thus on either side of the melting bath
as are the regenerators associated with them that are generally
located on the opposite side to the active burners.
[0010] It is not possible to use regenerators to reheat the oxygen.
Generators are commonly the receptacle for deposits of particles
carried by the combustion gases, even if these have been subjected
to a dust removal operation beforehand. Contact of the hot oxygen
with these deposits is not without risk. Moreover, it is difficult
to guarantee a perfect seal of these regenerators. The passage of
air and possible leaks are not dangerous, but this is not the case
with respect to oxygen.
[0011] The use of heat exchangers also raises sensitive problems of
resistance of the materials to contact with hot oxygen. In view of
these difficulties and taking into account economic constraints,
the use of hot oxygen has not found significant application in
large installations such as those for large-capacity glass melting
furnaces.
[0012] Therefore, the aim of the invention is to propose solutions
that encourage the use of oxygen or hot gases rich in oxygen in the
burners of glass melting furnaces, and in particular in
large-capacity furnaces. The invention also proposes to provide
solutions that make this use sufficiently safe despite the special
technical requirements associated with the use of oxygen at high
temperature.
[0013] Since the risks associated with the use of oxygen occur in
the installation from the instant the temperature increases, the
inventors have directed all their attention to the devices in
contact with this hot oxygen and in particular to the exchangers,
which contain significant quantities of this oxygen and whose
construction must allow operation without failure for very long
periods, because of the difficulties raised by any intervention for
maintenance or repairs.
[0014] The above considerations relating to the use of hot oxygen
also apply to gas mixtures, in particular with air, in which the
oxygen content is sufficiently high. In practice, for these gas
mixtures to remain advantageous in use, their oxygen content should
not be less than 50%. This condition applies to the invention
presented below. The invention preferably applies to gas mixtures
with an oxygen content of at least 80%.
[0015] According to the invention, the heating of oxygen or gas
rich in oxygen for supplying the burners of the furnace is
conducted in exchangers, in which the exchange power is
deliberately reduced without minimising the temperature to which
the oxygen or gas rich in oxygen is brought.
[0016] To sufficiently benefit the advantage for the energy
balance, the temperature of the oxygen or gas rich in oxygen at the
outlet of the exchanger is not less than 300.degree. C. and
preferably not less than 400.degree. C., and the power exchanged in
the exchanger to bring the oxygen to these temperatures according
to the invention lies between 20 and 300 kW, preferably between 40
and 250 kW and particularly preferred between 80 and 170 kW.
[0017] Power levels lower than those recommended are not economical
and impair the competitiveness of the system. Higher power levels
are likely to lead to difficulties with respect to convenience and,
above all, the safety of use of these exchangers.
[0018] In traditional glass melting furnaces, at least those of
large capacity, the burners used generate significant power levels
in the order of 1 to 6 MW resulting in an oxygen consumption in the
order of 200 to 1200 Nm.sup.3 of oxygen per hour.
[0019] In view of the power, and therefore consumption of each of
the burners typically used in these furnaces, the exchangers
according to the invention are only associated with a small number
of burners. Each exchanger advantageously only supplies at most
three separate burners simultaneously with hot oxygen or gas rich
in oxygen, wherein each burner can have several injection to
nozzles, depending on the specifications, such as those presented,
for example, in EP 1 194 719.
[0020] This arrangement that leads to an increase in numbers of
exchangers assures an improved safety of the installation by
limiting the dimensions and capacity of each exchanger, but also
significantly limiting the length of the ducts downstream of the
exchangers that lead to the burners.
[0021] In order to remain advantageous from the economic viewpoint,
the use of exchangers according to the invention must preferably
result in limited dimensions, which involves a quite specific mode
of operation and in particular allows the required power to be
generated while also keeping the exchange area as small as
possible.
[0022] According to the invention the exchangers for heating the
oxygen or gas rich in oxygen advantageously have a power per unit
area of contact of the oxygen with the exchange walls in the range
of between 5 and 15 kW/m.sup.2, and preferably between 7 and 12
kW/m.sup.2. The area in question is that of the wall separating the
oxygen or gas rich in oxygen from the heat transfer gas.
[0023] The construction of the exchangers used according to the
invention must provide as simple a structure as possible to prevent
risks of erosion and leakage as a result of the aggressiveness of
hot oxygen towards materials used. For convenience, the exchangers
according to the invention are preferably tubular, wherein the
oxygen or gas rich in oxygen circulates in a bank of tubes with the
heat transfer gas circulating on the outside of these.
