Construction

Dryer Drum
Fabricated in carbon, stainless or other alloy steel plate with reinforced bands for fitting of tyres and drive rings. Flights or lifters are welded or bolted internally to provide the required degree of contact between the material and the drying air.

The drum rotates on cast iron or steel tyres, supported on forged,  cast steel or polyurethane support rollers with shaft mounted spherical roller bearing plummer blocks, mounted on fabricated carbon steel roller support frames with locating rollers to limit any lateral movement of the dryer drum. All roller assemblies are fitted with safety guards and lubricators where appropriate.

The dryer drum is rotated by an electric motor through V-belts, gearbox, pinion and either heavy duty chain to a chain-wheel or spur gear drive ring bolted around the dryer drum. Alternatively by shaft mounted geared motor units located directly onto the support roller shafts. For high temperature operation the drive ring is secured to the dryer drum by tangent plates to allow for differential expansion.

All drive components are mounted on a common baseplate with guards as required. Integral low speed auxiliary drives can be supplied for emergency or maintenance purposes.

End Enclosures & Seals
Both ends of the drum may be enclosed by carbon or alloy steel fabricated hoods with air inlet/outlet chutes and suitable seals between the hood and the dryer drum.

Where slight ingress of air is acceptable simple labyrinth type seals are used. These comprise of a flexible fabric element, bolted to a stationary endplate and positioned between two steel rings welded to the dryer drum.

Where leakage of air is unacceptable, friction seals are provided, comprising a stationary ring or rings of semi-rigid compressed fibre or non-ferrous metal loaded onto the machined surface of the rotating element by springs, pneumatic or hydraulic cylinders. For processes involving hazardous materials multiple seals are used with inert gas or vacuum purging of the seal assembly.

Air Heater
In direct fired systems, the drying air is heated by an oil or gas burner firing into a refractory lined combustion chamber with dilution air being introduced through an annulus between the combustion chamber and air heater outer casing. Burner systems for a range of fuels can be provided. All gas and oil burners are supplied with complete valve trains, spark ignition, modulating control and flame failure control panel.

In indirect systems, heat may be transferred to the drying air by means of air to air, steam to air or thermal oil to air heat exchangers or finned tube electric heating elements.

For some applications, particularly where the direction of material and air flow are counter-current, the burner may be mounted onto the inlet end hood or endplate to fire directly into the dryer drum.

Air Handling System
In Rotary Cascade dryers the flow of air through the dryer is induced by a centrifugal fan located at the air outlet end of the dryer. In Rotary Louvre dryers the drying air is provided by a centrifugal fan located up stream of the air heater with filters at the fan inlet, as required. The exhaust air is removed by a second centrifugal fan located at the outlet end of the dryer. All fans are supplied complete with electric motor, V-belt drive and drive guard, all mounted on a common baseframe for ease of installation.

The exhaust air from the dryer is ducted via carbon or alloy steel ducting from the dryer to the cyclone, fan, dust collector or fume scrubber and, where required, exhaust stack. Butterfly type dampers mat be fitted in the exhaust air duct to provide for control of the air flow through the system and test/sampling points are also provided for pressure, temperature and emission measurement.

Particles entrained in the exhaust gas stream may be removed by passage of the gases through high efficiency cyclones, bag filter or wet scrubber or a combination of same, depending upon the nature and quantity of dust involved. All dust outlets are fitted with either rotary seals or double flap discharge valves to prevent excessive ingress of air. Materials of construction for the dust collection equipment are in line with overall plant requirements.

Process Control
For any system there is a relationship between the product moisture content and exhaust air temperature. Therefore product moisture can be controlled through control of exhaust air temperature. This control is achieved by regulating the flow of fuel to the burner by means of a temperature controller with a thermocouple located in the exhaust air duct.

Also, to achieve optimum performance the dryer must operate as close as possible to design conditions and with constant evaporative load. Any variation in evaporative load due to variations in feed rate or moisture content results in a corresponding variation in exhaust air temperature which in turn increases or decreases burner output. This raises or lowers the inlet air temperature. Thus the evaporative load may be controlled by measurement of inlet air temperature using a second controller with temperature probe located in the inlet air duct and output signal to a variable rate feeder.

This two step approach generally provides for adequate control of the drying process. When a more sophisticated control system is required it is sometimes possible to control dryer performance from direct measurement of either product or feed moisture content, or both.

In addition, high and low temperature safety interlocks, with or without alarms, may be provided and output signals from controllers may be fed to continuous chart recorders. All control instruments together with the necessary motor control gear, relays, overloads etc. are housed in a control cabinet, pre-wired for the correct sequential start-up and safe operation of the plant.