The temperature control of resistance furnaces involves several methods to ensure the furnace maintains or varies the temperature according to the requirements of the heat treatment process. These methods include:
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Automatic Control Systems: The temperature of the resistance furnace is automatically controlled by adjusting the heat source energy based on the deviation from a given temperature. This is achieved through various control laws such as two-position, three-position, proportional, proportional integral, and proportional integral derivative (PID) control. The PID control is the most widely used form, generating control functions based on the proportion, integral, and derivative of the deviation.
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Voltage and Current Adjustment: The temperature can be controlled by varying the applied voltage to the heating elements or the current flowing through them. This can be done by changing the transformer tappings, using an auto-transformer or induction regulator, or in some cases, an independent generating set for variable voltage supply.
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Resistance Variation: Adjusting the resistance of the heating elements is another method to control the temperature. This can be particularly useful in scenarios where the resistance of the elements can be altered to influence the heat developed in the furnace.
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On/Off Time Control: Controlling the ratio of on and off times of the power supply to the furnace is another effective method. This technique, often referred to as duty cycle control, helps in maintaining the desired temperature by cycling the power supply.
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Thermocouple Feedback: In split tube furnaces, thermocouples are used to monitor the temperature and provide feedback to the control system. This helps in mitigating issues such as thermal drift, which can lead to temperature fluctuations.
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Insulation: The type and quality of insulation in the furnace chamber play a crucial role in temperature control. Good insulation reduces heat loss and helps maintain a stable temperature. Ceramic fiber insulation, for example, is commonly used due to its high-temperature capabilities and low thermal conductivity.
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Gas Flow and Heat Transfer Method: The design of the furnace, whether it uses radiant or convection heat transfer, also influences temperature control. Radiant resistance furnaces are better for high-temperature applications, while convection resistance furnaces, often called air circulation resistance furnaces, are more suitable for lower temperature ranges.
These methods collectively ensure that resistance furnaces can operate efficiently and effectively, maintaining precise temperature control as required by various industrial processes.
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