My Attempt at an IFB Domed Furnace Lid

Discussion in 'Furnaces and their construction' started by Melterskelter, May 6, 2019.

  1. The problem I see is experts in flame and furnace design are experts at a totally different scale, and live in a world where cost of fuel is a major factor. Furnace efficiency is more important than speed of melt. A lot of people here seem bent on minimum melt times. In order to minimize melt times you have to sacrifice materials. It may be a profitable tradeoff but I doubt we could find an expert who would be well versed in what we're doing.

    The flame is the hottest part of the furnace, if the flue gas is not cooler then heat has not been taken out of the gas to heat contents (and the walls).

    I'm impressed by the variety of furnaces this group has which operate well. Just about every method seems to work. Low mass is faster but not as durable. Pick your poison.
     
  2. PatJ

    PatJ Silver

    Yes but I recall those experiments in science class where it seems like they stuck a Pyrex tube in the center of a flame, and siphoned off unburned gasses.
    And when you try to light a candle, it really lights better away from the center.

    I think if you can get the furnace interior very hot, then you will get pretty complete combustion inside the furnace, except in front of the burner tube.

    As I see it, you are pretty much blowing a whole lot of fuel and air through the furnace, and so much so that I am doubtful that it has time to cool down much.
    The efficiency of an oil fired crucible furnace is extremely low.
    One source says the efficiency is between 7 and 19%, and it is obvious why that is so.
    The reason is because you are blowing most of the air and fuel mixture out the lid.

    I think the reason you have to blow that much heat out the lid is to keep the differential temperature as high as possible (difference between the temperature of the combustion gasses and the temperature of the crucible).
    I am just speculating at all this.

    The guy knows a lot about this is Art B, and I think I saw him post the other day somewhere.
    I actually saved several of his posts; they were most informative.

    Edit:
    When I opened the furnace at FIRE the other night, I left the burner running (yes I know, spare me the safety lectures), and I discovered there is a tiny vent (about 1/8" diameter) in the arm pit of my leather jacket, and even though I was using a pretty long skimmer, I still got an 1/8" burn in my arm pit.
    Not a serious burn, but most annoying.
    A little metallic tape will help with that.
    So the moral of this story is, yes, those flue gasses are very very hot with an iron melt.

    .
     
  3. While the armpit method is a tried and true pyrometric solution, it appears you may have been over the acceptable operating range.
     
  4. PatJ

    PatJ Silver

    Would it be safe to say that if the furnace is 10% efficient, then 90% of the heat energy of the combustion of the oil is going out the lid opening, and only 10% of the energy is actually being used to convert the metal from a solid to a liquid, and then superheat the liquid ?
    A steel skimmer begins to get soft really quick in the flue stream, but steel loses most of its strength above 1,000 F.

    What if a steel plate were placed over the flue, up about 6", and at a 45 degree angle.
    The an optical pyrometer could take a reading from the steel plate.

    Some metals I think tend to luminesce more than others at a given temperature, so maybe that would throw off an optical reading ?

    If the melting point of mild steel is around 2,600 F, and a piece of mild steel held in the flue does not melt off, then the flue gas is less than 2,600 F ?
    If the steel does melt off, then the flue gas is above 2,600 F ?

    Something seems to be inherent in the 3 gal/hr fuel flow number, because it seems like fuel flows above and below that level are not as hot.
    I assume it is related to the interior surface area of the furnace, and X amount of hot surface area will somewhat fully combust 3 gal/hr of fuel oil, assuming the correct amount of combustion air is added.

    Above 3 gal/hr, it seems like the furnace is not able to fully combust this flow, and so the partially combusted fuel is cooling the surface ?

    Just a few random musings.

    What we need is for someone to put the tip of their iron-rated pyrometer in the flue stream and measure the temperature.
    If the tip melts off, you have made a sacrifice for the team (no pain, no gain, as they say).

    .
     
  5. PatJ

    PatJ Silver

    I am just catching up on reading here.
    So if the grill did not melt in the flue gas, then it is below 2,600 F or as MS says, more like 1,600 F ?

    And once the gases have exited the furnace, up about 6" perhaps, then there is no more combustion occurring, and stretching that hot gas over a long surface like the interior of a tall chimney I guess would cool it significantly.

    I still think somebody needs to put the tip of their iron-rated pyrometer in the flue stream about 6" above the furnace and see what it reads.
    That would tell the tale for sure.

