Go With the Flow

June 1, 2002
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A new micro-flow dynamical combustion system can improve temperature distribution and uniformity in the firing process.

The key issue when considering the design of a kiln is how to obtain the best uniformity and distribution of temperature and atmosphere during the complete firing cycle (heating and cooling). In general, three major areas must be considered:
  • General design of the kiln
  • Uniformity of thermal inertia
  • Heating and cooling equipment
With regard to general design, it is important to consider that a kiln, including its load, is mainly heat-regenerative—i.e., at the end of the heating cycle, the maximum amount of energy that has been put into the kiln is released during cooling. The setting geometry, position of the burners and flues, and design of the insulation are important issues in reducing energy consumption. However, achieving improved performance in temperature uniformity and fuel economy also requires an increase in the exchange rate between the flow of gases entering the kiln and the flow of gases entering the load, as well as an efficient and uniform exchange of radiant energy between the kiln and the load. For this reason, the thermal behavior of both the lining and product must be considered.

During the heating phase of the firing cycle, energy flows into the product, its supports, kiln cars and lining because the products of combustion flue gases import thermal energy through the relevant surfaces into the mass. During cooling, the heat flow reverses its direction due to the input of cold gases. A perfected kiln design, including the kiln cars, must have uniform thermal inertia to obtain a well-balanced energy exchange with the product and a consequential improvement in temperature uniformity.

These goals have been difficult to achieve with conventional heating and cooling equipment. Recently, however, a new technology was introduced that uses a “diffusion air system” to provide a fast, uniform firing process.

Figure 1. The MDS improves the shape of the burner jet, as seen in this burner velocity profile.

Developing the New Technology

Ceric’s Research and Development Division first studied the functions required for heating equipment using sophisticated computational fluid dynamics (CFD) software (3D FLUENT‚*). The software simulates the flow of gases inside the kiln and predicts the behavior of certain heated areas of the kiln and load. Based on extensive modeling, researchers developed a new micro-flow dynamical combustion system (MDS) that improves the burner mixing effect, gas flow uniformity and atmosphere recirculation, thereby improving temperature distribution and uniformity.

To reach the optimum distribution of temperature around the product, the best possible mixing action between the injected flow and the surrounding atmosphere in the kiln must be obtained. The development of standard diffusion burners, operating in either a continuous or pulse mode, has improved temperature distribution. However, a large burner orifice is required to mix the combustion gases with the secondary air as the mixture enters the kiln. As a result, the outlet diameter of the gases is typically oversized, and the impulse required to achieve the fundamental mixing effect of the products of combustion exiting the burner is too low to produce a uniform temperature distribution.

Figure 2. The MDS achieves a high level of uniformity, as seen in this profile of the temperature distribution in the burner jet.
Both the primary combustion air and the secondary air are typically exhausted from the kiln through a flue. The MDS, which adds the secondary air inside the kiln using a small nozzle at high velocity rather than mixing the air inside the burner block, increases the speed of the exhaust gases by two to three times that of standard diffusion burners. Since the mixing effect of the burner depends on the impulse factor, which is proportional to speed, the result is a vast improvement in temperature distribution and uniformity (see Figures 1 and 2).

Figure 3. The double well of pressure provided by the MDS allows improved mixing, as seen in this static pressure profile.
The MDS also features high-speed convergent air nozzles that accelerate the flow of the combustion gas and increase the mixing capacity of the burner. This action improves the temperature uniformity in the kiln and throughout the setting by more evenly distributing the dynamic and static pressure gradients (see Figure 3). The position and size of these high-speed air nozzles also provide the ability to increase the total flow of air required to accelerate the rate of cooling, whether the process operates at high or low temperatures. The system can be used conventionally (with ambient air) or with a recuperative system to enable a faster rate of temperature increase at higher temperatures (over 1600C [2900F]).

If the application requires a reducing atmosphere, the MDS can maintain the improved velocity of combustion gases. The additional gas is injected through a small nozzle at high velocity in the same way as the secondary air, allowing the high mixing action to remain constant through this important part of the firing cycle.

In addition to improving firing efficiency and temperature uniformity, the MDS also provides another important benefit. By profiling the flame to the highest possible degree (see Figure 1) and continuously recirculating the flue gases, the system reduces the amount of oxygen available for combustion, thereby lowering NOx emissions.

Optimizing the Firing Process

The MDS can control temperatures from below 70 to 1800∞C (150-3272∞F) with high quality and accuracy, allowing perfect control of the kiln throughout the firing cycle. The level of control provided by the MDS can be especially advantageous during the early stages of firing at low temperatures, where a tightly controlled pre-heating or post-drying cycle can help improve product quality.

For More Information

For more information about the MDS and how it can be applied to meet specific firing requirements, contact Krzysica at Ceric Inc., 350 Indiana St., Suite 550, Golden, CO 80401; (303) 277-0404; fax (303) 277-0506; e-mail info@cericus.com; or visit http://www.cericus.com.

For more information about FLUENT software, contact Fluent USA Inc., (800) 445-4454, e-mail sales@fluent.com or visit http://www.fluent.com.

*FLUENT is a registered trademark of Fluent, Inc., headquartered in Lebanon, N.H.

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