[0024] To guarantee the power levels indicated above, and in
particular the power levels per unit area, while keeping these
areas as small as possible, it is necessary to optimise the heat
exchange conditions.
[0025] A first way to benefit this exchange consists of increasing
the circulation rate of the gases and in particular the oxygen or
gas rich in oxygen. However, from this viewpoint it is necessary
once again to take into account that the increase in circulation
rate is a risk factor. The risk is all the more significant as hot
oxygen is likely to entrain particles that can react with the
oxygen and/or whose impact on the walls promotes rapid erosion in
addition to that resulting from the friction of the oxygen
itself.
[0026] In order to take this risk into account, according to the
invention the dimensions of the elements of the exchanger are
advantageously defined so that in order to obtain the necessary
power level, the circulation rate of the oxygen or gas rich in
oxygen is not higher than 120 m/s at any point in the exchanger,
and preferably is not higher than 100 m/s.
[0027] Besides the temperature and circulation rate of the oxygen,
it is known that the risk of ignition in particular is also
dependent on pressure. If an increase in pressure leads to a
reduction in volume and therefore in the circulation rate in the
exchanger, a modification of this type is also dependent on the
operating characteristics of the burners that can be used in the
furnaces in question. In practice, it is a very delicate matter to
proceed towards a pressure reduction of the hot oxygen after
passage into the exchanger and before it reaches the burner.
Because of its nature, it does not reliably allow passage into
pressure reducing means, and even the use of diaphragms to conduct
this operation is not recommended, both as a result of its
aggressiveness. A high pressure at the level of the injection
nozzle or nozzles of the burner to maintain an adequate delivery
rate would result in the opening of this or these nozzles to be
reduced. This is not desirable because of the risk of fouling
and/or wear of these nozzles, which would quickly lead to defective
operation.
[0028] To take these factors into account, the exchangers according
to the invention are also dimensioned such that for the power
levels sought, the pressure of the oxygen or gas rich in oxygen in
the exchanger does not exceed 3 bar, preferably not 2 bar and
particularly preferred 1.5 bar.
[0029] Studies of the operating specifications of these
installations show that the best energy balances require that the
fuels and oxygen carriers are brought to an elevated temperature.
The temperatures to which the gases must be brought are as elevated
as the installations used permit.
[0030] While it is advantageous from the point of view of
efficiency of the burners to bring the oxygen to the most elevated
temperatures, these are limited in practice by the temperature of
the heat transfer fluid taking part in the exchange as well as by
the need to limit the risk of corrosion or even ignition of the
materials in contact with the oxygen.
[0031] The energy supply to heat the oxygen or gas rich in oxygen
comes from the combustion gases either directly by circulation in
the exchanger or preferably indirectly by means of a fluid that has
itself been reheated beforehand by an exchange with the combustion
gases.
[0032] This latter manner of proceeding provides additional safety
in the case of deficient tightness in the circulation of the
oxygen. Rapid fouling of the exchanger can also be prevented as a
result, since the combustion gases entrain a significant quantity
of dust with them that it is difficult to separate completely.
[0033] The intermediate gas, should such a double exchange occur,
is advantageously inert with respect to oxygen. This is preferably
air, nitrogen, CO.sub.2, steam or a mixture of these gases.
[0034] If necessary, the intermediate gas can be formed from a
mixture of the inert gases indicated above and a portion of the
combustion gases that have undergone dust separation
beforehand.
[0035] The temperature of the fumes can increase to 1550.degree. C.
and most frequently lies between 1250.degree. and 1450.degree. C.
and is higher than the temperatures, to which oxygen can be brought
without too severely degrading the material of the walls with which
it comes into contact.
[0036] In the case of an installation comprising a double exchange,
in which the heat transfer fluid is formed from air, the
temperature of this latter after being reheated by the combustion
gases is preferably in the range of between 450.degree. and
1000.degree. C. and particularly preferred between 600.degree. and
800.degree. C.
[0037] As clearly evident from the studies of the inventors, which
form the subject of the tests outlined below, the corrosion of the
materials forming the heat exchanger increases rapidly with the
temperature such that it is preferable to keep the oxygen in
conditions that ensure against excessive corrosion.
