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Title:
INSERTS FOR BURNERS AND RADIANT TUBE HEATING SYSTEMS
Document Type and Number:
WIPO Patent Application WO/2015/062619
Kind Code:
A1
Abstract:
A heating system comprising a tubular member (10, 20, 30), such as a radiant tube, configured to define a combustion chamber and a path for combustion exhaust gas current (G), comprising at least one porous insert (16, 16A, 16B, 16C) which is located in the path of said combustion exhaust gas current, and is adapted to recover heat from said current and transfer said heat to said tubular member or is adapted to transfer said heat to incoming combustion air; applications of the invention include radiant tubes and recuperative burners.

Inventors:
GIANELLA SANDRO (CH)
ROMELLI LUCA (CH)
ORTONA ALBERTO (CH)
TRIMIS DIMOSTHENIS (DE)
UHLIG VOLKER (DE)
EDER ROBERT (DE)
GRÄMER TOBIAS (DE)
WÜNNING JOACHIM G (DE)
CRESCI ENRICO (DE)
Application Number:
PCT/EP2013/072487
Publication Date:
May 07, 2015
Filing Date:
October 28, 2013
Export Citation:
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Assignee:
ERBICOL SA (CH)
International Classes:
F23D14/12; F23C3/00
Domestic Patent References:
WO2008036515A22008-03-27
WO2012160095A12012-11-29
Foreign References:
US6484795B12002-11-26
US4355973A1982-10-26
CN201531881U2010-07-21
US6484795B12002-11-26
EP1591429A12005-11-02
EP1382590A22004-01-21
Attorney, Agent or Firm:
ZARDI, Marco (Via Pioda 6, Lugano, CH)
Download PDF:
Claims:
CLAIMS

1 . A fired heating system comprising a tubular member (10, 20, 30), said tubular member being configured to define a combustion chamber and a passage for a combustion exhaust gas current (G), which is downstream said combustion chamber or is coaxially arranged around said combustion chamber, characterized by comprising at least one heat transfer enhancing porous insert (16, 16A, 16B, 16C) which is located in said passage of combustion exhaust gas current, and is adapted to recover heat from said current.

2. A heating system according to claim 1 , said porous insert having a porosity between 20% and 95% and preferably between 50% and 90%.

3. A heating system according to claim 1 or 2, said porous insert having a foam structure or having a lattice structure.

4. A heating system according to any of claims 1 to 3, said porous insert being a ceramic porous insert.

5. A heating system according to any of claims 1 to 4, comprising a plurality of porous inserts located in the path of the exhaust gas current.

6. A heating system according to any of the previous claims, wherein said tubular member provides a passage for combustion air and a passage for exhaust gas, which may be coaxially arranged, and the heating system comprises at least one porous insert located in the passage of the exhaust gas and at least one porous insert located in the passage of the combustion air.

7. A heating system according to any of claims 1 to 6, wherein said tubular member (20, 30) has a coaxial structure comprising at least an inner tube (21 , 31 ) and an outer tube (22, 33) coaxial to said inner tube, said path for a combustion exhaust gas current comprising an annular space (23, 37) between said inner tube and outer tube, and said at least one insert (16B, 16C) being placed in said annular space.

8. A heating system according to claim 7, said tubular member comprising a combustion air inlet passage (35) and an exhaust gas passage (37) which are in the form of coaxially arranged annular spaces, and comprising one or more of said porous inserts (16C) placed in both the air inlet passage and exhaust gas passage.

9. A heating system according to claim 7 or 8, said at least one insert or each of the inserts (16B, 16C) being shaped as a ring or tube or a sector of a ring or tube.

10. A heating system according to any of the previous claims, wherein the porous insert or each one of the porous inserts (16A, 16B, 16C, 16D) has a cross section smaller than a gas passage section which is available in the location of the porous insert, in such a way that the porous insert or each one of the porous inserts covers only a portion of said passage section.

1 1 . A heating system according to claim 10, comprising at least one plurality of porous inserts (16A, 16B, 16C, 16D) which are located next to each other in a gas passage of said tubular member, and wherein adjoining porous inserts are arranged in such a way that a portion of said gas passage which is covered by any porous insert is left free or is only partially covered by the adjoining porous insert(s).

