GB2197939A

GB2197939A - Gas heated tank furnaces

Info

Publication number: GB2197939A
Application number: GB08727396A
Authority: GB
Inventors: Rudolf Strohmayer; Werner Ritter; Carl Werle
Original assignee: Didier Werke AG
Current assignee: Didier Werke AG
Priority date: 1986-11-25
Filing date: 1987-11-23
Publication date: 1988-06-02
Also published as: NL8702164A; DE3640213C1; IT1211884B; IT8748543A0; GB8727396D0

Abstract

A gas heated tank or tunnel furnace for firing moulded articles of coarse ceramic material or oxide ceramic material comprises a combustion zone (2) connected at one end to a cooling zone (3). Combustion gas enters the combustion zone (2) in the region of its connection with the cooling zone (3). The gas flow (G) is directed away from the cooling zone (2) towards an inlet (6). Articles to be fired enter the furnace via the inlet and travel in the direction (D) for the length of the combustion and cooling zones. The combustion gases are drawn out of the furnace through a combustion gas outlet (14) adjacent the inlet (6) and connected to an exhaust fan (15). Sudden changes in temperature in the longitudinal direction of the furnace and non-uniform temperature distributions in the cross-section of the furnace are thus minimised in order to avoid cracks in the fired articles. <IMAGE>

Description

GAS HEATED TANK FURNACES The invention relates to gas heated tank furnaces, in particular for firing moulded articles of coarse ceramic material or oxide ceramic material having a firing zone and a cooling zone.

Conventional furnaces, such as tunnel wagon furnaces, for firing moulded articles of coarse ceramic material or oxide ceramic material, operate with throughput times substantially in excess of 20 hours so that crack formation as a consequence of rapid heating and rapid cooling is avoided. Ovens with a crosssection which can accommodate many moulded articles or piles of moulded articles are particularly prone to causing crack formation since the fired articles are not subjected to a uniform temperature over the crosssection of the furnace. This effect could be reduced by a further increase in the throughput time. However, too long a throughput time is also undesirable.

Rapid firing furnaces for the firing of porcelain are known. Experience shows that these cannot be used for the firing of coarse ceramic material or oxide ceramic-material.

DE-PS 2800895 discloses a tunnel furnace with a preheating zone, a combustion zone and a cooling zone.

In this furnace there is disposed a plurality of inlet openings for gas or air as seen in the longitudinal direction. In the regions around the openings nonuniformities in the desirably uniform temperature profile are to be expected which promote crack formation.

It is an object of the invention to provide a gas heated tank furnace in which crack formation in the fired material as a consequence of relatively sudden changes in temperature in the longitudinal direction of the furnace (throughput direction) and irregular temperature distribution across the oven cross-section is avoided and which operates with a comparatively short throughput time.

According to the present invention there is provided a gas heated tank furnace comprising a combustion zone, having an inlet at one end, a cooling zone connected with the other end of the combustion zone, a combustion gas inlet adjacent said other end of the combustion zone and a combustion gas outlet arranged in the region of the inlet to the combustion zone. The combustion gas inlet preferably comprises two ports. There is in any event preferably an even number of such ports and these are preferably arranged in pairs on opposite sides of the combustion zone, i.e.

equispaced around the combustion zone.

In this manner a rising temperature profile in the combustion zone is achieved which is not disturbed by further incoming combustion gas streams, as seen in the longitudinal direction of the furnace. There is also a substantially uniform temperature distribution over the cross-section of the furnace. Thus excessive temperature peaks or other temperature non-uniformities do not occur in any of the articles being fired which would otherwise promote the formation of cracks. The exhausting of the combustion gas flow near to the furnace inlet reinforces the uniform temperature distribution and allows a high throughput of combustion gas.

When the furnace is in use the throughput velocity of the articles is preferably between 0.5m/hour and 1.5m/hour, in particular about 1m/hour. This ensures, one the one hand, that the articles being fired are in the hotest region of the combustion zone for a sufficiently long time and, on the other hand, that the transit time of articles in the furnace is minimised.

In a tank furnace in accordance with the invention a conventional preheating zone in which the articles to be fired are preheated from about 1000C to the temperature in the combustion zone is superfluous.

Associated with the omission of the preheating zone is a consequent shortening of the length of the tank furnace. This shorter length reduces the throughput time still further.

The tank furnace can be constructed with a small furnace cross-section relative to tunnel furnaces without losing its advantages. It is thus possible to fire smaller batches economically and rapidly. The breadth of the articles to be fired can be e.g. 25cm and up to about 75cm.

In a preferred embodiment of the invention an overpressure is formed in the cooling zone by virtue of the pressure of the combustion. gas flow entering the combustion zone, which pressure drops off towards the outlet of the cooling zone. Thus, the temperature in the cooling zone decreases in the direction of movement without temperature peaks occurring in the articles to be fired. Also, by reason of the overpressure, no atmospheric air is sucked in at the furnace outlet or, by reason of any leaks, through the furnace wall in the region of the cooling zone.

The present invention can be put into practice in several ways one of which will now be described by way of example with reference to the accompanying drawing in which the sole Figure is a plan sectional view of a tank furnace in accordance with the present invention.

The tank furnace is intended for firing refractory materials, such as compositions with up to 90% A1203, compositions with 40% to 90% SiC content or compositions with up to 96% Zr02 content at temperatures of up to 16500C.

The tank furnace has a tunnel 1 in which a cooling zone 3 is immediately downstream of a combustion zone 2 in the throughput direction D. The combustion zone 2 is approximately twice as long as the cooling zone 3.

