The present invention relates to a method and a device for cooling a rotary kiln having a cylindrical outer jacket.

Rotary kilns with a cylindrical outer steel jacket and an inner ceramic lining are, for instance, used as cement kilns and lime sludge reburning kilns. They may also be employed for burning waste, e.g. hazardous waste. When a rotary kiln is used for combustion, the steel jacket and the lining are worn as a result of the high temperature in the kiln and the considerable variations in temperature that arise when the kiln rotates. In addition, both the steel jacket and the lining are mechanically worn as a result of the rotary motion, and the lining is also worn by chemical attack. In a rotary kiln employed for burning chemical waste, the lining and the steel jacket have to be replaced, wholly or partly, after about 6,000 and about 25,000 hours of operation, respectively. However, if the temperature of the jacket is reduced and maintained at a relatively constant level, the service life of both the jacket and the lining is much prolonged.

Efforts have therefore been made to cool rotary kilns. In prior-art methods, the jacket of the rotary kiln is cooled with the aid of water. Water-cooling is, how¬ ever, extremely difficult to carry out in practice, since it requires an extremely water-tight enclosure of the kiln in order to make it possible to make use of the water steam or hot water formed upon the cooling, and also in order to protect the surroundings and the device which rotates the kiln. Naturally, water-cooling also involves problems of corrosion.

One object of the present invention is, therefore, to provide a simple and efficient method for cooling a rotary kiln.

According to the invention, this object is achieved by a method which is characterised in that cooling air is blown at high speed substantially radially towards the jacket from a supply-air chamber which surrounds said jacket over the main part of its circumference and extends over at least part of its axial extension, and which has an inner wall located at a slight distance from said jacket and formed with substantially radially directed perforations which are distributed over the supply-air chamber and through which the cooling air is blown towards said jacket, and that the cooling air is sucked out substantially radially from the space between the jacket and the inner wall of the supply-air chamber to an exhaust-air chamber which is separate from and surrounds the supply-air chamber and communicates with said space via tubes which are distributed throughout the supply-air chamber and extend substantially radially therethrough, the cooling-air flow through the chambers being adjusted by supply-air and exhaust-air devices connected to said chambers.

Another object of the invention is to provide a simple device for implementing this method.

According to the invention, this object is achieved by a device which is characterised by a supply-air chamber which surrounds said jacket over the main part of its cir¬ cumference and extends over at least part of its axial extension and which has at least one cooling-air inlet and an inner wall located at a slight distance from said jacket and formed with substantially radially directed perforations which are distributed over the supply-air chamber and serve to blow cooling air towards said jacket in a substantially radial direction; an exhaust-air cham¬ ber which is separate from and surrounds the supply-air chamber and communicates with the space between the jacket and the inner wall of the supply-air chamber via tubes which are distributed throughout the supply-air chamber and extend substantially radially therethrough and which

serve to conduct cooling air substantially radially from said space; as well as supply-air and exhaust-air devices which are connected to said chambers in order to adjust the cooling-air flow therethrough. In a preferred embodiment, the cross-sectional area of the supply-air chamber decreases in the circumferential direction away from the respective cooling-air inlet.

The su -y-air chamber is preferably annular, and the supply-air and exhaust-air devices are conveniently fan devices.

The invention will now be described in more detail below, reference being had to the accompanying drawings, in which

Fig. 1 is a schematic cross-section of a device according to the invention, and

Fig. 2 illustrates a portion of a perforated wall of the device.

The drawings show a rotary kiln having a cylindrical outer steel jacket 1 and an inner ceramic lining 2. The rotary kiln is surrounded by an annular casing 3.

The casing 3 has an annular supply-air chamber 4 which is defined by an outer wall 5 and a perforated inner wall 6. The inner wall 6 is located at a slight distance from the jacket 1, and its perforations 6a, which are evenly distributed across its entire surface, are directed substantially radially towards the centre of the rotary kiln. The supply-air chamber 4 has two cooling-air inlets 7a and 7b which are connected to a cooling-air-supply system including a fan device 7. Further, the casing 3 has an annular exhaust-air chamber 8 which surrounds the supply-air chamber 4 and is defined by the wall 5 and an outer wall 9. The exhaust-air chamber 8 has two cooling-air outlets 10a and 10b, which are connected to a cooling-air outlet system including a fan device 10. The exhaust-air chamber 8 communicates with the annular space 11 between the inner wall 6 of the sup¬ ply-air chamber 4 and the jacket 1 via a plurality of

tubes 12 which are evenly distributed throughout the entire supply-air chamber 4 and extend substantially radially therethrough.