12. A heating system according to claim 1 1 , wherein said gas passage comprises an alternation of porous inserts covering different portion of the available cross section, so to define a sinuous path for a gaseous stream throughout said gas passage. 13. A heating system according to any of claims 10 to 12, said at least one porous insert having substantially the shape of a solid cut from a horizontal cylinder, or of a sector of a ring or tube when placed in an annular passage.

14. A heating system according to the claims 10 to 13, comprising a plurality of porous inserts (16A) having the shape of horizontal cylindrical segments and where each insert is opposed by 180 degrees relative to adjoining inserts.

15. A heating system according to any of the previous claims, said tubular member being a radiant tube (10). 16. A heating system according to claim 15, wherein said at least one insert is located in a low emissivity region (14) of said radiant tube (10).

17. A heating system according to any of the previous claims, comprising a recuperative burner comprising an annular exhaust gas passage (37) for an exhaust gas flow and an annular air passage (35) for a stream of combustion air, said annular passages being coaxially arranged and separated by a tubular wall (32) to provide preheating of the combustion air, and said porous insert(s) (16C) having the form of one or more closed rings or sectors of rings inserted in said exhaust gas passage (37) and optionally also in said air passage (35).

18. The use of a porous insert to enhance the efficiency of an industrial heating system, wherein said heating system comprises a tubular member defining a combustion chamber and a path for a combustion effluent gas, said porous insert being located inside said tubular member on the path of exhaust gas and/or of combustion inlet air.

19. A porous insert made of a ceramic or metal material, having a porosity between 20% and 95% and preferably between 50% and 90%, having a foam or lattice structure, and having the form of a plug, ring, or portion thereof, for use as heat transfer enhancer in an exhaust gas passage or inlet air passage of a radiant tube or recuperative burner.

Description:
Inserts for burners and radiant tube heating systems

DESCRIPTION

Field of the invention

The invention relates to the field of radiant tubes, recuperative burners and more generally to heating systems including a tubular member which defines a combustion chamber and a passage for the exhaust gas.

Prior art Radiant tubes are known in the art. They are typically used as indirect heating systems for heat treatments to be carried out under a protective gas atmosphere. For example, radiant tubes are commonly used for providing heat to an annealing process.

In a radiant tube, combustion takes place within the tube and exhaust gas are discharge at a tube end. There are several known embodiments of radiant tubes according to the geometry of the tube, including for example straight tubes, U-tubes and W-tubes, as well as embodiments with exhaust gas recirculation such as the so-called P-tube, double-P tube and A-tube. Another known embodiment of a radiant tube is the single-ended tube, featuring a coaxial structure with two tubular walls and exhaust gas discharge through the annular space between them.

All the above embodiments of radiant tubes are well known in the art and a related description can be found in the literature. An overview of the various embodiments of radiant tubes can be found in "Industrial Combustion Testing", 201 1 Taylor and Francis Group, chapter 24.

A purpose of a radiant tube is to transfer as much heat as possible to the process. To do so, it is desired to maximize the transfer heat from combustion flame and from the exhaust gas to the surrounding walls of the radiant tube. It is known that heat transfer takes place primarily by radiation in the first portion of the tube in the presence of the combustion flame, while heat transfer in the remaining part of the tube is substantially governed by convection. The first portion of the tube, around and near the combustion chamber, is usually termed "high emissivity portion" while the second portion of the tube, until the outlet section, is termed "low emissivity portion".

For example, in a radiant U-tube the first leg of the tube is typically under high emissivity regime, while the second leg (after the U-turn) is typically under low emissivity regime.

Various attempts have been made in order to maximize the heat transfer of radiant tubes and, hence, the efficiency of the tube burner.

Many of the known solutions try to enhance the convective heat transfer in the low emissivity portion of the tube which, in absence of the strong radiation from the flame, is relatively poor. It has been proposed, for example, to introduce spiral-like inserts in the tube, to provide a swirling motion of the hot gas and increase the amount of surface area available.

WO 2008/036515 and US 6484795 disclose examples of spiral-like inserts for radiant tubes according to the prior art. Said inserts, however, are not fully satisfactory. In particular, it has been noted that despite the swirling motion and the larger contact area with the gas, the convective heat transfer in the presence of said spiral-like inserts remains relatively low.

The present invention also relates to burners which do not require radiant tubes, e.g. because a protective atmosphere is not required.

