EP2926623B1 - Heating element and process heater - Google Patents

Description

The present invention relates to a heating element for heating gases to high temperatures, comprising at least one pipe 1 designed for the flow of hot gas and an electric heating wire in the pipe, which is used to transfer heat to gas flowing past the heating wire is designed.
Likewise, the present invention also relates to a process heater having a housing with a gas supply and a gas outlet, a heating space between gas supply and gas outlet for receiving a heating element and electrical connections for at least one heating element, corresponding heating elements have long been known. They consist, as already mentioned, of at least one gas to be flowed through pipe, which is open on both sides for the purpose of flow, wherein in the tube, a heating wire is arranged, at which the gas flows past and heated by the direct contact with the heating wire.

The document DE 16 15 278 A1 discloses such a heating element of the prior art.

Usually used as heating wires helically wound, fine wires whose cross-section is much smaller than the pipe cross-section and are traversed by electricity and thereby heat. The electrical energy converted into heat by the heating wire naturally depends on the available electrical voltage and the resistance of corresponding heating wires, whereby the length of a coiled wire can be adapted to achieve desired resistance values or several corresponding heating wires can be connected in parallel or in series. Of course, the heat energy transferred to the gas flowing along the heating wire depends on the maximum temperature reached by the heating wire, on the flow velocity and on the surface available for heat exchange as well as on the exact flow conditions in the heating element. The maximum gas temperatures, which can be reached regularly in continuous operation with such process heaters in practice, are in the order of 700 ° C.
Although occasionally also heating elements or process heaters are offered, which allow the production of higher gas temperatures up to about 900 ° C, but they have only extremely short lives. At the gas flow rates required for many processes, the Heating wire itself necessarily always a more or less significantly above the gas temperature temperature, even the smallest inhomogeneities in the heating wire or in its cross section or even unfavorable local flow conditions and turbulence can cause some sections of the heating wire heat up more than the rest Part, which then quickly leads to breakage and failure of the heating wires. Since the heating wire typically contains aluminum in small amounts, contact with oxygen initially results in the formation of a protective aluminum oxide layer around the wire. However, after consuming the aluminum portion, other alloying components such as iron and chromium react with the oxygen, which generally means the end of the life of the heating wire. Other chemical reactions of the heated or hot process gas with the material of the heating wire can accelerate the failure or breakage of the heating wires. Small irregularities in the material or cross-section of the heating wire due to chemical changes quickly lead to local overheating of the heating wire and breakage. Since the stability of the very thin, coiled heating wires is relatively low, especially at high temperatures, the heating coils can easily collapse in a vertical tube, causing short circuits, which also reduce the life of such helical wires. Such a failure by overheating, especially local overheating, occurs the easier the smaller the cross section or diameter of the heating wires. On the other hand, however, a large surface-to-volume ratio of the heating wires is considered advantageous for an effective transmission of the heat energy generated in the heating wire on the gas flowing past, so far the short life of such heating elements is accepted when gas temperatures in the range of 900 ° C or above.

Process heaters and heating elements that generate gas temperatures of 900 ° C or even more, but have for the aforementioned reasons regularly only a service life of a few hours.

Against this background, the present invention has the object to provide a process heater and a corresponding heating element, which allow a generation of gas temperatures up to 1000 ° C or even above, so that extremely large amounts of energy can transfer to the gas and still have a relatively long life , which is at least 10 times the life of conventional heating coils in the generation of gas temperatures up to 1000 ° C usually.

This object is achieved in that the heating wire is formed as extending along the tube axis heating rod whose maximum clear distance to the inner wall of the tube over at least 80% of the circumference and / or at least 80% of the overlap length of the tube and heater a value of 10 mm does not exceed.

In other words, the heating wire is not a coiled wire whose material cross-section is substantially smaller than that of the tube, but rather a rod, for which in turn one can define a corresponding longitudinal axis which extends substantially along or parallel to the axis of the tube and while the tube fills so far that between heating element and tube wall only a relatively small, clear distance remains, which is at most 10 mm and preferably still significantly less, even if he punctually, ie in areas which account for less than 20% of the overlap length of tube and heater bar or less than 20% of the heater rod circumference. The term "heating wire" is therefore used in the present description as a generic term for both relatively thin coiled wires and for heating rods according to the present invention, wherein the different thickness is not the primary distinguishing criterion.

