Transparent Tube Furnaces – Design, Operation Benefits and Applications

It is possible to determine visually whether the processes are reacting at operating temperature or not by using a transparent tube furnace. During the fall of 2001, Thermcraft obtained technology that enabled developing a transparent tube furnace that allows users to visually check how their processes are reacting while at operating temperatures.

They began developing furnaces with a limited range of sizes and slowly went on to create the TransTemp product line, with sizes ranging up to an inner diameter of 8 in. and heated lengths of 40 in.

Innovative Design of the Transparent Tube Furnace

The transparent tube furnace comprises three glass tubes and a coaxially mounted, exclusively designed heating coil.

The Outer Pyrex Tube

The outer tube is made of Pyrex that has been coated with a non-toxic material. The coating is transparent to specific visible light wavelengths, enabling them to pass through while acting as a mirror and reflecting a considerable percentage of generated infrared energy back into the process chamber.

The coating efficiency allows for high process temperatures up to 1100°C with relatively lower outer tube temperatures. The transparent tube furnace does not have any insulation except for the hard refractory end plates that support the tubes.

The Second Quartz Tube

The second tube is made of quartz that enables protection of the mirror coating from getting contaminated and is transparent to both infrared energy and visible wavelengths.

The Third Quartz Tube

The third tube is also made of quartz is the actual process tube and allows isolation of the heater coils from the product being heated. Like the second tube, this process tube or muffle is also transparent to both infrared and visual wavelengths and typically extends some distance beyond the refractory end plates to provide loading of the product.

Operation of the Transparent Tube Furnace

The furnace can be operated either vertically or horizontally. Before delivery of the product the direction of mounting needs to be specified. It is possible to bolt the user’s fixtures to the unit by using an integral mounting plate.

Temperature Measurement for the Transparent Tube Furnace

The measurement of temperature by the Trans Temp furnace is done using an optical pyrometer or a standard thermocouple inserted from one of the process tube ends.

When optical pyrometers are used, it may be essential to perform calibration tests to allow for any offset between the heater coil temperature and the actual sample temperature.

Since the heating alloy used for coil manufacture has stable resistance characteristics, a simple power supply using a phase angle fired SCR can be used and is normally mounted remotely from the furnace.

Formation of Volatile Gases

Several inert atmospheres, including argon or nitrogen can be sent into the center process tube using exclusively designed end caps. It is essential to ensure that the gases present do not interact with glass and cause etching or form an opaque layer on the central quartz process tube.

In applications where the by-products are volatile there is a need to provide venting because any condensation forming on the process tube causes interference with the proper transfer of infrared energy. Any of the conditions will have an adverse impact on the furnace operation and could obstruct visual inspection of the process.

Benefits of the Transparent Tube Furnace

The advantages of the transparent tube furnace by Thermcraft are listed below:

  • In the TransTemp design, recurring reflection of infrared energy by the mirror takes place along the entire length of the heated chamber.
  • The mirror coating is applied in agreement with accurate thickness standards yielding uniform radial heat losses. With an appropriately designed heater coil and plugged ends, a transparent tube furnace can deliver a temperature evenness of ±1°C over the central 60% of the heated chamber under steady state conditions.
  • The furnace's heating element comprises an insulated metallic heating alloy formed into a series of fixed coils. The coil lengths, which include spacing between the coils combined with turn-to-turn spacing of the various coils, enables it to obtain temperature uniformity of the heating chamber or a thermal gradient.
  • It is possible to approximate zoning through heater coil spacing and design. In case, true independent zoning and control is required, individual furnaces with common end plate supports and a common process tube can be used
  • The heating chamber can be rapidly heated up as well as rapidly cooled down.
  • The transparent tube furnace features inherent cleanliness. The hard refractory material of the end pates supporting the tube are non-dusting in nature which leaves the client-supplied or specified end caps (if required) as the only source of refractory dust during operation. Subsequently, the furnace is well suited for use in clean room applications and requires minimal, if any, alterations for fairly stringent part per million (ppm) contamination requirements.

Applications of the Transparent Tube Furnace

The transparent tube furnace was originally developed as a research tool for corporate research centers and university laboratories. It is still used widely in these fields for applications such as ceramics, crystal growth, glass development, gas analysis, environmental product development and metallurgy. Since the furnace is now available in large sizes, the product line has branched out into production applications like wire annealing. The furnace may also be used for fiber conditioning, an application that is currently being investigated.


The transparent tube furnace from ThermCraft is supplied in a comprehensive range of sizes and has a large number of benefits when compared to previous types and models. It is unique in that it enables the operator to visualize what is happening to a sample in real time rather than after the process, providing valuable insights into material behaviour at elevated temperatures.

About Thermcraft

Thermcraft Inc. manufacturers custom industrial ovens, laboratory furnaces, and other furnace related supplies and is a recognized leader in high temperature heating equipment. Their state-of-the-art facility and design software will allow their staff to meet your requirements for any furnace or oven configuration that you may need.

This information has been sourced, reviewed and adapted from materials provided by Thermcraft.

For more information on this source, please visit Thermcraft.


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