The Thermal Disc is the only transport type dryer which can handle flotation, thickener underflow and pond recovery feed streams without the addition of a coarse fraction.
The principle of operation differs from all other types of indirect dryers in that it is a volumetric displacement apparatus rather than first in first out. An operating level of mechanically fluidized coal is maintained in the dryer body at all times and the product rate is determined by displacement of dry product by the incoming wet cake; i.e., a cubic foot of filter cake displaces one cubic foot of dry coal.
On a 12 disc shaft, four parallel paths are used, or there are four entry points to the discs. Each entry point, shown as a short nipple, takes oil flow through three discs in series and discharges through an internal header pipe to a rotary union.
The inlet section generally operates at a slower rotational speed than the discharge section. The drive is comprised of a variable pitch pulley having a 3:1 variation. The flat belt is sheaved to a speed reducer. The output shaft of the speed reducer is fitted with a torque limiting device which is coupled to one of the four disc assemblies. The additional shafts are connected by chain and sprocket from the driven shaft assembly.
Hot oil enters the individual hollow disc assemblies where it exchanges heat through the metal walls to the conveyed coal. The cooled oil is then returned to the fired heater to be reheated to the dryer inlet temperature. Oil is heated to 450 to 550°F, depending on the volatility of the coal.
The Thermal Disc indirect drying system has two advantages over existing drying methods:
– installed cost and operating costs are lower when compared within applicable tonnage parameters, and
– air pollution control equipment is less sophisticated and energy intensive due to the small amount of dust gases produced in the process.