Go With the Flow
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
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.
Developing the New TechnologyCeric’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.
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 ProcessThe 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 InformationFor 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 firstname.lastname@example.org; or visit http://www.cericus.com.
For more information about FLUENT software, contact Fluent USA Inc., (800) 445-4454, e-mail email@example.com or visit http://www.fluent.com.
*FLUENT is a registered trademark of Fluent, Inc., headquartered in Lebanon, N.H.