A cooling-air flow for cooling the jacket 1 is gene- rated with the aid of the fan device 7 connected to the cooling-air inlets 7a and 7b of the supply-air chamber 4, as well as the fan device 10 connected to the cooling-air outlets 10a and 10b of the exhaust-air chamber 8. The cooling air is blown into the supply-air chamber 4 through the inlets 7a and 7b, and is evenly distributed over the axial length of the chamber 4. The cooling air is driven away from the inlets 7a and 7b in the circumferential direction. When passing over the perforated inner wall 6, the cooling air flowing in the circumferential irection gives rise to radially directed cooling-air jets which at high speed impinge upon the jacket 1. The velocity of the cooling-air jets is maintained at such a high level that there is a satisfactory heat transfer between the jacket 1 and the cooling air. The heated cooling air is drawn into the exhaust-air chamber 8 through the tubes 12. Then, the heated cooling air, which may have a temperature of about 100°C, is drawn from the exhaust-air chamber 8 through the outlets 10a and 10b to be used e.g. as preheated air blown into the rotary kiln at the burner. The heated cooling air may also be employed for heating various premises.

Since the cooling air is blown into and sucked out of the space 11 in a substantially radial direction and even¬ ly distributed over the surface of the jacket 1, there is no cooling-air flow over the jacket 1 in its circumferen- tial or longitudinal direction, and the jacket is thus effectively cooled.

The radial extension, and consequently the cross-sec¬ tional area, of the supply-air chamber 4 decreases in the circumferential direction away from the cooling-air inlets 7a and 7b. Thus, the velocity of the cooling-air flow in the circumferential direction can be maintained constant or substantially constant around the entire supply-air

chamber 4, despite the fact that the flow is gradually reduced as a result of the formation of radially directed cooling-air jets. In the embodiment shown, e decreasing cross-sectional area has been achieved by imparting a stepwise decreasing radius to the outer wall 5 of the supply-air chamber 4. Naturally, the outer wall 5 may have a continuously decreasing radius, but this requires a more complicated manufacturing technique.

The casing 3 illustrated in Fig. 1 is made up of two identical halves 3a and 3b which, in a manner not shown in detail, can be parted (laterally in the embodiment shown) to uncover the rotary kiln, e.g. when a power failure has interrupted the cooling-air supply, and enable air-cooling of the jacket 1 by natural convection. At each end, the casing 3 has an end wall (not shown) extending transversely of the axis of the rotary kiln from the outer wall 9 c the exhaust-air chamber 8 towards the jacket 1. No particular sealing elements have been provid¬ ed between the end walls and the rotating jacket 1. To prevent heated cooling air at the jacket ends from flowing out between the end walls and the jacket 1, the pressure P. in the space 11 between the inner wall 6 of the supply- air chamber 4 and the jacket 1 as well as the pressure P _n outside the casing 3 are measured, whereupon the fan device 10 connected to the cooling-air outlets 10a and 10b of the exhaust-air chamber 8 is so controlled that P is maintained slightly lower than P _Q .

The device shown in the drawings may extend over part of the axial length of the ro ^t ary kiln, conveniently over its central portion where the cooling requirement is the highest. Naturally, the device may also extend over the entire length of the rotary kiln, but several devices of the type described may also be positioned axially after one another so as to cover part of or the entire length of the rotary kiln.

In the latter case, i.e. when several axially succes¬ sive devices are used, an increased or reduced cooling effect may, if necessary, be applied to certain sections of the kiln length. The need of such application may, for instance, be established by means of a contactless tem¬ perature sensor (not shown), such as an IR sensor, which, when the kiln rotates, is moved to and fro along the kiln, e.g. in an upper gap 13 formed between the two halves 3a and 3b of the casing.