The maximum clear distance between the heating element and the tube is in many practical cases between 1 and 2 mm, slightly above or even below, down to minimum values of 0.02 mm. The maximum diameter of the heating element is rarely over 10 mm, because even larger diameters, the efficiency of energy transfer due to a relatively large volume / surface ratio of the heating significantly decreases, which can only be partially compensated by a larger pipe and Heizstablänge. In principle, however, the use of heating elements with larger diameters is possible, although not preferred. A diameter range for heating rods apparently favorable in practice for the purposes of the present invention is between 0.5 mm and 5 mm.

The term "tube" is to be interpreted broadly in the context of the present invention and ultimately defines only one cavity with an inlet and an outlet opening, which allow a flow to be heated with gas. Not even the cross section across the length of the pipe must be constant, although this is of course preferred in order to produce a largely constant gap, in particular a constant annular gap, between the heating element and the pipe wall by simple means. The annular gap can be interrupted by elevations, which are distributed over the circumference on the Heizstaboberfläche or on the inner surface of the tube, to allow a centering of the heating rod and to ensure a homogeneous heat transfer.

As tubes, for example, through holes are considered in a solid block, such a block may have a plurality of parallel holes.

Since the heating rods according to the present invention are relatively thick compared to the coiled wires in respective tubes of conventional heaters, they can heat better internally transferred and distributed, which helps prevent local overheating, and they have already for this reason at high thermal load or high heating rod temperatures beyond 1000 ° C a significantly longer life and durability or allow only the heating of gases to over 1000 ° C. with metallic electric heating elements.

An alternative condition instead of the maximum clear distance between the heating element and the tube can be expressed by a minimum ratio of the cross-sectional area of the heating element to the free inner cross section of the tube. Accordingly, the heater should, at least as far as it extends within the pipe, have a cross-sectional area which is at least 30% and more preferably at least 50% of the free pipe cross-section. In specific embodiments, which were tested with positive results, this aspect ratio was about 80%, wherein the maximum clear distance was 0.2 to 0.5 mm and a correspondingly uniform annular gap between the heating element and the tube wall about 0.1 to 0.25 mm amounted to.

Generally speaking, the preferred proportions between the cross section of the heating element and the inner cross section of the tube are expediently in the range of 0.2 to about 0.95. A cross-sectional ratio of 0.2 results, for example, in a very thin heating rod diameter of 0.2 mm and a tube diameter of 0.45 mm ,. A cross-sectional ratio of 0.9 is obtained, for example, with a heater rod diameter of about 4.75 mm in a tube with 5 mm inner diameter, wherein it does not matter in terms of the cross-sectional ratios on the unit or on the absolute dimensions, as long as the Heizstabdurchmesser within the above and below ranges. A preferred range of cross-sectional ratios is between 0.3 and 0.8, corresponding to a diameter ratio between about 0.5 and 0.9 with absolute diameters of the heating rods between 0.5 and 5 mm.

At the same time it has been found that with a substantially laminar flow of gas through an annular gap between a rod-shaped heating rod extending along the tube axis and the inner wall of the tube, the heat transfer between the heating element and the gas flowing through is surprisingly effective, so that with such a heating element readily Process gas temperatures of up to 1200 ° C or even more, while the lifetime of these process heaters and in particular the heating rods is a multiple of the life of conventional process heaters or heating wires, which are designed for the generation of gas temperatures of 900 ° C or more. The annular gap along the circumference of the heating element also does not necessarily have to have a constant width, but can vary between 0 (contact) and the maximum value (in the case of circular cross sections, ie twice the uniform gap width.

The absolute pipe diameter and Heizstabdurchmesser can vary widely, for example between an inner diameter of the tube from 1 mm to 20 mm or more, z. B. 60 mm, again depending on the other dimensions, such. the length of tube and heater, the desired width of the annular gap, the gas flow rate and the electrical resistance of the heater and the available voltage.

Of course, the heating rod has a correspondingly smaller diameter for small pipe diameters, and in an extreme case also 0.5 mm or less, e.g. May be 0.2 mm. He is so compared to conventional helical wires or Heizfilamenten but still significantly thicker and especially not coiled, but extends parallel to the tube axis and along the tube axis. The difference between the "heating wire" according to the prior art and the "heating element" according to the present invention is thus not primarily (or not only) in the different thickness, but rather in the defined longitudinal extent and relatively stable shape of the heating rod, the extends, as far as practicable, exactly along the axis of the tube, so that its length within the tube corresponds exactly to the length of the tube and thus the heating element does not run along an artificially extended path in the tube. Nevertheless, the heating element of a heating element according to the present invention is usually also thicker than the heating wires in conventional heating elements with the same tube cross-section and a heating element in the overall comparable heating element according to the prior art.