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  6. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I can understand how flue gas could lose heat rapidly after exiting the furnace exhibiting lower temperatures, but is it possible that the flue gas is lower than the temp of the melt immediately before it exits the furnace? Wouldn't that say that the top of the furnace is the coolest region of the furnace? Seems like this could exist as a transient condition at start up but not in steady state.

    Hard to say what pulls more heat out of the combustion gas.....the massive less conductive surface of the furnace or the smaller more conductive crucible full of very conductive metal but a simple comparison of mass and specific heats will say how much energy is required to elevate the temperature of each but how they actually heat in the furnace can be another matter.

    Just not sure I can see a means for the average furnace temp (if that is what exits) to be below that of the melt as the melt temp is rising.

    Best,
    Kelly
     
  7. Melterskelter

    Melterskelter Gold Banner Member

    The luminous color of hot steels is very well worked out. Bright cherry is accurately known to be 1600 plus minus no more than 100. This was 2” above the standard (no chimney) Vent opening.

    Denis
     
  8. PatJ

    PatJ Silver

    Are we talking about 1,600 C, or 2,912 F ?

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  9. Melterskelter

    Melterskelter Gold Banner Member

    1600 F
     
  10. PatJ

    PatJ Silver

    Well I find that very curious.
    I am assuming that my furnace can get a melt to 2,800 F, judging from what the Navy Foundry Manual says about iron at that temperature.

    My furnace does not have the flanges at the top and bottom of the hot face like most furnaces do, and I think these flanges act like huge heat sinks.
    My hot face is up on top of IFB's with some ceramic blanket for a cushion.
    The outside of my stainless furnace shell remains cool to the touch throughout the melt, so I know I have a well insulated furnace with low losses.

    So it seems very odd that if I can hit 2,800 F in the crucible, then the hot gases can cool from at least 2,800 F to 1,600 F in the space of perhaps 8".
    I find that hard to believe, but I don't pretend to understand the thermal dynamics of an oil fired furnace either.

    .
     
  11. Melterskelter

    Melterskelter Gold Banner Member

    Very simple to test. Just bend a 1/2” bar into an L shape and drape it into the furnace to see the color changes related to proximity to the vent opening.

    Just at the opening there could be significant convection currents that mix outside air with furnace air cause cooling. All I can tell you is what I saw, though.

    I think that experiment would be interesting as the steel would graphically show its temp at various levels in the furnace top and opening.

    Better segregation of temperature might occur if the bar were turned in a lathe at, say, 1” intervals with a parting tool to 1/4” or less diameter so that conduction of heat along the length of the bar was somewhat reduced so that each “link” of bar was somewhat isolated from its neighbors. Using SS would also significantly reduce heat conduction. I think the test is very much worth doing and I would do it myself if I did not have the chimney in the way.

    Denis
     
  12. I went ahead and stuck my thermocouple in the flue opening and found about 400F (very roughly) above the crucible temperature from initial melt at 1,200F on up to 1,500F. Thermocouple in the flame never approached highest flame temperature.

     
  13. PatJ

    PatJ Silver

    I wish I could recall all the conversations with Art B.
    There was a lot of thermo talk.

    We know there are two components of heating the crucible, and that is by direct contact (convection I guess is the term), and conduction, which I think is any heat conveyed from the plinth directly up into the crucible (the crucible probably heats the plinth more than the plinth heats the crucible), and the heating of the crucible via infrared radiation from the walls of the furnace. And conduction when the hot gasses contact the crucible.

    I use a coat of ITC100 which increases the amount of energy that is transmitted from the hot furnace wall into the crucible.
    IR heating does not require any media between the hot and cooler surfaces in order to transmit energy.

    So I would guess the hot gasses are heating both the furnace interior surfaces and the crucible, and the furnace walls transmit some of that energy back into the crucible.

    You can tell how high the radiant energy is when you open the furnace while it is hot.
    Without full leathers you cannot remain in front of an open furnace at iron temperature for very long, and even with leathers you cannot remain in front of an open furnace indefinitely.

    ESC mentioned taking some optical readings on the furnace walls, and that would be interesting to see what temperature that operates at.

    Below is a color temperature chart.
    I wear one or more layers of shaded lenses when working around the furnace, but without any eye protection, the interior of the furnace is rather blinding, but I don't know how "blinding" falls on the chart.
    I assume anything above about 2,350 F is white technically speaking.