The length of the combustion zone 2 is for example 8m.

The cross-section of the tunnel 1 is approximately rectangular. The maximum breadth of articles which can be fired is about 25cm (with an unobstructed overall furnace aperture breadth B of 37cm). Accordingly, individual bricks up to a weight of about 25kg may be transported one after the other through the furnace.

The tank furnace is provided with a refractory and thermally insulated lining 4,5. The lining 4 in the region of the combustion zone 2 is thicker than the lining 5 in the cooling zone 3, as the amount of insulation required in the cooling zone 3 is smaller than is necessary in the combustion zone 2. At the beginning of the combustion zone 2 is an inlet opening 6 to the tunnel 1. An outlet opening 7 is provided at the end of the cooling zone 3.

The tank furnace has a burner 8,9 on each side of the tunnel 1. A plurality of burners can be distributed on the periphery of the tunnel 1. These burners should, however, not be offset from one another in the longitudinal direction of the tunnel 1.

Connected to the burners 8,9 are combustion gas passages 10,11 which are formed in the lining 4 and which discharge into the combustion zone 2 of the tunnel 1 through ports 12,13 each of which is broadened into the shape of a funnel. The ports 12,13 are laterally opposite one another in the cross-section of the tunnel 1 so that combustion gas enters the tunnel at the same point in the length of the combustion zone 2.

In the vicinity of the inlet opening 6 an exhaust channel 14 which surrounds the tunnel 1 is constructed in the lining 4 to which an exhaust fan 15 is connected.

Combustion air channels 16,17 extend within the lining 4 parallel to the combustion zone 2. If desired, the combustion air can be supplied preheated to the burners 8,9 via these channels.

A feed table 18 is arranged in front of the inlet opening 6. A roller track 19 extends parallel to the tunnel 1 between the inlet opening 6 and the outlet opening 7.

The mode of operation of the described tank furnace is as follows: Byvirtue of the combustion gas passages 10,11 and the construction of their ports 12,13 and the operation of the exhaust fan 15 a combustion gas flow G forms in the combustion zone 2, which flow is directed from the ports 12,13 towards the inlet opening 6 against the throughput direction D. The highest temperature of about 16500C is obtained over a length of, for example, 1.5m in the combustion gas flow, in the region of the ports 12,13. The temperature in the combustion gas flow G falls towards the inlet opening 6 but is substantially uniform over the cross-section of the tunnel 1. At the exhaust channel 14 and thus also the inlet opening 6 the temperature is between 5000C and 8000C, for instance 6000C to 7000C.

The material to be fired is introduced in the throughput direction D through the inlet opening 6 with a velocity of about 1m/hour and is moved through the tunnel 1. The material to be fired is thus exposed immediately after the inlet opening 6 to a comparatively high temperature. The articles being fired are, however, subject to uniform heating from all sides and cracks do not occur in the material of the articles.

The selected transport velocity ensures that the material to be fired is exposed to the highest temperature of about 16500C for a sufficiently long time.

After the articles to be fired haze moved past the ports 12,13 they reach the cooling zone 3. In the cooling zone 3 there is a gas flow K in the direction of the outlet opening 7. The gas flow K is a result of the overpressure of the combustion gas flow G in the region of the ports 12,13 with respect to the atmospheric pressure at the outlet opening 7. The flow velocity of the gas flow K is substantially smaller than that of the combustion gas flow G.

In the cooling zone 3 there is a gradual reduction of the temperature of the fired articles and the fired articles are exposed to a uniform temperature on all sides as they are cooled. In particular, the positive pressure with respect to atmosphere maintains the cross-sectionally uniform temperature as atmospheric air cannot enter the cooling zone 3. Thus stress cracks are also prevented from occurring in the fired articles in the cooling phase.

Various experiments have shown that even with comparatively short transit times of about 12 hours stress cracks are not noted in the fired articles.

Claims

1. A gas heated tank furnace comprising a combustion zone having an inlet at one end, a cooling zone connected with the other end of the combustion zone, a combustion gas inlet adjacent the said other end of the combustion zone and a combustion gas outlet arranged in the region of the inlet to the combustion zone.

2. A tank furnace as claimed in claim 1, wherein the combustion gas outlet comprises an exhaust channel surrounding the combustion zone in the region of the inlet to the combustion zone, which channel is connectable with an exhaust fan.

3. A tank furnace as claimed in claim 1 or 2, wherein the combustion gas inlet comprises at least two ports in the combustion zone.

4. A tank furnace as claimed in claim 3, wherein the ports are of increasing cross-section and directed towards the inlet to the combustion zone.

5. A tank furnace as claimed in any of the preceding claims, wherein the length of the cooling zone is substantially half the length of the combustion zone.

6. A tank furnace as claimed in any of the preceding claims, wherein combustion air passages extend parallel to and in heat exchange relationship with the combustion zone.

7. A tank furnace as claimed in any of the preceding claims, in use firing moulded articles in which the articles are moved through the furnace at a speed of between 0.5 and 1.5 metres/hour.

8. A tank furnace as claimed in claim 7, in which by reason of the combustion gas entering the combustion zone a super-atmospheric pressure is created in the cooling zone which decreases towards the outlet in the end of the cooling zone remote from the combustion zone.

9. A tank furnace as claimed in claim 7 or claim 8 in which the highest temperature in the combustion zone is substantially 16500C and the temperature adjacent the inlet to the combustion zone is between 5000C and 8000C.

10. A tank furnace for firing moulded articles substantially as specifically described herein with reference to the accompanying drawing.