Gas Forge "Efficiency"

[Randy C]

Has anyone tried this bio-soluble ceramic fiber?

"SAFE" Non-Ceramic Fiber Blanket 1" 8# dens. SW607 HT - eBay (item 140328938749 end time Sep-21-09 07:05:02 PDT)

I've used the used the Safe-Wool a couple of times to build pottery kilns and as the back-up liner on forges. It seems to work just as good as the fiber that is not lung soluble. I have not used it as the hot face in a forge because I have not found the Safe-Wool rated any higher than 2300 F.

[Frosty]

Who carries it in the USA or did you get it from overseas?

I've had pretty good luck using lower temp Kaowool when the higher temp stuff wasn't available. I'd like to give the stuff a try seeing as I'm phasing the soft fire brick out of my current forge it's too susceptible to thermal shock and too expensive to keep replacing.

Thanks,

Frosty

[Randy C]

Who carries it in the USA or did you get it from overseas?

I bought it from the ebay store that Nakedanvil mentioned earlier. They have really good prices.

[Frosty]

Ah HAH! I say as I slap myself in the head.

Thanks,

Frosty

[Michael Kellough]

I'm new so this may be a little off topic. I've been reading....

Kaowool insulates and ITC-100 consolidates and protects the Koawool.
But ITC-100 is prized for it's thermal reflectivity, so, what stores the heat?

Is this why many forges have a floor of firebrick?

I'm planning to make a bean can forge for heat treating very small knives.
Would I need to leave room for some kind of heat sink or would the air and
insulation retain enough heat to soak a small blade?

[pkrankow]

Heat is stored by the part, and wasted when no part is present. Idle circuits are nice for that reason.

Firebrick for a floor is to prevent damage to the rest of the forge. The brick is durable, resistant to flux agents, resistant to impact and poking, and is relatively cheap to replace if it gets damaged. That it provides an energy sink is debatable about its purpose since you cannot get most of the energy back out of it for work.

I take the stance of energy used to heat your forge is generally wasted energy. There are specific types of work contrary to this, annealing is one, and sometimes the thermal mass will help cycle small parts, but both require long operation time to get the benefit. If you are firing for 1-4 hours and it takes 30 min to heat the forge to working temp, you are on the loosing side of efficiency. If you work 8 hours a day this might be different. If the forge is running 24/7 energy loss in heating the walls and floor is not important as overall energy loss is the driving factor.

This is my opinion based on having studied thermodynamics a number of years ago. You are encouraged to form your own opinion.

Phil

[JNewman]

I would argue that idle circuits often WASTE gas. I installed one on my first gas forge so I have used one. When you idle the forge the forge temperature drops so that when you put your steel back in and turn the forge up you have to heat the forge back up before you heat the steel. The steel then takes longer to heat up so you have the forge running for 5 hours instead of 4 which uses more gas. While the forge is doing the 30 minute initial heat up I can be checking my email, cleaning up and organizing the day before's work or even cleaning up the shop. Any time spent standing waiting for a piece to reheat is wasted time. I also find that an idle circuit is a waste as well because most of the time that I am using a gas forge I have more than one iron in the fire, when do you turn it down?

Edited September 15, 2009 by JNewman

[Frosty]

I think an idle circuit is something that sounds better than it actually is. I know if I'm going to be away from the forge long enough an idle circuit would be helpful I just shut it off and light it up when I need it again.

Then again my forge goes from stone cold to forging temp in about 6-7 mins and welding heat in about 10-11. Conversely it's still mid orange half an hour after shutting it off if I slide a brick in front of the opening.

Frosty

[Nakedanvil - Grant Sarver]

pkrankow: Your point about time the forge is running is quite right and applicable to many here. Although I've had small forges made from castable that got to temp pretty quickly.