    IMG_0041 (2).jpg
     
  14. PatJ

    PatJ Silver

    A plot of the metal temperature was posted years ago by Adam on another forum, and as I recall it started out as a linear increase in temperature, then there was a plateau as the metal changed state from a solid to a liquid, and then the temperature started another linear increase as the melt went into the superheat range.

    I still don't understand how something with a theoretical flame temperature of 3,820 F could run so cool inside a furnace when the combustion appears to be fairly complete.
    There is no smoke or hint of incomplete combustion when the furnace is up to full temperature.

    I am going to have to do more research.
    I have no way to explain what is happening at this point.

    Edit:
    We also know that the introducing ambient air into the furnace via the burner tube limits the ultimate furnace temperature.
    Increasing the combustion air temperature increases the furnace temperature significantly, but I don't know if it is a linear relationship or what.

    So is the theoretical flame temperature of 3,820 F measured with a flame in open air, with completely atomized fuel?
    At what fuel and air temperature?
    Lots of unknowns about the 3,820 F figure.

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    Last edited: May 30, 2019
  15. PatJ

    PatJ Silver

    From a white paper titled "PERFORMANCE IMPROVEMENT OF AN OIL FIRED FURNACE THROUGH OSCILLATING COMBUSTION TECHNOLOGY", International Journal of Scientific & Engineering Research Volume 3, Issue 7, July-2012 1ISSN 2229-5518IJSER © 2012.

    https://www.ijser.org/researchpaper...THROUGH-OSCILLATING-COMBUSTION-TECHNOLOGY.pdf


    A rather interesting piece of research, and the only thing I could find that is remotely related to oil-fired metal melting furnaces.

    In a nutshell, from the paper:

    The melting time when compared to the initial state of the furnace have improved economically through the introduction of the oscillatory valve.
    This is because when the load heats up faster , the heat transfer from flame to the load increase due to more luminous fuel-rich zone and increased turbulence created by the oscillations produced by the valve.
    The decrease in melting time of the load is the result of higher rate of heat transfer due to the oscillating combustion....



    Some important parameters mentioned here too:

    within a reasonable short time the furnace all temperature becomes relatively uniform because time scale of the flame propagating is less and its velocity is faster due to more luminous flame from the fuel-rich zone of the flame

    the oscillating valve oscillates the air-fuel ratio of 14:1-17:1 into above and below the stoichiometric ratio resulting in improved efficiency


    IV.CONCLUSION

    The objective of this research work was a demonstration of the “Oscillatory Combustion Technology” on a diesel fired crucible furnace to melt aluminium for process operations.
    Oscillating combustion valve was installed in the testing furnace as a retrofit and carried our experiments to study the performance of the oil fired crucible furnace at different combustion modes through the study of different parameters (such as melting time, fuel consumption, heat transfer rate) and also through visual observations of various flames and emissions.

    The experimental results obtained using the test stand of oscillating valve in the furnace are very promising. The main conclusions that were drawn based on the data are:

    There is an increase in furnaceefficiency up to 6%.

    Fuel savings vary 7% to 39% in the oscillating combustion mode depending upon the condition.

    There is a huge increase in efficiencies for all loads during the oscillating combustion modes.

    The melting time observed with oscillating combustion operation is lower than the non oscillating combustion mode.
    However there is a change in melting time within the oscillating combustion mode operation.
    The maximum efficiency, minimum fuel consumption and melting time of the stock are observed atoscillations at minimum frequency.


    .
     
  16. There are three basic methods of heat transfer: conduction, convection, and radiation.

    Conduction is heat flow from a one body to a cooler one. If you flame is heating your plinth then it will conduct from the plinth to the crucible and to the floor. My furnace does this because my burner is pointed down.

    Convection is in two forms:

    Natural convection where a liquid rises when warmed and falls when cooled. This is how a "radiator" warms a room - it warms the air around it and that rises, cooler air on the far side of the room falls and moves across the floor to the radiator. The crucible warms the outside of the molten metal and it rises to the top and cooler metal in the middle falls and moves against the bottom of the crucible. This is why I want my plinth hot. Convection of the molten metal will warm the melt faster than conduction through the metal.

    Forced convection is where you have a fan or pump to move the fluid (liquid or gas). The blower and flame in an oil burner are a form of forced convection. An electric kiln has natural convection by comparison. Both forms of convection also rely on conduction of the heat from the fluid to the body being heated or cooled through the boundary layer of fluid which is not moving.