[0038] In practice, the temperature of the hot oxygen or gases rich
in oxygen as results from the heat exchanges remains within the
limits where the choice of materials made according to the
invention can prevent excessive corrosion of the installation. This
temperature does not ordinarily exceed 900.degree. C. and
preferably is not higher than 700.degree. C.
[0039] The materials forming the exchanger, and primarily those in
contact with the hot oxygen, must be selected in order to assure a
good resistance to oxidation by the gases and in particular the
oxygen in these temperature conditions.
[0040] The steels most commonly used are not sufficiently
resistant. Consequently, it is necessary to make a particularly
careful selection in order to guarantee long service life of the
installation.
[0041] The selection of materials not only includes consideration
of resistance to the highest temperatures reached in these
installations, but also a good resistance to temperatures that are
lower, but are also known to cause a change in state of the
material which will make it particularly sensitive to possible
degradations. During an increase in temperature, some steels in
particular pass through transition temperature zones that will
cause embrittlement of the metal.
[0042] There are several aspects to the desired resistance. It is
not just a matter of preventing vigorous oxidation of the materials
in the form of their combustion, it is also a matter of preventing
any modification of the surface in contact with the oxygen, which
can eventually not only lead to perforation of the walls but much
more, it is advisable in some cases to prevent the detachment of
particles that are likely to disturb the subsequent reaction and/or
pollute the products prepared by means of the combustion maintained
with this hot oxygen.
[0043] In all cases, the installation in which the gases rich in
oxygen circulate must be made from materials that naturally provide
the desired resistance to these gases that are particularly
aggressive by nature, and all the more so when the temperature,
circulation rate and pressure are more elevated.
[0044] So that the exchanger used to heat the oxygen or gas rich in
oxygen can meet the conditions of use outlined above in a lasting
manner, the inventors have established characteristics that the
materials forming them must meet.
[0045] In general, according to the invention the exchanger that
must receive gases containing at least 50% oxygen at a temperature
not less than 300.degree. C. is made, at least in the case of the
walls directly in contact with these gases, of a metal alloy that
complies with the following test protocol.
[0046] A sample of metal alloy according to the invention placed in
an atmosphere corresponding to the gas rich in oxygen that has to
circulate in the installation and at the most elevated temperature
encountered in the installation does not exhibit a weight gain of
more than 0.1 mg/cm.sup.2 of surface exposed after 1000 cycles each
including maintaining the anticipated maximum temperature for 1
hour, each phase at this temperature being followed by a return to
ambient temperature.
[0047] The choice of repeated passage through the elevated
temperatures followed by a return to ambient temperature is
particularly exacting. The structures of the metal can encounter
repeated phase modifications in particular that cause the highest
stresses, and therefore increased risks of degradation. In
practice, this test is such that it subjects the metal to more
significant trials than those withstood during use of the
industrial exchanger itself, which is intended to operate
continuously, this operation only being interrupted occasionally
for maintenance operations at the longest possible time
intervals.
[0048] As indicated above, since the burners of the glass furnaces
are preferably supplied with a gas with an oxygen content that is
preferably higher than 80% and can reach 100%, the test indicated
above must advantageously be passed for these oxygen contents.
[0049] To best assure a long service life of the exchanger, the
chosen metal alloy goes through the same test, but here the control
temperature is at least 500.degree. C., and to meet the envisaged
extreme conditions, the alloy went through the test in which the
most elevated test temperature is at least 600.degree. C., and can
pass this test even at temperatures of 800.degree. C.
[0050] In addition, in a test for spontaneous combustion in an
oxidising atmosphere in accordance with standard ASTM G-124, the
alloys most suitable for forming the exchanger according to the
invention resist this combustion at least up to pressures of 3 bar
and preferably at least up to pressures of 10 bar.
[0051] Those alloys advantageously used and having a positive
response to the corrosion test when used in temperature ranges
above 550.degree. C. include ferritic type non-oxidising alloys, in
which the Cr content is in the range of between 12 and 30% by
weight and which simultaneously contain Al at the rate of 1 to
8%.
[0052] Ferritic alloys are subject to embrittlement when in
temperature ranges between 400.degree. and 500.degree. C. For these
reasons, the use of these alloys must take into account the
considered factors and conditions, in particular temperature
conditions, prevailing in the exchanger.
