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19th Oct 2016, 09:41 AM #1
Pros and cons of electricity vs gas for crucible furnace
Gas (LPG) and electricity prices keep changing. From that point of view, which would currently be best if I want a furnace capable of melting about 5.5kg of LG2 gunmetal bronze? That's an A4 or A6 size crucible, and LG2 fully melts at about 1000oC.
One problem is that I don't know how long to expect the furnace would need to run, and I don't know the propane consumption of a crucible furnace of that size. I do know that here a 45kg bottle of propane costs $118 delivered from Supagas. But how much of that will the 5.5kg melt use?
On the other hand, I did a calculation based on an electric furnace design that was published by Colin Jones in Australian Model Engineering back in 1989-1990, and figure I'd use about 3kW maximum for a suitably sized furnace (slightly larger than his). I really don't know how many hours it would take to melt the bronze though. But if it was four hours, then at my "shoulder rate" (middle day) of 17 cents/kWh, I'd only be using 4 hrs * 3 kW * $0.17 = $2.04. Which is peanuts, and although a gas furnace would be easier and cheaper to build, the gas would need to come in at a good price per melt to compete with that.
-Ian -
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19th Oct 2016, 03:47 PM #2
Hi Ian
I think the calculation goes something like this
heating the alloy to its melting point requires 0.38 kJ/kg/degree Celsius. 0.38 kJ/kg/degree is the specific heat of manganese bronze
PLUS
the latent heat of melting bronze Taking the alloy from solid to liquid at the melting point, which is about 170 kJ/kg
PLUS
some additional heat to lift the melt to say 100 degrees above it's melting point
The melting point of LG2 bronze is 1010 degrees celsius
so for a 5.5 kg melt, where the ingot has a starting temperature of 10 degrees, the sums would be
0.38 x 5.5 x 1000 + 170 x 5.5 + 0.38 x 5.5 x 100 = 3243 kJ, which is about 900 Watt hours. Allowing for some significant heat loss and inefficiencies in the furnace, I think your 3kW furnace should be able to generate your melt in about an hour.
It's a very long time since I had to do these sorts of sums, so please have someone check my mathsregards from Canmore
ian
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19th Oct 2016, 04:05 PM #3
Thanks Ian -- I'll go through it more carefully when I have the concentration required for the task - which I will do shortly, thanks very much for the solution. If you are right on these calcs, and I myself haven't slipped up on the far simpler costing calculation I did above, that comes out at just 50 cents for the melt. Can it be really true? If so, that's amazing.
From the Alloyavenue.com metalcaster's forum someone has said that a furnace of the type I'm talking about ("Dave Gingery" style with 8" bore) took most of a 20lb barbeque-size gas bottle (that's a 9kg bottle) and two hours to melt gunmetal, but he seems to have since gained more efficiency by aspirating the burner. So if I say 6kg of LPG for the melt, and if an 8.5kg refill (the actual refill volume) costs $20 at Barbeques Galore, then that's about $14 in gas ... compared to 50 cents if I use electricity. What a difference!
- Ian -
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19th Oct 2016, 06:39 PM #4
Well I checked your work as a homework exercise, really just clarifying the sums, and got more or less the same answer.
Heat required to melt LG2 gunmetal bronze:
Two phases are involved:
Q1 = the heat required to raise the bronze from room temperature to melting point
Q2 = the heat required to melt the bronze once it is at its melting point
Plus, to get the bronze to pouring temperature requires a third amount of heat, Q3.
Input data:
Room temperature = 20°C
Melting point of LG2 gunmetal = 1010°C
Casting temp of LG2 gunmetal = 1120 to 1200°C (Foseco handbook)
Specific heat of LG2 gunmetal = 0.38 kJ/kg K
Latent heat of fusion, bronze = 170 kJ/kg
Mass of bronze to melt = 5.5kg
Q1 = m * Hs * ΔT
where Q is the required heat, Hs the specific heat (kJ/kg), ΔT is the temperature change (°C) and m is the mass (kg)
so Q1 = 0.38 * (1010-20) * 5.5 = 2069 kJ
Q2 = Hf * m
where Q2 is required heat, Hf is the latent heat of fusion (kJ/kg) and m is the mass (kg)
so Q2 = 170 * 5.5 = 935 kJ
Q3 = Hs' * m * ΔT
where Hs' is specific heat required to raise the temp of the molten bronze to pouring temperature
so Q3 = 0.38 * (1200-1010) * 5.5 = 397 kJ
Total heat energy required = Q1 + Q2 + Q3 = 2069 + 935 + 397 = 3401 kJ
1 kJ = 0.278 Watt hour, so 3401 kJ = 945 Wh
If furnace is 30% efficient (η = 0.3), then energy required = 945/0.3 = 3150 Wh = 3.15 kWh
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19th Oct 2016, 07:19 PM #5
But there is one thing that bugs me in this calculation, and that is the efficiency figure.
