Reducing Noise from an Oil-Fired Furnace

That is why I lined the pipes with rigidized kaowool rated 3009F. Seems to be holding up ok. Cheap and easy to replace if it needed.

BTW, I like the sliding pipe idea mentioned above. I’d like to try it.

Denis

 

Well, just a thought, but what about a refractory muffler chamber next to the furnace, or better yet, a second furnace next to the first one, and do two melts at the same time, with the second furnace used as a muffler.
Use a side exhaust on the first furnace so that it will not interfere with the (solid) lid.

 

That would certainly work, you could even slot the inner tube to allow cooling air in, jet engine combustor style. My mild steel skimmer gets orange hot in the exhaust stream so that would be a rough indication of temperature: 900 deg C or 1700 deg F. The sliding reflector could be cast from dense refractory to line the hot side.

 

A muffler may work till charging the furnace

The best way is use a concrete fence/pit and or change the burner

Dave

 

I wanna melt bronze with that sucker! Only if I can get it to burn the black goo.

 

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I do not know how loud other burners are for comparison as I have never been near anther one in operation. I have no reason to think mine is unusual. It would be very interesting if folks did measure theirs using a 15 dollar meter or a free app on their phones.

I can say that putting the stack on quiets it down to a remarkable degree. I melted again today and measured sound 120 feet away at 56dB (you can just hear a low pitched sound over ambient) with the stack. The stack seems to be holding up well. Though the furnace itself might get as hot as 3200 or 3400 degrees (can't really measure it) the gases quickly cool as they exit. Simple expansion of the gas stream likely plays a role. I think those gases must be on the order of 1500 to 2000 degrees. I base that guess on the observation that a preheat grill placed right over the exit gets to a nice medium cherry red or perhaps 1200 to 1400 degrees F after a long exposure. The stack consists of galvanized heating duct (cheap and readily available to me) lined with 1" of rigidized ceramic wool. I has gotten hot enough to oxidize the galvanized coating but does not glow.

Denis

 

The down side to a stack is the sound is up off the ground making the sound go farther just like a load speaker
A stove is very quiet but the blue flame is over 3,000° F[1,650°C] so furnace burn can quiet.

Dave

 

But unlike a stove, a furnace has to have combustion air forced into it to reach the right temperature, and I think it does act much like a pulsajet engine, with the associated noise.

 

It takes energy to make noise, so the quieter the basic process is, the more efficient it is. That being said, well designed large industrial furnaces require ear protection up close. They are very efficient but still make a lot of noise so it's not surprising that a home designed furnace without many years of research would be noisy. Silencers don't add to efficiency but some changes to make more efficiency will result in quieter operation. But when you take a given furnace and push the temperature up you generally lose efficiency to make noise and increase the temperature.

 

One interesting part of that pulsejet video I linked to, is the pulsejet main exhaust is running cool despite the blue flames and hot exhaust gasses coming out of it. There is some physics phenomena occurring when the combustion chamber is glowing yellow hot yet the exhaust jet is cool/black. Pulsejets have to be tuned to very specific pipe lengths to work at all: some sort of pressure standing wave at work? or maybe high speed boundary layer flow? or even thermoacoustics?. Thermoacoustics is the study of sounds waves to generate useful work like compression or heating or cooling from sound waves. Sound waves can be thought of heat energy in gasses: alternating hot and cold zones in gasses (alternating low and high pressure zones in gasses).

Denis' length of large bore stove pipe is smoothing the pulsations out so that there's minimal pressure pulsation/sound energy at the open end, kind of like an electrical capacitor/condenser smoothing a pulsating DC voltage.

I'll put a rounded corner on the furnace lid hole on the inside edge to cut down the high frequency whooshing sound at higher settings. It also removes a hot spot that can cause refractory to break off and fall into the crucible.

 

I have been told that too.
But the color of flame till temperature.
Yellow flame is low temperature
Very blue flame is hottest temperature

The noise from burner is just to make the operator happy thinking it more power.

LP and NG with a blue flame is over 3,000° F[1,650°C]

Dave

 

I am trying to recall how loud porositymaster's Ursutz burner was at Soule.
It seems like it was considerably more quiet than a standard oil burner, but I am not certain of that.

I will ask Chirpy about that, he was there working next to the burner/furnace.

Patent drawing here:
https://patents.google.com/patent/US2387420

Video of on running here:

 

It may be useful to review flame temperatures of common fuels here:

https://en.m.wikipedia.org/wiki/Adiabatic_flame_temperature


Denis

 

I think you hit the nail on the head here. The long column of exhaust gas has a fair bit of momentum and is directly proportional to column length. I Observed that column length correlated directly with noise suppression. There also may be a fractional wavelength tuning effect in play as well. That could be more easily determined if, as suggested in a prior post above, my stovepipe had a "trombone slide" in it. That would be really fun to experiment with.

Denis

 

I think a movable shallow 20-30 degrees cone, with the open end pointing towards the combustion and say a 3-4" hole would reflect any speed of sound pulses if you got the length right. The only trouble is you'd have to remove your insulation packing. Is there an audio spectrum app for your phone that would let you measure the dominant frequency and place the cone at the half wavelength point?. You'd have to add a bit of length for the increased speed at high temps too.

 

So I dug up this graph on Wikipedia and extrapolated an extra 3.16 metres per second speed of sound increase for every five degrees C increase. So if the exhaust if say 1500 deg C
that would translate to a speed of sound of (1500/5) x 3.16 = 948 metres per second speed of sound. If the main sound frequency is 50 Hz then a full wavelength is 948/50= 18.96 metres full wavelength
so the reflector would have to be at the 9.5 metre mark or 31.5 feet!!!. So a quarter wavelength might be practical at 15.75 feet.

 

I do have an app that shows frequency distribution of sound. I will try t measure that with the next melt.

Removing the insulation from the chimney will be problematic. I think the tubing will easily reach bright red temps and will soften and scale. Since the tubes are quite thin, they won't tolerate much scaling or weakening due to heat. Stainless would be more heat-tolerant with respect to scaling. But it would be pricey.

The cone idea is similar to the tuneable intakes n supersonic jet engines?

Denis

 

Not sure about the practicality f a 15 ft stack for my setup. Since the stack needs to move with the top when I lift and rotate the furnace top, I would need to do some serious building to make that work. As it is, the stack weight is approaching a comfort limit. Having the stack topple onto me or, much worse, a visitor would not be good.

I will look for predominant frequency outputs. I have an idea the frequencies will be on the low end.

Incidentally, I did feel the spider-like legs that support the furnace and could not detect any vibration in them. My furnace is pretty heavy (200 pounds?) which may tend to reduce its mass vibration and therefore any transmission into the soil on which it rests.

Denis

 

Similar to two stroke tuned pipes, that link I posted earlier had an animated GIF which was too large to directly display here. Hmm I may have a lead on some 6" stainless pipe that I could fit baffles to.

 

Here is a bit of an update on what my chimney looks like in its current iteration.

I have attached handle to the lower section to make removal (with welding gloves) possible while it is still 450 degrees F

I have punched screws into the wool to keep it from sliding within the steel tube. I used about 30 on the bottom and twenty on the top. No more sliding problems. Size was 5/8” number 8 pan head square drives. Good seals at the top to bottom tube juncture and at the lid is important as a hot jet of escaping exhaust will degrade the tube in any a few melts. I use wool to effect seals.



Denis