Flame temperature of Propane vs Oil

[kelvin2164]

I have a small propane furnace that I use for casting aluminium and bronze. It works fine but I need to do cast iron.
Research suggests a waste oil burner, but why? The flame temp of propane is 1960C. 600 degrees higher than the melting point of cast iron. The flame temp of oil is 2100C. Only 140 degrees higher. Why wont propane work with cast iron?
Also, if I preheated my air supply, with a heat exchanger, by 200C, would my flame temp on propane then be 2160C.

 

[Cogsy]

It's about the energy contained in the fuel, rather than the temperature it burns at, plus the energy required to melt the cast iron, rather than it's melting temperature.

By weight, oil contains more energy per unit than propane, so can deliver more heat from the same amount (heat is a measure of energy, not temperature). Think of the energy released by burning a kilogram of wood vs a kilogram of petrol, the difference is the difference in energy density.

The specific heat capacity of the material determines how much energy is required to melt it. Iron needs a lot of energy to melt it so you need to provide a lot of energy, which is easier with oil. Think of aluminium - it will melt around the 600C range but if turn a hand-held propane torch onto the bottom of a 1kg lump of the stuff you'll never get it to melt because you're not supplying enough energy, even though your flame temperature is high enough.

So oil is recommended because you get enough energy out of it to melt the iron. If you had a big enough propane burner and an efficient furnace you could melt iron with it, but it's going to be more difficult and cost more in fuel than using oil, especially if you burn used motor oil that you may be able to get very cheap or even free.

Edit to add: If you preheat your air supply the air will expand and contain less oxygen by volume, significantly affecting combustion. This is why turbochargers use intercoolers, trying to get the air as cold and dense as possible so they can burn more fuel and make more power.

 

[kelvin2164]

Thanks for that. Makes a lot of sense.
I have my oil burner working. Just need to fine tune it. Hope it works.

 

[kelvin2164]

I've just tested my oil burner with mixed success. Ran it on propane to warm up then changed to kerosene (it was my only available oil).
My compressed air/oil combining nozzle is almost at the entry to the furnace and I noticed that it looked like it didn't start burning until across the other side of the furnace. How far up the tube should this nozzle be? (I was afraid of melting the main tube)
Also my oil is picked up by suction only and I feel it could use more oil so will rig up a pressure feed.
Any advise appreciated.

 

[master53yoda]

My large furnace uses 2 waste oil burners firing about 5 to 6 GPH on high fire, I use it to process aluminum engine parts into to ingots and sell them. Each pour is between 140 and 200 lbs. from cold start to 1500F takes about one hour.

the flame temps your are mentioning are the temp at perfect air/fuel ratio with zero excess air. As more excess air is introduced it lowers the flame temperature. if you control your excess air you can get to melting temps for iron. The problem then becomes the insulting efficiency of the furnace and length of time to melt the metal. Flame color for perfect combustion is blue, yellow flame is a cold flame and is burning to carbon monoxide instead of carbon dioxide. you get the blue flame for vapor fuels like propane and natural gas because the heat needed to change from liquid to vapor is not needed. the yellow flame you see on oil is the first stage of combustion. it is far brighter then the blue flame that is also in the combustion process but we can't see it without filters.

Tuning in order to get the highest flame for a vapor fuel is to start with a yellow flame and add just enough air for it to turn blue. you then add more fuel and air until the flame just starts to leave the furnace, add a little more air and the flame will either pull back inside the furnace if it still needs more fuel air, if the flame leaves the furnace further and the sound changes you have to much air and cut the air back until the flame just leaves the furnace. At that point you are burning the most fuel possible with that particular furnace considering the size of the combustion area in the Furnace. the temps will be high enough to melt iron. The limiting factor is the size of the combustion chamber. my small furnace will max fire at about 225kbtu on propane and also a little over 1.75 GPH when using waste oil.

Tuning for the highest efficiency with oil is to add fuel and air until the flame starts leaving the top of the furnace. When the furnace temp reaches 1000F The oil is vaporized buy the air in the furnace and the flame becomes more blue and less yellow but again you need filters to really see the difference. If there is any smoke in the flame leaving the furnace it does not have enough air for the fuel that is being used. You will see the flame pull back inside the furnace as you add air until you have enough air for the amount of fuel you are using, again increase fuel and air until the flame is just leaving the furnace .

Remember that any fuel burning outside the furnace is wasted fuel.

The pictures are a cold furnace firing at about 4.5 GPH, 140 lbs alum cylinder heads etc., waste oil pressure tank and filter, I fire at about 8 PSI in the tank and 12 to 15 PSI on the siphon type nozzle. The Arduino controls for variable firing control using a t-couple sensor in the melt pool. and servos on needle valves in the oil feed lines. Oil preheat to 150F this allows me to fire in the winter time

Art B

 

[SmithDoor]

I have a small propane furnace that I use for casting aluminium and bronze. It works fine but I need to do cast iron.
Research suggests a waste oil burner, but why? The flame temp of propane is 1960C. 600 degrees higher than the melting point of cast iron. The flame temp of oil is 2100C. Only 140 degrees higher. Why wont propane work with cast iron?
Also, if I preheated my air supply, with a heat exchanger, by 200C, would my flame temp on propane then be 2160C.

