When last we left this subject, I indicated we would discuss the nuts and bolts of how in fact does the control work for this Pulse Firing. Well, I’m not nearly smart enough to get into the “nuts and bolts” of the control, so to speak. I’m going to enlist the help of a guest blogger, Mike Shay, here at HEAT Combustion Solutions, LLC for the next installment. He’s a controls guy, much more so than me when it comes to the technical qualities of Pulse Firing Technology in as much as he will get into the algorithms, saw tooth graphs and…………….. sorry, fell asleep just talking about that.
Anyway, here we go with what I term “The Everyman’s Guide to Pulse Firing Control.”
Pulse Firing Technology was a system designed to operate High Velocity Burners at their maximum capacity during the part of the heating cycle when the combustion system input must be reduced below 90% in response to the heat demand as dictated by the temperature controller for each zone. In contrast to a Cross Connected Modulated motorized air control valve for a zone that will drive to a lower position in response to a zone temperature controller, the Pulse Fire system utilizes a fast acting solenoid actuated air butterfly valve and a fast acting ratio regulator for each burner. Much like the Cross Connected system, the ratio regulator for a Pulse Fire System receives a pressure signal from an impulse line which is connected to a point downstream of the solenoid– actuated –butterfly– air– valve.
It should be noted that both the solenoid actuated butterfly air valve and the ratio regulator are tested for the severe duty and high cycle times required for Pulse Firing. The cycles for these components can be as frequent as 10 times/minute. To put this in some sort of perspective, we use what could be termed an extreme use example. Let’s say we have a furnace that is heat treating a super secret special alloy shaft for the next generation of nuclear propulsion submarines. I just like submarines and this is my blog so, hey, roll with it. This new shaft requires a heat treat cycle of ramp/soak for 24 hours, 7days a week for 4 years with a pulse cycle time of 10/minute. This would equate to 10 cycles x 60 minutes x 24 hours x 365 days x 4 years resulting in a total number of 21,024,000 cycles. Granted, this is a wildly theoretical suggestion but the point is that the Solenoid- Actuated- Air- Butterfly- Valve and the Ratio Regulator have been tested by the manufacturer of this equipment to withstand these amount of cycles without failure. The robust construction of these two components ensures many years of life before replacement.
Continuing on, rather than controlling the heat input of the High Velocity Burners by modulating all of the burners in that zone to some lower firing rate which will decrease the velocity of the burners, the Pulse Fire System operates in a High/Off or High/Low mode. The heat input is controlled by varying the off-time of each burner while maintaining a constant on-time. This on-time usually ranges between 6 and 10 seconds. It is also controlled by calculating the number of burners controlled in that zone and a certain amount of burners being fired at any given time.
For example, we will compare two 4 burner zones. One 4 burner zone is controlled with Pulse Firing Technology and the other 4 burner zone is controlled in the traditional Cross Connected Modulated scheme. Each of these respective zones has a heat requirement, as dictated by its zone temperature controller. In the Cross Connected system, all four burners will be modulated down to an output of 25% of the designed capacity. This kind of negates the reason for using High Velocity Burners as their output will only be 25% which reduces their velocity exponentially. This will result in a much reduced ability to stir up the furnace atmosphere and negatively impact uniformity. In contrast the 4 Pulse Fire zone burners will each individual burner will fire thusly. One burner will be at high fire for 10 seconds while the other 3 burners at their off or low fire position awaiting their turn in the sequence. The burner that was at high fire will, after 10 seconds, go to an off or low fire position for 30 seconds. Then a second burner will sequence on to high fire for 10 seconds while the remaining burners that have yet to go to high fire wait their turn and so forth. If we were to monitor the PLC as it goes through its sequencing, we would see that each burner takes its turn at 100% output while the other three remain at low or off and await the signal from the PLC to go to 100% thereby achieving our 25% output while being able to still take full advantage of the velocity of the burner by timing and sequence. This is what will allow us to achieve that stirring or Convective Heat Transfer we have been discussing throughout this series. In summary, at 25% heat demand, one High Velocity Burner is at 100% output at any given time while the other three burners remain at low or off, thus achieving the desired 25% heat demand.
Continuing with our example, the zone temperature controller has determined that the process requires 50% heat input. The same scenario as described for a heat demand of 25% is followed for 50%. However, instead of one burner firing at 100% at any given time, we would now observe two burners firing at 100% while the other two burners would remain at low or off until their turn came. So, it follows that at 50% heat demand, two burners firing at 100% while the other two are at low or off, we can achieve the 50% heat requirement as well as maintain the Convective Heat Transfer. In contrast, the Cross Connected system will drive all four burners to 50% of their rated capacity. While this is somewhat better and the velocity is increased to a degree, we are still not enjoying the full benefit of the High Velocity Burner as far as Convective Heat Transfer is concerned.
To summarize, the idea behind Pulse Firing Technology is to utilize the High Velocity Burner and its ability to entrain and stir up the hot gasses in the furnace to obtain Convective Heat Transfer which is the most efficient way to transfer heat to load up to 1200F. Further to that Forced Convection augments Radiation above that temperature. You may refer to the Radiation Convection Curve by IHEA which appeared in the Pulse Firing – Part Three blog post.
That’s it for this time folks. As always, work safe and be safe.