Johnson furnace, uses how much fuel per session?

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PapaLion
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Johnson furnace, uses how much fuel per session?

Post by PapaLion »

I acquired an older nice Johnson crucible furnace recently. I believe it is similar to current model 900 on size and capacity. It was hooked to Natural Gas but I'm figuring on using propane.

the 900 model shows a gas input of about 130,000 BTU /per hour with a capacity of about 16 pounds of Aluminum. Could someone with more experience gauge about how much propane per session that would need? I doubt I'll do 16 # every time of course but if I go buy a tank about how large might I need?

thanks much,
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Post by Harold_V »

One thing I recommend is that you either get a large tank, or several small tanks, so you can draw from them as a unit. The problem comes from drawing fuel too quickly, so the temperature in a single, small tank drops, and the liquid fuel doesn't evaporate in keeping with demand. That will limit the amount of heat you can generate, and may even prevent you from melting a given heat.

There are some knowledgeable people on the forum that should be able to provide some guidance as to the requirements.

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Post by PapaLion »

good idea, I've seen fittings for switching tanks ... but not ones to draw from simultaneously. I'll check that out. I'm still trying to figure out how big they have to be?
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Post by tomc »

What you need to do is find out how many BTU's are in a pound of Propane. Google may be your best friend.

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Post by steamin10 »

Just from my experience I have used the 100# tank for my feed. It is portable enough to chuck in the truck and drive up to a supply just blocks from home and buy any amount of Propane up to a full fill, and take it home to wrestle back inot the shop.

The 100# tank wont freeze or loose pressure until it gets run hard down to the last coupla gallons, then it will how frosting at the base. Teh fact that the tank is so big it is a pretty good radiater and maintains good pressure, even at my rediculous firing rates for a furnace.

This particular tank, does double duty, as it is the core of my Oxy-fuelgas rig for the low pressure industrial torch in the garage. It will blast 1 inch plate without a nod. ( NO, the LP mark on the torch, does not stand for Liquid propane. It stands for Low Pressure.) I can hook it to the house supply of natural gas and do the same thing. The low feed on the house will not adequately run my furnace.
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Post by PapaLion »

thanks much Steamin10, I'll take a looksee at 100# tanks and see how that works out. It comes home to my ranchita tomorrow on a pallet and then we can better ID it. I thik it is a 520-K model Johnson with 250,000 BTU on the tag. My guess is it'll hold about 20# of Al.
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Post by Harold_V »

PapaLion wrote:thanks much Steamin10, I'll take a looksee at 100# tanks and see how that works out. It comes home to my ranchita tomorrow on a pallet and then we can better ID it. I thik it is a 520-K model Johnson with 250,000 BTU on the tag. My guess is it'll hold about 20# of Al.
I'm familiar with McEngelvan's furnaces, but no others. I have always built my own. I'd enjoy seeing a picture of the Johnson if that's possible.

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Post by PapaLion »

pics of the Johnson as it came home. We are deciding whether to use NG or LP.
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Post by Harold_V »

Thanks for the pics, although they only created more questions for me.

It appears to have all of the safety devices, so it will be nice to operate.

I'm somewhat confused by what I see in the first photo. Why is there a disruption in the walls in two places?

How does the protrusion on the right side, second picture (the square portion) relate to the furnace chamber?

In general, the design reminds me a little of the McEnglevan furnaces, but then there's only so much you can do when building such a furnace. I expect all of them resemble one another to some degree.

I like it!

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Post by PapaLion »

Hi Harold,
If I understand your ? there are actually two inputs for the blown gas... swirls in. The larger hole is for exhaust and outside it is the box hanging on the unit... UR supposed to put an exhaust tube there.
couple more pics today
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Post by Harold_V »

Thanks for the additional pictures, and the comments. I hadn't considered the extra port as a discharge. Furnaces I've built, and those I've seen, discharge through the lid.

One thing concerns me. Depending on how long you run a furnace, they get pretty hot at the bottom. If you allow yours to bear on concrete, you may invite a steam explosion. You might see if the manufacturer suggests such a mounting. Otherwise, things are looking good for you.

Where I live, you wouldn't want that thing outside. We get rain more than we see the sun. It would be wet constantly, and limit my ability to put it to work. You folks must enjoy a lot of dry weather.

One of the furnaces I built was for reducing waste materials that contained precious metals. Instead of melting in a crucible, the furnace was built as a tilting reverberatory type, with the charge placed directly in the furnace. It was lined with a 94% alumina refractory. It performed to perfection, but I had 1 pound natural gas delivery. Indications were that I needed everything I had, so you might consider that if you decide on natural gas (which would always be my first choice, assuming it was available). Running propane presents a risk of running out of fuel just at the wrong moment.

Don't know that you'd be interested, but here's a picture of that tilting reverb I built, many years ago.

Harold
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Re: Johnson furnace, uses how much fuel per session?

Post by PapaLion »

The cranking wheel is overwhelming must be fun to do the pour!

We get about 7" of rain /year, sandy dry here... uh oh? next thing u know you'll join the other 100,000 winter visitors who come here to escape the wet and cold:) UR welcome, come down, we could mess about with hot metal for fun.

I figure I will shade the thing some and add a tarp/cover for when not in use and then heap sand around it while in use... could build a metal angle iron rack so it sits up an inch or so and ventilates under it too, that might be an idea.

I have info that I might be a good candidate for an oil burner fuel delivery system. You would use discarded oil. If this is something you fellas have some experience in I would like the input as a choice.

Calculations based on 100 Therms per year use.

SoutWest Gas costs: $1600 to install a main (I dig the 3' deepx 80' long trench.
I need to add the line to the foundry estimated at $500
Monthly cost $12 minimum always even if not in use... and figuring approximately 100 therms per year cost would be about $275/year to run.

Propane: 1 used but nice 50 gallon tank with new regulator and accurate gauge $300
fill for one year to equal 100 therms is about $375 delivered, cheaper if I drag the tank to them.
So it would take me running the Natural gas system 12+ years (I'm 61 now:) to just break even on the install costs vs the higher annual run cost... and a forever monthly bill while with propane when I'm done someday I turn it off/ sell it etc.
3rd choice might be the used oil burner?
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