EP0117029A1

EP0117029A1 - Single-ended radiant tube

Info

Publication number: EP0117029A1
Application number: EP84300103A
Authority: EP
Inventors: David William Collier
Original assignee: Wb Combustion Ltd
Current assignee: Wb Combustion Ltd
Priority date: 1983-01-18
Filing date: 1984-01-09
Publication date: 1984-08-29
Also published as: GB8301274D0; DE3461951D1; CA1239865A; EP0117029B1

Abstract

To reduce heat transfer to the flame tube 2 adjacent to the burner 5, vanes 19 are associated with the burner which cause combustion air to swirl before it mixes with gas from the burner and produce a vortex which encourages the gas to flow, and the flame to extend, centrally of the flame tube adjacent to the burner, and also cause air to surround the flame adjacent to the burner. In one embodiment the air is divided by a cowl 23 into a primary stream which passes through the vanes and a secondary stream which passes overthe cowl and surounds the flame adjacent to the burner. By means of an air shroud 23 the air is caused to converge towards the central axis of the flame tube as it approaches the vanes. The flame length can be varied by adjusting the distance between the shroud and the burner.

Description

This invention relates to a single-ended radiant tube which is heated by a gas burner.
A problem with known single-ended radiant tubes, particularly those of small diameters, has been that their flame tubes have tended to fail after a relatively short working life. The flame tubes have failed primarily as a result of the burner flame's impinging on them too close to the burner after combustion and causing excessive heating of the flame tube in front of the burner.
Usually the combustible air and gas mixture has been caused to swirl in front of the burner to enhance the mixing of the air and gas for improved combustion. However, the swirling action has encouraged the mixture to spread outwardly away from the central axis of the burner and flame tube so that on combustion the flame impinges on the inside surface of the flame tube close to the burner and causes the undesirable excessive heating of the flame tube.
The present invention seeks to overcome, or at least reduce, the problem.
The present invention consists in a single-ended radiant tube comprising an outer tube closed at a first end and connected, or adapted to be connected, at a second end to exhaust means, a gas burner at or near the second end and connected to gas supply means, combustion air supply means at the second end by which air is supplied to be mixed with gas discharged from the burner for combustion, a flame tube in and spaced from the outer tube extending from the burner towards the first end, in which flame tube the flame burns on combustion of the gas/air mixture, and an annular exhaust gases passage being defined between the flame tube and the outer tube which communicates with the interior of the flame tube at the first end and along which exhaust gases pass to the exhaust means, characterised in that swirl inducing means is provided adapted to cause combustion air supplied to the flame tube to swirl and produce a vortex in front of the burner which encourages the gas to flow, and the flame to extend, centrally of the flame tube adjacent to the burner, and to cause the flame adjacent to the burner to be surrounded by air which has still to be used for combustion, thereby to reduce heat transfer from the flame to the flame tube in that region of the flame tube.
By encouraging and flame to burn centrally of the flame tube adjacent to the burner, impingement of the flame on the flame tube adjacent to the burner where the combustion is most violent is prevented or reduced, and hence the extent to which that part of the flame tube is heated is reduced. This together with the surrounding of the flame adjacent to the burner with combustion air to reduce heat transfer from the flame to the flame tube can extend appreciably the working life of the flame tube.
The swirl inducing means preferably comprise vanes disposed about the burner which cause the air to swirl as it flows past the burner. The vanes may be on the burner. They may extend fully across the entry for the combustion air into the flame tube. They may be contained within a tubular cowl which extends over and is spaced from the burner and around which an annular passage for air is defined opening into the flame tube. The cowl divides the air supply to the flame tube into a primary air stream which passes through the cowl between the vanes, and so is caused to swirl, and a secondary air stream which passes through the annular passage to provide an envelope of air which surrounds the flame immediately in front of the burner. In both arrangements the swirling air encourages the flame to burn centrally of the flame tube adjacent to the burner and adjacent to the burner the flame is surrounded by air which reduces the heat transfer to the flame tube, but the cowl provides a more accurate division of the air supply to perform these functions.