    Radiation is confusing to most people. The amount of heat radiated from one surface to another is based on the fourth power of the temperature difference between the two surfaces. If they are at the same temperature, or nearly the same temperature, no energy transfer takes place. The flame will radiate (hot molecules) to the walls and crucible. If the walls are hotter than the crucible they will radiate to the crucible. This takes place when you add charge. If you heat an empty crucible it may warm as fast as the walls and no radiation energy is transferred. Generally the flame and hot gas swirling around the firebox will warm the walls before the crucible because centrifugal force is forcing it a little toward the walls. When you have an open flue the walls and crucible and metal in the crucible will radiate to the cold surfaces above. This is a significant amount of heat because the surroundings are so clod compared to the crucible. At night the night sky looks like absolute zero to the crucible and will suck the maximum heat from the crucible through radiation.
    My understanding is ITC-100 is touted to reflect radiant energy back. It could only increase radiation from the wall if it made the surface of the wall hotter. It might do that if it insulates the wall from the flame. I don't know. The easiest example of radiation we have is the sun. It is very hot and we are pretty cool (at least I am) so we receive it's radiation. The hotter we are the less we receive. There are many wavelengths of radiation, and they do not pass through solids easily. A thin piece of paper effectively blocks the sun's radiation. Thick clouds block most of it, while thin clouds block some but still let wavelengths through which can give you a sunburn. It is not only infrared radiation. The thick flame in a furnace will do two things, one it can block radiation from the walls, but more importantly, it is hot, maybe hotter than the wall. The wall will not radiate to a hotter surface and I doubt it will radiate anything into a flame hotter than it is even though there is a cool surface on the other side of the flame. Just like clouds, where the flame has thinned radiation will take place. Remember the crucible will only warm when it is cooler than it's surroundings. I like to charge the crucible as fast as I can and keep it as cool as possible so it will absorb the most energy to make a quick melt. Waiting until you have a melted pool is fun to stick stuff in and watch it melt fast, but you have lowered the rate of energy transfer into the crucible by letting it get hot thereby extending your melt time.

    The radiant energy you feel when you open the firebox is only because it thinks you are very cold. If it is 2,000F and you are 130F you are very cold by comparison. You get a lot more radiant energy if you have on a dark apron instead of a reflective one as well.

    Energy always goes from hot to cold. The gas has to be hot to warm the crucible. But the gas has to give up energy to heat the crucible. The more efficient the furnace the more the gas will be cooled going through it.
    Notice in the color chart that hot gas will still radiate light even down to 1,000F. There is only a brief period when an air molecule gets to maximum temperature in an efficient burner. Then it is rapidly cooled by it's surroundings. Rapidly. But it still glows, very much. A lot of heat in oil but a propane flame looks hotter. I do not understand that. But I do understand glowing gas being cooler than maximum combustion temperature. In fact, I would be surprised if any of us were able to achieve maximum combustion temperature anywhere in our furnaces. It is an elusive pinnacle. That is why I encouraged you to put your thermocouple into the flue gas. If your entire furnace was at 3,500F then you might get 3,500F flue gas. Even if you did, it takes several seconds for a thermocouple to melt and what I suggested was stick it in and pull it out as it gets to 2,000F. No damage. I don't know whether you looked at the video I posted, but it showed when the furnace is cold it sucks all the heat out of the flame, even though the flame is still hot enough to glow.

    That beings us back to the original point: if you are bringing your melt to 2,600F and don't hold it there for an extended period, and the gas is exiting at 3,000F, the refractory in the flue will never get to 3,000F, it will only be heating toward that. Energy it is absorbing will be sucked away by it's cooler parts. When you run the temperature up to melt the refractory is still warming like the crucible and as soon as you kill the flame it starts right back down. Denis is a little different in that his stack will prevent radiation but a normal flue opening can see a lot of cold surfaces and will be radiating a lot of energy to those. That energy being radiated by the refractory comes out of the energy the gas is putting into the refractory. Like trying to warm your wife in a football stadium on a cold windy day. Since you bought the cheap seats there is nobody around you and you can't supply enough heat to get her warm. Same deal, the flue opening is in a bad spot to try to heat to the limit. It will never get to gas temperature.

    Try a thermocouple in your flue for a few seconds on your next heat. Pull it out before 2,000F. You won't have to pull it out before the crucible hits 1,600F I would guess.
     