Ideally, the heating element is arranged as accurately as possible in the center of the tube, wherein the outer cross section of the heating element substantially coincides with the shape of the inner cross section of the tube, resulting in the result that the annular gap between the heating element and the inner wall of the tube has a substantially constant width. Possibly, however, the inner surface of the tube and / or the outer surface of the heating element could also be structured, i. For example, have a running in the longitudinal direction of the rod and the tube rib or groove structure, which may also have a small helix angle. Such superficial structures may, for a given annular gap width, expand the laminar flow region to larger gas flow rates if desired.

The concrete width of the annular gap always represents a compromise between maximum heat energy transfer and pressure loss at the desired gas flow rate. That is, the narrower the annular gap, the more effective is the heat transfer from the heating element to the gas flowing between the heating element and the tube, with a narrow gap but also limits the gas flow and / or requires a large pressure difference between inlet and outlet.

In addition, the reasonable width of the annular gap but also depends on the length of the tube and also from the converted in the heating element electric heating power.

In a specific embodiment, the average width of the annular gap is about 0.1 mm, in another example, 0.2 mm but it is not always possible to really concentrically arrange the heating element in a tube, so that the annular gap width at least at some axial positions in the circumferential direction can vary between zero and twice the average annular gap width.

In one embodiment, spacers are therefore provided at some positions along the circumference and / or over the length, which center the heating element in the tube. The spacers may be integrally formed with the heating element or the tube and are in particular designed so that they impede the gas flow between the heating element and the pipe as little as possible. The spacers are preferably made of heat-resistant ceramic and are ideally realized via the tube geometry.

Ideally, heating rod and tube are arranged coaxially with each other, d. H. their axes collapse.

In this case, the heating element and the tube but by no means have a circular cross section, they could for example also have the cross section of a preferably equilateral polygon and it could, for example, a pipe with hexagonal or octagonal cross-section or outer contour, which receives a cylindrical heating element. In particular, a square or hexagonal outer contour of the tubes allows a very compact arrangement of the tube bundle and a resulting minimal bypass flow between the tubes.

In one embodiment of the invention, a plurality of parallel tubes are combined to form a tube package and the heating element, more precisely the heating rods of the individual tubes of the tube package have the shape of a meandering passed through the heating wire, which is inserted at the end of a tube and from the outlet side In this case, the number of tubes through which a single heating wire is passed as a heating rod, preferably straight, so that the heating element in the form of a back-and-forth through the plurality of tubes wire on the same side as the inlet end parallel to this exits and thus can be connected at one end of the tube package with corresponding electrical connection contacts. It is understood that a tube package can consist of several groups of tubes, which are each traversed by a single continuous heating wire. Should it be the electrical Required connection performance, a division into several electrical zones has proven, which allow a connection in delta or star connection.

Appropriately, a tight packing of such tubes is arranged in a common housing, wherein between the housing wall and the outside of the dense packing of individual tubes additionally insulating material is arranged.

The insulating material is preferably a high temperature resistant, ceramic material which has sufficient stability for the production of dimensionally stable tubes. Between a plurality of parallel tubes bundled together, a high-temperature resistant ceramic insulating material such as sold by the applicant under the trade name "Fibrothal" can be disposed.

Instead of side by side also several of the heating elements according to the invention and corresponding packages of heating elements can be arranged axially one behind the other.

The tubes should consist of an insulating and high-temperature-resistant ceramic, which in particular aluminum oxide (Al2O3) comes into consideration.

The heating element is preferably made of an iron-chromium-aluminum alloy or of a nickel-chromium-iron alloy. Optionally, in particular, a thicker heating rod in turn consist of a bundle of parallel, possibly also twisted together individual rods or wires, in such an embodiment, the above-defined clearance by the clearance of an envelope of the bundle of rods or wires to the inner wall of the tube is defined.

The heating element may have a diameter in the range of 0.2 to 50 mm, preferably between 0.5 and 10 mm.

Further advantages, features and possible applications of the present invention will become apparent from the following description of a preferred embodiment and the associated figures.

Figur 1

eine stirnseitige Draufsicht auf ein Heizelement, welches aus einem Bündel von Rohren mit hindurchgeführten Heizstäben besteht.