The invention can be applied for example to recuperative burners which typically comprise an elongated tube member with a coaxial structure, including a tubular recuperator and an exhaust gas guide tube coaxially arranged around the recuperator. Exhaust gas is passed in the annular space between said tubes, in order to pre-heat the combustion air and/or dilute the fresh charge, improve the temperature uniformity and lower the emissions. Designers of said burners seek for maximization of the heat transferred from the exhaust gas to the tube-like recuperator, without drawbacks such as complication, cost or excessive pressure drop which may affect the combustion.

WO 2012/160095 discloses a solution to this problem by a textile-coated heat exchanger. The heat exchanger surface is coated on one or both sides, over a ceramic intermediate layer, with a ceramic structure which is produced using a textile material soaked in a ceramic slip.

Summary of the invention

The aim of the invention is to further improve the above prior art. In particular, one of the goals of the invention is a better heat exchange between a hot combustion gas and a tubular body, possibly with a coaxial structure, like for example a radiant tube or a recuperative burner. For application to recuperative burners, a goal of the invention is a better heat transfer between hot gas and an inlet stream of combustion air.

This aim is reached with a fired heating system according to claim 1 , comprising a tubular member configured to define a combustion chamber and a passage for combustion exhaust gas current, characterized by comprising at least one porous insert which is located in the passage of said combustion exhaust gas current, and is adapted to recover heat from said current. Said tubular member may be a radiant tube or a tubular body of a burner. In embodiments involving a radiant tube, said radiant tube can be made according to various known geometries including but not limited to: non recirculating tubes such as straight tube, U-tube, W-tube, Trident® tube; recirculating tubes such as P-tube, double-P tube, A-tube.

Said passage for the combustion exhaust gas is typically downstream said combustion chamber, for example in a radiant U-tube, or is coaxially arranged around said combustion chamber, for example in a single-ended radiant tube or in a recuperative burner.

Said porous insert is preferably positioned in a low-emissivity region of the tubular member, particularly in the case of radiant tubes. The high emissivity region is understood as the portion of the tubular member around the combustion flame and where heat is transferred primarily through radiation. The low emissivity region, on the other hand, is understood as the portion of the tubular member where heat transfer takes place primarily by convection.

Said porous insert has preferably a porosity (or vacuum degree) between 20% and 95% and more preferably between 50% and 90%. According to some preferred embodiments the porous insert has a foam structure; according to other preferred embodiments it has a lattice structure.

Said porous insert is preferably a ceramic porous insert, for example made of ceramic foam or lattice.

A suitable ceramic porous material for said insert is disclosed, for example, in EP-A-1591429. An open cell foam ceramic material and a method for producing the same are disclosed in EP-A-1382590.

A particularly preferred material for the ceramic porous insert is silicon carbide SiC. Other preferred materials are materials with similar coefficient of thermal expansion to the tube material. Other preferred materials are ceramic oxide materials, like cordierite.

According to other embodiments, the porous material may be a metal porous material, e.g. made of metal wires.

Further preferred embodiments of the invention are as follows. More than one porous inserts can be provided. In the embodiments with multiple porous inserts, the inserts may be identical or different.

In certain embodiments of the invention, one or more porous inserts are positioned in the path of exhaust gas and also in the path of a stream of combustion air, possibly mixed with recirculated exhaust gas. This is particularly the case of recuperative burners where inlet combustion air and a stream of exhaust gas pass through coaxial annular passages. By provision of one or more inserts in the annular passages, the heat transfer from the exhaust gas to the combustion air is enhanced to the advantage of pre-heating of the combustion air.

In some embodiments, one or more porous inserts may have a cross section smaller than the available gas passage area. For example, one or more porous inserts which partially cover the gas passage area can have the shape of a solid cut from a horizontal cylinder. Said solid cut may be a horizontal cylindrical segment, which is defined as a solid cut from a horizontal cylinder by a plane parallel to the axis of cylinder, or may be different, e.g. having a cross section like a sector of a circle. In some preferred embodiments, the porous inserts are substantially half-moon shaped, thus covering around 50% of the gas passage area. The porous inserts designed to partially cover the available passage area can be arranged in such a way that they alternately cover different portions of the gas passage area. Hence, a portion of the gas passage area which is covered by a certain porous insert is left free or is only partially covered by the adjoining porous insert(s). In the embodiments of the invention where the tubular member has a coaxial structure, said at least one insert can be formed, by way of non- limitative examples, as a ring or sleeve inserted within an inner tube and an outer tube which are parts of the tubular member. A tubular member with a coaxial structure is found for example in single-ended radiant tubes or in recuperative burners comprising a burner tube, an air guide tube and an exhaust gas guide tube which are coaxially arranged.

