Oil fired crucible furnace

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Re: Oil fired crucible furnace

Post by Harold_V » Thu Sep 02, 2021 4:11 pm

John Hasler wrote:
Thu Sep 02, 2021 7:26 am
I'm assuming, perhaps incorrectly, that the mixture of air and atomized oil enters the fat end of the tapered tube. If so you have a blowback hazard because the flow rate (not the pressure) toward the fat end may be low enough to support combustion.
Your assumption is, as you suspect, incorrect. The atomizer (nozzle) would be at the opposite end of the tube, right where it discharges in to the furnace.

My simple test of the necked down tube was as you described. The flame (in to atmosphere, not in to a furnace) was still outside the tube, but a good portion of the atomized oil collected on the tube and ran to the ground. That most likely would not have occurred had the tube been hot enough. That was a good learning experience, however, as one does not wish to dump raw oil in to a furnace. Likewise, it won't occur if the nozzle is placed in the end of the reduced tube.
Do you have some drawings or photos? I'm havig trouble visualizing this.
I do not. Pretty much all of this is just visualized in my head. I'll likely make a sketch of the required part when I conclude the design, to facilitate machining. I find that helps eliminate mistakes.

Mean time, visualize a 4" diameter tube, short in length (no more than 3/4"), just long enough to enter the existing burner assembly. It immediately reduces size via the funnel shape (likely a 45° angle per side) reducer, to the 1½" stainless pipe in which the nozzle is mounted. The length of the pipe would be in the vicinity of 4". If I incorporate a gate valve, it would be longer. The bulk of this pipe would be outside the furnace, with a port cast in the furnace in which the pipe is mounted. Same design that has been so successful in gas fired furnaces I've built. Combustion, in all cases, has been in the furnace, never in the pipe.

If I conclude that the existing blower isn't adequate, there will be no need for the reducer. That is highly likely.

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Russ Hanscom
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Re: Oil fired crucible furnace

Post by Russ Hanscom » Thu Sep 02, 2021 8:48 pm

Assuming the burner assembly uses a commercial spray nozzle, you may want to investigate as to whether they are available in different spray patterns. One with a lower cone angle might reduce the amount of oil hitting the sides of the reducer.

Most of the nozzle manufacturers I have checked in the past have different volume rates and different spray patterns, so you have the option of selecting one that best suits your needs.

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Re: Oil fired crucible furnace

Post by Harold_V » Fri Sep 03, 2021 1:58 am

Thanks, Russ. That's a good point. The burner DOES use typical nozzles, as I replaced the original with one I had on hand, removed from the heating system of our shop and house. I'd likely need the narrowest angle available, as the spray must enter the furnace though the same diameter as the pipe in which the nozzle is mounted. That will be a relatively short distance, though, likely about a couple inches. I also must explore the concept of introducing the air in a swirling pattern rather than just a straight discharge. The existing burner has a plate that accomplishes that function, and it is sized according to nozzle capacity (the slots in the plate vary in size according to the required nozzle). This plate reduces the area of the discharge such that I suspect that the reducer I'll make will result in similar area, so the squirrel cage blower my work.

By the way, the squirrel cage blower in the burner assembly has two adjustable ports for air intake. Very little opening is needed to support combustion, so I suspect that the blower will be adequate for my needs. The reduced bore may slow it down some, but it has far greater capacity than is required for normal operation.

This burner is almost identical to the one in my heating system for the shop and house.

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Re: Oil fired crucible furnace

Post by liveaboard » Fri Sep 03, 2021 8:38 am

Good point about the nozzle pattern.

You will need to control airflow; if your air velocity is too high, the flame will blow out. If you have any sort of upward exhaust flu, convection is surprisingly powerful and as heat increases you need to dampen the flow, which is easiest done at the intake.
If the air velocity is too slow, too much combustion will happen too close to the nozzle and things might get too hot there.

You can let secondary air in around the burner tube, that will allow combustion inside the furnace. My burners worked that way, as power increased I'd pull the tubes away from the ports and let convection suck it all in. It sounded like a jet plane taking off. I pinched the top of the chimney with a pair of fire bricks to control the gas flow at high heat, but I needed all that was available during the first phase when temperatures were moderate.

Maybe you can have some control over the fuel flow by controlling the fuel pressure. A simple needle relief valve would do it. But those nozzles have a certain range of acceptable pressure, you'd need to check that.
I don't know if you need to control the power for a foundry like I had to for the ceramic kiln.

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