GB2111178A - Improvements in rotary kilns - Google Patents

Abstract

In rotary cement kilns from which hot air is withdrawn through the kiln door for drying purposes, the invention provides a heat-resistant plate (5) supported by the kiln door (3), the conduit for hot air (10) passing through both plate and door. Preferably the space (7) between the plate and the door is maintained, by means of a suction fan (11) at substantially the same pressure as the kiln so as to minimise the inflow of cold air through gaps (8) and (9). <IMAGE>

Description

SPECIFICATION Improvements in rotary kilns The present invention refers to improvements in rotary kilns. The invention is primarily concerned with such kilns as used in the cement industry, but is also applicable to rotary kilns employed for the calcination of other solid materials.

In such rotary kilns calcined material is withdrawn from the bottom end of the kiln and is passed to a cooler. In many cases the calcined product is cooled by air blast in coolers which rotate with the kiln.

In rotary kilns equipped with planetary coolers, independent of the purpose for which the kiln is employed, there is a stationary bottom end door, through which the fuel feed pipe for the selected fuel (gas, oil or solid fuel) extends into the kiln.

While the gap between the bottom end door and the rotating kiln shell is maintained as small as possible, nevertheless the gap at that location is usually of the order of 25 mm or larger, because irregularities in the kiln shell and the need to allow for thermal expansion, renders it impracticable to maintain a smaller gap between the stationary door and the end of the rotating shell.

The bulk of the combustion air is pre-heated by being drawn into the kiln through coolers for the calcined product (e.g. cement clinker) which rotate with the shell. With coal-fired kilns a minor proportion (about 30%) of the combustion air is injected with the fuel through the barrier pipe.

As already stated the gap between the bottom end door and the rotating shell is maintained as small as possible to hold down the intake of cold air, which can interfere with combustion efficiency.

In many instances it is desired to have an available source of heated air for the operation of ancillary equipment; for example in the cement industry hot air is required for drying coal fuel in coal mills and for other drying duties. It is becoming increasingly common practice to withdraw such air from the lower end of the kiln through a suitable outlet in the kiln door. However the temperature of such withdrawn air is rather low, because of the close proximity of the point of withdrawal to the gap between the shell and the door through which cold air is drawn in. The withdrawn air tends essentially to be the air drawn in between the shell and the door with relatively slight heat up-take from the kiln atmosphere.

It is an object of the invention to provide a simple system to allow hot air to be withdrawn from the bottom end of the kiln at higher temperature than can conveniently be achieved by the existing practice.

According to this invention this is achieved by providing a heat resisting plate, supported by the door and extending into the kiln space to define a space between the door and the plate, the edge of the plate being spaced away from the refractory lining of the rotating shell to provide a small clearance, the take-off conduit for heated air extending through the heat resisting plate and door for take off of hot air from the side of the heat resistant plate which is remote from the door. This system reduces the flow of cold air directly from the gap between the kiln and the door because the cold air draught now flows axially of the kiln instead of radially.

While the provision of such an auxiliary plate may in some instances be sufficient in itself, it is greatly preferred to provide a means of withdrawing inflowing air from the space between the door and the heat resisting plate so as to reduce the flow of cold air up the kiln.

Accordingly in most cases a suction off take will be provided to withdraw air from this sqace and return it to the kiln via the produce coolers, so that its-thermal content is not wasted. The suction fan for this off take is preferably operated in such a way that the pressure conditions on opposite sides of the heat resisting plate are essentially balanced so that there is effectively zero air flow around the edges of the heat resisting plate. The suction fan would ideally be controlled by means of a controller which measures the differential pressure on opposite sides of the heat resistant plate. However in most instances it will be sufficient to control the fan in response to the temperature of the air withdrawn by the suction fan.

One form of cement kiln door system in accordance with the invention is diagrammatically illustrated in the accompanying drawing.

The rotating kiln shell is indicated at 1 and satellite clinker coolers 2 rotate with it.

Combustion air is drawn into the kiln shell through the clinker coolers 2.

The kiln is provided with a stationary door 3, through which a fuel pipe 4 extends upstream into the kiln. A circular heat resistant plate 5, faced by a layer of insulation 6, is supported by the door 3 so as to define an air space 7. The gap 8 between the edge of the plate 5 and the kiln shell 1 is of the same order of size as the gap 9 between the door 3 and the kiln shell 1 and is typically about 25 mm but can be larger where the shell has become distorted.

The hot air off take is via a conduit 10 which extends through the plate 5 and door 3, while cooler air is withdrawn from the space 7 by means of a fan 11 to restrict upstream air flow from the space 7 between the kiln shell 1 and plate 5.

