GB2198507A

GB2198507A - Stock heating furnaces

Info

Publication number: GB2198507A
Application number: GB08629427A
Authority: GB
Inventors: Neil Fricker; Kevin Fred Pomfret
Original assignee: British Gas PLC
Current assignee: British Gas PLC
Priority date: 1986-12-09
Filing date: 1986-12-09
Publication date: 1988-06-15
Also published as: GB8629427D0

Abstract

A furnace for rapid heating of stock such as billets, comprises a tunnel burner assembly having at least one burner and a heating chamber 1 of at least part annular form, support means for supporting the stock, and means for causing relative rotary movement in opposite directions between the tunnel burner and the support means, wherein the heating chamber and at the least one burner are adapted and arranged such that in use said at least one burner emits a stream of burned gas which moves through the heating chamber in a direction counter to the direction of relative movement of the stock thereby to effect transfer of heat predominantly by convection through the heating chamber to the stock. <IMAGE>

Description

"STOCK HEATING FURNACES" The present invention relates to furnaces, and more particularly to continuous furnaces.

Pre-cut metal stock, for example short billets or bars, often need to be reheated before undergoing forming operations such as forging, pressing or extrusion. These operations may be carried out by using fossil fuel fired batch furnaces, or continuous furnaces such as rotary hearth furnaces, all of which depend overwhelmingly for their heating effect on thermal radiation from the combustion gases both to the furnace walls and to the stock. The furnace walls are heated mainly by hot gas radiation whereas the stock is heated by a combination of wall radiation and hot gas radiation. In order to achieve acceptable heating rates, these furnaces have been designed with large working chambers and therefore use large quantities of refractory material which have substantial thermal inertias.This leads to difficulties in controlling stock temperature, and to scaling and decarburisation problems, particularly for steel stock. In an attempt to alleviate such problems the furnaces needed to be kept hot irrespective of whether or not stock is present in the furnace, resulting in significant thermal inefficiency.

In the past, thermal efficiency has been improved by employing a continuous counterflow method whereby the combustion gas flow is counterflow with respect to the direction of stock movement through the furnace thereby preheating the stock before exhausting the waste combustion gases to a flue. However, such a system has a lower specific output (eg. weight of stock heated per unit hearth area per unit) than a non-counterflow furnace, and in order to save space and reduce capital costs, it has been practice to fire such furnaces with a plurality of burners along substantially the whole length of the furnace. Although this has the desired effect of increasing the output from a given furnace hearth, it reduces the effectiveness of the stock preheat zone.

An object of the invention is to alleviate or substantially eliminate at least some of the problems associated with knwon furnaces.

To this end the present invention provides a furnace for rapid heating of stock such as billets, comprising a tunnel burner assembly having at least one burner and a heating chamber of at least part annular form, support means for supporting the stock, and means for causing relative rotary movement in opposite directions between the tunnel burner and the support means, wherein the heating chamber and at the least one burner are adapted and arranged such that in use said at least one burner emits a stream of burned gas which moves through the heating chamber in a direction counter to the direction of relative movement of the stock transfer thereby to effect transfer of heat predominantly by convection through the heating chamber to the stock.

In known burners the tunnel dimensions are chosen such that the burner can emit a high-velocity stream of burned gas at the tunnel exit.

Burners of this kind have generally been termed in the art as "high-velocity tunnel burners" or "jet burners" or "high-intensity combustors".

Compared with known furnaces the inner widthwise cross-sectional areas or the inner lateral dimensions of the heating chamber of tunnel burner furnaces maybe sufficiently small to enable a relatively high combustion gas velocity within the chamber to be obtained to limit mixing between hot gases from the burner or burners and cooler gases within the furnace; and to reduce the transmission of heat by radiation from the "hot" and (or burner gas discharge end) of the furnace to the other end. As a result heat transfer occurs in the heating chamber predominantly by convection and temperature differentials along the length of the heating chamber are reduced.These advantages together with the lower thermal inertia which accompanies a furnace having a reduced mass and the promotion of convective heating also leads to better control of stock temperature and significant reductions in scale and decarburisation of the stock.

Preferably, the heating chamber is of complete circular or toroidal form so that the burned gas can circulate and recirculate around the heating chamber, thereby increasing the velocity of the gases around the heating chamber and further promoting convective heat transfer while at the same time reducing gas temperatures. This arrangement is particularly suitable for the re-heating of metals having a relatively low surface emissivity and a relatively low melting point, such as aluminium.