[0053] The parts of the exchanger exposed to hot oxygen can also be
made from alloys rich in Ni and Cr having Ni contents higher than
25% by weight and simultaneously containing 10 to 30% Cr. The Ni
content can rise to 75% or more.
[0054] These alloys differ from one another in particular in their
mechanical properties. Moreover, their selection must possibly take
into account any limitations specific to the envisaged use. While
alloys with a high Ni content work well in flat glass production
installations, it is important to take into consideration the risk
posed by the presence of Ni, as any entrainment of particles by the
Ni must be carefully avoided because of the risk of nickel sulphide
forming in the glass sheets that generates fractures.
[0055] These alloys have a good resistance to corrosion at elevated
temperature due to the formation of a protective film of chromium
or aluminium oxide. The chromium content of the alloy must be
sufficiently high in order to prevent the formation of nickel oxide
nodules that increase rapidly and, if entrained, would be capable
of forming nickel sulphide in the glass sheets that generates
fractures.
[0056] With oxygen temperature ranges higher than 500.degree. C.,
and in particular between 500.degree. and 700.degree. C., it is
preferable to use alloys in which the chromium content is only 10
to 20%, particularly preferred between 10 and 16%. Alloys rich in
Ni that meet the specified requirements are in particular those
usually referred to by the names Inconel 600H, 600L, Incoloy
800H.
[0057] With oxygen temperature ranges higher than 100.degree. C.,
and in particular between 100.degree. and 600.degree. C., it is
preferable to use alloys in which the chromium content is higher
than 16%, particularly preferred between 20 and 30%. Alloys rich in
Ni that meet the specified requirements are in particular those
usually referred to by the names Inconel 600H, 600L, 601, 617, 625,
Incoloy 800H and 800HT.
[0058] It is also possible to use an alloy such as stainless steels
316L and 310, which are easy to work, but whose service life is
less assured.
[0059] While the circulation rate of the highly oxidising gases at
elevated temperature is a risk factor with respect to erosion, this
can be increased by particles carried by these gases. Initially,
the gases are substantially free of solid particles, but these can
come from the installation itself. The walls of the ducts and the
heat exchangers exposed to corrosion by these gases can in fact
release particles, which as they impact the elements downstream
also generate erosion and to a much higher degree, as the flow rate
of the gases increases.
[0060] The surface condition of the walls of the exchanger can
affect the resistance to corrosion. The more pronounced the surface
irregularities are, the more the alloy is corroded with otherwise
identical conditions. For this reason, the surfaces of the walls of
the exchanger according to the invention that come into contact
with the gases rich in oxygen are polished and have a roughness of
not more than 6 micrometres (.mu.). The roughness is preferably
less than 4.mu. and most advantageously is at most equal to
2.mu..
[0061] Examples of practical details of the invention are given in
the description below with reference to the set of drawings:
[0062] FIG. 1 is a schematic sectional illustration of a gas
exchanger usable according to the invention to reheat oxygen or gas
rich in oxygen;
[0063] FIG. 2 is a partially enlarged view of the end of the
exchanger shown in FIG. 1;
[0064] FIG. 3 shows a detail of part-section A taken from FIG.
2.
[0065] The general structure of the exchanger is the conventional
type for gas exchangers. It comprises a chamber 1 enclosing a bank
of tubes 2. The tubes are secured inside the chamber by plates 3,
4.
[0066] The plates form a sealed wall delimiting the zone of the
chamber 1, in which the heat transfer gas circulates.
[0067] The chamber is closed at its ends by two covers 5, 6. These
covers are tightly secured to the chamber by means of flanges 7, 8,
9, 10 and seals. These flanges can be removed to give access to the
ends of the tubes 2, where necessary.
[0068] To obtain the best possible exchange, the circulation of the
heat transfer gas and the oxygen or gas rich in oxygen is
advantageously conducted in reverse flow. The hot heat transfer gas
passes into the chamber through conduit and exits through conduit
12 after having passed through the circuit created by the baffles
13, 14, 15 inside the chamber.
[0069] The oxygen or gas rich in oxygen circulates in the tubes 2
along a substantially rectilinear course. It passes cold through
end 16 and exits hot at end 17 to be conducted to the burners.
[0070] The exchange is all the more effective when circulation
rates are higher. Nevertheless, the flow rate and pressure of the
oxygen must be held within the limits that assure the operating
safety of the device. The circulation of the oxygen must be
prevented from resulting in excessive corrosion of the walls which
it comes into contact with. It should also be ensured that the
oxygen does not strike the walls. The use of rectilinear tubes thus
restricts erosion.