The rate of air flow through the conduit 12, in which the fan 11 is located, is governed by a damper valve 14, the position of which can either be controlled by a differential pressure measuring controller 15, which detects the pressure on both sides of plate 5, or by means of a temperatureresponsive controller 16, which measures the air temperature in the conduit 1 2. The air withdrawn from the space 7 is typically at a temperature of 3000C when there is a zero air flow at the edge of the plate 5 and the rate of air withdrawal from space 7 may be controlled to maintain a substantially constant air temperature in the space 7.Too high a temperature would indicate upstream air leakage around the edge of the plate 5, while too low an air tempurature would indicate excess intake of air through gap 8 and excess heat loss through the plate 5.

The warm air from the conduit 12 is returned to the clinker coolers 2 for further heating before entry into the kiln as combustion air.

The plate 5 is preferably made of a heat resistant stainless steel alloy and is preferably relatively thin (about 10 mm) to avoid cracking problems.

Claims (Filed on 3-12-82) 1. A rotary kiln comprising a generally cylindrical rotatable kiln shell, a stationary door adjacent one end of the kiln shell and a conduit extending through the door for removing hot air from the interior of the kiln, characterized in that there is provided a heat-resistant plate supported by the door and extending into the kiln to define a space between the door and the plate, the edge of the plate being spaced away from the kiln shell to provide a small clearance, the conduit extending through the heat-resistant plate and the door for removing hot air from the side of the heatresistant plate which is remote from the door.

2. A rotary kiln as claimed in claim 1, wherein there is also provided a means of withdrawing inflowing air from the space between the door and the heat-resistant plate.

3. A rotary kiln as claimed in claim 1 or claim 2, wherein the kiln is provided with planetary product coolers which are rotatably with the kiln shell.

4. A rotary kiln as claimed in claims 2 and 3, wherein the means for withdrawing inflowing air comprises a suction fan to draw air from the space between the door and the heat-resistant plate and return it to the kiln via the product coolers.

5. A rotary kiln as claimed in claim 4, wherein the suction fan is operable to maintain the pressure conditions on opposite sides of the heat-resistant plate essentially balanced.

6. A rotary kiln as claimed in claim 5, wherein the suction fan is controlled by means of a controller which measures the differential pressure on opposite sides of the heat-resistant plate.

7. A rotary kiln as claimed in claim 5, wherein the suction fan is controlled by the temperature of the air withdrawn by it.

8. A rotary kiln as claimed in any one of claims 1 to 7 and substantially s hereinbefore described with reference to and as illustrated in the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (8)

**WARNING** start of CLMS field may overlap end of DESC **. space 7 may be controlled to maintain a substantially constant air temperature in the space 7. Too high a temperature would indicate upstream air leakage around the edge of the plate 5, while too low an air tempurature would indicate excess intake of air through gap 8 and excess heat loss through the plate 5. The warm air from the conduit 12 is returned to the clinker coolers 2 for further heating before entry into the kiln as combustion air. The plate 5 is preferably made of a heat resistant stainless steel alloy and is preferably relatively thin (about 10 mm) to avoid cracking problems. Claims (Filed on 3-12-82)

1. A rotary kiln comprising a generally cylindrical rotatable kiln shell, a stationary door adjacent one end of the kiln shell and a conduit extending through the door for removing hot air from the interior of the kiln, characterized in that there is provided a heat-resistant plate supported by the door and extending into the kiln to define a space between the door and the plate, the edge of the plate being spaced away from the kiln shell to provide a small clearance, the conduit extending through the heat-resistant plate and the door for removing hot air from the side of the heatresistant plate which is remote from the door.

2. A rotary kiln as claimed in claim 1, wherein there is also provided a means of withdrawing inflowing air from the space between the door and the heat-resistant plate.

3. A rotary kiln as claimed in claim 1 or claim 2, wherein the kiln is provided with planetary product coolers which are rotatably with the kiln shell.

4. A rotary kiln as claimed in claims 2 and 3, wherein the means for withdrawing inflowing air comprises a suction fan to draw air from the space between the door and the heat-resistant plate and return it to the kiln via the product coolers.

5. A rotary kiln as claimed in claim 4, wherein the suction fan is operable to maintain the pressure conditions on opposite sides of the heat-resistant plate essentially balanced.

6. A rotary kiln as claimed in claim 5, wherein the suction fan is controlled by means of a controller which measures the differential pressure on opposite sides of the heat-resistant plate.

7. A rotary kiln as claimed in claim 5, wherein the suction fan is controlled by the temperature of the air withdrawn by it.

8. A rotary kiln as claimed in any one of claims 1 to 7 and substantially s hereinbefore described with reference to and as illustrated in the accompanying drawings.