SPECIAL SECTION/BRICK & CLAY RECORD: The Evolution of the Low-Set Brick Plant
February 1, 2008
It has long been recognized that drying and firing brick a few courses high can provide many benefits compared to conventional practices. The low-set concept allows for faster firing cycles, which provides manufacturers with the ability to dry and fire in approximately a quarter of the time required for traditional methods. Simpler, less expensive automated setting and unloading machinery is another important factor, as is reduced manpower requirements. The more uniform temperature distribution results in improved physical properties, and low-set eliminates bottom course deformation with recovery approaching 100%. In addition, the concept’s flexibility can complement the adaptability of robotic setting or unloading, and low-set also offers the potential for dramatic energy savings.
The evolution of the low-set brick plant has occurred incrementally over the last 30+ years as improving technology allowed the development of three key elements: a post and fiber lightweight kiln car to minimize the mass to be heated, a unique top-fired gas combustion system (or solid fuel) to eliminate car refractory thermal abuse, and ceiling-type recirculation fan systems for temperature uniformity and fuel and horsepower efficiency.
Early ChallengesIn the early 1970s, the General Shale Brick Co. recognized the potential benefits of a low-set brick plant. General Shale, along with other brick manufactures and the Harrop Kiln Co., designed and built the first production low-set plant in North America. This kiln was low-set and top-fired with a continuous push of the kiln cars through the kiln. The plant operated for five years before it was determined that it was not economically competitive with a conventional brick plant. One of the primary reasons for the plant’s failure was the kiln car’s ratio of heavy car refractory to the relatively small mass of the product to be fired, which created firing uniformity problems as well as high gas consumption per pound of product.
The energy crisis of the 1970s caused most ceramic plants to be concerned with heavy refractory kiln cars and prompted them to look for an energy-saving alternative. For many ceramic manufacturers, the roller hearth kiln seemed like the perfect solution since it eliminated the kiln car completely. The roller hearth kiln was already successful in other sectors of the ceramic industry, having virtually taken over the firing of ceramic floor tile. The roller hearth is the kiln of choice when the product can be fired directly on the rolls, like floor tile, with no support slab or deck.
However, most ceramic products require a support deck, which means manufacturers must buy the decks, use energy to heat and cool the decks, reduce production rates because of the heat sink of the deck on the product, and supply a return conveyor for the decks. Even with the disadvantage of the support deck, many ceramic industries started to fire their products, such as sanitaryware, on a deck in a roller hearth kiln to avoid the heavy kiln car. This worked well, but the inherent design of a roller hearth limits the kiln width and the weight of product that the rolls can support. As a result, many roller hearths were required for large production volumes.
Refractory ReformThe refractories on a tunnel kiln car have two functions: they must support the load, and they must serve as the fourth wall of the kiln to minimize heat transfer through-and heat storage in-the kiln car. Conventional kiln cars use the same heavy refractories for both functions. This refractory design is adequate for supporting the load, but the heavy refractory suffers from high heat storage and high thermal conduction that cause it to absorb considerable heat while traveling through the kiln.
The breakthrough solution was to separate the two functions of the kiln car refractory. First, the car was built with very lightweight refractory fiber to be the fourth wall of the kiln and insulate the car steel. Second, lightweight hollow posts were installed through the fiber to attach beams to support the load. The new refractory system eliminated over 80% of the heat storage and cut the conduction heat transfer in half. The result is a kiln that is essentially as thermally efficient as a roller hearth, but can be designed to any kiln width and product weight.
In the mid 1980s, Canada Brick (now Hanson Brick) built a semi-low-set (eight high) plant with the Swindell Dressler kiln company. This was the first brick plant to employ this first key element of a lightweight ceramic fiber kiln car base with posts and deck to support the load, along with a continuous push. This allowed the plant to produce uniform brick with low fuel consumption. However, the under-deck firing system resulted in high post and deck replacement costs.
Combustion and CirculationThe early 1990s brought an innovative client to Swindell Dressler. Brick and Pipe (later Nubrik and now Austral Brick) from Australia wanted a 30-ft-wide low-set kiln, which was too wide to use an under-deck firing concept. As a result, Swindell Dressler hired an outside fluid flow expert from the aeronautical industry who developed and built a cold model from which Swindell Dressler then built a full-scale hot model.
