Constructing An Efficient Tunnel Kiln

February 1, 2001
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A new generation of high temperature refractory and insulating materials, a patented top-firing system and an innovative low-mass kiln car design have been combined to create a tunnel kiln construction that can slash energy consumption and produce superior products.

Thermal Ceramics' new tunnel kiln.
Any company thinking of building a new tunnel kiln or relining an existing one should remember that the choice of refractory and insulating materials for the kiln and kiln cars is no longer limited to traditional dense firebrick or castable materials that absorb heat and waste energy. By transitioning to structurally sound, yet lighter and more thermally efficient products, manufacturers can cut their fuel costs and boost their bottom line.

That’s what Thermal Ceramics, a supplier of high temperature insulating products, and kiln manufacturer Swindell Dressler did when they partnered to develop a more efficient system for firing insulating firebrick (IFB) at the Thermal Ceramics plant in Augusta, Ga.

The project began when Thermal Ceramics decided to build a new kiln for the manufacture of its 2000 to 2500°F grade IFB (K-20, K-23 and K-25). The kiln would replace two existing tunnel kilns built in the early ’60s. One operated full time, while the other was used as needed. Each was fired by four to five large overhead burners, vented through the roof and operated at a maximum temperature of 2550°F.

Both existing kilns had a sidewall hot face lining of 9-in. thick firebrick with a density of 154 pounds per cubic foot (pcf) and a thermal conductivity of 13.3 BTU x in/hr x F x ft2 at 2000°F. This traditional hard brick lining was backed up by 41⁄2-in. thick 2000°F IFB.

The old kilns had sulfur emission problems, and their 1960s technology refractory design was far from energy efficient. They had an estimated cold face temperature of 265°F, a heat loss of 492 BTU/ft2/hr and heat storage of 67,016 BTU/ft2.

Thermal Ceramics decided to replace these units with one new kiln with a production capacity roughly equal to the capacity of both the old kilns. The company wanted a more environmentally friendly way of firing its IFB, and it also wanted to incorporate state-of-the-art refractory design techniques and use the latest generation of insulating products.

Thermal Ceramics selected Swindell Dressler, the largest tunnel kiln builder in the U.S., from among four contractors who submitted bids for the project. “We received four totally different kiln concepts but chose Swindell Dressler, in part, because the firing design they presented was closest to the design of our old kilns,” says Fred Fidler, a senior technical service engineer with Thermal Ceramics.

Matching Materials to the Operating Parameters

Rather than relying on standard refractory materials, the two companies worked together to match materials to the operating parameters of the kiln. “By considering process requirements on the IFB product being fired in the kiln, such as heat up, cool down, weight, time in kiln, operating temperature and other factors, Swindell Dressler was able to design and build a kiln tailor-made for our operation,” says Fidler.

According to Jim Bushman, technical director of Swindell Dressler, the kiln system can be customized for various applications and temperature ranges, allowing other ceramic manufacturers to use similar techniques and products to reduce their heat loss and energy consumption.

Construction on the new kiln began in November 1999, and the new 256-ft long kiln went into operation in August 2000. The kiln is insulated with lightweight, energy-saving products that minimize heat loss during its continuous operation. Unlike the old kilns that only had 13-1/2-in. thick sidewalls, the walls of the new kiln are up to 28-3/4-in. thick, with a 6-3/4-in. hot face layer of 2800°F IFB that has a density of 55 pcf and a thermal conductivity of 2.43 BTU x in/hr x F x ft2 at 2000°F. This material is more than five times more thermally efficient than the traditional firebrick used on the hot face layer in the old kiln.

The 2800°F IFB layer is backed up by 4-1/2-in. thick 2600°F IFB, and bulk fiber is sandwiched between two types of ceramic fiber blankets, one for the hot face and another for the inner layer. The outside wall is made of face brick.

Table 1. Comparative thermal efficiencies of the old and new kiln linings.
With its structural soundness and greater thermal efficiency, the IFB proved an ideal choice for the tunnel kiln application. By using these lightweight materials rather than traditional firebrick and then backing them up with ceramic fiber materials, Thermal Ceramics now has a kiln with a cold face temperature of 148°F in the firing zone. The heat loss is an estimated 136 Btu/Ft2/hr, and the heat storage is 32,260 Btu/ft2. “The new kiln’s cold face temperature and heat storage is approximately half that of the old kilns, and its heat loss is less than one-third,” says Fidler (see Table 1).