  17. A few more comments if you will:

    Just how ice melts too. When it gets to 32F the mixture stays at 32F until all the ice is gone then the water can start to warm. Different that ice in your iced tea. That system never stabilizes, ice continues to melt, tea continues to cool to melt the ice but warms from the 110F room it is in. However it won't be a linear increase in temperature. It will be asymptotic to the furnace temperature but may appear linear over short distances.
    I thought you were getting good combustion. I missed what you are referencing here. Sorry.
    In industry it is common to use air preheat but the heat for the preheating is taken from the exhaust. This is for efficiency only. And the fireboxes are designed for efficiency.

    I don't know of any benefit to preheating air other than raising the temperature so the flame does not have to use it's energy. Humid air does take a lot more energy to warm than dry air, and also has less free oxygen to burn. However a little more fuel in the firebox can do the same thing unless the firebox is too small. You could put a small propane burner in the inlet air tube to preheat the air just before it gets to the furnace. If you really want the heat look at using oxygen. Just add another line to your burner and start kicking in some oxygen while cutting back on the air. Over 5,000F flame temperature for pure oxygen.
    I think if you look up any references to the flame temperature they give you the conditions, typically 15.55C.

    But if you are only after quicker melt times you need a bigger firebox so you can put a lot more fuel in and make the crucible a smaller part of the system. And, make sure the cooler gas can get out, don't pressurize your firebox holding gas that has already heated the crucible in the furnace. For instance, see how fast iron will melt in an A2 crucible in your furnace. That's why I gave my furnace some elbow room.
     
  18. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Gee Andy, felt like I was back in my college heat transfer class for a moment there. Definetely have all modes of heat transfer going on in a fuel fired foundry furnace. All good but not sure I can agree with this bit as a blanket statement.

    To illustrate, you can wave your hand around in 200F air with no problem (free to mild forced convection) but if you stick your hand in a bucket of water at 200F you'd be severely scalded and change your mind about conduction's ranking in rate of heat transfer. In the furnace/metal melting scenario, I think you can transfer heat more quickly to a crucible full of molten metal than solid for a given temperature differential........the latter being an important factor.

    The intimate contact of the molten metal with the interior crucible and the excellent conductance of the metal compared to a sheltered bunch of solid chunks of metal in a crucible with generally low contact is why. Where forced convection wins is high delta Ts and gas velocities (cutting torch for example), or if the conductive circuit has small contact area or narrow cross sections reducing heat flow. In the furnace you typically have big temperature differentials initially when the metal is solid. But when the metal melts, there is typical smaller differences in temperature between the metal and furnace environment.

    The one I scratch my head about a bit is the ITC coating. The emissivity part is easy to understand, that being a reflective coating on the furnace puts more radiative energy into the black body like crucible, but you still have to raise the temperature of the furnace wall, which has a lot more area and mass) above the crucible, via forced convection in order to heat the melt. So what difference does it make whether you do that with convection of radiation?

    Slightly more on topic, I've never fully gotten on board with reducing melt times. If it reducing it from an hour or more to 15 minutes, I do get that, especially for hobbyists who typical only do one heat. But, I don't get trading equipment durability for to go from 20-15 minute melts, and if you do multiple heats/melts per session, I think you're probably better off with more furnace mass......and for iron guys, wouldn't you always trade the ability to achieve a higher ultimate furnace temperature over melt time.....at least within reason? ....and on the common sense front, isn't the fastest route to quicker melt times just preheating subsequent metal additions in the flue gas?

    I'm looking forward to see MS' lid in action.

    Best,
    K
     
  19. Wow Kelly! You actually read this drivel.:eek:

    It is very basic thermodynamics, but sometimes the easy stuff is the most valuable because you can grasp it and apply it to your current situation. You know how fiberglass insulation works. All the fibers inhibit convection within the walls. And the fibers are long so conduction down the fibers isn't very good. And they block low level radiation. They also inhibit infiltration flow but not very well. The more insulation you pack into a space the less insulation value you have. If you pack it tight enough you have solid glass, which is OK insulation but not good insulation.

    It's splitting hairs, but I think we agree. The quoted post was only saying when the crucible is full of melted metal that the hot crucible wall will cause that metal to rise and cooler metal in the center of the pool to fall and circulate out to the hot wall to be heated. I think that is a larger driver for heating the already molten pool than conduction through the pool,. In other words if the molten metal had high viscosity and would not readily flow then it would take longer to heat the interior of the pool. That is convection of already molten metal, not air convection within the crucible full of chunks which would certainly be negligible. I think a lot of the heat transfer into a crucible of chunks is also by radiation, from warming crucible walls and from a hot arch. I think I see chunks at the top melt and run down to form a pool to start the real heating.