The goal was to harness the full recirculation ability of high-velocity burners to create near-perfect temperature uniformity throughout the load. High-velocity burners were installed in a proprietary angled kiln roof arrangement to force 100% of the recirculated gases down through and under the load, and back up through the load, six times. It is well known that high-velocity burners can recirculate six times their volume, but until this application high-velocity burners were used primarily to circulate over the load and under the load, such as in an under-deck-fired kiln or within the lane for a lane-fired kiln.
The plant was the first time high-velocity burners were implemented to recirculate this effectively through the load. This second key element of the low-set brick plant was the perfect complement to the continuous push, lightweight kiln car and low-setting pattern. The firing system eliminated the kiln car refractory failures and subsequent replacements caused by the temperature and thermal shock from the under-deck-fired combustion system.
It is important to note that solid fuel could be used in place of gas burners and would complement the low-set kiln concept. It took a fluid flow expert a great deal of time and money to develop the concept to distribute the concentrated heat release of a high-velocity gas burner uniformly throughout the load. Particles of solid fuel, by their nature, will distribute the heat release uniformly throughout the load. A well-designed arrangement of top-fired solid fuel lances would create exceptional temperature uniformity and efficiency with the fast moving, continuously pushed low-set load.
In the late 1990s, Pacific Clay Products selected a Swindell Dressler low-set kiln, which was later lengthened to increase production. At the time the kiln was lengthened, low-horsepower ceiling type recirculation fans were added to the dryer and preheat and cooling zones to improve temperature uniformity. These 1.5 horsepower (HP) fans recirculate 10,000 cfm per fan. Recirculating fans proved to be the third key element in the total design concept of low-set. Convective heat transfer is especially compatible with the low-set pattern.
Bringing It All TogetherThe best example of all three key elements of the low-set brick plant working together is the 2006 Swindell Dressler installation at Rocky Ridge Brick in Maryland. This is a molded brick plant set four high in a cross-set pattern to match existing products. The plant demonstrates that solid (or cored) brick can be set four high in a conventional pattern that any brick manufacturer can relate to.
Even though this kiln is set only four high, it is shorter in length than recent conventional lane-fired kilns of the same production. Much faster cycles combined with the elimination of the firing lanes make this possible. The plant has resulted in record-setting performance, including the achievement of fuel consumption of 650 gross BTU per pound of fired brick (325 net KCAL/KG).
It is interesting to note that excellent temperature uniformity, low fuel consumption, low horsepower and a small-volume scrubber (if required) are all results of this kiln design. A large space should be left over the load to allow for maximum preheat vertical recirculation of the ceiling-type recirculation fans. This arrangement results in significant heat being effectively removed from the exhaust gases (reducing the temperature and volume of the gases) and transferred into the load. The preheat fans are so effective in transferring the heat into the load that the amount of natural gas required in the furnace zone is significantly reduced, which, in turn, reduces the exhaust gas volume.
In addition, the recirculation fans are equipped with variable-speed drives that allow the speed of the fans to be adjusted to shape the preheat curve and create temperature uniformity through the load. These fans replace the added air volume and energy costs of preheat gas burners and cold air roof jets as used in a conventional kiln.
The pressure drop in a low-set kiln is less than 10% of the pressure drop of a conventional kiln, which minimizes undesired ingress of outside air into the kiln, eliminates the requirement of forced-air under-car cooling, and allows for an efficient axial design of the products of combustion exhaust fan. Instead of the 50-HP centrifugal exhaust fan included in a conventional kiln, the low-set kiln can be designed with a 5-HP axial exhaust fan due to the low volume and low pressure requirements. The dryer can be designed with similar dramatic savings in horsepower due to the same principles.
The combination of the three key elements-a lightweight post and fiber kiln car, a top-fired gas combustion system and ceiling-type recirculation fan systems-places the low-set kiln and plant at the top of the list of brick plant concepts to consider when planning a new plant or retrofit project. With this approach, fuel consumption can be reduced to approximately three-quarters and the horsepower about one-half that of a conventional kiln and dryer.