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*Designed and manufactured by Deltech, Inc.
Basic ConceptPositive-pressure furnaces look a bit odd; they've been variously called "diving bells," "spaceships" and "beer kegs." Every furnace starts with an ASME-certified vessel of either mild or stainless steel that has been manufactured to custom specifications. Once certified, the vessel cannot be altered in any way. From a manufacturing standpoint, this unfortunately eliminates the opportunity to make any last-minute design changes on the production floor, because to do so could violate the structural integrity of the vessel and would, in any case, invalidate the certification.
In some instances, the furnace itself is built into the vessel (see Figure 2), while in others the vessel is designed to enclose a separate furnace. The overall furnace design is largely unaffected by its integration with a pressure vessel. That is, fiberboard insulation and resistance heating elements can still be used, and the same thermodynamic principles apply. A gas delivery and pressure regulation system is used to pump oxygen or other gases into the furnace workspace. The gases go through the porous insulating materials used in the construction of the furnace itself and are maintained at the desired pressures by the steel vessel.
Development MilestonesOf course, the presence of the pressure vessel does lead to some occasionally unanticipated consequences. One early-model positive- pressure vessel was designed with a 24-in. cubed workspace for operation at 1600°C in 40 psi of pure oxygen. The unit was a "top hat," in which the furnace lifts away from the load platform.
In this case, the load platform was replaced by a set of two removable kiln cars to allow for the processing of one load while another was being cooled, unloaded and reloaded in another area. The ports located around the circumference of the furnace housed individual heating elements, allowing for both efficient element terminal cooling and convenient heater replacement, as needed.
As is the case with all of the 1600 and 1700°C units mentioned here, this furnace was heated with U-shaped molydisilicide heating elements with a 90° bend in the cold zone. The unanticipated effect of the pressure vessel was that the element terminals stayed so cool that moisture condensed in the element ports. This problem was solved by sloping the ports down and inward to ensure that any condensate would rapidly evaporate in the furnace heat.
Another project called for multiple units for use in solid oxide fuel cell manufacture. These low-temperature (1000°C), 75 psi furnaces, which were used as part of solid oxide fuel cell test stand apparatuses, included heating elements that were wire-wound ceramic modules. Ceramic modules, which are rated for use at temperatures up to 1200°C, provide durable, lower-cost heating when operating temperatures are relatively low. The wires are embedded in the ceramic, which also functions as the hot face insulation layer.
Another positive-pressure vessel project incorporated a new element. The furnace was similar to the early positive-pressure design, with a top hat and kiln car, an 8-cubic-foot working volume, and 1700°C operation in oxygen. However, this unit also included a vacuum-assisted evacuation feature to remove the room air prior to the introduction of the oxygen. Doing so minimizes the presence of other gases to ensure that the product is not exposed to elements that might alter features such as color, composition and performance.
One extremely large positive-pressure furnace was commissioned by a company in Great Britain that had the task of supplying all of the equipment needed for a new factory in China, including a production-size 1700°C positive-pressure furnace for sintering ITO tiles in an oxygen atmosphere. In addition to a 90-cubic-foot clear workspace, the system had multiple temperature zones controlled by a master/slave system, a human machine interface (HMI) system, an exhaust for precise pressure maintenance, and an electrically actuated rail system for the kiln cars.
Beyond size and complexity, this project served as a reminder that design and manufacturing don't present the only challenges. The system had to be humidity-proof packaged for ocean shipment, and, due to its size, experienced home construction framers were employed to expertly build a crate around the furnace. The unit also had to be shipped from Denver, Colo., to the East Coast via special truck routes, requiring a special permit for every state.
Another design went to the opposite size extreme with a 12-in. cubed workspace, but to much higher pressure (300 psi). This furnace was also designed for use in pure oxygen at 1700°C. The relatively small workspace size required the development of a special tool for the installation and replacement of the heating elements, both during the furnace's manufacture and for use by the end user. In addition, a miniaturized version of the standard element port design, which had proved so successful, was used.
Continued EvolutionCurrently underway is a project involving another small workspace furnace, 6-in. cubed, but the pressure requirement has more than doubled to 750 psi. This furnace will also feature custom throughways for high-temperature data acquisition.
Specializing in custom furnace design and manufacture involves the same process of fits, starts and surprises as those experienced by ceramic researchers and manufacturers. This has certainly proved true of the evolution of positive-pressure furnaces. Only time will tell whether materials science will continue to head in directions that will require furnaces with similar features-or if even more innovation is on the horizon.
For more information, contact Deltech, Inc., 1007 East 75th Ave., Unit E, Denver, CO 80229; (303) 433-5939; fax (303) 433-2809; e-mail email@example.com; or visit www.deltechfurnaces.com.