Propane 2100°C is 3,560°F.
I have used both for melting Aluminum and Brass.

You want look at Charcoal It was used production of iron since Roman times and steel in modern.
It is about same temperature as coke. Blacksmith use coke or charcoal.

Dave

 

[GreenTwin]

I can honestly say that everything of value that I have ever learned concerning conbustion engineering and burners used for backyard foundry work has come from Art B (master53yoda). (thanks a lot Art for sharing your knowledge; much appreciated).
I consider Art to be the master of combustion engineering, and the name suits him well.
I think he has worked in that field for a very long time, and so his knowledge comes from work experience, not just tinkering with backyard foundry burners.

I have also purchased aluminum ingots from Art, and the quality has been consistently high with every purchase.
I highly recommend Art's aluminum ingots.

Popane will melt gray iron, and I have seen more than one backyard casting person demonstrate that online.
You have to add combustion air to the propane; I have never been able to melt cast iron with a naturally aspirated propane burner, no matter how high the fuel flow rate, or how large the naturaly aspirated burner.

The trick with propane is to keep the tank warm enough to keep your vapor pressure high enough to get the high flows required to melt gray iron.
I don't like heating propane tanks, and so I use propane sometimes for small aluminum melts only. Some place their propane tank in a tub of warm water. A larger tank helps a bit, but even a 100 lb tank tended to get too cool when trying to melt iron.

I initially started backyard casting with propane, trying to melt iron, and then built an oil burner, and I have never used propane for anything since except for melting small batches of aluminum.

As with any foundry burner, you must learn to tune it correctly to get the maximum heat and effiency, as described by Art above.
It took me a long time to learn to tune an oil burner, but it is not difficult once you know how to do it.
Any setting other than an optimally tuned oil burner will run cooler (as Art mentions).

Oil burners (when run with diesel or kerosene) will light (without propane) in cold weather without any preheating of anything.
I have operated my oil burner in 34F, and it lit and operated with no problems at all (using diesel).
You don't need propane with a siphon-nozzle style oil burner running diesel or kerosene.
I have never had to heat my diesel as Art is doing with the pipe-looking device (I think), but we have pretty mild winters around here.

The folks who use waste oil (I don't due to heavy metals, contamination, etc.) generally seem to mix it with perhaps 20-30% diesel, to thin it down a bit, and waste oil won't necessarily light easily without propane like diesel and kerosene will.

If you are using a spray nozzle type burner, the nozzle should be about 1" back from the furnace-end of the burner tube.
The nozzle does not overheat because it has fuel flowing through it, which cools it, and combustion air flowing around it, which also cools it.
If you turn off your burner, you can either keep the combustion air running, to keep the burner nozzle cool, or you can withdraw the burner from the furnace, so it does not overheat.

.

 

[GreenTwin]

A few pointers on oil burners.

I use a siphon-nozzle burner, which is what Art uses.
The brand I use is Delavan, but there are others.
A siphon-nozzle burner works in a very similar fashion to a spray painter, but in a more compact form.

The idea is to introduce oil into a swirling stream of compressed air, which atomizes the oil, and then introduce combustion air around the nozzle, via the burner tube.

The burner tube should not protrude into the furnace.
The nozzle should be close to the furnace end of the burner tube, perhaps 1" or less from the end of the burner tube.

I run a pressurized fuel tank like Art's, and use 10 psi compressed air pressure on the tank, with a 30 psi safety valve.
The tank uses its own dedicated pressure regulator.

The siphon nozzle also has a pressure regulator, and this feeds compressed air to the nozzle.
I use 30 psi compressed air to the nozzle, but based on what Art says above, I will probably reduce my compressed air to the nozzle to 15 psi.

I use ball valves on the compressed air and fuel to the burner, and these are for on-off, and emergency-fuel-off.
These valves should not be close to the furnace, since you can get hot gasses leaking around the burner tube, which will melt them.

For fine tuning, I use a needle valve downstream of the fuel ball valve.
I calibrate my oil burner with it off, and with no combustion air or atomizing air, and just discharge a stream into a measuring cup for one minute.
Calibrating the burner ahead of time allows you to get close to the fuel flow that you need before you light the burner.

I burn about 2.7 gal/hr of diesel in my iron furnace.

If you preset your fuel flow to the correct amount, then you can use a combustion air dump valve (or variable speed blower motor control) to fine tune the burner. With a needle valve, and a pressurized fuel tank (10 psi), you should never have to adjust the fuel needle vavle again.
You should never need to adjust the needle valve during a melt, and a sign of a burner that has poor control is that it needs to be adjusted during the melt, and adjusted as the fuel tank level varies.
I literally have not touched the needle vavle on my burner for as long as I can remember.

A siphon nozzle burner requires a fuel filter in the fuel line just as it goes into the burner.
A siphon nozzle burner will not tolerate any trash in the fuel, such as sludgy gloppy waste oil.
I use clean automotive diesel in my burner, and an inline automotive fuel filter at the burner.

.