Overcoming Thermal Management Challenges

The companies faced several problems in designing and building the new kiln. For example, the roofs in the old kilns were self-supporting sprung-arched roofs made of a mullite composition firebrick that was not very energy efficient. However, the new kiln was too wide for this type of construction, so it was designed with a suspended flat roof. To minimize the need for steel support and increase energy efficiency, the roof was lined with the same 2800°F IFB used in the sidewalls. This structural insulating material was suspended from the roof by a hanger system. It was backed up by a 3-1/2-in. thick “R” grade ceramic fiber* with a density of 10 pcf.

This IFB/ceramic fiber combination construction made the new roof three times more energy efficient than the old ones. Heat loss through the roofs of the old kilns measured 752 BTU/ft2/hr, whereas the heat loss through the roof of the new kiln is approximately 254 BTU/ft2/hr.

The kiln design team also had to find a thin but highly efficient material to insulate certain sections of the wall because the new kiln vents exhaust gas through hollows in its walls rather than through the roof. “This is more efficient than bringing the exhaust out through the roof. It forces the hot gases down in the kiln, where they can aid the firing process,” Fidler says.

Figure 1. Thermal conductivity curves for selected refractory/insulation materials.
But venting through the wall leaves those sections that contain vents much thinner than other sidewall areas. Therefore, the walls in those vented areas required a more thermally efficient insulation. “Microporous insulation proved to be a real problem solver in this regard,” Fidler says. Swindell Dressler used 1-1/2-in. thick 1800°F rated BTU block, which offers approximately the equivalent thermal efficiency as 4-1/2 in. of fiber or 6 in. of IFB. Microporous insulation is the most thermally efficient insulating material on the market and is ideal in areas such as this, where space or weight constraints are critical (see Figure 1).

Advanced Features Equal Better Efficiency

The kiln is designed around Swindell Dressler’s patented top firing system. Thermal flywheels, each containing an assembly of burners, are placed throughout the roof of the kiln. More than 50 overhead burners create a virtual curtain of evenly distributed heat. The housings around the burners, or fireboxes, are lined with layers of ceramic fiber, with the hot face layers being high-temperature mullite fiber.

The kiln cars and their conveyor system are also innovative. The cars in the old kiln were made of a heavy, dense castable that absorbed heat from the kiln. However, this was not a major problem since the cars cycled rapidly and were generally outside the kilns for approximately thirty minutes, not long enough for them to lose much heat. In the new kiln, however, the cars remain out of the kiln for approximately six hours, so they cool to ambient temperatures between cycles.

The new cars had to be made of a lightweight material that would not absorb much heat from the kiln, could withstand the potential thermal shock from the fast cycling and yet still support the weight of the brick they would carry. To meet these criteria, Swindell Dressler designed a virtually all-fiber kiln car made of fiber modules. The cars are covered by thin firebrick setting slabs, manufactured by Thermal Ceramics, which are designed to withstand the thermal shock potential. The mass of the old cars was about 162 lb/ft2, while the mass of the new cars is approximately 36 lb/ft2. Potential heat storage savings should reflect these mass proportions.

Under the new system, the cars move on a continuous conveyor with an innovative mechanical turning system that shifts 90 degrees for loading. The IFB ware to be fired are placed on end on the setting slab, maximizing surface area exposure for uniform heat transfer as they pass through the tunnel kiln firing system.

Advantages of the New Kiln

Although the new tunnel kiln has not been operating long, it is expected to significantly reduce Thermal Ceramics’ overall manufacturing costs. By using a new kiln lining design built around the energy efficient IFB and ceramic fiber products they manufacture, the company has cut its energy consumption in this area by approximately 10%.

The overall tunnel kiln construction project cost approximately $3.5 million, including the in-house demolition work and insulating materials supplied by Thermal Ceramics. But the company expects a fairly quick payback from the anticipated reduction in fuel usage.

What’s more, the IFB products fired in the new tunnel kiln are more consistent and have higher strengths than those fired in the old kiln. Early testing indicates that the average cold crushing strength of the brick is as much as 20% higher, based on values for a standard 2300°F IFB (K-23).

For More Information

For more information about the new kiln and its insulation, contact Thermal Ceramics, P.O. Box 923, Dept. 167, Augusta, GA 30903; (706) 796-4200; fax (706) 796-4328; e-mail tceramics@thermalceramics.com; or visit www.thermalceramics.com.

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