Imitation Experiment of Using a Bimetal Thermometer to Detect the Correct Temperature of a Coal Mill Bearing
After analyzing the layout and temperature measurement principle of the bimetal thermometer, as well as the characteristics of the bearing structure in coal mills, it was concluded that the bimetal thermometer may not be suitable for accurately measuring the bearing temperature of a coal mill. A laboratory simulation experiment was conducted to verify this assumption and propose an alternative solution.
In 1998, the Meixian Power Plant experienced two instances of bearing failure in its coal mill due to high temperatures. The bimetal thermometer used to monitor the bearing temperature failed to provide accurate readings or trigger the necessary protective measures. This raised concerns about the reliability of using bimetal thermometers for such applications.
The working principle of a bimetal thermometer is based on the differential expansion of two metals with different coefficients. When the temperature changes, the bimetal strip bends, and this displacement is translated into a temperature reading. However, this method is designed for immersion in movable media like liquids or gases, not for solid surfaces.
Measuring the temperature of a coal mill bearing using a bimetal thermometer involves point contact with a solid surface, which may not yield accurate results. To investigate this, a lab experiment was conducted to simulate real-world conditions.
Lab Calibration: A new WSSX-401 bimetal thermometer (with a sensing length of 450mm) was tested. It passed verification by comparing its readings with a standard reference in warm water, showing acceptable accuracy under controlled conditions.
Simulation Experiment: An electric kettle was used as a model for the coal mill bearing. A foam layer was placed around the kettle to reduce heat loss, and small holes were made to insert the bimetal thermometer and a platinum-rhodium thermocouple. As the kettle heated up, data from both devices were recorded.
Results:
- The bimetal thermometer did not reflect the actual temperature of the object being measured.
- It showed a delay in responding to temperature changes.
- The difference between the actual temperature (e.g., 84°C) and the bimetal reading (e.g., 46°C) was significant (38°C).
Conclusion:
- The bimetal thermometer is not suitable for measuring the temperature of coal mill bearings, as it fails to provide accurate or timely readings.
- This limitation also applies to other types of bearings, such as those in induced draft fans and blowers.
Recommendations:
- Drill a hole in the bearing housing and install a resistance thermometer instead of the bimetal thermometer.
- Use a fast-response armored thermoresistor with a smaller sensing area as an alternative to the bimetal thermometer.
These solutions can help ensure more accurate temperature monitoring and effective protection against overheating in critical equipment like coal mills.
Bearing Related Knowledge
- Detailed analysis of bearing damage and countermeasures
- Understanding the preload effect of TIMKEN bearings
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