About Ceramic Furnace Tubes

Thermal Expansion of Process Tubes Used in Furnaces

Alumina tubes will expand uniformly when heated homogenously. But when they are used to process materials in furnaces, they develop thermal gradients due to factors such as the presence of the substance being processed inside the tube. While gradients can be controlled through expert furnace design and limiting the tube diameter, it is not possible to eliminate all variations.

Gradients will also affect the tensile strength of tubes. For 998 alumina, temperatures over 1000 degrees Celsius will accelerate the loss of tensile strength.

Recommended Ways to Limit Tube Loss

• Limiting furnace ramp rates to 60 degrees Celsius per hour will reduce the risk of thermal shock from the production of gradients.

• Tubes are subject to mechanical stresses. To help prevent tubes from being restrained during thermal cycling, firmly secure the tube at one end only. Alternatively, Deltech’s water cooled end caps are designed with O-rings that allow for tube expansion and contraction in addition to ensuring effective sealing to maintain the integrity of inert atmospheres inside the tube.

• Large diameter, thick-walled tubes should be avoided whenever possible. The elastic strain that develops because of thermal gradients increases with the size of the tube. When tubes larger than 100mm in diameter are necessary for the process, Deltech provides furnace design solutions that help to mitigate the effects.

• Thermal gradients are more pronounced at the point or points where the process tube passes through the furnace wall(s). Deltech minimizes their occurrence by using insulation designs that allow for variations in thermal expansion without producing significant heat loss from the furnace.

• For applications in which parts must be placed inside the tube of a horizontal furnace, it is best to place objects on a fixed bed of small diameter ceramic rods lining the bottom of the tube. Crucibles, trays, or boats can be placed onto refractory supports or D tubes pre-installed inside the furnace tube. Deltech’s engineering team can assist you in the choice of an optimal solution for your process.

• When end seals are used, it is best to use internal refractory tubes or discs to protect the seals from radiant heat. Deltech’s engineering team can recommend the best design and optimal placement.

Contact our Sales and Engineering team to discuss your application

Guide adapted from the McDanel Advanced Ceramic Technologies Application Sheet, “General Considerations for the Use of Ceramic Furnace Tubes”.

Deltech vertical tube furnace and control system