What’s Inside Your Ducts?

February 1, 2001
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The refractories on the inside of your kiln’s air recirculation ducts can make all the difference in how long those ducts last—and how clean the air is in your preheater.

The vacuum forming process enables the fabrication of both extremely large and relatively small duct linings.
Do you know what’s inside your kiln’s air recirculation ducts? If the ducts are made of stainless steel, chances are you’ve had to replace them at least once if you’ve owned your kiln for any period of time. And if you replaced those ducts with more stainless steel, you can bet you’ll probably have to replace them again. The steel tends to fatigue easily and fail after a few years of service at high temperatures. The ducts can be lined for protection, but in the whitewares industry, where the quality of the kiln atmosphere is imperative, a failure in the refractory lining can cause contamination problems as the air is recirculated back into the preheater.

“Manufacturers in the whiteware business, making pots and plates and cups or catalytic converters, for example, can’t have any speckles on their whiteware, yet these furnaces are running, on some of the cooler units, from 1500 to 1600°F,” said Vic Davis, general manager of Danser Inc. Prior to 1990, the company supplied stainless steel ductwork to the ceramic industry. “But we thought there had to be a better high-temperature solution,” Davis said. “At first we started wrapping the interior of the ductwork with blankets and pins. This worked, but it was very time consuming—and it wasn’t a real good-looking product.

“About that time, a vacuum-former came through and we got to talking to him. He thought we could make round cylinders out of vacuum-formed ceramic fiber that would go into our ductwork. Although we were successful, again, this was quite labor-intensive. So we figured out another way of actually making a vacuum-formed tube inside steel ductwork, then putting them together as one to dry. When we found out we could do that, we patented and trademarked the product in 1992.”*

The duct is high-temperature insulation consisting of a low-density ceramic fiber sleeve or custom fitting (about 17 lbs/cu. ft.) cured inside a metal skin. It withstands temperatures up to 2800°F continuous; provides instant on/off thermal protection to minimize thermal shock; and at just 20% the weight of castable, requires not only less support structure but fewer people and far less time to install than does castable ductwork.

Gerald Amick, production manager of the company's vacuum-forming operation, first developed the steel die that could be practicably modified for custom sleeves and fittings. “The first vacuum-forming dies were built for the plastics, pulp and paper industries,” said Amick. “They weren’t practical for the ceramic industry application that we were undertaking—they were over-designed and over-engineered because they were built for a mass-production product. We needed to make dies that could be modified for a custom part.

“In our dies now as a general rule, once the flow and vacuum surface area are established, the superstructure of a given die is the next major concept to be looked at. When you have a very large-cylinder die, it becomes a vacuum vessel when inserted into a ceramic fiber slurry. You have a very thin screen surface that has to withstand full vacuum. Otherwise you would have an implosion such as the crushing of a submarine. So the superstructure has to be calculated to withstand the actual pressure exerted to the outside. On a day-to-day basis we have a continual upgrade of die prototypes due to custom applications.”

The insulation can be used in any industrial application where managing extreme production temperatures can provide substantial savings.

The Vacuum-Forming Process

In the vacuum-forming process, ceramic fiber and its binders are suspended in a water solution, and the solution is drawn onto a product-shaped filter die using a vacuum system. While the water is pulled through the filter, the ceramic fiber is deposited on the die, forming the product. The water is then recycled.

Any of three mix tanks are then filled with a predetermined amount of white or clean water. Fiber, fillers and binders are added to make the appropriate mixture for a particular product, and the mixture is then fed into draw tanks through a network of piping. A custom-built filter die with the appropriate vacuum connections is attached to a vacuum system.

To achieve maximum vacuum at the die, which can be a major problem due to the large surface area of the dies, vacuum connections are made to a header system, which can be valved according to die configuration. The headers are located at “start” water level to allow the hoses to accumulate a full volume of water in each hose, which reduces air bypass, providing maximum vacuum at both the header and the die.

