Temperature monitoring and control of heat treatment furnaces

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This post was last edited by lyyifeng on 2016-4-7 at 15:22. The heating furnace temperature monitoring system is primarily composed of a PLC (programmable controller) and sensors. The process implemented by a temperature detection system involves sensors converting temperature into electrical signals; these electrical signals are then fed into the PLC through its input module. The CPU of the PLC uses A/D conversion to transform these electrical signals into values that can be recognized by the PLC. Subsequently, the PLC performs automatic control based on the difference between the set temperature (SP) and the actual temperature (PV), using PID calculations. The data related to temperature measurement can be recorded as historical curves, which serve as a basis for determining whether the products from the heating furnace meet the required standards. Selection of testing equipment: For heating steel pipes, we generally use thermocouples to monitor the temperature inside the heating furnace, thereby allowing real-time tracking of the heating effect in the furnace (www.rclkz.com). The furnace is controlled according to the temperature rise curve required for heating the metal. An infrared thermometer is used outside the furnace to measure the temperature of the material being heated, which helps to determine the degree of heating of the metal within the furnace and to detect any abnormalities that may occur during the heating process. The potential difference signal generated by the thermocouple, as well as the analog signal produced by the infrared thermometer, can be acquired by the PLC. After being processed by the PLC, these signals are used as control signals to regulate the temperature of the heating furnace, as well as to record and store this temperature data. (1) For in-furnace measurements, temperature measurement devices are primarily used; currently, sensors employed for online in-furnace temperature monitoring both domestically and internationally generally rely on thermocouple technology. Principle of thermocouple temperature measurement: When two conductors with different compositions are connected at both ends to form a circuit, an electromotive force is generated in the circuit when the temperatures at the connection points differ. This phenomenon is known as the thermoelectric effect, and such an electromotive force is called a thermoelectromotive force. The thermocouples used in the heating furnace during the heat treatment process are mainly K-type and S-type thermocouples. Type K: made of nickel-chromium–nickel-silicon; suitable for temperature measurement over a range of 0 to 900°C over extended periods; generally made of low-cost metal materials. Type S: made of platinum-rhodium 10–platinum; suitable for temperature measurement over a range of 0 to 1,300°C over extended periods; made of precious metal materials and thus is more expensive. Taking into account the temperature measurement range, accuracy grade, and cost-effectiveness of thermocouples, it is recommended to use Class II S thermocouples in quenching ovens during heat treatment, with a measurement error of 2.25–2.75°C; for tempering ovens, Class I K thermocouples should be used, with a measurement error of 2.4–3.2°C. (2) For external furnace temperature measurement, temperature sensing devices are primarily used. The principle of infrared temperature measurement is as follows: Objects with a temperature above absolute zero emit infrared radiation. An infrared detector converts the power signal emitted by these objects into an electrical signal, which is then converted into a temperature signal, thereby enabling remote temperature measurement of the target object. As shown in Figure 1, within the same wavelength range, the higher the temperature, the more energy an object emits. Infrared temperature measurement devices utilize this principle to detect the amount of radiation emitted, and then calculate the actual temperature of the object. There are many factors that affect the accuracy of measurements, mainly including the precision grade of the measuring equipment, the measurement distance, and the wavelength, among others. Objects with different temperatures have different distributions of their radiant energy across various wavelength ranges; the higher the temperature, the more the region with higher radiant energy shifts toward shorter wavelengths. Therefore, for measuring objects with lower temperatures, temperature measurement devices with longer wavelengths should be used, while devices with shorter wavelengths are appropriate for measuring high-temperature objects.