In previous months, this series has explored the geometry of a tube, why radiant tubes matter, what happens inside the tube, and radiant tube control systems. For the first three installments, check out Heat Treat Today’s digital editions in November 2022, December 2022, and February 2023. For the month of May, we will continue our discussion of different modes of control for radiant tube burners.
This column is a Combustion Corner feature written by John Clarke, technical director at Helios Electric Corporation, and appeared in Heat Treat Today’s May 2023 Sustainable Heat Treat Technologies print edition.
If you have suggestions for radiant tube topics you’d like John to explore for future Technical Tuesdays, please email Bethany@heattreattoday.com.
Technical Director
Helios Electric Corporation
Source: Helios Electric Corporation
High/low and on/off controls require different control strategies from a proportional mode of control. In all cases, we assume the temperature control will be provided by a proportional-integral-derivative loop (PID loop). The function can be provided by a stand-alone instrument or a PID function in a programmable or process controller. The PID algorithm looks not only at the temperature of the process as indicated by the control element (thermocouple or RTD) and compares it to the setpoint — but it also considers the offset and rate of change as well. When properly tuned, a PID control loop can provide control accurate enough to match the process (actual) temperature to the setpoint within a degree or two.
For the lay person, another way of describing a PID loop is to consider how a driver regulates the speed of his automobile. Assume you are driving and want to catch up with and follow the car ahead of you — to do so, you need to match that car’s speed and maintain a safe distance. What you don’t do is floor the automobile until you get to the desired following distance and then hit the brakes. What you do is first accelerate to a speed faster than the target car to close the gap, then you instinctively take your foot off the accelerator when you get close, slowing gradually until your speed and position are as you desire. In this example, you have considered your speed, how close you are to the car you are attempting to follow, and the rate at which you are closing the gap. A PID loop is nothing more than a mathematical model of these actions.
The PID control loop provides an output — the format can vary, but it is in essence a percent output. It is a percent of the maximum firing rate the system needs to provide to achieve and maintain the desired furnace temperature. This percent output can be translated directly into a proportional output for proportional control — where the firing rate is proportional to the loop’s output.
On/off or high/low controls require a different approach where a time proportioning output is provided in which the burner fires on and off on a fixed time cycle. In this mode of control, the PID loop’s output is multiplied by the cycle time to determine the on or high fire period and the on or high fire time is subtracted from the cycle time to determine the off or low fire period. Cycle times can run from as little as 30 seconds to as much as a few minutes. Obviously, the shorter the cycle time, the more responsive the control, but also the more wear on the control components. The cycle time should be as long as possible but still meet the needs of the process control.
Don’t confuse these pulses with other control methods that are marketed as pulse firing. When people speak of pulse firing, they often mean a pattern with alternate burners firing to provide greater temperature uniformity and heat transfer. This is a very interesting subject and the topic for another day.
