Ceramic Industry

SPECIAL SECTION/BRICK & CLAY RECORD: Firing in a Crisis Economy

August 1, 2009
For the most efficient results, a kiln’s heat input should be compared with the heat absorption capability of the brick setting in all of the kiln’s functional zones.

In this economic climate, many brick manufacturers are cutting their plants’ output in order to survive. The method presented below allows manufacturers to redefine their optimal firing schedules based on lower output levels.

Equation

The kiln’s heat input should be compared with the heat absorption capability of the brick setting in all of the kiln’s functional zones: preheating, firing and cooling. The required heat input (Hi) is defined by the heat balance of each functional zone of the kiln. The heat absorption capability (Ha) is found through the following equation:

Ha = h x S x ∆T

where h = the heat transfer coefficient, which depends on the flow velocities along the kiln channel and is limited by flow passing resistance; S = brick setting, which occurs when the surface is opened to direct contact with airflow and is defined by the package machine and quantity of carts in the kiln channel; and ∆T = the temperature difference between the airflow and brick, which depends on the firing curve.

When Hi > Ha, the kiln works with energy losses; in the case of Hi < Ha, not enough heat is available for accurate firing. For the optimal firing program, Hi must be equal to Ha.

Figure 1. Defining optimal flows and heat consumption when the kiln’s output has changed.

Example

To find the most efficient firing schedule, the firing parameters in each functional zone should be adjusted to reach Hi = Ha. In one example, the kiln was 336 ft long with 28 cars carrying 29,600 lb of brick each. The manufacturer first analyzed the brick setting geometry to define the surface that was open for heat flow (S = 1165.8 sq ft), as well as the area available for airflow passing through the brick setting (A = 21.66 sq ft).

Next, the manufacturer studied the kiln’s heat balance to determine the theoretically required hot gas flow in the preheating zone, cold air in the cooling zone, and fuel consumption in the firing zone. Calculations regarding the heat absorption capability of the brick setting were then performed and compared with the heat required for each zone in order to determine if heat was lacking or excessive.

Calculations varying the hot and cold air input were then performed for different firing schedules in order to reach the Hi = Ha state. (The quantities of the cars in the zones did not vary.) Finally, the company developed a graph that illustrates the optimal firing schedule when the kiln’s output has been changed (see Figure 1). By following this graph, the manufacturer has found it simple to define the optimal input and appropriate heat consumption at different output levels.

For additional information regarding improved firing performance, contact Credo Engineering, 7920 N. Caldwell Ave., #16, Niles, IL 60714; (847) 912-8940; e-mail valtitov@gmail.com; or visit www.vtcredo.com.

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