Home Foundry

dazz · Jul 11, 2021

Hi
Having never seen a package burner, never done any burner experiments and never cast iron, I looked at the packaged burner option and rejected it. They are too expensive to buy and to ship. They are all designed to operate on the cold side of an insulated furnace wall so they have meltable plastic fittings. They are not designed to be modified, so any sort of experimentation could require a lot of work.

High pressure low swirl nozzle burners work differently to commonly used backyard casting furnaces, in almost every respect. I anticipate experimentation will be required to end up with a usable design. That is where my complete lack of experience will help because I will be unencumbered by the irrelevant knowledge gained from other burner types. At least that is the theory.

Reality tends to wreck theory.

GreenTwin · Jul 11, 2021

The logical response of some to prevent the cool spot in front of the burner tube inside the furnace is to use what I call a pre-combustion style burner.
One example of a pre-combustion burner is the Ursutz type.
I built and tried an Ursutz, and did not care for the bulkiness of it, the radiant heat from it, the rapid degradation of the burner metal due to oxidation, the poor control, etc.

Problems with the Ursutz design:

1. If made from metal, the metal will degrade over time and fail, often in a dangerous way.
2. The Ursutz burner at full output works too well, and it causes rapid crucible and plinth failure, which defeats any gains from using this style of burner.
3. The Ursutz is heavy, bulky, difficult to handle and store.
4. An Ursutz burner made from a high-grade, high-temperature refractory will last for a while, but then will need to be rebuilt.
A good siphon or pressure nozzle burner never needs to be rebuilt.

Here is a photo of my Ursutz-style burner in operation:

GreenTwin · Jul 11, 2021

dazz said:
That is where my complete lack of experience will help because I will be unencumbered by the irrelevant knowledge gained from other burner types.

This is absolutely true, and I have seen more than once where someone comes up with a better design simply because they were unaware of what what others told them would not work.
I have done this myself.
Better to start without being encumbered by what other's think works and does not work.

That being said, the folks that routinely cast a lot of iron successfully that I am aware of use either a siphon nozzle or drip-style oil burner, with an occasional propane burner guy jumping in to prove that cast iron can be melted with propane/natural gas.

I think more if not most would use a pressure style burner like you are designing if they had that design in front of them, especially if you could demonstrate that it functions well.
I have seen at least one video of someone demonstrating a pressure-style burner.
I will look for that.

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GreenTwin · Jul 11, 2021

Here is an example of a pressure nozzle setup, with a commercial heating unit.

My setup for my pressure nozzle will be very similar, but my gear pump will run at 1725 rpm, and will send 100 psi to the nozzle.

Good filtration is a must with a pressure nozzle.
I am going to put my spin-on fuel filter on the low pressure (intake) side of the gear pump.
I am not sure a spin-on fuel filter is designed to operate under pressure, and I will not operate mine on the 100 psi side of the gear pump.

I will have to study this video a bit more to get a full understanding of how he is using all of his components, but this is how you can remotely install the gear pump from the pressure nozzle/burner.

I think this person is using the gear pump more as a means to send a constant pressure to his packaged pressure nozzle burner, but with this arrangement, you could omit the packaged burner completely, and just use a pressure nozzle in a burner tube, as jazz is proposing (he would still need the combustion air blower).
He is basically running two gear pumps in series.
The gear pump at the packaged burner increases the fuel pressure to 100 psi, and the gear pump at the fuel tank provides a consistent low fuel pressure to the input side of the packaged gear pump.
More complex than it needs to be, but it does look like it accomplishes what he is trying to do, which it not have burner fluctuations due to temperature changes, and so a successful project build.

Edit:
I see what looks like a compressed air pressure regulator at the packaged burner, which seems to indicate that he is using a siphon-nozzle burner with his packaged unit? I will have to study this, but anyway, it does demonstrate a gear-pump in action.

dazz · Jul 11, 2021

GreenTwin said:
...
This is a view of the interior of a furnace operating on two siphon nozzle burners at 180 degrees.
The flame is even around the crucible, and the combustion air velocity is divided by 1/2 for each burner tube.
The lower combustion air velocity helps keep the flame low in the furnace, and reduces the tendency of the flame from striking the back wall of the furnace opposite the burner tube and then climbing upwards towards the lid.
...

I suspect that a low swirl pressure nozzle burner, that does not require a high velocity compressed air flow, will achieve the same benefits of a two burner siphon nozzle burner. A short wide (high intensity) flame should help reduce wall climbing.

GreenTwin said:
...
The wall climbing does not seem to adversely affect the furnace operation, but it makes me wonder if I could save a bit of time with a dual burner 180 degree arrangement that did not wall climb.
...

With a closed container like a furnace, it is relatively easy to calculate the thermodynamic heating effects of surface radiation. It turns out the thermal radiation from the furnace walls is really important. The time required for a burner to heat the furnace walls is dependent on the thermal mass of the hot face and beyond. If you want to save time heating metal, try building a furnace with a thin light hot face, backed by really good insulating material ( = low thermal mass). At least, that is the theory.

GreenTwin · Jul 11, 2021

One guy online uses a high-pressure relief valve on the pressure side of the gear pump, so if the nozzle clogs, then the relief valve opens and returns fuel to the fuel tank.

I think I will add one of these to my pressure nozzle setup.

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GreenTwin · Jul 11, 2021

dazz said:
If you want to save time heating metal, try building a furnace with a thin light hot face, backed by really good insulating material ( = low thermal mass). At least, that is the theory.

Yes that is exactly what I did with my second furnace, and it heats up much faster than my first high-mass furnace.

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GreenTwin · Jul 11, 2021

The pressure nozzles come in solid and hollow cone styles, with varying angles.

I am going to try a solid cone (blue), 2.5 gal/hr, 45 degree angle first, and see if that works.

