GB2294535A

GB2294535A - Flame trap electrically heated for regeneration

Info

Publication number: GB2294535A
Application number: GB9513243A
Authority: GB
Inventors: Michael John Davies; Norman Jorgensen
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1994-07-06
Filing date: 1995-06-29
Publication date: 1996-05-01
Anticipated expiration: 2015-06-29
Also published as: GB2294535B; GB9513243D0

Abstract

A flame trap (10) for use in the exhaust system of a device in which combustion occurs, such as a diesel engine, consists of metal sheets (16, 18) forming a honeycomb-like structure with many parallel flow channels (20) through it. The flow channels (20) are less than 5mm wide. The sheets (16, 18) are connected to electrode terminals (24) so they can be heated electrically to about 600 DEG C at intervals, so as to burn off any carbon particulates which settle on them. The metal sheets are corrosion resistant at temperatures of up to 600 DEG C. Two flame traps may be used in an exhaust system. The two flame traps may be connected in parallel and valves (36, 38, figure 3) may be provided to isolate the flame traps from the outlet during regeneration. <IMAGE>

Description

Flame Trao This invention relates to a flame trap for use in the exhaust from a device in which combustion occurs1 such as an internal combustion engine, and to such a device incorporating such a flame trap.

The use of flame traps in the air inlet and the exhaust of internal combustion engines which are to be operated in an environment in which the atmosphere may be inflammable is known. Such flame traps typically consist of a stack or array of flat metal plates defining narrow channels between them; any burning gases or incandescent particles are rapidly cooled and extinguished as they pass through these channels. Flame traps used in the exhaust typically experience a gradual build-up of deposited carbon particles (even though they are not intended to trap such particles) and so must either be cleaned or replaced at intervals.

According to the present invention there is provided a flame trap for use in a duct carrying exhaust gases from a device in which combustion occurs, the flame trap comprising metal sheets arranged to form a honeycomb-like structure defining a multiplicity of flow channels therethrough each of width no more than 5 mm, the metal being resistant to corrosion at temperatures up to at least 6000C, and the trap also including electrical means whereby it may be heated to a sufficient temperature to cause combustion of any carbon particles deposited thereon.

Preferably the channels are of width no more than 3 mm, for example 1 mm or 2 mm. The metal is preferably an iron/chromium/aluminium alloy, in particular an aluminium-bearing ferritic steel containing chromium and yttrium (of composition: carbon not more than 0.03%; silicon 0.2-0.4%; chromium 15-22%; aluminium 4.0-5.2%; and yttrium 0.05-0.4%) as such steels have very good oxidation resistance at high temperatures, to 12000C in air and up to at least 9500C in exhaust gases. Such steels are available under the trade mark FECRALLOY from Resistalloy Ltd.

In order for ignition to occur the particles must be heated to about 550-6000C, which is well above the normal operating temperature of an exhaust gas stream, which is typically no more than about 2000C. If the flame trap is electrically heated with the normal flow of exhaust gas through it then the exhaust gas will also have to be heated and this will require considerable power consumption. Alternatively the flame trap may be regenerated while the engine is off so there is no flow of exhaust gas or with a much reduced air flow through a bleed valve.

During the electrical heating and regeneration of the flame trap it clearly must not be exposed to an inflammable atmosphere. This may be ensured by carrying out this regeneration in clean air, or alternatively by providing two such flame traps through which the gas flows in succession, and regenerating the flame trap further from the inflammable atmosphere.

The present invention also encompasses the use of such a flame trap in a duct carrying exhaust gases; and also a device in which combustion occurs incorporating such a flame trap in a duct carrying its exhaust gases.

The invention will now be further described by way of example only and with reference to the accompanying drawings in which: Figure 1 shows an axial sectional view, partly broken away, of a flame trap; Figure 2 shows a view in the direction of arrow A of part of the flame trap of Figure 1; and Figure 3 shows a longitudinal sectional view of an exhaust system incorporating two flame traps of Figure 1.

Referring to Figure 1 a flame trap 10 is of cylindrical shape and is located within a tubular section of a duct 12 carrying exhaust gases from a diesel engine (not shown). The flame trap 10 is of a ferritic steel containing chromium, aluminium and yttrium (of weight proportions 15%, 5%, and 0.4% respectively). It comprises an electrode rod 14 on the axis of the trap 10, and two 0.3 mm thick sheets 16, 18, one of which 16 is initially flat and the other of which 18 is corrugated into sinusoidal corrugations of wavelength 2 mm and height (from trough to peak) of 2 mm. Referring also to Figure 2, the two sheets 16, 18 are wound together around the rod 14 so as to follow spiral paths; successive turns of the flat sheet 16 are thus 2 mm apart, and the gaps between the sheets 16, 18 define a large number of flow channels 20 parallel to the longitudinal axis, and each about 2 mm wide.The outermost turn of the sheets 16, 18 is connected to an electrode strip 22. Electrical connections 24 enable the electrode rod 14 and the electrode strip 22 to be connected to a power supply (not shown).