[0071] The arrangement of the ends of the tubes 2 is shown in
detail in FIG. 2.
[0072] To prevent turbulence at the ends of the tubes, with the
risks of increased erosion at the point where these tubes connect
to the plate 4 most frequently by suitable welds, these tubes
terminate with a widened section. This arrangement facilitates the
flow of oxygen and its expansion and subsequently some
deceleration. This widening is in the shape of a truncated cone in
the figure with an angle of opening .alpha..
[0073] For the same reason, the covers, and above all cover 6
arranged at the oxygen outlet, are located at a distance from the
ends of the tubes 2. In this way, the flow rate of the oxygen along
the walls of the cover is substantially reduced in relation to that
at the outlet of the tubes.
[0074] The general shape of this cover 6 is also chosen so that the
advance of the hot oxygen encounters the wall of the cover at a low
incidence, thus minimising impact. For example, the wall of the
cover is at an angle of about 20 to 30 degrees relative to the
direction of the tubes 2. The profile of the cover decreases
progressively up to the connection with the outlet duct.
[0075] It is also advantageous to ensure that there are no sharp
angles or welds in this section.
[0076] The dimensioning of the tubes and their distribution are
such that the flow rate and pressure conditions indicated above are
met by the delivery rates implemented.
[0077] Since the exchanger must operate continuously over very long
periods, it may eventuate that a tube no longer has the necessary
tightness in spite of precautions taken to prevent wear of the
elements of the exchanger. The assembly of the exchanger is such
that the defective tube can be blocked at these two ends. The
operation requires that the covers be removed. After the defective
tube has been taken out of service, the exchanger is once again
usable with an efficiency that is little changed in proportion to
the remaining active tubes.
[0078] The tightness at the level of the flanges of the covers 9,
10 of the exchanger or at the connection of these covers with the
oxygen intake or outlet ducts is advantageously obtained by means
of a metal annular seal 18 lined with a material 19, 20 resistant
to oxygen. The material in question is mica or a compressible
mineral material, for example. Seals of this type are produced in
particular by Garlock under the brand name "Vitaflex".
[0079] In order to determine the alloys that comply with the
implementation conditions according to the invention, the inventors
have conducted tests that are discussed in the following
description.
[0080] For these tests, the samples are formed from 2 mm thick
plates of metal alloy measuring 20.times.20 mm.
[0081] The condition of the surface of the samples indicates its
clear importance with respect to sensitivity to oxidation. For this
reason, one face of each of these plates is polished with an
abrasive sheet of SiC to grain size 1200. The other face is left in
its original state as produced by the industrial rolling
process.
[0082] The composition by weight of the samples of alloys tested is
specified in the following table:
TABLE-US-00001 Alloy Fe C Si Cr Al Ni Mn Others I 6-10 0.15 0.5
14-17 72 0.5 Cu II comp <0.04 <1.0 19-23 0.15-0.6 30-34
<1.5 0.15-0.6 Cu III comp 20 5.5 0.5 Ti 0.5 Y.sub.2O.sub.3 IV
comp 1-2 24 1-2 V comp 22 5 VI comp 0.15 23-27 <1.0 1.5
[0083] The measurement of the oxidation of the samples is evaluated
by the increase in their mass after testing over a thousand cycles.
The results for different phase temperatures are indicated in the
following table:
TABLE-US-00002 Alloy I V Temperature 550 650 800 550 650 800
mg/cm.sup.2 0.013 0.06 0.347 0.004 0.02 0.099 Alloy III IV
Temperature 550 650 800 550 650 800 mg/cm.sup.2 0.097 0.10 0.232
0.026 0.05 0.103
[0084] It is evident from these tests that the oxidation is more
significant when the phase temperature is higher. At 550.degree. C.
the increase in all cases remains well below 0.1 mg/cm.sup.2. At
650.degree. C. only one sample reaches this value. At 800.degree.
C. the most resistant samples are those of alloys IV and V.
[0085] The metallographic observation of the samples shows a much
lower tendency towards oxidation in the polished face of the
samples.
[0086] The above measurements at the same time include the
oxidation of the two faces of the sample. Since only one face is
polished, the oxidation measurement obtained is thus higher than
that which would be observed in practice when the surface in
contact with the oxygen is polished.