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dazz · Jul 12, 2021

Gear Pumps
I have noticed that shaft driven gear pumps are normally fitted to package burners. I think that is probably so that if the fan stops, so does the fuel flow. A really good safety feature for unattended boiler operation.

My electric pump will continue to run if the fan stops. I think that will be OK for attended casting operation. It also means if I have got the wrong sized fan (highly likely), I can make a change without needing to modify other items. Other than that, as long as a pump of any type can reliably deliver fuel at pressure, it doesn't matter.

Propane Fuel
This is something I do know about. Any form of gas fuel would be a poor choice for a DIY furnace because the gas takes up more space than a heavier liquid fuel. As a result, there is less volume available for air, leading to low energy density in the furnace. Diesel has greater peak energy density than natural gas-by a factor of more than of three (generally 129btu versus 37btu). It will take longer to heat a melt with gas.

The solution is to pressurize the furnace to raise the energy density. Not practical for a home furnace. It is done at a larger scale. The GE LMS100 gas turbine generator has a strengthened CF6 engine at its core. On liquid aviation fuel, the CF6 engine puts out around 50MW when installed on a 747. The LMS100 includes a compressor and inter-cooler (like a turbo-charged car engine) and puts out over 100MW burning natural gas. So yes I can see that burning propane to melt iron is possible, but so is poking a stick into your eye.

Burner nozzle set back

I think it will be desirable to locate the nozzle about 5cm to 10cm from the interior of the furnace hot face (set back), so the flame has time to heat the air before hitting the base of the crucible. This may require some bell mouth shaping of the turere to accommodate the combustion zone, a little like a rocket engine. What is the best setup? The answer is that I don't think anyone knows the answer, so this will be something that requires a lot of experimentation.

The same for nozzle spray pattern. I have gone for a hollow cone type nozzle because this will maximise the fuel in contact with the air. The volume where the fuel and air will burn (pure air doesn't burn. pure fuel doesn't burn) is a thin sheet. Is that the right choice?? Who could know without trying it.

Fuel Tank / Line
I agree with your choice of steel fuel tank and protected fuel line. I plan something similar. Safety first.

GreenTwin · Jul 12, 2021

dazz said:
Burner nozzle set back

I think it will be desirable to locate the nozzle about 5cm to 10cm from the interior of the furnace hot face (set back), so the flame has time to heat the air before hitting the base of the crucible. This may require some bell mouth shaping of the turere to accommodate the combustion zone, a little like a rocket engine. What is the best setup? The answer is that I don't think anyone knows the answer, so this will be something that requires a lot of experimentation.

The same for nozzle spray pattern. I have gone for a hollow cone type nozzle because this will maximise the fuel in contact with the air. The volume where the fuel and air will burn (pure air doesn't burn. pure fuel doesn't burn) is a thin sheet. Is that the right choice?? Who could know without trying it.

I have noticed that nozzles are very prone to drip into the furnace if they are not about 1/2" from the furnace end of the burner tube.
You may get a combustion zone working, but I could never get that to work without dripping into the furnace, and I seen others getting the dripping when they deviate from the 1/2" dimension.

I suspect it will not make any measurable difference between a solid and hollow cone, but I have not tested that.

It may make more of a difference with the spray angles, especially if it is so wide that it causes dripping.
I have not tried a wide angle nozzle, so I am not sure if it would work with a furnace.
My guess is that the flow of combustion air coming out of a 2.5" diameter burner tube will tend to create a narrow slip stream that would probably drag along the nozzle discharge at a wide angle.

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dazz · Jul 12, 2021

Hi
A dripping nozzle would be a safety issue and something to watch for.

Unless the nozzle spray shape causes some significant problem or deficiency, I don't think it will be practical to see any meaningful difference.

I am thinking of achieving burner set back by adjusting the shape of the furnace in the immediate vicinity of the burner as conceptually shown in the attached sketch. The attached sketch is a view of the bottom section of a proposed furnace. The furnace is truncated in the drawing for clarity.
Above this section, the furnace would be the conventional cylindrical shape.

The aim would be to provide a reasonably clear volume for the flame to form and burn before impinging on the furnace wall or crucible. That should improve the performance of the furnace.

Dazz

GreenTwin · Jul 12, 2021

There was one guy who used a pool noodle (round foam) to make a spiral in the cast refractory low in the furnace.
I considered that, but did not use it since it made the furnace design much more complex, and more difficult to repair.

I think his idea was to give the air and fuel one good spin to reach full combustion, while holding that first spin low in the furnace.
I never heard back from him how it operated.

I think you are using a similar technique.
If I were making such a shape, I would extend the tuyere out, but not change the round shape of the furnace.
I sort of did this with my 2nd furnace, but it was from the standpoint of having a longer tuyere so that the burner tube would have enough area to seal correctly.

My tuyere extension was removable, which can be a good and bad feature.
If you used a removable tuyere extension, then you could try different sizes and shapes.

That is my 2 cents on giving more combustion area.

If you did use your design below, you should extend out the end of the spiral slightly, so as to start to turn the air before it strikes the flat wall on the left, otherwise you could disrupt the combustion air flow.

I will post some photos of my 2nd (120 lbs of cast refractory) furnace, which was a fraction of the weight of my first furnace.

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GreenTwin · Jul 12, 2021

The only things I would change on this furnace are:

1. Bring out a lip on the top of the hot face, to make a 2" wide sealing joint, where the top of the hot face contacts the lid refractory.

2. Make the tuyere extension from soft 2,600 F fire bricks, and use a hole saw to drill the burner tube opening in them.
I am going to retrofit this change to my furnace #2.
The bricks will be flat on the bottom, and so will rest on the base, and not rely on the hot face or a strap for support.