Empirically (I wish I had more and better data) as I mentioned above, it seems that 5.5 kg gunmetal takes about 6 kg of LPG (propane) to melt and raise to casting temp.
The Elgas website informs that 1kg LPG yields 49 MJ, so 6 kg LPG = 294 MJ or 294000 kJ. If the actual energy used to melt is just 3200 kJ , then efficiency is just 1%.
The efficiency of converting gas to heat may not be the same as converting electricity to heat (different losses in the differently constructed furnaces), but still, something is grossly amiss here. Unfortunately the 1989-1990 article by Colin Jones in AME did not provide any performance data.
Any ideas?
- Ian -
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19th Oct 2016, 08:50 PM #6Member: Blue and white apron brigade
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Is this of any use?
Energy Efficiency | Metal Casting
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19th Oct 2016, 09:33 PM #7
Interesting, Bob. Two things about that link -
1. Their comment - "Electric melting has some overall advantages [over gas] in thermal efficiencies and in lower metal melt loss. Combustion inefficiencies, exhaust waste heat loss, and combustion and oxidation contaminants in the form of dross and slag metal melt loss can contribute to fuel-fired furnace lesser efficiencies."
2. Even at their lowest efficiency cited for gas-fired crucible furnaces (7%), that works out at needing just 1kg LPG for 5.5kg bronze (3400kJ theoretical energy input / 0.07 = 49000 kJ approx = 49 MJ, and 1kg LPG converts to 49 MJ). No doubt they are talking about industrial furnaces, not tiny home-built furnaces that are even more leaky. So I guess a home-built gas furnace really could be only 1% to 2% efficient.
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20th Oct 2016, 06:31 AM #8Senior Member
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I used to run an electric kiln for melting bronze.
Pros. They are simple, neat and clean to run. No fiddly adjustments to make, just plug it in and away you go. Mine was made to suit an A8 crucible, and used to take about 3-4 hours to get my bronze to pouring temps. I made most of mine with a handsaw and a round file, (and an angle grinder). Extremely light, (apart from the heavy steel shell.........)
IMG_5021.JPGIMG_5025.JPGIMG_5028.JPGIMG_5030.JPG
You really need to use Al. Oxide firebricks in construction for their ability to reflect the heat back into the crucible. Regular heavy fire bricks or castable refractory soaks up to much heat.
Cons. A Nichrome element, (15 years ago) cost $120. They are extremely fragile, difficult to repair and susceptible to being splashed with flux and dross. I went through 3 elements in rapid succession.
After burning out the 3rd element I went researching gas, (and oil) burners.
Pros. Simple to make, provided you have access to a lathe, (I did), cheap to fuel, (mine is a waste oil burner). You only need castable refractory. Very fast to melt. From a cold start I can have the furnace hot in 15 minutes, place the crucible inside and be melting bronze in 10 more minutes. (To fill an A8 will take about 30 minutes) It will also melt cast iron.
IMG_5386.JPGIMG_5391.JPGIMG_6004.JPGIMG_6007.jpg
Cons. A bit more fiddly than electric. My (oil) burner requires compressed air to atomize the fuel and burner air, (vacuum blower, leaf blower etc) for combustion. Can get messy when you have a brain fade. Extremely heavy castable, (not forgetting the heavy steel shell also) It will also melt cast iron........
Dont believe all the carp you hear about smoke. Mine runs very clean. When you have the fuel/air ratio right there is no soot or smoke, and these pics show it running on used diesel engine oil. It is noisier to run. I use a vacuum motor, and they tend to be whiny. Others use jumping castle blowers or squirrel cage fans. The compressed air for atomization is minimal, about 60 kPa, so the compressor doesn't cycle very often.
You need a few more bits and bobs to make a burner, fine metering valve, regulator, fuel tank, (old bbq bottle), some handyman skills to put it all together. Mine is called a "Nudge" burner, after the inventor/designer, but there are other styles as well, Moya burner, Brute burner, Hot shot, Babbington etc.
I have no experience using a gas foundry. I did research it but the oil won me over because the fuel is free and you get higher temps. The calorific value of oil is higher than gas. I have heard from others that do run gas that its pretty easy to go through a 9kg bottle and still be not ready to do a pour. But that could be due to poor design and/or efficiency of their system.
