How to heat only what is needed to a specific temperature and maintain that temperature.
Heating derived from electrical sources is used extensively in industrial processes for thousands of widely varying applications. They include:
- Heating water or other liquids for cleaning parts or to transfer the heat to other environments.
- Increasing the temperature of metal pieces.
- Environmental conditioning.
While the primary goal of these and other applications may vary, overriding objectives include to heat only what is needed, heat it to a specific temperature and maintain that temperature for the period of time required by the application.
Coarse temperature regulation can be achieved by simply turning the circuit that is powering the elements on and off using a switch (mechanical or solid-state relays) or contactor. Adding a thermostat will improve the consistency of the amount of heat applied, and adding a temperature sensing element (RTD, thermocouple or associated sensor) will further improve regulation of the heating process.
For the most precise and controlled impact on the treated material, the most common method is to control the power applied using silicone-controlled rectifiers (SCRs). These solid-state devices permit the electrical circuit to be turned on and off as needed, allowing only part of the potential power to be applied to the elements (figure 1).
The SCR arrangement commonly used for heating controls combines two diodes with sufficient capacity to switch the full current of the load. One diode allows conduction of the positive half of the alternating current and the other diode allows the negative current to conduct. The extra tails (marked as 1 and 2) shown in figure 1 are trigger signal connections (gates) that tell the SCR to conduct or block that portion of the AC sine wave. This cycling of the circuit on and off can be accomplished several times each second, thereby allowing the heating element to be energized for only part of each cycle, or for several cycles on and several cycles off.
The method of adjusting the power applied by varying each cycle is commonly referred to as phase-angle firing. By contrast, the approach of allowing full or half cycles to conduct is called burst- or time-proportioned firing.
Either method has advantages and disadvantages. Selecting which method to employ depends upon the type of heating element being controlled and the need to manage issues such as poor power quality or reduced element longevity.
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