GB2210681A - Continuous kiln - Google Patents

Abstract

A continuous kiln has a preheating run 22, a heating zone 26, and a cooling run 23. The preheating run is above the cooling run and air is circulated between the two runs so heat lost during the cooling run is used to preheat during the preheating run. Conveyors in the heating zone transfer ware trollies 12 from the preheating run to the cooling run. The trollies are stepped through the kiln. The axes of the ware trollies project through the kiln's side walls and means are provided to seal around the axles at least when the trollies are stationary. The heat source in the heating zone preferably directs the heat up through an open bottom of the trollies. The bricks can be tightly stacked on the trollies so that after passage through the kiln the bricks can be banded without rehandling. <IMAGE>

Description

CONTIIZOUS KILN FIELD OF THE INVENTION The present invention relates to a continuous kiln for firing articles made for example from clay.

BACKGROUND OF THE INVENTION When making articles such as bricks from clay, it is necessary to form the malleable clay into the desired shapes or green articles, to dry the articles in drying kilns, and then to fire the articles for a prolonged time. Such firing can take place in a batch process in intermittent kilns or in flow production in continuous kilns. Continuous kilns are used for large production quantities and are usually called tunnel kilns since the articles suitably stacked are moved from an inlet end of a tunnel to the outlet end. It is possible to save on heat by using the exhaust gases to pre-heat the combustion air and the heat in the articles and their trollies at the outlet end can be partially recovered and used in the green article drying kilns.However as much as 40% of. the total heat input can be lost, which is of course an expensive waste of fuel. Normally bricks stacked in an open stack with gaps between the bricks are heated by high velocity åets directed horizontally at the stacks but even so it is difficult to heat the stack evenly and the bottom of the stack is normally cooler than the top entailing a long soak time to get all the bricks up to the firing temperature. This prolongs the firing time and is expensive in capital costs and adds to the heat loss.

After firing the bricks have to cool and then be restacked ready for binding and despatch, the restacking is a relatively expensive step. The aim of the present invention is to provide a continuous kiln allowing articles to be fired more economically.

SUMMARY OF THE PRESENT INVENTION The present invention is one aspect provides a continuous kiln having pre-heating, firing, and cooling zones wherein a heat source in the firing zone directs hot gases vertically upwards at articles to be fired.

The articles would of course have to be supported on carriers or kiln cars with an open base to allow the heat direct access to the articles. Thus the articles would be heated from the bottom up and thus will tend to heat more evenly possibly with the aid of auxiliary heat sources to even out the temperature.

Some styles of brick have holes through them and these can be stacked in an array suitable for despatch with the holes in line for the passage of the hot gases rather than the more conventional open stack and then it is only necessary to have slots in the carriers to allow binding straps to be passed below the stack for the bricks to be bound ready for despatch direct from the carrier.

Another aspect of the present invention provides a continuous kiln wherein the kiln is in the form of a blind-ended tunnel, and wherein articles to be fired are moved into the kiln towards the blind-end in a pre-heating zone, undergo firing at the blind-end and then exit through a cooling zone with the two zones being adjacent so heat can be transferred from the articles which are being cooled to those which are being pre-heated.

This forms a form of counter-current heat exchanger enabling the heat in the articles and their carriers to be transferred more completely to the articles being pre-heated. This aspect of the invention combines usefully with the first aspect in that the two zones can be one above the other, preferably the pre-heating zone above the cooling zone, with air circulators aiding the beat transfer from the cooling articles to the articles being pre-heated by directing vertical draughts of air through the articles.

The blind-end of the kiln can be in the form of a well containing a furnace, possibly a fluidised bed burner containing a refractory sand, into which the articles are lowered from the end of the pre-heating zone and then raised to the cooling zone.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic vertical axial section of a kiln, Figure 2 is a section on line II - II of Figure 1, Figure 3 is a section on line m - m of Figure 1, Figure 4 is a worm's eye view of a trolley used in Figure 1, Figure 5 is a detail showing a way of closing a slot in the wall of a kiln, and figure 6 is a detail of'a way of minimising heat loss through a slot in a wall of a kiln.

DESCRIPTION OF AN EXEMPLARY EMBODIMENT Packs or stacks 11 of say three to five hundred bricks are mounted on trollies or kiln cars 12. Each trolley as shom in Figure 4 has a base 14 formed from crossing members so as to define an open structure through which air can flow to the bricks. The trolley is supported on wheels 15 or skids on the ends of longish axles 16 which run on rails 17 outside the kiln with the axles penetrating the side walls of the kiln through slots 19.

The kiln is in the form of a tunnel 20 of refractory insulating material with an external structural support, and has a blind end 21. There are two sets of rails 17 so the trollies can move into a pre-heating zone on a higher run 22 and leave through a cooling zone on a lower run 23. When a trolley on the upper run reaches the blind end of the tunnel, its wheels rest on portions 24 of the rails which are displaceable downwardly on an intermittently operated continuous belt conveyor 25 (Fig 3) to lower the trolley either to the lower run or as shown into a well 26. If a well is used, there is a short rail for the trolley wheels so the trolley can be moved away from the blind end by one or more trolley lengths onto rail portions which are movable vertically by a second conveyor similar to conveyor 25.