An air shroud may be positioned rearwardly of the burner over which the combustion air passes to the swirl inducing means. The extent of the mixing of the air with the gas immediately in front of the burner can be adjusted by varying the distance between the burner and the air shroud. This enables the length of the flame to be adjusted. It is desirable to arrange it such that combustion takes place over the length of the flame tube to its discharge end, but not beyond the discharge end. Ideally, the distance of the burner from the air shroud is such that the air is converging towards the central axis of the flame tube as it approaches the swirl inducing means. The air then continues to keep close to the central axis as it is caused to swirl and flows past the burner.
When the radiant tube is relatively long and is to have a high input the distance of the burner from the air shroud may be shortened with the result that the air, whilst converging towards the central axis of the flame tube, passes through the swirl inducing means further away from the central axis. In consequence there will be reduced initial mixing of the air with the gas, and the flame length is increased. Conversely when the radiant tube is relatively short and is to have a low input the distance of the burner from the air shroud is increased so that the air passes through the swirl inducing means closer to the central axis of the flame tube, and hence passes the gas port or ports of the burner at higher velocity, thereby inducing faster mixing of the air and gas and producing a shorter flame.
This method of varying the flame length differs from the normal method which is to increase the air in a supply of pre-mixed air and gas to the burner to shorten the flame, and to reduce the air in the supply of pre-mixed air and gas to lengthen the flame. A radiant tube in accordance with the present invention may be operated without a pre-mixed supply of air and gas to the burner. Shortening the flame length with the air and gas swirling radially outwards in the previously known radiant tubes has generally resulted in serious overheating of the flame tubes adjacent to the burners in use and early failure of the flame tubes.
The most suitable position of the burner relative to the air shroud for a particular length and diameter of flame tube and gas flow can be ascertained empirically without undue difficulty, and once ascertained can be fixed for production. It is possible that the radiant tube may be produced incorporating adjustment means for varying the distance between the air shroud and burner as required by the user, but it is envisaged that normally the desirable distance will be ascertained empirically by workshop or laboratory testing of prototypes and then be fixed accordingly during production.
The radiant tube may include a recuperator. Thus there may be an air passage by way of which the combustion air passes from the air supply means to the flame tube, and to which heat is imparted to pre-heat the air for combustion from exhaust gases passing through an exhaust gas passage which is connected to the annular exhaust gases passage defined between the flame tube and the outer tube. In the preferred embodiment the flame tube extends rearwards past the burner and serves as part of the recuperator to separate the exhaust gases passage from the air passage. The air shroud previously mentioned may conveniently be tubular and be disposed in and spaced from the rearward extension of the flame tube such that the air passage is defined at least in part by the space between the air shroud and the rearward extension of the flame tube. The air passage may be defined by other parts.
The present invention is applicable to radiant tubes having flame tubes of a range of diameters. The advantages of the invention are especially noteworthy, however, when the flame tube is of small outside diameter for example 4.5" (11.43 cm) or less.
An embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which:

Figure 1 is a longitudinal axial section through a single-ended recuperative radiant tube in accordance with the invention;
Figure 2 is a similar section showing a modification, and
Figures 3 and 4 are enlarged views respectively of a burner and associated cowl.

Referring to Figure 1 of the drawings, the radiant tube comprises an outer tube l, a co-axial flame tube 2 inside and extending for a substantial part of the length of the outer tube, a tubular air shroud 3 extending co-axially within a rear section of the flame tube 2, and a burner tube 4 extending co-axially through the air shroud 3 to a burner 5 spaced in front of the air shroud within the flame tube.
The outer tube 1 has a closed forward end 6 and an externally flanged rearward end 7 by which an exhaust housing 8 is fixed co-axially at one end, at an external annular flange 9, to the outer tube. An exhaust gas outlet 10 opens radially from the exhaust housing 8. At an opposite end the exhaust housing 8 has fixed to it, at a further external annular flange 11, a burner inlet housing 12 having a corresponding external annular flange 13 at one end. The opposite end of the burner inlet housing 12 is closed by a radial end wall 14 axially through which extends, and to which is fixed, the burner tube 4. The air shroud 3 is also fixed, at its rear end, to the end wall 14 of the burner inlet housing. A combustion air inlet 15 opens radially into the burner inlet housing 12. At its rear end the flame tube has an external annular flange 16 which is clamped between the adjacent annular flanges 11 and 13 of the exhaust housing 8 and the burner inlet .housing respectively, thereby to secure the flame tube in position with respect to the exhaust housing and the outer tube 1. The forward, discharge, end of the flame tube is open.
An air passage 17 defined by the annular space between the air shroud and the rear section of the flame tube extends forwards from the burner inlet housing 12 towards the burner 5. An exhaust gas passage 18 is defined by the annular space between the outer tube and the flame tube and extends rearwards from the forward end of the outer tube to the exhaust housing 8. The annular flange 16 of the flame tube separates the exhaust gas passage from the interior of the burner inlet housing. The part of the radiant tube from the rearward end of the exhaust housing 8 to the burner 5 constitutes a recuperator in which combustion air passing along the passage 17 to the burner is heated by the exhaust gases passing along the exhaust gas passage 18 to the exhaust gas outlet 10.
The burner 5 has a cylindrical body from the circumferential surface of which project several, equi-angularly spaced, vanes 19 of air screw form designed to induce swirling movement of air passing between them about the axis of the flame tube. The vanes 19 extend to the internal surface of the flame tube. In front of the vanes 19 the burner has a ring of gas ports 20 which communicate with the bore of the burner tube 4 and open radially through the circumferential surface of the burner's body.
A spark rod 27 of known kind provides the means of ignition for the burner. The spark rod is secured to the end wall 14 of the burner inlet housing, extends through the air shroud 3, through one of the vanes 19 of the burner, which supports the'forward end of the spark rod, and terminates just in front of the gas ports 20.
In use of the radiant tube gas supplied to the burner by the burner tube 4 is discharged radially into the flame tube from the gas ports 20. Combustion air supplied to the burner inlet housing 12 at the air inlet 15 flows forwards through the air passage 17 and then between the vanes 19 of the burner which cause the air to swirl before it meets the gas discharging from the gas ports. The distance of the burner from the forward end of the air shroud is set to suit the length and diameter of the flame tube and the gas flow to the burner, and is such that when the air meets the vanes it is converging towards the axis of the flame tube. The swirling of the air caused by the vanes produces a vortex in front of the burner which encourages the gas to flow centrally along the flame tube at least next to the burner. In consequence on combustion of the gas/air mixture the flame is contained centrally of the flame tube adjacent to the burner and so impingement of the flame on the part of the flame tube adjacent to the burner, where combustion is most violent, is prevented or substantially reduced. Heat transfer to the flame tube is also reduced by the envelope of air still to be used for combustion surrounding the flame adjacent to the burner. Impingement of the flame on the flame tube further along the flame tube is acceptable.
Reference will now be made to Figures 2, 3 and 4 of I the drawings in which parts similar to those of the radiant tube described above are identified by the same reference numerals. Except for the modification which relates to the burner 5, the radiant tube is in accordance with that described above.
The burner 5 has equi-angularly spaced vanes 19 of air screw form projecting from its cylindrical body behind a ring of radial gas ports 20, Figure 3, communicating with an axial blind bore 21 with which the bore of the burner tube 4 connects. However, the vanes 19 are radially shorter than before and do not extend to the internal surface of the flame tube 2. Instead they extend to the internal surface of the bore 22 of a tubular, circular section, cowl 23 which extends co-axially over the burner. A reduced diameter portion 24 of the bore 22, Figure 4, presents a shoulder 25 against which the vanes 19 abut at their rear edges adjacent their outer ends to locate the burner correctly in the cowl. The outer ends of the vanes are secured to the cowl. The external diameter of the cowl 23 is smaller than the internal diameter of the flame tube, and the cowl is positioned co-axially with the flame tube so that an annular passage 26 is defined between the cowl and the wall of the flame tube. As shown in Figure 2 the external diameter of the cowl is similar to that of the air shroud, but that is not essential. The cowl is longer than the burner which is positioned so that it is fully contained within the cowl.
The vanes could possibly be formed integrally with the cowl, rather than with the burner, and extend to the burner.
The cowl is spaced axially from the discharge end of the air shroud 3. It serves to separate the air issuing from the air shroud in use of the radiant tube into two streams. One, primary, stream passes through the cowl and is caused by the vanes to swirl before it meets and mixes with the gas discharging from the gas ports 20 for primary combustion, and the other, secondary, stream passes through the annular passage 26. The swirling movement imparted by the primary stream to the gas/air mixture emerging from the cowl, contains the flame centrally of the flame tube adjacent to the burner. The secondary stream of air forms an envelope around the flame adjacent to the burner which, as before, reduces the heat transfer to the flame tube in that region. It will be appreciated that in this case the envelope is more clearly defined than in the above-described embodiment. Further along the flame tube secondary stream provides secondary air for combustion.
As in the above-described embodiment, the distance of the burner, with the cowl, from the discharge end of the air shroud can be adjusted to suit the gas flow and to vary the length of the flame according to the length of the flame tube.
In a typical example of the modification the cowl 23 projects forwards from the burner by a distance of about 0.375" (9.5 mm).
Typically with and without the modification the flame tube of the radiant tube has an outside diameter of 3.25" (8.255 cm).