"Heat is stored by the part, and wasted when no part is present". In your studies of thermodynamics, did you not understand that ANYTHING with mass will absorb heat? Did you learn that heat is transfered from hot bodies to cold bodies according to the square of the difference in their temperatures? How about: that at forge temperatures 80% of the heat transfer is by radiation from the incandescent walls? You can hardly feel the flame from two feet away, but you can feel the radiant energy from ten feet away.

What does all that mean in a forge? When you take your part out of the forge to work on, the forge body (regardless of what material it's constructed of) will get hotter. When you put the part back in it will heat MUCH faster because the forge is now hotter. Using an idle circuit would do what? Are you going to switch it to idle every time you pull your piece out. And then turn it back on when you put your part back in? And wait twice as long for it to heat up? It would work if the flame was doing most of the heating.

Did you ever try to heat a piece as soon as you turned on the forge? Doesn't work very well, does it? The forge really doesn't work good until the walls become incandescent. That's where the real heating comes from.

Edited September 16, 2009 by nakedanvil

[pkrankow]

My understanding is when idle is set properly the forge will remain at temperature, but have much less blast. Maybe calling it "overdrive" instead of "idle" would be better since I understand it as maintaining temp using less fuel. That analogy would compare better to a car driving down the road. When the car is in "overdrive" it does not accelerate well (heat increase) but it maintains that speed using less fuel with a lower throttle setting. When a car is at "idle" it is sitting stationary (lower temperature), but ready to move (still lit). Most talk of idle circuits is very short and not detailed in the use of one. Is lit just the presence of fire, or is lit at operating temperature?

On a solid fuel forge, do you maintain blast the entire time that parts you are heating are in the fire, or do you let parts soak at a low air flow while you work, them pump up the blast after they have some temperature? I read about leaving blast on with other parts in the fire can waste fuel and burn steel in a solid fuel fire if the soak time is too long.

If you are cycling multiple parts, then idle is not used. There would always be a part to accept energy. The exception would be if your work takes longer than the heat, then blast *could* be reduced (and probably should be reduced too), but without an automated temperature control (or someone else watching the fire) not many people would do this. If a pyrometer and an automated control with a gas solenoid was only a few dollars then everybody would have their forge turn its blast down at temperature.

A properly insulated body will not loose temperature since energy is still added, unless your idle is just a pilot light. I hold the stance that a large mass body for the forge is a waste for short duration firing, and while you will get some stored heat out of it but most heat absorbed is wasted because it cannot be extracted effectively. Energy transfer across a temperature chance causes inefficiency. Granted that is what we are doing in the first place, moving energy across a large temperature difference.

A thin hard protective layer on ceramic wool will not have enough mass to store an appreciable amount of energy that can be returned. A thicker shell will obviously have more energy to return, but it takes more to heat to incandescent too. Yes the forge needs to be incandescent to transfer energy well. Heating the forge beyond the required temperature however is still a waste of energy, especially since the forge is already set hotter than the target temperature to reduce cycle time regardless of the insulation used. The only time I expect forge temperature to not be set higher is when a time at temperature soak is needed as part of a heat treat cycle.

Part of the problem is which efficiency are we talking about? Thermal efficiency or business efficiency? If your energy costs significantly less than your time, you will set your equipment to operate differently than if your time is of similar value as your fuel.

I fired my forge without all the proper plumbing on it this weekend, and only one burner. I threw steel in to heat as soon as I had fire. Since I was curing the coating, the steel became incandescent long before the walls on the forge. After running till the walls became incandescent I could turn the gas down and keep temperature. I also made a sparkler after leaving steel in a while as I was drawing out handles on a pair of tongs, one in/one working. (nice burner Frosty) After shutdown the forge remained incandescent for more than 10 minutes, the brick, as expected, more so than the sides. I left it to cool some before putting it away. For my case having a short cool down time is desirable too as it allows for quick clean up. My forge is not permanently located, but has to move every time it is used.

