In the modern world of industrial control and automation, there are many different ways to control electric heaters. Electric heaters are not a cheap device, and therefore the controls should be robust enough to provide adequate control and safety over the electric heater. The ideal way to control an electric heater is to use a PID controller in conjunction with a contactor and Solid-state device. Unfortunately, though, most companies choose the cheapest option which is on/off or bang-bang control with a contactor. As we dive deeper into why this option is not the best, first we must explore what PID, Solid state devices, and Contactors are.
So, what is a PID controller? PID stands for Proportional, Integral, Derivative controller. Essentially the controller is taking a steady stream of data points and performing proportional, integral, and derivative calculations multiple times a second to ensure that the controller output matches the desired setpoint. So, what does this mean? Well layman’s terms, the proportional term of the controller controls how fast the output reaches the setpoint, or the rise time. The integral term controls under and overshoot of the setpoint and the derivative term controls how much the control will “bounce” above and below the setpoint before it settles in on the exact setpoint this is also referred to as the settle time. Figure 1 below depicts a typical PID closed loop response.
On/Off control is another type of heater control method. Basically, the system just turns on or off based on a temperature setpoint. When the temp is below the setpoint, the system is on, and when the setpoint is reached, the system turns off. This can be achieved by sending a signal to a contactor or relay, or an SSR. Figure 2 below shows a typical system response for on/off control. As you can see from the image, the system is defined by upper and lower setpoint. This is due to the system being unable to control to one specific setpoint.
So lastly, what are contactors and solid-state devices? Both contactors and solid-state devices are essentially just big, enclosed switches, but they way they operate as a switch is what makes them essential to controlling electric heaters. Contactors are great for coupling and decoupling large loads. They use a magnetic coil to close the switch. Solid state devices are designed to open and close very rapidly. These devices are typically controlled by pulse width modulation, or a 4-20ma analog signal.
Now that we understand a little better the terminology, and the devices in question, we can better explain why using PID control is the ideal method for controlling your heater. By using PID control, you have the ability to pick a desired temperature setpoint, and just press start and allow the controller to do its thing. If one were to use on/off control, then there would be an under and overshoot of the desired setpoint, and a lot of tweaking of values. Now that we understand a little better the terminology, and the devices in question, we can better explain why using PID control is the ideal method for controlling your heater.
By using PID control, you have the ability to pick a desired temperature setpoint, and just press start and allow the controller to do its thing. If one were to use on/off control, then there would be an under and overshoot of the desired setpoint, and a lot of tweaking of values. PID control is also much more reactive to process changes. If the flow rate through the heater changes, then PID control is able to sense the change, and adjust the output to the heater to ensure the desired setpoint is maintained at all times. PID control also greatly increases the lifespan of the heater by minimizing the temperature the heater runs at, based on the setpoint.
PID control also has other built-in features. Custom program setpoints, depending on specifics with the customer process, built in process timers, and data logging capabilities. PID controllers also have built in alarms and relays, that can be adjusted to customer specifics. PID controllers can also be configured for Hi-limit control as a secondary safety mechanism for processes, to ensure the process does not go over a hi-limit, or also as a failsafe to ensure the heater does not have an internal failure do to over-temperature. PID controllers can also be controlled and monitored remotely. This is accomplished via 4-20ma remote setpoint, rs485/422/232 serial communication, or via Ethernet communication, via Modbus TCP/RTU or Ethernet IP protocol. With PID controllers you can also change all of the PID values individually, to optimize your process, or you can use the autotune feature to let the controller figure out the optimal values based on the heater inlet conditions. You can also purchase PI or PD controllers. These with typically perform better than on/off control but do lack when compared to full PID control.
It is important to point out that PID control cannot be used with just a contactor. The average PID controller has a refresh rate of more than once per second, and the contactor is not capable of turning on and off that quickly, and has a relatively low rated cycle count, when compared to a solid-state device. Due to this, the PID controller must be connected to a solid-state device, to enable the heater to fire on and off many times a second. Solid state devices though have a tendency to fail in the closed position, when they are overworked, or reach the end of their service life. Due to this, the proper way to setup a control system for an electric heater, is to run power through a contactor, with a safety circuit wired into the contactor coil, that typically contains a flow/pressure switch, and Hi-limit/overtemp relay. Then out of the contactor, is the solid-state device, with the PID controller sending it the signal to open or close. The benefit of designing the system this way, is you get all the benefits of PID control, while also having a safety circuit to protect against all type of potential system failures.
Does all of this sound very technical and complicated? At Farnam, we design and build in house, custom process air electric heaters, we also design and build heater control panels to UL508A standard. We design these panels around the customers heaters, as well as any other process i/o capabilities necessary to fully integrate into your system.
Works Cited:
https://www.ni.com/en/shop/labview/pid-theory-explained.html
https://control.com/textbook/closed-loop-control/onoff-control/