A series of vessels, situated out of plum so they completely drain during a cycle and are self-cleaning, make up the vacuum storage system. The horizontal header vessel is located at the low point of the draw system, at the discharge of the headers. It allows maximum vacuum at all headers, while permitting discharged water to be diverted to, or stored in, other tanks. A primary vacuum system of no less than 27 hg provides vacuum to a vertical ballast vessel. This vessel, located above water level, keeps a steady vacuum draw on all other vessels, achieved by means of piping connected to the top of the tank. Also, the vacuum take-off is at the top of this vessel, which never has water introduced into it. If any accumulation or overflow water occurs within the vessel, a bottom flow line allows it to drain.

“Through a network of valves and piping, the vacuum is distributed from the ballast vessel to the other vacuum vessels in the system,” said Amick. “Each of these vessels can be isolated from the system, thus allowing it to be vented, its contents pumped back to any mix tank, re-vacuumed, and reintroduced into the draw system.” In addition, any of these isolated vessels can be used as storage or can be valved off and not used at all within a draw cycle.

After a product has been formed to the die’s configuration and removed from the slurry, it is placed on a material handling cart. At this stage in processing, the product is about two-thirds water weight and needs to be cured in drying ovens to remove most of the water. “One of the major problems in curing a vacuum-formed product is that it is such a good insulating product that oven-drying time is one of the major bottlenecks,” Amick said. “Although we use conventional ovens for drying thinner and production parts, our new facility has a microwave with the capability of penetrating the ceramic fiber to dry the core of large components faster. After drying, products move to a finishing area for cutting, sanding, anchors, or anything else that needs to be finalized on the product before it is packed for shipping. Since our product is often used in furnaces in order to cure other products, there is a large need for custom parts such as heating elements as well as products with anchors or hangers, usually for a secondary process.”

Amick has been employed by Danser for 19 years, starting in electrical maintenance and proceeding to welding, fabricating and installing the company’s industrial heating and combustion systems nationwide. When the pilot vacuum-forming operation was introduced in 1990, he became production supervisor, in addition to designing, fabricating and modifying dies. “The initial system was intended primarily to determine production bottlenecks and customer demand,” said Amick, “and a fundamental modification took place about a year later. Product formulation was one of the key issues, particularly in terms of structural strength.

“The Unifrax Corp., our supplier of bulk fiber, also vacuum-forms a board product thinner than our boards, so there’s no problem with competition. Their engineers helped us somewhat with flow configuration and the actual formulation of our own product, even though their draw system is a totally different concept from ours. Our system relies more on a look-at-the-product-and-see-what-it-takes-to-build concept. To manufacture our large custom sleeves and fittings, we use much deeper-than-average draw tanks, large vacuum vessels, and mix tanks that deliver continuous flow to the draw system. In designing the new facility, we were looking at large volumes of water and vacuum, whereas most others in the industry were vacuum-forming large-volume commodity parts using small batches.”

Vacuum-formed, ceramic fiber blocks.

Insulation Applications

The steel-jacketed, vacuum-formed insulation is used in brick and ceramic kilns for flues, stacks, ducts and exhausts, and in any industrial application where managing extreme production temperatures, particularly in the range of 2300 to 2800°F, can provide substantial savings. For example, an older kiln might be retrofitted so that a corroded, possibly crumbling brick stack is replaced by flanged 5-ft sections of steel-jacketed ceramic fiber. In another retrofit application, the escalating cost of natural gas could be offset by a heat re-circulation system, which conveys extreme heat from the hottest to the coolest part of the kiln. Fabricating this system with the vacuum-formed fiber product eliminates not only the speckling associated with stainless steel, but also the expansion joints.

In other situations, all high-temperature kiln ductwork within the reach of employees could be replaced by the steel-jacketed ceramic fiber to bring it within cold-face compliance. Additionally, fractured or cracked burner blocks made of castable refractory can be replaced by instant on/off ceramic fiber blocks to eliminate thermal shock. (These are usually just vacuum-formed ceramic fiber but can be steel-jacketed if necessary.)

Although few companies are burdened with all of these needs in one facility, the sheet metal fabrication and vacuum-forming operations have satisfied each of these requirements for many different kiln and ceramic manufacturing companies.

For More Information

For more information about the steel-jacketed, vacuum-formed ductwork, contact Danser Inc., Vacuduct Division, Route 50, Murphytown Road, P.O. Box 4098, Parkersburg, WV 26104; (304) 679-3666; fax (304) 679-3354; e-mail danser@wirefire.com or visit http://www.vacuduct.com.

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