Drying drum: design and operating principle
When the clay mixture is prepared in dry condition, it is preliminarily dried to 8 to 10% moisture content, and then it is ground. Infirm clays also need drying. It is produced in drying drums most often.
The drum rotates slowly (5.5 rpm) driven by a drive. The drum rotation axis is inclined at 5 to 6°, so the lumps of clay fed to the upper end of the drum move through cells to the bottom discharge end as the drum rotates. Flue gases from special auxiliary kilns are a heat-transfer medium.
It is recommended to hang metal chains in the discharge section of the drying drums. They facilitate loosening of clay during drying. It allows producing clay with more uniform moisture content and increasing the drum output.
Characteristics of drying drums
A drying drum is a welded metal cylinder the internal cavity of which is drying space. The drum is installed at 5 to 6° inclination towards dried material output end.
The ends of the drum housing rotating at 4 to 6 rpm are inserted into chambers – gas chamber through which hot gases are fed into the drum, and discharge chamber through which the exhaust gases are removed and the dried clay is discharged. A fire chamber is attached to the gas chamber. The fire chamber design depends on the type of fuel to be burnt. Before charging into the drum gases are usually diluted with cold air in the chamber to decrease temperature to a constant value. The exhaust gases are exhausted by a flue-gas fan and are fed through a centrifugal dust collector cyclone where fine particles of material suspended in gases are removed. The temperature of gases fed into the drying drum is 600 to 880 C; at the outlet – up to 120 C. Drying drums with recoiling fire chambers are increasingly used recently.
Gas is burnt in them directly in receiving section of the drying drum where the combustion products are diluted with outer air fed into the drum through gaps between end wall and the shell. The operating principle of such a drum is described below. The combustion products from burners are delivered directly into the rotating drum housing where they are diluted to the set temperature, for example, 800 С by the outer air drawn through the gap.
Before mixing with the combustion products the air is heated due to heat exchange with the end wall, back-up ring and shell shaped as a rotational surface which is used as a guide for the air drawn into the drum. The end wall can move along the drum axis on a cart with a lock. Some portion of the air is drawn to the slot of the burner and spout integrated into the vertical wall due to ejecting action of the combustion products and cools them down. The material is fed to receiving blades through the charging spout and then, being poured over, moves to the discharging chamber with the heat-transfer medium contacting with it and drying to the set temperature. The exhaust gases pass through the dedusting system and a flue-gas fan and are emitted to atmosphere.
Drying clay in the drying drum
Clay is dried in drying drums by direct flow, i.e. both material and heated gases move in the same direction, as in case of a counterflow the clay may be overheated which would result in the loss of its plastic properties. When the clay passes through the drying drum, its range changes. Small pieces dry and break up to form dust, large pieces are steamed and conglobate. It induces high heterogeneity in terms of moisture content both between different pieces, and within one piece. The moisture content is averaged using chain curtains inside the drum. They grind the clay to some extent and create an intermediate stock of dried clay in which moisture is redistributed with the course of time.
The drying drum lowering procedure is as follows: the drum drive is switched on; hot gases are fed; feeding devices and conveyors for dried material are switched on.
If the drying drum slides up along the axis during operation, operating surfaces of the upper support rolls are lubricated; if the drums slides down, it is necessary to wipe dry the operating surfaces of the support rolls and shell; if the drum continues to lower, it is necessary to wipe rolls on the bottom support also.
The drying drum shall operate at constant temperature of flue gases. As the material moisture content or the rate of the material delivery into the drum changes, the drying mode shall be only regulated by the amount of hot gases fed into the drum; it is only recommended to change the gas temperature within a small range by increasing or decreasing the cold air delivery into the mixing chamber. The moisture content of the material charged into the drum and discharged from it is monitored by the laboratory, and the analysis results are reported to the drying drum servicing personnel.
The drying drum is shut down as follows. The raw material delivery is stopped 30 minutes before the drum shutdown. Then the delivery of the heat-transfer medium into the drum is stopped. After that the drive and the dried material conveyor are stopped. In case of sudden shut-downs the fuel delivery is stopped.
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