Guest Post: Atmosphere Furnaces & Temperature Uniformity (Part 2)

Nate Sroka, Mechanical Design Engineer
As a Mechanical Design Engineer, Nate Sroka focuses on the mechanical design of atmosphere heat-treating systems. Before joining Ipsen in 2014, he received his Bachelor of Science in mechanical engineering from Northern Illinois University.

Nate also brings with him a significant amount of knowledge and training on combustion safety, NFPA 86, furnace maintenance and heat treatment evaluations.


Part Two: Managing the Burner Control Unit

In the first post of this series, we discussed how the burner functions, as well as the key components of the fuel delivery and air supply. While knowing how the equipment functions is an important part of its operation, it is also essential to know how the burner control unit (BCU) works so you can manage it in a safe and effective manner.

To start, what is the BCU?

The BCU is a safety device that controls, ignites and monitors the burners during on/off cycling and constant operation. It ensures that the air/gas mixture is combusted into heat.

When the furnace temperature falls below a certain point, the BCU starts a sequence that calls for heat; this begins a ten-second window called the trial for ignition.

What happens during these ten seconds?

The first progression is to open the control valves on the individual air and gas supply at each burner head. Once the air and gas control valves are opened, the mixing chamber of the burner is filled with the volatile mixture. The quicker that this mixture is converted into flame, the more controlled this light-off will be.

Recon® III Burners are direct ignited, meaning that a flame ignitor is used as opposed to a standing pilot.  This ignition is controlled through the BCU as it sends a signal to the transformers to operate. This signal begins the second stage of the progression, which consumes approximately six to eight seconds of the ten-second window. High voltage is supplied from the transformer to the ignitor where it is directed along the electrode and into the mixing chamber. There, it is forced to jump an air gap where it is seen as a spark.

From here, the sequence enters the third stage. This period of time is when the electrode transitions from igniting the flame to monitoring the stability of the flame.  This is accomplished with the BCU sending an AC voltage to the electrode. If a flame is present, ions travel to the electrode and electrons travel to the burner ground. The positive and negative half waves are unequal and, therefore, can be processed as a DC signal. Typically known as the flame signal, it can be measured using a multimeter. You can measure the voltage in mV across a 1 KΩ resistor in series with the electrode giving 1 mV per μA. The flame signal strength is also displayed visually on the BCU unit.

If the flame signal is not achieved during, or is lost at any point after, the trial for ignition, the BCU will shut off the air and gas supply to that burner. If this occurs, the operator will also receive a visual and auditory alarm. The BCU can then be manually reset and burners fired.

What else can the BCU do?

The BCU can be used to purge the burner tubes with air prior to opening the gas valve. This may be required if multiple unsuccessful attempts are made to ignite the burners. The BCU is also capable of performing a controlled cooling event. This is initiated when the control temperature is lowered (i.e., as the recipe transitions to a different segment). In this case, the control valve for just the air is actuated. This allows cooler air to enter the burner and reject heat through the burner exhaust.

Beyond understanding how the system works and how to safely manage it, the final step is knowing how to maintain, troubleshoot and correct any issues that may arise when operating the heating system.

Make sure to check back! The final post in this guest series will cover the topic of burner maintenance and performance.

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