Inductive Stub Drier


Stub drying machine for CD200 and P69 cells

The Comtec inductive stub drier is a compact, high efficiency induction heater designed to dry stubs of aluminium reduction cell anode rod assemblies before they are cast. This ensures that molten metal explosions do not occur during the casting process. The stub surface is coated in a water-based graphite slurry prior to casting, so proper drying is very important. Traditionally the stubs are heated using gas burners which do not heat the entire stub surface and rely on thermal conduction through the stub to ensure all surfaces are dry. This approach does not produce consistent results and wastes a large amount of energy. The use of induction heating allows even and consistent heating of the stub surface in very short periods. Because heat is generated in the stub surface and the machine only runs when it is heating a stub, very little energy is lost. By utilising the latest semiconductor technologies, a surface temperature rise of 105 degrees Celsius on a150mm diameter in 25 seconds can be achieved consistently with a power input of only 35kVA and efficiencies above 90%. This high efficiency has allowed us to mount the cooling system on the machine without compromising the overall footprint. 


Consistent, Efficient Heating

A stub in one of our test rigs, which is used for coil optimisation

Induction heating offers significant advantages over heating using burner or radiant sources. The heat is generated directly in a thin layer on the surface of the stub, so very little heat is lost into the surrounding air. Because the entire surface that is to be cast can be heated simultaneously and evenly conduction losses into the stub and rod assembly are minimised. This results in higher efficiency and less thermal expansion in the stub when it is cast, which leads to higher contact pressures during service. Induction heating also tends to concentrate energy around cracks and surface imperfections, so areas that would normally trap water are dried thoroughly.

A stub in one of our test rigs, which is used for coil optimisation

Reducing Running Cost

Because of the high efficiency and minimal losses of the Comtec inductive stub drier, running cost can be reduced by as much as 85% (based on 2010 LPG and electricity costs in New Zealand). By using the latest semiconductor technologies we have also been able to reduce inverter losses to a fraction of the power our competitors lose in their inverters. This not only reduces energy bills but allows far smaller cooling systems to be used, which reduces running and maintenance costs.

Low Cycle Times

FEA analysis of a concept induction coil. This technique allows
extremely effective coils to be developed quickly and cheaply

The use of induction heating can greatly reduce heating cycle times compared to LPG systems because the surfaces that are to be cast are heated uniformly, which eliminates soak time required for surfaces that are not directly heated by a burner to reach the target temperature. Our system also measures the stub surface temperature during heating, which ensures that the target temperature has been reached and allows cycle times to be reduced on warm days.

FEA analysis of a concept induction coil. This technique allows extremely effective coils
to be developed quickly and cheaply

Minimal Cooling Requirements

As anyone familiar with water cooling systems is aware, ongoing monitoring and maintenance represent a significant portion of a machine’s running costs. Our high efficiency inverter technology requires less cooling than other inverters of a similar power rating, so the cooling system can be made smaller, simpler and cheaper to maintain. No external water system is necessary because the cooling system fits inside the machine.

Modular Design

Comtec Consulting works closely with maintainers and operators when designing equipment, so we realise the importance of maintainability. Our induction heaters are designed to be as modular as possible, so that replacement parts can be “dropped in” with minimal effort. Each inverter can be unbolted and lifted out for servicing without removing sub-components, which not only allows a spare inverter to be connected very quickly, but also makes bench testing the inverters very easy.

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