GB2197938A

GB2197938A - Creation of flame fronts at charging openings of heat-treatment furnaces

Info

Publication number: GB2197938A
Application number: GB08725906A
Authority: GB
Inventors: Cornelius H Luiten; Werner Gohring
Original assignee: Ipsen International GmbH
Current assignee: Ipsen International GmbH
Priority date: 1986-11-26
Filing date: 1987-11-05
Publication date: 1988-06-02
Also published as: FR2607233B1; GB8725906D0; DE3640324A1; FR2607233A1; ES2008312A6

Abstract

A flame front (8) is created at an opening (4) of a charging duct (3) of a heat-treatment furnace (2), through which opening workpieces are inserted into and/or removed from a furnace chamber, which has a combustible protective atmosphere. The flame curtain, which has been used until now, is replaced by a steep flame front which is substantially perpendicular to the axis of the duct and which separates the protective atmosphere in the furnace chamber from the ambient air and by means of which ambient air or burnt gases can be prevented from entering the charging duct and/or furnace chamber. The flame front is created using a substantially reduced gas volume outflow from the furnace by feeding a comparatively cold non-combustible gas or gas mixture to the flame front via a pipe (6). <IMAGE>

Description

CREATION OF FLAME FRONTS AT CHARGING OPENINGS OF HEAT-TREATMENT FURNACES The present invention relates to a method and apparatus for creating a flame front at a charging opening of a heat-treatment furnace, through which opening workpieces are inserted into and/or removed from a furnace chamber, which has a combustible protective atmosphere. The invention also relates to the application of the method to a certain type of furnace.

The term "charging opening" refers to any opening for either or both charging and emptying the furnace, and the charging operations can take place either through one or different door openings.

Heat-treatment furnaces which are operated with a combustible protective gas atmosphere have an almost horizontal charging duct with an opening through which workpieces can be inserted into or removed from the furnace chamber. In order to do this the furnace door has to be opened, thereby enabling the combustible protective gas to come into contact with the external atmosphere. The gas will therefore normally ignite at the moment the door is opened. In many cases an ignition source is provided at the charging opening for safety reasons, so as to create a stable flame front which visibly separates the protective atmosphere in the furnace chamber from the ambient air.It is necessary to keep the volume of the gas flow entering the charging duct from the furnace chamber at a level which prevents the flame front from entering the furnace chamber of the heat-treatment furnace at the internal end of the charging duct.

The burning flame front in the charging duct raises the temperature of the emerging gas mixture. This causes the latter to become lighter, to move into the upper area of the duct cross section and draw in cold ambient air in the lower area of the duct cross section. The undesirable result is that the flame front in the lower area of the charging opening is drawn a considerable distance into the charging duct in the direction of the furnace chamber. The flame front which, in order to be effective, should extend in front of the opening, as nearly perpendicular to the axis of the opening as possible, takes up an unfavourable flat path as it extends far into the charging duct. In order to prevent the flame front from entering the actual furnace chamber, a relatively large outwards gas volume flow has to be maintained in the charging duct.

It has been proposed to use flame curtains to reduce the gas volume flow required in the charging duct. For this purpose a number of flames are produced at the charging opening by using a pipe carrying fuel gas which pipe is provided with a plurality of jets. The fuel gas is burned at the lower, downstream end of the charging duct. The flames or hot gases which are thus produced flow across the end of the charging duct in a plane perpendicular to the axis of the duct. However this entails the disadvantage that, due to their high temperature and the turbulence which this creates, the curtain cannot prevent the cold ambient air from entering the charging duct through the opening to any appreciable extent.The flame front also extends axially into the charging duct, which means that the danger ramins of air entering the furnace chamber, which is filled with a protective atmosphere, if the furnace chamber door is kept open for too long or there is an unsuitable geometric door opening cross section:shaft depth ratio. Apart from the additional, undesirable consumption of the atmosphere, another disadvantage lies in the fact that the workpieces in the furnace chamber which are to be treated may be damaged. For example, the workpieces may come into contact with waste gases due to the circulation in the furnace chamber. These gases cause oxidation and discoloration of the workpiece surface or other defects to the surface of the workpieces in the batch.

On the basis of this prior art, the object of the invention is to render the flame curtain, which has been used until now, unnecessary and create a steep, essentially perpendicular flame front which separates the protective atmosphere in the furnace chamber from the ambient air and by means of which ambient air or burnt gases can be prevented from entering the charging duct and/or furnace chamber with a substantially reduced gas volume outflow.

The present invention accordingly provides a method for creating a flame front at a charging opening of a heat-treatment furnace, through which opening workpieces are inserted into and/or removed from a furnace chamber, which has a combustible protective atmosphere, comprising feeding to the flame front a comparatively cold gas or gas mixture which plays no part in any combustion process.

Using such a method, the non-combustible gas initially flows into the charging duct instead of the ambient air and extinguishes the flame front in the duct. The gas is ignited again in the lower area of the charging opening outside of the charging duct, which the ambient air can freely enter. The unsuitable flat flow is isolated by extinguishing the flame front inside the charging duct. The flame front which forms outside of the charging duct when the method according to the invention is applied is substantially steeper. It is possible, by forming a steep flame front of this kind, to prevent air or burnt gases from entering the furnace chamber with significantly smaller gas volume outflow in the charging duct.

The gas or gas mixture is preferably supplied to a vertical charging opening from the bottom towards the top. It is advisable for the gas or gas mixture to be at ambient temperature.