 

[GreenTwin]

A single oil burner will stratify, ie: you will have a cold spot in front of the burner tube, and they a burn area that arcs upwards at about a 45 degree angle as is swirls around the furnace wall.

The stratification is not a problem, since the entire furnace interior will reach a very high temperature very quickly, in spite of the stratification.

Here is my oil burner after just starting it.
This was a test, and so I don't have any combustion air connected.
It should be noted that a siphon nozzle burner using diesel will run quite well in naturally-aspirated mode, with no combustion air blower, but for melting iron, you need a combustion air blower (I use a variable speed Toro leaf blower, operating on its lowest setting, with 100% of that low setting output going into the furnace).

 

[GreenTwin]

And this is the furnace after about 10 minutes, operating at 2.7 gal/hr diesel.

When you close the furnace lid, the inside surface temperature is pretty evenly distributed, with the only cold spot being directly in front of the burner tube (which is ok).

Note that you should never see any smoke when operating a siphon nozzle burner (perhaps a little smoke at startup).
The burn should be clean and clear.

.

 

[GreenTwin]

Here is my furnace operating at 2.7 gal/hr on diesel.
There is a slight amount of flame coming out the lid.
Very clean burn, with no smoke.
This will melt 25 lbs of gray iron in under 1 hour from a cold start.

The valve tree that can be seen on the right was used to quickly change from one fuel flow to another, back when I was trying to figure out what the optimum fuel flow should be, and it is not necessary for the operation of the burner.

The compressed air and fuel ball valves are on the end of the burner tube, away from the furnace, were they can easily be reached, and far enough away from the furnace to prevent damage if there is leakage of hot gasses around the burner tube.

Generally, the burner tube should make a tight seal to the tuyere.
The tuyere is the hole in the side of the furnace where the burner goes.

This burner will operate indefinitely without needing any adjustments whatsoever.

.

 

[master53yoda]

Pat I was wondering if Green Twin was you, I had lost track during the time I was down with the heart Valve replacement.

I have processed 18 tons of engines parts since I started This last furnace build is based on a 200 gallon fuel tank, it is probably as big as i will go it is the 3rd version since I started. . The oil heater is needed in the winter, it also gets the waste oil thinned out so the flow stays the same no mater what temp it is outside. I get the waste oil from the Local transfer station in 55 gal drums, The furnace is in a container with a large evap cooler in the end and I fire with the doors open, The propane I use is part of the oil burners and is used as a pilot on the waste oil burners until the furnace temp gets into 1000 F range on the flue. then I shut down the propane .. I run a pretty high excess air ratio in order to burn the garbage off the parts without the furnace smoking. I high fire until every thing is melted then the control cuts the burners back as needed to maintain the pool temp at 1450 low fire gets down to about 1 GPH and will shut off if the temp gets over 1525.. It relights at 1425 with the propane as a pilot for the first five minutes and then goes back to auto control. It normally doesn't go through the on off cycle unless I had a really large pour like 200 lbs.
I do an inventory pour on Wednesdays most weeks, that pretty well keeps up with the the normal orders although when I get 2 or 3 4 box orders at the same Time I end up firing on Monday and Friday as Well. The biggest single order was for 600 lbs from an art Gallery.

The problem with the ingot business is that I don't have the time to do the other stuff that I like, as in fishing etc.

Art b

 

[ddmckee54]

Don't ya just hate it when work gets in the road of the fun stuff?

 

[master53yoda]

I have time to tie them but don't seem to find the time to put them in the water.
Art b

 

[GreenTwin]

Hey Art, great to hear from you.
Yep, "greentwin" = "PatJ".

I forget to even put my name on posts these days.

Glad to hear that you are still at it.

Great setup you have there !

Very cool what you are doing with the servos and such.

.

 

[master53yoda]

The servos are connected to the oil needle valves and vary the firing rate on the oil. it takes full fire to get the melt done in a reasonable time frame 140 lbs in about 25 minutes once the furnace gets to 1000f. but then i need to back the burners off to keep the furnace from over heating. at full fire it could hit over 2000F in about 10 minutes ounce the metal is melted, it starts cutting back the burners at 1350 and will be down to one burner and the other firing at about 1 GPH at 1450. It takes about one hour to pour the ingots once it hits 1450, I have three sets of molds holding 12 lbs per set I dump them into water cooling tanks on a rotating bases. They set about 6 or 7 minutes between pouring and dumping. the furnace is set on linear actuators that are used to control the pour. The control system is based on an Arduino MEGA2560 every thing but the combustion fan and a on off valve in the main oil feed is 12 VDC
I have been lurking on this site for a couple years mostly looking at setting up my lathe with an electronic lead screw, and finishing getting my mill capable of CNC. using GRBL with an Arduino on the mill.
Art b

 

[100model]

Also, if I preheated my air supply, with a heat exchanger, by 200C, would my flame temp on propane then be 2160C.

No need to preheat your intake air because my small propane furnace uses room temp air to burn propane and will melt cast iron quickly. Have a look at my video of that furnace.

 

[Rocket Man]

Propane about 2000°F first picture and 2300° second pictrue.