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My normal course of consulting routinely involves analyzing periodic and tunnel kilns, adjusting combustion systems and controls, or modifying kiln designs to improve performance. Data loggers that produce the true thermal history of the firing process are some of the most effective tools available for these purposes. However, to fully take advantage of this excellent technology, we have to set up the hardware and software so the diagnosis of problems becomes easier.
The EquipmentSeveral systems are commercially available for this task. Since most of my clients use the Datapaq system, my comments will refer to this device. It consists of a data logger and thermal protection, as well as software that allows for the setup of collection, data download and tools for analysis. This system's use of analysis tools makes it far more useful than the old time "traveling thermocouple" curve. Additionally, a new enhancement-real-time monitoring-allows users to see the data as the firing schedule progresses, and if you wish to share the data, you need only e-mail the file to others.
The ProcedureWhen you run a thermocouple (TC) curve for the first time, you will probably see a variety of temperature transients, hot and cold spots, and a whole range of firing anomalies. Since everything happens for a reason (believe it or not!) it is imperative to have a picture of every kiln input and output to figure out what is creating the range in uniformity, as well as the various transients in temperature.
Use of the mimic diagram-a kiln map-is critical to understanding the Datapaq curve. The location of all of the devices-exhaust offtakes, recirculation fans, burners, etc.-is essential to understanding the results and then making improvements. Often, the reason for an improper temperature transient can be traced to a kiln device such as an exhaust port, air jet, hot burner, etc.
The mimic diagram should be accurately prepared and saved as a "process." It is then loaded (selected) in the run and need only be developed one time for the kiln that is being studied.
Setup of the thermocouples is straightforward. Develop a TC map based on setting. The first trial may be general-simply test the outer boundaries of the setting combined with a few points inside the setting. More detailed tests can also be designed to test isolated trouble spots where losses occur. If ware cracks in a particular portion of the kiln car, that area should be scrutinized by locating thermocouples in that position.
Analysis ToolsNumerous tools are built into the software, and these tools calculate critical data for problem solving. In the Datapaq software, for example, the following tools are standard.
- Max/min temperature is useful in determining the relative amount of time at a given temperature. It helps define the soaking time or cumulative time for oxidation.
- Time at temperature helps to further define the soaking time by providing a calculation of time above a certain temperature.
- Rise and fall time allows for the calculation of heating and cooling rates.
- Slopes provides a calculation of the rate of heating and cooling for zoomed areas on the graph. This is very helpful for diagnosing heating and cooling cracks.
- Peak differences allows for a quick calculation of differential temperature.
- Q1 slope shows the heating and cooling rates through the quartz inversion-valuable information to avoid heating cracks and cooling cracks.
- HWI (heat work index) provides the relative heat work applied to all of the TC-monitored positions.
Another feature is the system's capability to overlay curves, allowing for the comparison of different kiln tests. This provides the ability to accurately monitor your adjustments to see if you're making progress.
Interpretation and ImprovementMost of the analysis tools can be configured to look at distinct segments and rates of temperature. Often, when defects are developed in firing, they occur in different areas of the kiln car setting. Using analysis tools can tell us what is different in the thermal history of one part of the kiln car vs. another. You can often diagnose problems by comparing the thermal history of "good" setting areas vs. "bad" setting areas.
At that point, we can look at the mimic diagram and determine what devices are available to help improve the situation, or even evaluate modifications to the equipment.
Ralph Ruark is a registered professional engineer with degrees in ceramic engineering and business, and 28 years of experience in the ceramic industry. He formed Ruark Engineering Inc. several years ago and serves as a technical consultant to a number of ceramic manufacturers and kiln companies. He is dedicated to assisting ceramic companies with a variety of kiln and firing needs, leading kiln analysis efforts, providing training expertise, and improving operations. Ruark can be reached at (941) 360-3111, fax (941) 360-3211, e-mail email@example.com or online at http://www.ruarkengineering.com .