The second tube, which serves to protect the mirror coating from contamination, is quartz and is transparent to both visible wavelengths and infrared energy. The third tube, also quartz, is the actual process tube and serves to isolate the heater coils from the product being heated. Like the second tube, this process tube (or muffle) is also transparent to both visual and infrared wavelengths and typically extends some distance beyond the refractory end plates to facilitate loading of the product.
The furnace's heating element consists of an insulated metallic heating alloy formed into a series of fixed coils. The coil lengths, which include spacing between the coils coupled with turn-to-turn spacing of the various coils, determine the temperature uniformity of the heating chamber and can also be used to provide a thermal gradient. Due to the nature of the furnace's construction and its theory of operation, programmed thermal gradients can be rather difficult to obtain, mainly due to the uniformity of the reflected infrared energy.
Zoning can be approximated through heater coil spacing and design. If true independent zoning and control is required, individual furnaces with common end plate supports and a common process tube can be used. This joining of individual furnaces has been successful when applications have required process tube lengths greater than those that are commercially available, or when unsupported span restrictions come into play.
The furnace design also features a low thermal mass, which allows for rapid heat up and cool down of the heating chamber. A typical design with a center process tube inner diameter of 2.5 in. with a heated chamber of about 20 in. long can be expected to reach 1000°C in about 15 minutes and will cool to 600°C in approximately 10 minutes. (Process tube loading, the material being heated and the efficiency of the end cap seals can have a bearing on these numbers.)
A frequently overlooked benefit of this style of furnace, in comparison to a conventionally insulated furnace, is its inherent cleanliness. The hard refractory material of the tube support end plates is non-dusting in nature, which leaves the client-supplied or specified end caps (if required) as the sole source of refractory dust during operation. Subsequently, the furnace is well suited for use in clean room applications and requires very little, if any, modification for fairly stringent part per million (ppm) contamination requirements.
Temperature measurement for the Trans Temp furnace is achieved either by use of an optical pyrometer or a standard thermocouple inserted from one of the process tube ends. When using optical pyrometers, it may be necessary to run calibration tests to allow for any offset between the heater coil temperature and the actual sample temperature. Due to the stable resistance characteristics of the heating alloy used to manufacture the coils, a simple power supply that uses a phase angle fired SCR can be used and is typically mounted remotely from the furnace.
Various inert atmospheres, including nitrogen or argon, can be introduced into the center process tube through the use of specially designed end caps. (When using other atmospheres, such as forming gases, the customer should contact the factory for additional information.) Since the center process tube is generally quartz, care must be taken to insure that the gases do not interact with the glass and cause etching or the formation of an opaque coating. Similarly, in applications where volatiles may be present, some venting is required since any condensation forming on the process tube will interfere with the proper transfer of infrared energy. Either condition will have an adverse effect on the operation of the furnace and could hamper visual inspection of the process.
For more information about the transparent tube furnace, contact Thermcraft Inc., P.O. Box 12037, 3950 Overdale Rd., Winston-Salem, N.C. 27117; (336) 784-4800; fax (336) 784-0634; e-mail firstname.lastname@example.org ; or visit http://www.thermcraft.com .