In use the exhaust gases flow through the flame trap 10 which ensures no flames or incandescent particles can reach the surrounding atmosphere. The flame trap 10 does not create a large pressure drop because of the large number of flow channels 20. However after prolonged use the channels 20 will become partly blocked by deposited carbon particles. This may be detected by observing the pressure drop across the flame trap 10. At this stage the diesel engine is removed from the possiblyinflammable atmosphere, the power supply is connected to the connections 24 so that the flame trap 10 is heated up to 6000C (which is dull-red hot). The carbon particles ignite and burn off. The power supply is disconnected, and normal operation of the diesel engine can recommence.

Referring to Figure 3 there is shown a sectional view of an alternative exhaust system 30 which incorporates two flame traps 10 exactly as described above and which avoids the need to remove the system 30 from the possibly-flammable atmosphere before regenerating the flame trap 10. The system 30 forms part of an exhaust duct 32 through which the exhaust gases flow in the direction of the arrows S. The system 30 includes two flame traps 10 in parallel ducts 34, 35, an inlet control valve 36, and an outlet control valve 38, each control valve 36,38 consisting of an apertured valve plate 40, 41 slidable (in the plane of the Figure) between a pair of apertured side plates 42, 43.The inlet portion of the exhaust duct 32 branches to form two parallel ducts before reaching the inlet control valve 36; the ducts 34, 35 each branch into two ducts 44, 45 communicating with separate apertures in the inlet control valve side plates 42; the outlet ends of the ducts 34, 35 join to form a single outlet duct leading to the outlet control valve 38.

The valve plates 40, 41 are shown in their normal operational positions, in which the exhaust gases flow through two apertures 46 in the inlet valve plate 40, through the two ducts 44 to the flame traps 10 in the parallel ducts 34, 35, and through an aperture 48 in the outlet valve plate 41, as indicated by the arrows S. The two ducts 45 are closed off by the valve plate 40.

When it is desired to regenerate one of the flame traps 10, the valve plates 40 and 41 are both slid sideways. For example to regenerate the one in the duct 35, the two valve plates 40 and 41 are slid in the direction of the arrow X. This closes the outlet valve 38. The apertures 46 in the inlet valve plate 40 are offset, so that the flow path for the exhaust gas into the duct 35 (via duct 44) remains open while that into the duct 34 (via duct 44) is closed; at the same time the duct 45 is opened so the duct 34 communicates with the atmosphere. The exhaust gas now flows through the two flame traps 10 in series, first through the duct 35 and then the duct 34. A power supply 50 is then connected to the flame trap 10 in duct 35 to regenerate it.

After regeneration, the valve plates 40, 41 are slid back to the normal operational positions. If instead the flame trap 10 in the duct 34 were to be regenerated, the valve plates 40, 41 would be slid in the opposite direction (arrows Y) so the flow was through the duct 34 and then the duct 35 in series. Thus whenever one flame trap 10 is to be regenerated it is always separated from the atmosphere by the other flame trap 10.

It will be appreciated that an exhaust system may differ from that described above while remaining within the scope of the invention. In particular the inlet valve and outlet valve mechanisms might be replaced by other types of valve, interlinked so as to change the gas flows from parallel to series. Alternatively an exhaust gas system might use just one flame trap 10 during normal operation, but include a second flame trap 10 which is arranged in series with (and downstream of) the first when the first is to be regenerated.

Claims

1. A flame trap for use in a duct carrying exhaust gases from a device in which combustion occurs, the flame trap comprising metal sheets arranged to form a honeycomb-like structure defining a multiplicity of flow channels therethrough each of width no more than 5 mm, the metal being resistant to corrosion at temperatures up to at least 6000C, and the trap also including electrical means whereby it may be heated to a sufficient temperature to cause combustion of any carbon particles deposited thereon.

2. A flame trap as claimed in Claim 1 wherein the channels are of width no more than 3 mm.

3. A flame trap as claimed in Claim 1 or Claim 2 wherein the metal is an iron/chromium/aluminium alloy.

4. A flame trap as claimed in Claim 3 wherein the alloy is an aluminium-bearing ferritic steel containing chromium and yttrium of composition: carbon not more than 0.03%; silicon 02-0.4%; chromium 15-22%; aluminium 4.05.2%; and yttrium 0.05-0.4%.

5. An exhaust duct system for a device in which combustion occurs, including at least two flame traps as claimed in any one of the preceding Claims, and valve means to ensure that when one flame trap is to be electrically heated another, unheated, flame trap is downstream of it, in the flow path of the exhaust gases.

6. A flame trap substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.

7. An exhaust duct system substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.