The recirculation may be produced by the jet or stream of hot gases leaving the burner, or by using combustion air supplied to the burner to induce gases from the charging zone.

The means for supporting and moving the stock through the heating chamber may be a rotary hearth.

In one embodiment the heating chamber may have a stock charging station via which stock is loaded into the furnace and onto the support means and a separate stock discharging station via which stock is removed from the support means and the furnace.

In order that the invention may be more readily understood, reference will now be made by way of example, to the accompanying drawings, in which : Figure 1 is a view in elevation of one embodiment of a furnace according to the invention, Figure 2 is a top plan view of the arrangement shown in Figure 1; Figure 3 is a sectional view on the lines III-III in Figure 2.

Referring to figures 1 to 3 of the drawings the furnace has a circular or toroidal heating chamber 1 defined by cast refractory circular inner 2 and outer 3 side walls respectively; the upper support surface of a rotary hearth; and a ceramic fibre roof 4 which bridges the tops of the side walls. The roof and side walls are supported on a frame 5 having inner and outer side members connected to a roof member. The frame supports the roof and side walls and provides part of water seals 7 and 8 which extend around the heating chamber between the rotary hearth and the inner and outer side walls. The roof includes an opening 9 which communicates with a flue for venting burned gases and another opening 10 which serves to vent exhaust gases from tunnel burner (not shown).

The tunnel burner is mounted above the chamber and is arranged to fire downwardly, and at an angle, to initiate high combustion gas velocity within the heating chamber. The outer side wall is provided with charging door 11, disposed generally beneath the vlue, via which stock can be introduced into the heating chamber, and a discharging door 12 via which stock can be removed from the furnace.

The rotary hearth 13 includes a support frame with a circular drive tract 15 to enable the hearth to rotate (whilst supported on bearing 16).

The angle through which stock loaded via charging door travels to the discharging door is about 3300.

By way of example, the inside width of the heating chamber between the inner surfaces of the side walls may be 0.4m and the height between the stock supporting surface of the rotary hearth and the underside of the roof may be 0.15m. The outside diameter of the inner side wall may be 1.8m whilst the inside diameter of outer side wall may be 2.6m. The size of the billets which for example can be used with a furnace of such dimensions may be of the order of 5cm x 5cm x 7.5cm.

The tunnel burner and heating chamber arrangement and dimensions are such that in operation the burner emits into the heating chamber a stream of high velocity burned gas which circulates anti-clockwise (when viewed from the top) around the chamber whilst the rotary hearth transports billets, e.g. aluminium billets, in the opposite (clockwise) direction from the charging door to the discharging door. The stream of burned gas thus moves counterflow to the direction of movement of the billets which are heated predominantly by convection via the circulating gases.

The circulation of hot gases allows the temperature differential between the load charging and discharging regions to be reduced and also reduces radiant heat change between "hot" billets and "cold" billets within the heating chamber.

Furnaces in accordance with the invention are particularly suitable for reheating metals such as aluminium which have a low surface emissivity and a relatively low melting point.

Whilst a particular embodiment of the invention has been described above, it will be understood that various modifications may be made without departing from the scope of the invention. For example, contoured refractory surfaces to further promote convective heat may be incorporated to into the furnace. The flue being located near to the burner, whilst the major portion of the circumference is used to form the furnace chamber, allows the furnace to include recuperators to provide preheated air to the burners. This arrangement avoids having long ducts to transport hot air or flue gases from the flue zone to the burner zone.

Claims

1. A furnace for rapid heating of stock, comprising a tunnel burner assembly having at least one burner and a heating chamber of at least part annular form, support means for supporting the stock, and means for causing relative rotary movement in opposite directions between the tunnel burner and the support means, wherein the heating chamber and at the least one burner are adapted and arranged such that in use the at least one burner emits a stream of burned gas which moves through the heating chamber in a direction counter to the direction of relative movement of the stock thereby to effect transfer of heat predominatly by convection through the heating chamber to the stock.

2. A furnace as claimed in Claim 1 wherein the heating chamber is of circular or toroidal form.

3. A furnace as claimed in Claim 1 or Claim 2 wherein the furnace includes a rotary hearth for supporting and moving the stock through the heating chamber.

4. A furnace as claimed in Claim 1 and substantially as hereinbefore described.