In the coaxial embodiments, inserts covering only a portion of the available passage area may have an open-ring structure, being for example C- shaped.

An aspect of the invention is also the use of a porous insert to enhance the efficiency of an industrial heating system, according to the attached claims. Another aspect of the invention is a porous insert made of a ceramic or metal material, having a porosity between 20% and 95% and preferably between 50% and 90%, having a foam or lattice structure, and having the form of a plug, ring, or portion thereof, for use as heat transfer enhancer in an exhaust gas passage or inlet air passage of a radiant tube or recuperative burner.

The porous insert of the invention absorbs a substantial amount of heat from the effluent combustion gas, thanks to the porous structure. Some of the heat is then released by conduction and radiation, and the convective heat transfer, the turbulence of the flow and the residence time of the hot gas are increased, all the above effects concurring in a considerable enhancement of the heat transfer or heat recovery. The increased amount of heat transferred to a radiant tube, or to incoming combustion air in a recuperative burner, results in a reduced fuel consumption for a given duty and/or more heat delivered to the process for a given size of a radiant tube or burner. In particular, it has been found that the ability of the porous inserts to provide enhanced heat transfer is greater than the prior-art solutions.

Another advantage is a more uniform temperature profile of the tubular member, compared to the prior art, reducing the thermal stress of radiant tubes or burners. In the recuperative systems, the inserts increase the efficiency of the heat recovery. In a tube burner, for example, it has been found that ring-shaped porous inserts realized according to the invention and positioned between a burner tube and an exhaust gas guide tube increase the heat recovered from the exhaust gas in a significant manner, thus increasing efficiency and reducing fuel consumption.

Another advantage of the porous inserts of the invention is their low cost and easy mounting. Hence, most of the conventional burners can be modified in accordance with the invention. The porous inserts induce a certain pressure drop of the exhaust gas. This drawback will be normally overcompensated by the increased amount of heat transferred. Anyway, the embodiments of the invention with porous inserts covering only a portion of the available gas passage area have the additional advantage of reducing this pressure drop. Furthermore, the insert may be arranged in such a way that they provide an alternate path of the hot gas which is also beneficial to the heat transfer.

These and other features and advantages of the invention will now be elucidated with the following detailed description of some embodiments.

Description of the figures Fig. 1 shows an embodiment of the invention applied to a radiant tube. Figs. 2, 3 show a variant of the invention.

Fig. 4 shows an embodiment of the invention applied to a single-ended radiant tube.

Fig. 5 shows an embodiment of the invention applied to a recuperative burner.

Detailed description of preferred embodiments

Referring to Fig. 1 , a first example of a heating system making use of the invention is shown, comprising a radiant tube 10 connected to a burner 1 1 and installed in a furnace 12.

The burner 1 1 has inlets for fuel Fu and oxidant A (for example air) which produce a flame F within the tube 10. The tube 10 comprises a first leg 13 which contains the flame F and then substantially defines a combustion chamber, and a second leg 14 which defines a path for the hot exhaust gas G until they reach an outlet section 15 of said tube 10.

The first leg 13 substantially corresponds to a high emissivity region of the tube 10, whilst the second leg 14 substantially corresponds to a low emissivity region of the same tube 10.

According to an embodiment of the invention, a porous insert 16 in the form of a cylindrical plug is fitted in the leg 14 of the radiant tube 10, preferably near the outlet section 15.

Said porous insert 16 absorbs heat from the gas G, which is released by conduction and radiation to the tubular wall around, and increases convective heat transfer and residence time of the hot gas G in the tube leg 14. As a consequence, the amount of heat transferred from the effluent gas G to the tube 10 and then to the furnace 12 is increased.

Said porous insert 16 is preferably a ceramic porous insert with a foam structure or with a lattice structure.

The U-tube of Fig. 1 is shown for illustrative purpose as the invention is equally applicable to the various other radiant tubes which are known in the art, including straight once-through tube, W-tube, P-tube, double-P tube, Trident®, etc. In some cases (such as A-tube, P-tube and double-P tube), a recirculation is also provided, which means that a portion of the effluent gas G is mixed with the fresh mixture burning in the high- emissivity region.