    I agree with reducing melt times. Bragging rights for some people but I'm not into bragging, much better to keep ones mouth shut and be thought a fool...

    Hot charge is as good as higher furnace temperature for reducing melt times, and if it's heated in the waste flue gas it's free free. I had my warming try set too low and was dripping my preheated charge into the crucible. An excellent oxide generator.

    Denis is doing good!
     
  20. PatJ

    PatJ Silver

    I think I have totally hijacked melter's domed brick lid thread; my apologies for that.
    Maybe we need to peel off this discussion into a thermo thread.

    Kelly seems to be quite up on his thermo, and provides some very lucid examples, which helps a lot, as does OIF.

    My uneducated slant on this is that an uncoated wall will reflect X amount of radiant heat into the crucible, and convection will supply Y amount of heat into the crucible.
    A furnace wall coated with ITC will have the same amount of hot gasses passing it and the same Y convection, but the wall will be more radiant, and so could provide 1.3X of radiant heat into the crucible, and this is energy that would otherwise go out the lid opening.
    Bottom line is that the total amount of energy going into the crucible goes up when an ITC coating is used.

    I have not tried to melt iron in my high mass furnace since I built my new lower mass furnace, but looking at the furnace scavenger had, it was significantly higher mass than mine, and he routinely melted iron in it.
    So why use a lower mass furnace to melt iron?
    There are several reasons:

    1. In some cases, melting iron can operate on the edge of a melt/no melt situation, and I have introduced scrap into the melt that was on the larger size, and had the entire molten melt go solid (this happened with the high mass furnace).
    With the lower mass furnace, this sort of thing never happens, and I can drop large chunks of cold iron into the melt with impunity, and it is going to melt without any problems.
    One of the guys at the FIRE exhibit noted that he thought for sure when I dropped a large chunk of iron into the furnace that the melt would go cold, and was surprised when that did not happen.
    The lower mass furnace seems to have much more thermal momentum, and can overcome cool shocks easily.

    Its like pulling a heavy load with a truck that has either a 4 cylinder engine, or a big V-8 engine.
    Either engine will pull the load up a hill, but one engine does it with ease, and one engine does it with a lot of careful shifting and at a slower rate.

    2. The initial charge goes into a puddle much more quickly with a lower mass furnace.
    One of the tricks of iron melting is to get a puddle going, and then push subsequent scraps through the slag cover and under the surface of the puddle.
    If you don't do this, the slag cover will get physically very hard, and I have had it get so hard that I had trouble breaking through it with a steel rod.
    Pushing the scrap under the puddle also protects it from the atmosphere while it is melting, which reduces slag.

    3. The temperature ramp up of a lower mass furnace is much steeper than with a high mass furnace, and I feel like my well insulated lower mass furnace will achieve a higher end temperature than my high mass furnace, but that is probably due to it being better insulated, and not as a result of lower mass.

    4. Shorter melt time also means less fuel usage, and in the case of Clarke, greater productivity (he is in production with a commercial foundry).

    5. Melts during the summer tend to be very hot around here, and the temps can easily get above 100F in the shade.
    Inside of full leathers at 100F and 85% humidity is not a position I can stay in for long.
    I generally start getting into heat exhaustion by the end of the first melt, so a second melt pretty much exceeds my personal physical thermal limits.


    I have heard melter mention this, but have not really given it much attention.
    I hold my scrap in the exhaust stream for about 30 seconds and then drop it in, so I don't do any other preheating, but perhaps should be doing that.
    I don't like setting scrap on top of my IFB's and lid, since I don't want to risk cracking/failing the rather thin layer of Mizzou (a little more than 1" thick) that is in the lid.
    I don't like preheat racks over the top of the furnace either; I like clear access to the top of the furnace at all times.

    The only way to check this is to do a timed melt with and without preheating the scrap.
    I am not sure there is significant time to be gained by this, but have not measured it either.


    I have read that the iron should not be charged above the top of the crucible, and should always remain out of the hot gas stream for this reason.
    Preheating seems to be the same situation, but may not get into this phase unless the scrap is held long enough to start melting.

    But what about pulsing the fuel pressure as mentioned in the article above?
    That is an intriguing topic for sure.
    If you turn the atomizing pressure down on a siphon nozzle burner, you get very coarse droplets, and you can better see the flame dynamics of the fuel/air mixing and burning.
    So the discussion of creating a larger luminous fuel-rich zone makes more sense.

    .
     

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