Posted by: johnhennessy
Date: April 1, 2010 06:51AM
I read with interest the article by Geary Baese in the April edition of “the Steam Car”; at last a definitive explanation of an interesting phenomena that most of us have experienced at some time.
Sorry Geary I can not agree with your explanation.
In the opening statement “Stanley burners howl because the mixing tubes act like organ pipes”
I think not.
We can agree that, common with all wind instruments, an organ pipe has an oscillating device near where the air enters followed by a tube which acts as a resonating cavity (which determines the frequency of the note).
In the mixing tube assembly on most of the burners that I have come across, the air/fuel mixture is directed through the venturi straight down the tube. And I challenge anybody to blow straight down a 1 inch diameter tube 14 inches long and produce a note. Further, in my burners (Oterway type) the mixing tubes are closed at the end but have a series of holes along them; so the effective resonating cavity is only approximately 2 inches long, which would, in an organ pipe produce a note near the limit of audibility. But have you ever tried a burner on its own without a boiler on top? Howl they do not.(well mine didn’t)
So what is going on?
Here is my explanation for what it is worth.
First find your resonating cavity. Having eliminated the mixing tube we are left with the boiler tubes.
On Locomobile boilers they are around 13 inches (in my case 12.75) long. Based on Geary’s formula this would give a frequency of 258 (sounds familiar) organ note around C That sounds about right also considering that there are, in my case, 305 such tubes all making there contribution to the cacophony.
But now we come to the tricky bit and where I put my head above the parapet. What is triggering and maintaining the oscillation? I think it is instability in the combustion, or if you like “flame oscillation”.
And the cause? Partial combustion of the gasses in the burner flames. The flame oscillation is probably of a much higher frequency than the final pitch of the howl, and this is where the mixing tube parameters come in; this oscillation then maintains the fundamental of the boiler tube cavities.
Consider some of the conditions and parameters. Howling usually occurs at start up and then dies away. It is effected by, amongst other things, venturi size, jet to venturi gap, fuel type, fuel pressure. and jet size. In my case the last two are the most significant. At start-up the air/fuel mixture is relatively cold and often ‘wet’ after a while the mixture gets hotter and less dense, the combustion is improved and the flue gasses get hotter, and surpise,suprise, howling stops.
I like to run the fuel pressure at around 35psi. I can make my system howl continually, or not at all, by changing the jet size. I prefer to have howling at start up so when out on the road, and the burner cuts in, it is good enough it let me know we have hotness where it matters.
In fairness to Geary much of what he says about the characteristics of the howling phenomena I do indorse, but I think he is short of the mark.
So why do these burners howl? Answer, flame oscillation due to incomplete combustion coupled with the resonating cavities of the boiler tubes.
And when the pilot goes out I get my own back and howl instead.