At the end of the day I would say for minimal use and simple operation the electric wins hands down, but if you are going to use it more than twice a year then look into an oil, (or gas) burner.
PS I have a brand new, unused element for sale if you do go electric.
Cheers Phil
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20th Oct 2016, 07:05 AM #9
Well, so far we've only calculated the energy required to melt the bronze.
To that you have to add the energy required to heat the crucible and any tools in the furnace to the casting temperature and the energy required to heat the furnace itself. The principles will be the same as for the metal, but I've no knowledge of the masses and specific heats involved.regards from Canmore
ian
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20th Oct 2016, 08:24 AM #10
Thanks Phil for your comments, observations on pros and cons much appreciated!
Just to be able to gauge the efficiency of your electric kiln, can you tell me three more things:
- The power or current draw of the kiln?
- an A8 crucible can take 12.5 kg brass - would that be about how much bronze you were melting in the 3-4 hour period you mention?
- what are the inside measurements of the kiln - 'across the flats' and depth?
And on your oil-fired furnace, where do you source waste oil?
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20th Oct 2016, 09:58 AM #11Member: Blue and white apron brigade
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Used engine oil is well know to be mutagenic and carcinogenic so just handling it represents a risk and burning it makes things worse.
The main effects are environmental but burnt engine oil vapours has been associated with skin, bladder, lung and scrota cancers.
Even if you burn it away from anyone pregnant, a couple of times a year upwind in an isolated location I reckon its not worth the risk.
EG See
ENVIRONMENTAL IMPACT OF USED MOTOR OIL
The Science of the Total Environment, 79 (1989), 1 23
By RAFAELVAZQUEZ-DUHALT
Centro de Investigaciones Bioldgicas de Baja California, ApartadoPostal 128, LaPazB.C.S.,23000 (Mexico)
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20th Oct 2016, 10:30 AM #12
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20th Oct 2016, 11:00 AM #13Member: Blue and white apron brigade
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Yeah I hate sounding like nanny about this because folks have been using used engine for donkeys with supposedly little obvious effects. I have mind my old man who used it as chainsaw bar lube. In the late 1970's I studied the heavy metal content of used engine old at uni and was surprised at how much there was in the oil. Even when we went lead free with petrol didn't make that much difference, it just became easier to see all the other stuff.
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20th Oct 2016, 11:06 AM #14
Over on alloyavenue, 'cactusdreams' in Canada says he has a propane furnace with 7" bore, to melt approx 4.5kg 'Cu alloy' (bronze?) in a nr 4 crucible. The furnace has a 3/4" T-Rex burner at 27 psi using 2.41 kg/hr of propane. He uses a blower. He says that the time to achieve first melt is 45 to 60 min (say average 55 mins), and subsequent melt time is 25 to 30 min (say average 30 min). So:
1) He is using about 2.4 kg of LPG to melt 4.5kg bronze/brass
2) evidently about half the energy goes into heating the furnace itself, above and beyond energy vented out the exhaust and other constant losses.
Also, to put it in dollar terms, that's about $5 (AUD) of LPG refill for the first melt and say $2 or $3 for subsequent melts in the run.
Also on Alloyavenue, I dug up various posts by 'small potatoes' who a few years ago reported his 220V, 2400W electric furnace (top-loading 'pottery kiln' style with Kanthal elements - presumably Kanthal A1) melts 8.2 kg bronze to 1230°C in a no. 6 crucible in 2 hours, although when the elements were new it was 90 minutes. So that's about $1 (AUD) in electricity, at least it would be on my tariff.
On those figures, electric melting seems to be about 9 times cheaper than propane LPG melting, just on energy cost, but of course that's not the full story of costs - as 12bolts points out, the NiCr (or longer lasting more expensive A1) elements have to be costed in as a consumable especially if temperatures go up to bronze melting levels. On that point, I haven't dug up info on durability of A1 at 1200°C, so I can't estimate that cost yet.
- Ian -
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20th Oct 2016, 11:37 AM #15Member: Blue and white apron brigade
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I have a 4kW furnace sitting on my back veranda that I got from where I used to work for nothing about 3 years ago but I have not bothered to connect it because of precisely that reason. As you and 12bolts say the NiCr filaments cost a bomb and require very gentle treatment.
FWIW I made a turbocharged Twin torch mains gas forge that fair pumps heat.
I don't use it often and tend to save jobs up until I have enough to warrant using it.
Usually I just run 1 torch and worked out it costs me ~$6/hr to run
Adding the second torch doesn't seem to make that much difference cost or heat wise because the long pipe run from the front of the house to the shed limits the gas flow..
Lots of pictures here //metalworkforums.com/f266/t168...gas-forge-wip?
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