Figur 2

eine Seitenansicht des Heizelementes nach Figur 1,

Figur 3

eine Schnittansicht mit einem Schnitt entlang der Längsachse eines kompletten Prozessheizers mit einem erfindungsgemäßen Heizelement und einem Gehäuse mit Anschlüssen für Gas und Strom sowie einer Isolierung,

Figur 4

eine Stirnansicht von links auf den Prozessheizer nach Figur 3.

Figur 5

einen Schnitt durch ein Heizelement gemäß Figur 1 und 2 und

Figur 6

nochmals schematisch einen Prozessheizer mit der Lage der Schnittlinie der Figur 5

Show it:

FIG. 1

a frontal plan view of a heating element, which consists of a bundle of tubes with passed heating rods.

FIG. 2

a side view of the heating element after FIG. 1 .

FIG. 3

3 is a sectional view with a section along the longitudinal axis of a complete process heater with a heating element according to the invention and a housing with connections for gas and electricity and an insulation,

FIG. 4

a front view from the left on the process heater after FIG. 3 ,

FIG. 5

a section through a heating element according to FIGS. 1 and 2 and

FIG. 6

again schematically a process heater with the position of the intersection of the FIG. 5

One recognizes in FIG. 1 a dense packing of tubes 1 in a hexagonal arrangement, through which heating rods 2 are passed. The tubes 1 are made of alumina ceramic and have an inner diameter of about 1.7 mm, and an outer diameter of about 2.7 to 2.8 mm, resulting in a wall thickness of the tubes 1 of about 0.5 to 0.55 mm results. The heating elements are formed here by a continuous heating wire with a diameter of about 1.5 mm, which is passed alternately in opposite directions through a plurality of tubes of this tube package, wherein the heating element marked 2a, the insertion side of the heating wire in the tube 1 a marked, which is then returned through the pipe 1 b again, reintroduced into the tube 1 c and in this way passed through a plurality of tubes and substantially parallel to the axis until finally the end of the wire in the form of the heating element 2 z through the Tube 1 z exits again.

Some of the tubes are conduits 3, which z. As the inclusion of thermocouples or other thermometers serve, while the central tube may have, for example, a centering 4, with the aid of which from the tube package and the passed heating wire existing heating element 10 can be centered in the housing of a process heater.

FIG. 2 is a side view of the package or the hexagonal packing of tubes according to FIG. 1 ,

The length I of the tubes 1 is, for example, between 150 and 500 mm, while the length L of the entire heating element 10 (without the protruding terminal ends 2a and 2z) is greater by approximately 4-5 mm for the dimensions of tubes 1 and heating rods 2 indicated here ,

FIG. 3 shows a complete process heater 100 with a tubular housing 6, a gas supply pipe 7, a gas outlet nozzle 9 with outlet pipe 8 and a mounting flange 13, which in turn is mounted on a power supply flange 14.

The gas supply pipe 7 opens into a cylindrical cavity 18, through which extend two parallel power supply pipes 16, of which in the side view of FIG. 3 only one is recognizable. The power supply tubes form a passage for the connection of the wire ends 2a and 2z with electrical connection contacts on the electrical connection flange 14. The heating element 10, which consists of a tube package, for example according to FIGS. 1 and 2 is located in the center of the tubular housing 6, wherein between the inner wall of the tubular housing 6 and the heating element 10, a high temperature resistant, ceramic insulating material 17 is arranged, which typically consists of two heating elements 10 from opposite sides enclosing half shells 17a, 17b (see FIG. 5 ), whose inner contour of the outer contour of the heating element 10 is adjusted.

Alternatively, the half shells can also together form a simple cylindrical tube, in which case the remaining spaces between the heating element 10 are stuffed with present in loose fiber composite insulating material, which also fills the spaces between the tubes 1, 3 otherwise.

As an alternative to the plug of the tube interspaces, the gas inlet side of the heating element 10 could also have a corresponding perforated, circular cover, the diameter of which corresponds to the maximum outer diameter of the tube assembly of the heating element 10 and which has holes only at the position of the tubes or pipe openings and thus the Cover the entire face of the pipe pack, except for the holes, before passing the heating wire through the pipes. Such a cover plate could consist of the same ceramic insulating material, as it is also used for the half-shells 17a, 17b between the housing and heating element 10 and which is sold by the applicant under the brand name "Fibrothal". The ends 2a and 2z of the heating wire (s) 2 are connected by the insulating connecting pipes 16 to external electrical terminals 12 which are mounted to the supply flange 14 via a compression fitting 11.