Each conveyor consists of two endless members one each side of the kiln and each carrying two rail portions so only the wheels and the portions of the rails immediately below the wheels are moved. This vertical movement involves vertical slots 28. The rails 17 are enclosed in or form part of channels 29 bridging the horizontal slots 19 and these channels have to be cut away to allow the vertical movement. These channels form part of the structural support and hold the insulation in place despite the slots.

It is convenient to explain here that the trollies are moved through the kiln in infrequent steps with about an hour between movements. Each movement entails moving a cooled trolley out and moving all the trollies towards the exit, raising a trolley from the well (if one is there), moving the remaining trollies in the well towards the exit, dropping a trolley into the well and then moving the trollies being pre-heated towards the blind-end with new trolley adding the to the pre-heating run. This takes comparatively little time.

Thus it is not inconvenient to provide doors 30 at the open end of the kiln. This enables the kiln to be run at below atmospheric pressure so any leakage through the slots or elsewhere is into the kiln and does not involve hot air carrying heat from the kiln.

The pressure in the kiln can be reduced below atmospheric pressure by a pump (not shown) in an exhaust 31 drawing warmish air from the kiln adjacent the door for use in green article drying kilns.

Nevertheless it is desirable to minimise or prevent cold air entering the kiln for it would only have to be drawn out and would carry some heat with it. Figure 5 shows a way of sealing the vertical slots. Each slot 28 would have associated with it a flap 32 carrying insulating material 33 and pivotal about a vertical axis 34. This flap would normally seal the vertical slot and also the cut away portions of the channels 29 but when a vertical movement is needed it would be swung out to permit the movement. When the movement is complete the flap could again seal the slot.

A similar device could be used with the horizontal slots but it is thought that a trailing link type of device which would be displaced by movement of a trolley and then drop between the trolley axles might be better.

Figure 6 illustrates a slab 35 of insulating material with slots 36 towards the bottom so it can be clipped over the trolley axles on one side of a trolley and then slid along the trolley to be engaged by a suitable detent so it is held upright covering the entire side of the trolley and the articles thereon. This slab could not normally be allowed to rub against the inside wall which is normally mechanically soft but would be spaced from the inside wall so asto restrict the passage for air leakage and tend to reduce radiation through the slot. Any air leakage would not impinge on the articles being fired but would be directed along the kiln walls and so would be heated to a temperature which would not thermally shock the articles being fired.

The blind end of the kiln is the firing zone and in this zone a burner 37 provides an upwardly directed flow of combustion products with the desired amount of free oxygen both to the underside of the load and around the periphery; auxiliary heaters can be provided if tests show the packs are not being heated evenly. This burner can have associated with it a fluidised bed and if so a well 26 is desirable to minimise loss of the fluidised bed material such as refractory sand but without using a fluidised bed such a well is not needed and entails using the second conveyor.

A succession of high temperature fans 38 is provided to circulate air turbulently between the cooling run and the pre-heating run so the heat from the bricks being cooled is transferred to the bricks being pre-heated. These fans can merely circulate the gaseous contents of the kiln or if needed can introduce fresh air or further heat.to adjust the firing profile.

The trollies have shallow grooves 39 laterally andlongitudinally for use when bricks'having holes through them allowing air to pass through a stack can be fired in a close stack suitable for despatch. In which case it is only necessary to pass bands through these grooves and then tighten them up for the stack to be dispatched.

Claims (9)

1. A continuous kiln wherein the kiln is in the form of a blind-ended tunnel and wherein articles to be fired are moved into the kiln towards the blind-end in a pre-heating zone, undergo firing at the blind-end and then exit through a cooling zone with the two zones being adjacent so heat can be transferred from the articles which are being cooled to those which are being preheated.

2. A kiln according to claim 1 wherein the pre-heating zone is above the cooling zone and air circulators aid the heat transfer from the cooling zone to the pre-heating zone by directing vertical draughts of air through the zones.

3. A kiln according to claim 1 or claim 2 wherein slots in the kiln's walls enable ware to be carried on trollies supported on wheels outside the kiln.

4. A kiln according to claim 3 wherein the, horizontal, slots permitting movement of the trollies along the said zones are bridged by channels within which the wheels move and any vertical slots are normally closed by members which are removed to permit vertical movement movement of the wheels and thereafter replaced to seal the slots.

5. A kiln according to any one of the preceding claims wherein the blind end of the kiln is provided with a well into which the ware is lowered into and from which the ware is subsequently raised.

6. A kiln according to any one of the preceding claims wherein a burner generating a vertical blast of hot gases is provided at the blind end.

7. A method of heat treating bricks wherein the bricks are of the type having holes through them, wherein the bricks are stacked on trollies in a close array, wherein the trollies are of open nature, wherein a vertical blast of hot gases is directed through the trollies and through the said holes, and the trollies have slots in them for the passage of binding strips.

8. A continuous kiln having pre-heating, firing and cooling zones wherein a heat source in the firing zone directs hot gases vertically upwards at the articles to be fired.

9. A kiln substantially as herein described with reference to the accompanying drawings.