More than one porous insert can be provided, according to various embodiments of the invention.

The porous insert(s) may have a cross section smaller than the actual passage area available to the gas, in such a way that the cross section available for passage of the effluent gas is only partially obstructed by the porous insert or by each one of the porous inserts.

Fig. 2 shows an example of an embodiment with multiple porous inserts smaller than the gas passage. Said Fig. 2 shows the tube leg 14 fitted with a plurality of porous inserts 16A which cover around 50% of the gas passage area. The porous inserts 16A are shaped as a solid cut from a cylindrical plug and they have a half-moon cross section, as elucidated in the view of Fig. 3.

Said inserts 16A are advantageously alternated (Fig. 2) in such a way that the gas passage section covered by a porous insert 16A is left free by the adjoining porous insert(s). By doing so, the pressure drop is reduced and, at the same time, it is avoided to leave a straight path for the gas G which may cause undesired bypass of the porous inserts 16A.

Fig. 4 shows an embodiment of the invention applied to a single-ended radiant tube 20 comprising an inner tube 21 and an outer tube 22.

The inner tube 21 contains the flame F and defines the combustion chamber and then the high-emissivity region. The hot gases leaving the end section 24 of the inner tube 21 flow in the annular space 23 between tubes 21 and 22.

One or more porous inserts are provided in said annular space 23, preferably near the outlet section 25 thereof. In the shown embodiment, the porous inserts take the form of annular rings 16B fitted around the inner tube 21 .

Said ring inserts 16B have an inner diameter substantially equal to the outer diameter of the tube 21 , and an outer diameter substantially equal to the inner diameter of tube 22. Porous inserts may also be shaped as open rings to cover only a portion of the annular gas passage area in the space 23. For example C-shaped inserts may be used to this purpose.

Fig. 5 shows the application of the invention to a tubular burner generally denoted by numeral 30.

The burner 30 is a per se known kind of recuperative burner where the incoming combustion air is preheated by the exhaust gas, or at least a portion thereof.

The burner 30 comprises a tubular member with a coaxial structure made of several tube parts, namely a burner tube 31 , an air guide tube 32 and an exhaust gas guide tube 33.

Fuel is admitted in the combustion chamber, at the end of the burner tube 31 , via a gas lance inside said burner tube 31 . The combustion air, possibly mixed with recirculated exhaust gas, is admitted via an annular channel 35 between the outside wall of said burner tube 31 and the air guide tube 32, and enters the combustion chamber through a number of suitable holes 38 in the tube 31 .

Hot combustion gas stream leaves the burner tube 31 at an open end 36 and flows in a space 37 between said burner tube 31 and the exhaust gas guide tube 33.

In order to increase the efficiency of heat recovery and combustion air preheating, it is desired to maximize the heat transfer from the hot gas flowing in the annular channel 37 to the inlet air in the channel 35.

According to some embodiments of the invention, one or more ring- shaped porous inserts 16C are provided in the annular space 37 and/or in the annular channel 35, as shown in the figure.

In some other embodiments, ring-shaped porous inserts may be positioned e.g. around a ceramic body of a burner. Examples Example 1

The performance of a radiant U-tube having an inner diameter of 150 mm and outer diameter of 160 mm has been numerically tested with ceramic foam half-moon shaped inserts as depicted in Figs. 2 and 3. A gas inlet temperature of 1000 °C was adopted.

Calculations for the plain tube, i.e. without any insert in the low-emissivity region, give an outlet temperature of 723 °C and a calculated efficiency is 0.286. Said efficiency is defined as the ratio of outlet temperature over inlet temperature. The outlet temperature indicates the amount of recuperated heat.

The addition of two ceramic foam inserts leads to a lower outlet temperature of 708 °C and efficiency of 29.2%. Adding more inserts provides a lower outlet temperature and a better efficiency, with a substantially linear increase. Calculations with seven inserts give an outlet temperature of 670 °C and efficiency of 32.9%, thus an efficiency increase of 3.7%.

Example 2

A 40 kW recuperative burner has been tested with a bare tube and with SiC foam rings. Conditions were the following: length of tube 1 meter; 10 foam rings distributed all over said tube length and having a thickness of 1 cm; reference furnace temperature of 1000 °C; Oxygen (O 2 ) content in the exhaust gas 3%. An increase of the efficiency from 58% to nearly 70% has been observed.