Silicon nitride is often used for the heater protection tube because of its superior characteristics of high heat resistance and high corrosion resistance. TKK has been manufacturing silicon nitride (Si3N4) protection tubes in house since 2006. Immersion heaters popular to the molten-aluminum bath are commonly built with a paired heating element and protection tube. TKK manufactures both of these products and has commercialized the combination unit for the general market.
Immersion Heater Unit for Molten-Aluminum Applications
Silicon nitride is a material that has many excellent characteristics, such as high strength, high fracture toughness, high electrical insulating capacity and good thermal conductivity. It is used widely in many industry areas but particularly for aluminum processing. Common applications are as protection tubes for thermocouples in melting, holding and low-pressure foundry; protection tubes for heating elements; and as riser tubes. Figure 1 shows the major components and products used in the molten-aluminum bath.
Figures 2 and 3 show our company’s silicon nitride tube products for these applications. The size of the products varies from small to large, but typical dimensions are 20-150 mm (0.8-6.0 inches) in diameter and up to 1,200 mm (47.25 inches) in length for high-demand items.
Common heat sources for the aluminum foundry operation are natural gas and electricity. Gas firing has an advantage in its operation cost, but electricity has less emissions for a clean factory environment. For electric heating, metal and SiC are the two major materials used as the heat source. As mentioned previously, TKK has been a leading manufacturer of SiC heating elements for many years. With this background, it was natural for us to develop the combination unit of an outer silicon nitride tube and an inner SiC heater.
A user benefit of this combination unit is that the purchase of two different products – silicon nitride tube and SiC heater – from different manufacturers is no longer required. With this immersion heater unit, users’ procurement will be much simpler and less troublesome.
Another benefit is the efficiency in their technical resources. These two-component products – a protection tube and a heating element – require precise specifications to be matched to each other for use over the intended temperature range. Parameters, such as clearance between the two materials and dimensional tolerance required to accommodate thermal expansion, are designed into the combination unit. The unit can be efficiently controlled to a precise temperature in the molten-aluminum bath due to SiC’s high joule heating and resultant high heat output. Moreover, this unit’s compact design is easy to handle. It utilizes a 200-V power supply and can be protected from overload with a built-in thermocouple.
This combination unit won’t restrict your equipment design flexibility because we can produce custom-specified units. For example, we can provide a custom-design combination unit for a specific electric and space requirement that customers may encounter in their equipment.
Figure 4 is an example of custom assembly of a combined unit required to have a hot-zone length of 500 mm (19.7 inches), input voltage of 230 V and an output power of 18 kW. In this case, the spacer to hold the heating element to the protection tube was made with silicon nitride, which ensures mechanical integrity of the combined unit through the entire temperature range.
Technical Approach on the Immersion Heater Unit
To effectively provide sufficient energy into the entire molten-aluminum bath, a high-power heater is often required. To utilize such high power input, however, both components need to be designed to endure heat and power density. To fabricate such an efficient heating system, it is necessary to understand the mechanism of heat transfer in the bath.
For this mechanism of heat transfer, as shown in Figs. 5 and 6, the energy will move from the heating element heated with joule heat to the inner wall of the protection tube, then to the outer wall of the tube, then to the molten aluminum and finally to the furnace-bath walls. This flow of heat consists of thermal radiation, convection and conduction in series. It is difficult to figure out the exact value of energy required by simple calculations.
TKK approached this design problem by measuring the temperatures at heating elements at the inside and outside of the protection tubes and of the molten aluminum in the bath for different output heater units and different heater designs. Figure 7 shows the temperatures of those measurement points. As you can see, the temperature of the heater goes up with higher output power, but protection-tube temperatures remain at a constant value. This is because the relatively high heat conductivity of the silicon nitride tube quickly transfers the heat from the SiC element to the molten aluminum, which then convectively transfers the heat throughout the bath. Since the molten-aluminum temperature remains constant, the higher-input power elements will result in a faster transfer of heat and a more stable aluminum temperature as convective currents recirculate cooler aluminum from the cold furnace walls. For clarification, the heat conductivity of air is 0.024 W/cm2, aluminum is 237
W/cm2 and our silicon nitride protection tube is 40 W/cm2. It can be said from this observation that the immersion system is a method to efficiently introduce the energy into the bath.
Figure 8 shows the surface temperature of the heater by watt density, in which the heater temperature can go as high as 1450˚C (2642˚F). Metal elements cannot achieve this high of a surface temperature due to limitations of oxidation and melting. This means that the heat transfer of a SiC element to the aluminum bath can be much faster and therefore more efficient than for a metal element.
The technical data shown in this article is a small part of our body of knowledge. TKK continuously strives to produce a better, longer-lasting and more efficient combined heater and protection tube through practical studies of our materials and your processes. This is why we are confident that we can accommodate your heating needs whether it be dimensional, power or heating efficiency in the molten aluminum industry. IH