Understanding Infrared Heater Control: A Guide to Components and Optimization

How to Control Your Infrared Heater: Understanding Key Components

As a manufacturer of infrared heaters for a multitude of industrial heat processing applications, a question that we are frequently asked here at Casso-Solar Technologies is, "How should I control my infrared heater?" In the most common setup, there are 3 main components to an infrared heater control circuit:

• A temperature measurement device
• A temperature controller
• A power controller

The measurement device sends a temperature signal to the temperature controller, which then sends a control signal to the power controller, which regulates the power to the heater to maintain a desired temperature.

This post will cover the different options for those 3 components and how they can be best utilized for infrared heater control.

Temperature Measurement Devices

The three most widely used types of temperature measurement devices are thermocouples, RTDs, and pyrometers. While all can be utilized in an infrared heater control circuit, they differ in how they operate. Thermocouples and RTDs are similar in structure as they consist of a measurement probe that is then wired into a temperature controller. Thermocouples are more common in infrared applications because they are able to operate at higher temperatures than RTDs, which often cannot be used above 400°C. RTDs, however, are more accurate. The probe is installed in a location within a process to allow it to measure temperature at a specific location. It can either be installed to measure air temperature of a process environment or the element temperature of an infrared heater.

Pyrometers work in a very different manner. These are optical units that operate at a specific wavelength in the infrared spectrum and measure the temperature of the object in their line of sight. Depending on the material being measured, and the expected temperature range, a pyrometer of a particular wavelength is selected and then connected to a temperature controller. For example, pyrometers for glass processes typically operate at a wavelength of 5 microns. Pyrometers work best for control when the objective is to heat a product to a target temperature because they can directly measure the temperature of the object being heated. Web heating processes in particular are very well suited to pyrometer measurement as they have a constant product flow in view of the pyrometer. Thermocouples are often used when controlling the air temperature in a process environment or controlling the heater temperature is preferred. This could be because the product flow is inconsistent, and you wish to keep the oven environment at a constant temperature when the product is not present.

Temperature Controller

To maintain the temperature measured by one of the devices above, you would connect that device to a temperature controller. This could either take the form of a dedicated temperature controller or a PLC. For a simpler system with a single heater or single zone, a temperature controller is the most economical approach. For a system with multiple different control zones, a PLC often is the best option as you can connect multiple devices to a single PLC. Standalone temperature controllers come with a wide variety of options and configurations, some are single zone, some are multi-zone. The signal from the temperature measurement device is fed into the temperature controller which then uses a PID control loop to produce a control signal that it adjusts as needed. That control signal is sent to the power controller, causing the power to increase or decrease as directed to keep the measured temperature equal to the requested set-point temperature. This principle is similar whether the controller is a standalone controller or a PLC, with the difference being that PLCs can control numerous zones and other equipment as well.

Power Controller

The control signal is received by the power controller which then regulates the power that is delivered to the heater. The two primary types of power controllers that are commonly utilized are solid state relays (SSRs) and silicon-controlled rectifiers (SCRs). SSRs are fast-acting contactors which turn the voltage on and off to a heater to control how it heats up. For example, to run a heater at 50% power an SSR might cycle between being on for 1 second and then off for 1 second. The on-off pulses could also be much more frequent depending on the application and the heater. The cycling rate is adjustable within the capabilities of both the SSR and the temperature controller.

An SCR, on the other hand, provides a much more consistent power output. Instead of cycling on and off, an SCR regulates the voltage that is being sent to the heater. For a 50% output in a 480-volt system, an SCR would send 240 volts to the heater. The way in which the SCR controls the voltage differs depending on whether the SCR is phase-angle fired or zero-cross fired. Phase-angle fired SCRs produce the most consistent output, but this is at the expense of creating electrical noise in the system, which may not be desirable. While it might seem that an SCR is always the better option because it provides more consistent power output, they are considerably more expensive than SSRs so their use must be balanced with budget considerations.​ For higher mass panel heaters (such as Casso-Solar FB, C+, or FHT type), the on-off pulses of an SSR will not noticeably impact the process or the heat output because the heater has too slow of a response to cool down during the pulses where the SSR is off. On lower-mass heaters (tube heaters, especially shortwave infrared heaters) using an SSR would cause the heaters to "flicker" and is undesirable in most processes. In those cases and for high-speed processes where dwell time is short, an SCR is necessary to ensure consistent heat is delivered to the product.

Conclusion

The different components discussed above can be mixed and matched depending on the application and heater type to create an ideal control circuit, but the principle remains the same regardless of which types are utilized. Contact the Sales Team at Casso-Solar Technologies for help specifying which components are most suited to your application and chosen infrared heater.