Why is my furnace burning back into the tuyere? The burner is made from exhaust pipe and is plumbed to a 0.75 ID natural gas line running at at 3" of water pressure with a blower providing air. It is constructed using straight pipe and a straight tuyere. The burner is designed to slide into the furnace and originally ended inside the furnace. Over time, the end of the burner has oxidized and is now flaking away. More concerning it appears to be burning back into the tuyere. The gas line is plumbed 3ft away from the burner end and the air is being blown in from the end of the burner tube. Current end of the burner. The furnace was constructed as an inverted top hat with a hot face refractory and an IFB collar between the outside of the furnace and the refractory. Previously, the end of the burner exited just inside the furnace tuyere opening. I'm guessing it's exiting inside the IFB collar now. Fortunately, I can slide out the hot face and inspect/repair it but not sure what to do about it. The MIFCO furnace manual shows a slight restriction on their burner and then a cone shape exiting into the furnace. Would a reducing cone on the end of the burner tube increasing velocity and push combustion into the furnace and outside the tuyere?
To hazard a guess, I'd say the burner tube cross section is too large for the volumetric flow of air/fuel. That can cause the exit velocity to be less than the flame propagation speed for your fuel. You can up the fuel/air flow, or reduce the burner tube, or a very common feature is to install a baffle.......a plate with hole(s) in it. Maybe even perforated plate. It will reduce the cross sectional area, and increase the exit velocity to the point where the burn can only occur outside the tube. If you knew your volumetric flow you could look up the flame propagation speed and calculate the required area......or just experimentally arrive at same. Your burner may appear to burn properly outside the furnace but when placed inside the furnace, the additional pressure drop probably further reduces the fuel air flow producing the back burn.....especially sensitive with the very low pressure NG source. Best, Kelly
Thanks Kelly, its exhaust pipe so the easiest is to swap in different reducers or slot the pipe to drop in baffles with different size holes. I'll give a go this week and report back.