Claims

1. A single-ended radiant tube comprising an outer tube (1) closed at a first end and connected, or adapted to be connected, at a second end to exhaust means (10), a gas burner (5) at or near the second end and connected to gas supply means (4), combustion air supply means (12) at the second end by which air is supplied to be mixed with gas discharged from the burner (5) for combustion, a flame tube (2) in and spaced from the outer tube (1) extending from the burner (5) towards the first end, in which flame tube (2) the flame burns on combustion of the gas/air mixture, and an annular exhaust gases passage (18) being defined between the flame tube (2) and the outer tube (1) which communicates with the interior of the flame tube (2) at the first end and along which exhaust gases pass to the exhaust means (10), characterised in that swirl inducing means (19) is provided adapted to cause combustion air supplied to the flame tube (2) to swirl and produce a vortex in front of the burner (5) which encourages the gas to flow, and the flame to extend, centrally of the flame tube (2) adjacent to the burner (5), and to cause the flame adjacent to the burner (5) to be surrounded by air which has still to be used for combustion, thereby to reduce heat transfer from the flame to the flame tube (2) in that region of the flame tube (2).

2. A single-ended radiant tube according to claim 1 characterised in that the swirl inducing means comprises vanes (19) disposed about the burner (5).

3. A single-ended radiant tube according to claim 2 characterised in that the burner (5) has a cylindrical body and the vanes (19) project from and are equi-angularly spaced about the circumference of the body.

4. A single-ended radiant tube according to claim 2 or claim 3 characterised in that the burner (5) has a ring of gas ports (20) disposed forwardly of the vanes (19) and directed away from the central axis of the flame tube (2) into the flow paths of combustion air passing between the vanes (19).

5. A single-ended radiant tube according to any of claims 2 to 4 characterised in that the vanes (19) extend fully across an entry for the combustion air into the flame tube (2).

6. A single-ended radiant tube according to any of claims 2 to 4 characterised in that the vanes (19) are contained within a tubular cowl (23) which extends over and is spaced from the burner (5) and around which an annular passage (26) for air is defined opening into the flame tube (2), the arrangement being such that the cowl (23) divides air supplied by the combustion air supply means (12) into a primary air stream which passes through the cowl (23) between the vanes (19) and is caused to swirl and mixes with the gas discharged from the burner (5) for combustion, and a secondary air stream which passes through the annular passage (26) into the flame tube (2) to provide an envelope of air which surrounds the flame adjacent to the burner (5).

7. A single-ended radiant tube according to any preceding claim characterised in that an air shroud (3) is positioned rearwardly of the burner (5) over which the combustion air passes to the swirl inducing means (19), and the burner (5) and the. air shroud (3) are arranged such that the air passing from the air shroud (3) is caused to converge towards the central axis of the flame tube (2) as it approaches the swirl inducing means (19).

8. A single-ended radiant tube according to claim 7 characterised in that the arrangement of the burner (5) and the air shroud (3) is such that increasing the distance between the air shroud (3) and the burner (5) causes the combustion air to pass through the swirl inducing means (19) closer to the central axis of the flame tube (2) and increases the velocity at which it meets the gas discharged from the burner (5), thereby inducing faster mixing of the air and gas and reducing the length of the flame.

9. A single-ended radiant tube according to claim 7 or claim 8 characterised in that the flame tube (2) extends rearwards past the burner (5) and forms part of a recuperator, the air shroud (3) is tubular and is disposed in and spaced from the rearward extension of the flame tube (2), an air passage (17) is defined at least in part by the space between the air shroud (3) and the rearward extension, by way of which air passage (17) the combustion air passes from the air supply means (12) to the flame tube (2), and an exhaust gases passage is defined around the rearward extension and connects with the annular passage (18) to receive exhaust gases whereby heat is imparted to the air passage (17) to pre-heat the air for combustion.

10. A-single-ended radiant tube according to claim 9 characterised in that the exhaust means comprises an exhaust housing (8) having an exhaust outlet (10) and to which the second end of the outer tube (1) and a rear end of the flame tube (2) are connected and into which the annular exhaust gases passage (18) opens, and the combustion air supply means comprises a housing (12) which has an air inlet (15), is connected to the rear end of the flame tube (2) and to the air shroud (3), and with which the air passage (17) communicates.

11. A single-ended radiant tube according to any preceding claim characterised in that the flame tube (2) has an outside diameter of up to 4.5" (11.43 cm).

12. A single-ended radiant tube according to claim 11 characterised in that the flame tube (2) has an outside diameter of 3.25" (8.255 cm).