Over the last several months, the Combustion Corner series has challenged readers to spend some time researching opportunities to improve their use of radiant tubes — their performance, efficiency, and uniformity. So far, the series has explored the geometry of a tube, why radiant tubes matter, and what happens inside the tube. When it comes to radiant tube systems controls, what are your options? Read on to learn about the three modes of control.
This column is a Combustion Corner feature written by John Clarke, technical director at Helios Electric Corporation, and appeared in Heat Treat Today’s February 2023 Air & Atmosphere Furnace Systems print edition.
If you have suggestions for savings opportunities you’d like John to explore for future columns, please email Karen@heattreattoday.com.
Technical Director
Helios Electric Corporation
Source: Helios Electric Corporation
This month we will discuss the various modes of control that can be applied to radiant tube systems. We will consider three typical modes of control: on/off, high/low, and proportional control.
When a radiant tube is operated in an on/off mode, the burner is fired full on or completely off. Using this mode of control, the burner must be relit at the start of each cycle. The advantage of this mode of control is that the on firing rate can be optimized to provide optimum heat transfer, and when the burner cycle is off, the tube will idle. If the pulses are rapid enough, there is very little cyclical variation in temperature. The heat capacity (stored heat) of the radiant tube provides a flywheel effect to smooth out the temperature swings between on and off periods. The drawback of this mode of control is that the ignition system, most commonly a spark plug, is energized frequently, loading the transformer and wearing material off the spark plug and the valves that control the air and fuel are cycled frequently. If the cycle time is one minute — the burner must relight, and the valves must cycle over 500,000 times a year. Care must be taken to ensure the components used in this system are rated to survive this demand.
Another mode of control is high/low firing. With this mode of control, the burner cycles between the high firing rate and low firing rate, but instead of shutting down completely, the burners are returned to a low firing condition. In this mode of control, care must be taken to ensure the low firing rate does not overheat the firing leg of the radiant tube. Other than that, this mode of control is very similar to on/off control.
The last mode of control is fully proportional. In this mode of control, the burner fires between 0 and 100 percent of the maximum output depending on the burner demand. The air can be adjusted using a proportional valve or by varying the combustion air blower speed using a variable frequency drive, or in some cases, both. The fuel gas is regulated by a proportional valve or a regulator that matches the output pressure to an impulse or control pressure. Using this mode, the burner fires more or less on ratio (with a consistent level of excess air), or some systems will increase the excess air at low fire to ensure clean combustion and to reduce the available heat at low fire. When a burner has higher levels of excess air, more energy is used to heat the air not used to burn the gas; therefore, less energy is available to heat the furnace chamber. This provides greater turndown (the difference between high and low firing).
Which method is best for a given furnace? That is impossible to say without considering the burner type and geometry of the radiant tube used in the furnace. All three methods can provide good uniformity and efficiency, provided it is appropriate for the equipment in question. In fact, there are applications that blend proportional with high/low firing to meet very specific needs. These systems simply alter the maximum — or high — firing rate to better meet the systems’ requirements.
Again, the control approach is a function of the burner, the radiant tube, and the application. There is really no one-size-its-all; each application must be approached with an open mind. The next column will address the role of heat recovery to efficiency in greater detail.