The gases or gas mixtures which are used and which do not take part in a combustion process are preferably nitrogen and/or cooled burnt gases, preferably from the furnace chamber heating device, or all these gases mixed together and/or with another non-combustible gases.

According to the invention, the apparatus for carrying out the method described above comprises a pipe which is arranged at the charging opening or in the area of the lower, downstream end of the charging duct and comprises jets for delivering a cold, non-combustible gas or gas mixture.

The pipe is preferably connected via a cooling device to a burnt gas discharge pipe of a furnace chamber heating device. It is advantageous if the pipe is provided with closely arranged jets over the width of the charging opening, so as to create a curtain of non-combustible gases at location.

The method and apparatus according to the invention can be applied to all furnaces whose cross section is open all the time or for a brief period and whose burning atmosphere has to prevent air from entering the furnace chamber. These are, for example chamber furnaces, continuous pusher type furnaces and rotary-hearth furnaces. The invention is particularly important in hardening press furnaces, in which a plurality of workpieces are heated in batches and then individually removed and quenched in a hardening press. This operation is carried out through the flame wall partly by hand and partly using robots. For practical reasons the charging duct must be very short. This means that it is very difficult to prevent air or burnt gases from entering the furnace chamber so that the furnace atmosphere does not have a decarburization effect on the workpieces which are subsequently to be hardened. It is basically only possible to maintain a non-decarburizing furnace atmosphere in furnaces of this kind when the door is opened very frequently if the flame front in the short charging duct has an optimum shape.

A method and apparatus embodying the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings in which: Figure 1 is a diagrammatic sectional view of a furnace with a charging duct and a flame front according to the prior art; and Figure 2 is a view corresponding to Figure 1 but with the flame front formed according to the invention.

The furnace 1 shown in the drawing comprises a furnace chamber 2 with a heating device, which is not shown. A protective atmosphere of combustible gases, which are circulated during the heat-treatmet, is maintained in this chamber. Access to the furnace chamber for charging and/or removal purposes is through a charging duct 3, which extends horizontally at one side of the furnace. The charging duct 3 can be closed by a door, which is not shown and which may uncover or close a charging opening 4.

A pipe 5, in which a plurality of jets are provided over the width of the charging opening, is arranged at the charging opening 4 at the lower external end of the charging duct 3. The pipe 5 in Figure 1 of the drawing is provided for producing a flame curtain according to the prior art. For this purpose a fuel gas is burned at the lower, downstream end of the charging duct and the resultant flames or their hot gases flow across the charging opening 4 in the vertical direction. A thermal current causes a flame curtain according to the drawing to form which has a flat internal front 7 which extends quite a distance into the charging duct 3 in the lower area. The disadvantages of a flame front of this kind have already been described.

In Figure 2 the pipe 5 is replaced by a pipe 6, which is also provided with a plurality of jets over the width of the charging opening. However this pipe 6 is not formed or provided to produce a flame curtain, but rather to deliver a cold gas or gas mixture which does not take part in a combustion process, to the flame front 8 in the area of the lower, downstream end of the charging duct 3 at the charging opening. The pipe 6 is connected via a cooling device to the discharge pipe for the burnt gases from the heating device of the furnace chamber 2.

the cooling device may already be provided in the furnace as a device for utilizing the waste heat, for example in the form of a recuperator.

The drawing shows how, by delivering a cold gas which does not take part in the combustion process, the gases sweep over the charging opening 4 in the manner of a curtain, as a result of which only an extremely steep flame front can form in the area outside the charging duct 3. The flame front is no longer flat, but rather extends esssentially in the vertical direction. The penetration of air or burnt gases into the furnace chamber 2 can be prevented with gas volume outflow in the charging duct which is substantially smaller than that which has previously been necessary.

Claims

1. A method for creating a flame front at a charging opening of a heat-treatment furnace, through which opening workpieces are inserted into and/or removed from a furnace chamber, which has a combustible protective atmosphere, comprising feeding to the flame front a comparatively cold gas or gas mixture which plays no part in any combustion process.

2. A method according to claim 1, wherein the gas or gas mixture is supplied to a vertical charging opening from the bottom towards the top.

3. A method according to claim 2, wherein the gas or gas mixture is at ambient temperature.

4. A method according to any one of the preceding claims, wherein the gas or gas mixture which is used is nitrogen and carbon dioxide and/or cooled burnt gases, or some or all of these gases mixed together and/or with other non-combustible gases.

5. A method for creating a flame front substantially as herein described with reference to the accompanying drawings.

6. Apparatus for carrying out the method according to any one of the preceding claims, a pipe which is arranged at the charging opening or in the area of the lower, downstream end of the charging duct and comprises jets for delivering a cold, non-combustible gas or gas mixture.

7. Appartus according to claim 6, wherein the pipe is connected via a cooling device to a burnt gas discharge pipe of a furnace chamber heating device.

8. Appartus according to claim 6 or 7, wherein the pipe is provided with closely arranged jets in the area of the charging opening.

9. Apparatus for creating a flame front substantially as herein described with reference to the accompanying drawings.

10. Use of the method according to any one of claims 1 to 5 in furnaces which have an open cross section and in which the burning furnace atmosphere prevents air from entering the furnace chamber, i.e. chamber furnaces, continuous pusher-type furnaces, rotary-hearth furnaces and, in particular, hardening press furnaces.