I admit I have limited practical experience, and book knowledge does not always translate to practical use. I stand behind my opinion, even if it demonstrates that business efficiency is more valuable than thermal efficiency.

Phil

[Sweany]

There is a lot of info here recuperative forge pics alomost to the end of the page.



Here is a series of industrial articles on process burners. Good stuff.

Burner History 101 - Energy Notes - Process Heating

[JNewman]

I am reffering to business efficiency but also partially thermal efficiency. Yes I agree that if you are going to use the forge for only a short period of time having a forge that heats up quickly will save you gas, because the forge is on less time. I still think the idle circuit is a big waste of time and expensive plumbing fittings. The forge temperature cycles up and down as you put fresh pieces of steel into the forge and then they heat up, you want the temperature to rise in the forge while your stock is being worked so that the stock reheats quickly when it is returned to the forge.

Yes I often leave the blower running when I am using a coal fire and I am forging for exactly the same reason, the fire cools down while I am forging. You do have to be careful if there are more than one irons in the fire and sometimes rely on the eyes in the back of your head to watch it.

Good to hear that your forge is coming together.

[Nakedanvil - Grant Sarver]

Comparisons to solid fuel forges is not valid, in my opinion. There is no "chamber" that is part of the heat transfer cycle and the incandescent fuel is heating the work. Not to mention the much higher temperature of most solid fuels.

[pkrankow]

I threw the solid fuel reference out as an additional thought on fuel management related to blast, which is what I understand idle to be about. The IDEA of idle is a good one, but its implementation and use is very understated. Most of the references I have read typically use it as a pilot light, and spend one or two SENTENCES on the topic. Much has been said about fuel management in a solid fuel forge, there are chapters dedicated to it in the older smithing books, and I would expect that there are entire books on just the one topic. Fuel management on solid fuel vs gas is valid for some comparison even though the thermal process is different, there are similarities due to the same final results being desired.

That said there are probably more differences than similarities, but heat is still heat.

I reread this thread from the start since the idle circuit and forge body material is a digression, even though related. I have some thoughts that are more in line with the original post.

Grant stated initially that most energy loss is due to IR radiation through your openings, and high exhaust temperatures. On a small forge the exhaust temperature is a difficult issue and would involve building a new forge so it is not an operation discussion, but instead a design and construction discussion.

From a daily operational standpoint:

How can IR losses be minimized?
What is the best way to reduce radiant losses?
How much can you choke the exhaust (all openings) down and still have good combustion (low CO generation)?
Is there a better way to load few or multiple pieces of metal into a forge and still reduce losses due to IR radiation?
I wonder if loading the forge from a "corner" instead of an "end" would help with radiant losses, and also if using a baffle with a space around the edges would allow adequate breathing?

I have read, but cannot locate the reference, that for an aspirated 3/4 inch burner a 4x4 inch (or equivalent) opening is desired, but that can be choked down some at low pressure.

The concept of loading from a corner is the same as a corner cube reflector. Light entering a set of 3 planes at right angles to each other returns along an offset of the same vector it entered at. I suggest having a few bricks stacked so there is a small opening around the edges, but the direct opening of the forge is closed off. One edge will have a larger opening to load stock into, and this edge can be built to be in the center of the forge. It may be more simple to load from off to the side instead of directly in front, and having a flat stack of brick in the opening. I think many forges can be set up in this manner as a trial at NO COST if the porch is large enough. A movable stock rest may help too. If only a few parts are being cycled at a time, this seems a simple solution to limiting IR losses, but for many parts being cycled, it may not present enough access without being able to move the bricks easily.

I know I just asked 5 questions and offered 1 clarification and one thought, but no answers.

I got plumbing parts today and intend to try the corner loading with a brick baffle reducing the opening after my forge is properly plumbed. Yes I am installing an idle circuit, and I also can choose if the front or back or both burners are lit based on an earlier discussion of efficiency and convenience. While valves are not cheap, they are not budget breaking expensive for an extra one.

Phil