Forges, gas and btu’s

btu’s needed to hold my 1,944 cubic inch forge at 2000°

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JTknives

JTknives

Blade Heat Treating www.jarodtodd.com
Knifemaker / Craftsman / Service Provider
Joined
Jun 11, 2006
Messages
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So with my new forge build I decided to really nail down gas consumption. My forge has an inside dimension of 12” wide, 18” deep and 9” tall which is 1,944 cubic inches. how many btus do you think it requires to run at 2000°. I picked 2000° becaus it was just a nice round number. In the past I had heard numbers all over the map about the required btus per cubic inch. Well with the pid telling me the temp and an accurate flow meter on the gas line I was able to track the amount of gas needed to hold 2000°. I am interested in seeing what your guesses are when it comes to needed btus for my forge. After I give you pluntly of guessing time I will come back and tell you what my numbers where. I would also like to know what size your forge is and it’s comsumption rate. If you don’t know the btu amount that’s ok. Each gallon of propane has 91,600 btus. So how fast do you go through your tank? Thanks guys I look forward to seeing your guesses on my forge and hearing about yours.
 
I don't care. Mine gets plenty hot in 10 minutes and uses a small amount of lpg.

My buddy has one that uses 1.5 hrs to get up to temp a swallows lpg. It is over engineerd.
 
Hold 2000F or get up to and hold 2000F?

I'm interested in putting a flow meter on mine as well simply because I think it's rather inefficient but have nothing to gauge it off of. Although my gas use went down when I put a thermocouple in it and realized I could quit trying to melt the lining off just to weld.
 
Well I will let the cat out of the bag. After letting it warm up and adjusting the gas and air down to where it was holding a solid 2009 deg. I looked at the flow meter and I was right around 12CFH. Propane being 2488btu per cubic foot means I was pulling 29,856btu. That only comes out to a hair over 15btu per cubic inch. I know in the past numbers like 350-500btu per cubic inch where considered what to shoot for in a forge. I I put that many btus in my new forge I would be close to 1million and probably melt it down. According to the math my 100lb tank will give me a total of 21,548btu/lb x 100lb = 2,154,800. At my current draw that’s 2,154,800/29,856 = 72hrs. That seams crazy to me but that’s not trying to heat anything else. So considering steel has a specific heat close to .120btu per pound for every 1° Deg. If we say I put 5lbs of steel in, it would take
(5x(2000°-70°))x.120= 1158btus to heat that steel to 2000°. But that’s not taking into account the rate at which the btus are being added. I’m sure you could calculate all the math and figure it out using the thermal conductivity of steal and add in the rate at which you want to heat it. But that’s more math then I want to do.

All this comes back around to the fact that there are a lot more btus in propane then we are using in our forges when we use the simple ventury burners. Even the blown burner on my vertical forge did not see this kind of efficiency. I mean look at the atlas mini, it uses a 30,000btu burner to heat a forge that has a area of 54 cubic inches. Dont get me wrong I’m not knocking that forge. It’s a great little forge for guys starting out. But it falls right in line with normal numbers required to heat to the higher temps.

I am surprised that ribbon burners are not more popular then thy are. Knowing what I know now. And seeing it my self and hearing how quiet it is I will never go back to a normal burner ever agian. I would wadger a bet that all these other forges that require the 450-500btu/in^3 to reach temp don’t have a refractory problem. Thy have burner problems. And by that I mean the common burners are not extracting all the available heat out of the gas it’s using. When you see a ribon burner run the flame is just about if not completely invisible and there is zero “dragons breath” coming out of the forge. So just some food for thought if your wondering why your 20lb bbq tank is only lasting you a saterday of forging. Maybe it’s time to re-evaluate your forge needs.
 
What is your flowmeter calibrated in? LPG? water? Air? Most rotometers have the calibration info printed at top of tube, but I could never read what was there. Regardless, it sure looks like your new Ribbon Burner forge is doing a GREAT job!
 
Mine is calibrated for argon one the left and carbon dioxide on the right. According to the conversion charts for flow meters propane has the same conversion factor as carbon dioxide, which is 1.23.
 
I agree with you that the recommended 350-400 BTU per cu.in. seen on charts seems rather high.

Yes, sipping along at 12CFH would mean 1.4 pounds an hour, or 72 hours run time per 100 pound tank. However, that is the rate of a fully soaked forge with the ports shut as needed and the air/gas flow set to hold at 2000F. The amount used to get the forge up to 2000F, and the rate needed for heating up a couple pounds of steel in a billet are higher than the idle rate.

While your testing surely shows the new burner is efficient, the only realistic way to calculate your use is to start with a full tank and keep accurate records on the actual use time. Make a log sheet and log the turn on/turn off times of the forge. When the tank is empty, add up the time. This will give you the number of hours the tank provided. This provides the heat-up and hold times together. It also averages out the general forging and the welding times. It also is based on the actual yield from the propane, which is always less than the potential yield. Burner efficiency, heat loss through the refractory, port size and heat loss, etc. all lower the actual output from the theoretical values. I would guess that the actual in a forge is around half the theoretical.
If the 100 gallon( roughly 25 gallons) tank of propane has 2,000,000 BTU potential (rounded off to make the numbers easier to understand) and ran 40 hours of forge time - then you got 50,000 BTU per hour, from the 2.5 pounds of propane you use an hour. Trying to calculate from a theoretical value of sq.in and potential BTU value will never get a real world answer.

Idea for you ... because you are a mad-scientist type like me:
I have a 120VAC elapsed time meter in my box of parts that will go in the power line to the main shut-off solenoid for my 100 pound tank. When the tank it turned on, the meter runs. This will give me the info on consumption without keeping a detailed forge time log. I will just write the start time on the tank side when putting in a new tank, and see how many hours I got from it.
 
JTknives said:
Mine is calibrated for argon one the left and carbon dioxide on the right. According to the conversion charts for flow meters propane has the same conversion factor as carbon dioxide, which is 1.23.
Click to expand...

Good deal JT - looks like you've got it covered for decent accuracy with LPG. I know rotameters are good, but a person does have to pay attention to how it's calibrated.

Stacy - we're all looking forward to the results of your propane usage with this method.

Ken H>
 
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