[0087] Since the tests were conducted in a static manner, in other
words without circulation of the atmosphere in relation to the
sample, no "scale" appeared to have detached from the surface.
[0088] Analysis of the modification of the compositions at the
surface, and in particular the decrease in Cr content, is a means
of evaluating the risk of detachable particles forming. The
presence of Cr with a content of not less than 7% guarantees the
formation of a protective layer that prevents the formation of
scale.
[0089] The measurements outlined in the following table show that
the Cr content remains well above these values. After testing over
1000 cycles, in which the phases are at the maximum temperatures
indicated, the analysis in percentage by weight of the samples at
the surface (S) and at the core (C) of the product leads to the
results indicated in the following table:
TABLE-US-00003 Alloy .degree. C. Cr Si Al Fe Ti Mn Ni III 800 S
17.9 6.3 72.7 0.4 0.1 C 19 5.6 73.6 0.5 0.0 IV 800 S 22.1 0.8 2.1
72.1 0.0 0.4 C 23.1 0.8 1.9 71.9 0.3 0.4 IV 550 S 18.6 0.7 1.8 75.8
0.0 0.3 C 23.7 0.8 1.9 71.8 0.0 0.4 V 800 S 20 0.0 6.3 71.0 0.0 0.1
C 21.5 0.0 5.6 71.4 0.0 0.1 I 800 S 10.9 0.2 8.8 79.5 C 15.7 0.2
7.9 75.5 I 550 S 15.6 0.1 7.5 75 C 15.9 0.1 8.0 74.2
[0090] Considering the nature of the anticipated atmosphere, the
use of materials must comply with strict safety conditions. The
risk of combustion of the material brought to elevated temperature
in the presence of pure oxygen is thus evaluated in accordance with
the protocol of standard ASTM G 124.
[0091] In these tests, specimens of material placed in an
atmosphere of oxygen under pressure are subjected to a combustion
test. The results of these tests show that at 550.degree. C. and at
a pressure of 3 bar, combustion does not occur in any of the
samples.
[0092] When the pressure or temperature is increased, the tendency
towards combustion increases. Alloy III was found to be the most
sensitive to this test.
[0093] In general, at the temperatures envisaged above, the
pressure must not exceed 10 bar, whatever alloy is selected. On
this condition, the test in accordance with the standard shows that
use in supply installations for gas rich in oxygen does not cause
any risk of combustion.
[0094] On the basis of the results of these tests for resistance to
hot oxygen, a particularly interesting point appears to be that in
an exchanger according to the invention the thickness of the walls
can be relatively less thick than one would assume from the prior
art. Longevity simulations based on these results lead to walls for
the tubes of the exchangers according to the invention that have a
thickness that can be no more than 3 mm. This thickness can even be
equal to or less than 2.5 mm.
[0095] The relatively low thickness of the walls of the tubes of
the exchanger benefits the heat transfer and therefore increases
the available power for the same exchange area.
[0096] As an exemplary embodiment, an exchanger according to the
invention is configured in the following manner. It is formed by a
bank of 40 tubes of Inconel 600. The outside diameter of the tubes
is 17.2 mm and the thickness of the wall is 2.3 mm. The tubes have
a length of 4000 mm.
[0097] The exchange area in contact with the oxygen is therefore
8.4 m.sup.2.
[0098] Corning from a first exchanger, the heat transfer gas (air
with dust extracted) enters the exchanger at a temperature of
650.degree. C. The delivery rate of the heat transfer gas is set at
750 Nm.sup.3/h. The delivery rate of oxygen is 400 Nm.sup.3/h. As
it enters at ambient temperature the oxygen is heated to
550.degree. C.
[0099] The flow rate of the oxygen in the ducts is 67 m/s and the
load loss in the exchanger is less than 0.15 bar. A safety system
comprising a pressure controller maintains the pressure in the
exchanger at less than 1 bar.
[0100] The nominal power of the exchanger is 84 kW and per unit
area is set at 9.7 kW/m.sup.2.
[0101] The exchanger supplies a burner of a glass melting furnace
with a power of 2 MW with oxygen.
[0102] The full furnace is supplied with oxygen by 10 similarly
dimensioned exchangers. The power of each of these exchangers is
adjusted to better distribute the total power necessary to operate
the furnace.
* * * * *