The variant of a process heater shown here is designed for a Heizstab- or- Heizdrahtdurchmesser of about 1, 5 mm for a heating power of 3.5 kW, the clear inner tube diameter between about 1.7 and 2.2 mm can be and the heating wire or rods consist of an iron-chromium-aluminum alloy. Suitable heating wires are sold by the applicant, inter alia, under the trade name "NICROTHAL". It goes without saying that corresponding process heaters can be dimensioned arbitrarily so that the power range can extend between a few watts or a few 100 watts and 100 or more kilowatts.

The gas to be heated is supplied through the connection 7 and arrives in a substantially cylindrical antechamber 18, which is otherwise still separated from the two insulating tubes 16 of FIG Power connection is traversed and flows into the open annular gaps 5 between the tubes 1 and the heating wires 2 in and through the tubes, and then exit through the nozzle 9 and the outlet tube 8 from the process heater.

It goes without saying that you can also switch several heating elements or process heaters axially one behind the other.

FIG. 4 is finally after a frontal view of the process heater after FIG. 3 from the left, which in turn recognizes the nozzle 9 with the outlet end 8, as well as the housing 6, the gas supply pipe 7 and the connecting flange thirteenth

LIST OF REFERENCE NUMBERS

1

pipe

2

Heating rods, heating wire

2a, 2z

Ends of the heating wire or the heating rods

3

conduit

4

centering

5

annular gaps

6

casing

7

Gas supply pipe

8th

outlet pipe

9

jet

10

heating element

11

compression fittings

12

electrical connections

13

mounting flange

14

delivery flange

16

Power supply pipes / connecting pipes

17a, 17b

shells

18

anteroom

Claims (15)

1. Heating element for heating gas to high temperatures and comprising at least one tube (1) designed for the through-flow of gas to be heated and an electrical heating wire within said tube, which is designed for the transfer of heat to the gas passing along the heating wire, characterized in that the heating wire is formed as a heating rod (2) extending along the tube axis, the maximum clearance of which to the inner wall of the tube does not exceed an amount of 10 mm for at least 80 % of the extent and/or at least 80 % of the overlapping length of the tube and the heating rod.
2. Heating element according to one of the preceding claims, characterized in that the heating rod has a diameter in the range of between 0.2 and 50 mm, preferably between 0.5 and 10 mm.
3. Heating element according to one of the preceding claims, characterized in that the ratio of the diameter of the heating rod to the inner diameter of the tube is in the range of between 0.04 and 0.95 and preferably between 0.3 and 0.8.
4. Heating element according to one of the preceding claims, characterized in that the maximum clearance between the heating rod and the inner wall of tube is between 0.02 and 5 mm.
5. Heating element according to one of the preceding claims, characterized in that the clearance between the heating rod and the inner wall of the tube is defined by an annular gap which is in general constant over the overlapping length and the extent.
6. Heating element according to one of the preceding claims, characterized in that the clearance or the width, respectively, of the annular gap is in the range of between 0.05 and 1 mm, preferably in the range of between 0.1 and 0.5 mm.
7. Heating element according to one of the preceding claims, characterized in that the heating rod extends through a plurality of parallel tubes as a continuous massive heating wire.
8. Heating element according to one of the preceding claims, characterized in that it comprises a plurality of parallel tubes having heating wires, which are preferably arranged in a close packing next to each other.
9. Heating element according to one of the preceding claims, characterized in that the at least one tube consists of aluminium oxide.
10. Heating element according to one of the preceding claims, characterized in that the heating rod consists of an iron chromium aluminium alloy or a nickel chromium alloy.
11. Heating element according to one of the preceding claims, characterized in that the heating rod itself consists of a bundle of parallel single rods or wires, respectively, optionally twisted with each other, wherein the clearance is defined by the clearance of the envelope surface of the bundle to the inner wall of the tube.
12. Heating element according to one of the preceding claims, characterized in that spacers are provided between the heating rod and the tube wall, which preferably result from the tube geometry.
13. Heating element according to one of the preceding claims, characterized in that the inner surface of the tube is structured.
14. Heating element according to one of the preceding claims, characterized in that the space between a plurality of tubes and between the tubes and the housing is filled in and sealed by a high-temperature resistant ceramic fibre material.
15. Process heater having a housing, which comprises a gas supply and a gas discharge, a heating chamber between the gas supply and the gas outlet and electrical connections for at least one electrical heating element, characterized in that the heating chamber comprises at least one heating element according to one of claims 1 to 14.

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