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timgunn

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Posts posted by timgunn

  1. It's worth mentioning that "Kast-O-Lite" is a trade-name used for a series of Insulating Castables with a range of temperature ratings. Usually Kast-O-Lite is followed by a number, which denotes the rated temperature in hundreds of degF. For welding forges, Kast-O-Lite 30 (3000 degF) is the stuff to use. Also seen as Kast-O-Lite 30 LI, the LI stands for Low Iron: this may not matter much to "us", but there are some applications where the LI is important.

     

  2. The biggest issue is the thread type. If the imports are specced as NPT and you are in the US, there is unlikely to be any tangible difference between import and domestic. 

     

    If they are BSP threads. you may have trouble mating them with NPT threads. 

     

    The civilized world (defined as the part that includes me) tends to use BSP threads, which have a Whitworth, 55-degree, threadform, as opposed to the (uncivilized heathen) American, 60-degree threadform. There are some  differences in thread pitch between BSP and NPT at many sizes, but 1/2" and 3/4" use the same TPI.

     

    In many/most cases, just using more PTFE tape or a liquid PTFE sealant will let you join them together without leaks, especially at the sort of pressures (inches of water column or less) we see on burners. 

     

    The best advice I can give is to get some liquid PTFE or similar sealant and to be aware that there is a possibility of a problem when you assemble. If it rears it's head, then you'll need to decide on your next step. You'll probably be fine, but if things seem iffy, don't just reach for a bigger pipe wrench.

  3. Drilled-bar burners are used in Propane BBQs and the like, where the chamber being heated is relatively cool. I seem to recall from school that Radiative heat transfer increases as the 4th power of the temperature difference (though I have a nagging feeling it may be a bit more complicated than that). At forging temperatures, the heat transfer is MUCH higher than at BBQ temperatures and Jaro is right about the effect. To overcome this, a ceramic, insulating version of the drilled-bar is needed to reduce the heat flow and keep the inside surface of the plenum below auto-ignition temperature: hence the "ribbon burner".

  4. I think there are probably ways of using IR to replace R and S in certain production applications, but only if all the other variables can be eliminated. That ain't gonna happen in many of "our" applications. It doesn't seem likely it could happen in yours.

     

    The only smith I know who seems to have used one effectively just used it to check his billet temperatures were consistent coming out of the forge, before sticking them under the power hammer. He always measured straight from the forge and didn't care what temperature was read, as long as it was the same temperature every time. The main reason he used the IR was that the light in the shop was very variable throughout the day, making it very hard to judge temperature consistently by eye. Once the billet was hit and the scale came off, emissivity changed dramatically and the IR pyrometer was useless.

     

  5. I tend to regard a long hand-held Mineral Insulated type K thermocouple with a high-temperature sheath material as the first step. This can be moved about in the forge to establish where the hot zone is. If it is big enough and reaches the lining (which is primarily what will emit the IR), the thermocouple can be kept in the working zone while an IR reading is taken. If the IR and thermocouple agree, the IR measurement can be used for subsequent measurements.  

     

    • Like 1
  6. Interestingly, emissivity can just stop being an issue if the measurement is taken from a hole that is at least 6 diameters deep. Oversimplifying somewhat, the general idea is that anything emitted will reflect around within the hole before coming out and the effective emissivity within the hole will be very close to 1.

     

    In many cases, a forge is very similar to a hole and an IR temperature measurement taken through the mouth of the forge can be quite accurate. Obviously it will only work if there is not a rear opening to confuse things and the distance-to-spot ratio needs to be sufficient to get the whole measuring spot within the forge without melting the instrument.  

     

    IR units are already cheaper than type R and S thermocouples in many cases.  They are becoming cost-comparable with "proper industrial" type K and N thermocouples, over which they have some advantages. Admittedly they have some disadvantages as well.

     

     

    • Like 1
  7. On a single burner forge, a small leak is not usually a significant problem. 

     

    On a double-burner, it may be.

     

    The gas leaving the jet at high speed is what causes the air to be entrained. Gas from the leak is relatively slow and effectively non-directional, so doesn't help to induce air itself and just mixes with the air going past. This means that the "air", induced by the gas leaving the jet as intended, is actually a (lean) gas/air mixture and the overall mixture will be richer than that from an otherwise identical, but leak-free, burner.

     

    Flame temperature varies as the air:fuel ratio of the mixture changes: it peaks around the stoichiometric ratio and reduces as the ratio moves away from stoichiometric. We usually run fuel-rich to limit scaling. A leaky burner will therefore move the mixture further from stoichiometric and run cooler than a non-leaky one. 

     

    There are many other things that affect the mixture ratio and a small leak might well cause a negligible effect if other things are different (jet alignment, smoothness of the transition from reducer to pipe, etc, etc), but it's usually better to aim for zero leakage.

     

    I like anaerobic pipe seal myself: it sets in the joint, but the excess will wipe off. I use Loctite 567 when I have it to hand through work, but use the cheap stuff from the local plumbing/DIY supplier when I need to buy it myself. I'm in the UK, so can't offer experience-based advice on which cheap one to use in the USA. Read the spec and check it's rated for your fuel gas (Propane or LPG) when you buy.

  8. When I built my first HT oven, I made an angle-iron frame, put everything together and test-ran it. I was intending to clad the outside in sheet metal, but had no idea how hot it would get until I tested it. The available choices seemed to be plastic-coated steel, Aluminium or Stainless Steel, in increasing order of cost. 

     

    In the event, I found the surface temperature reached about 135 degC, 275 degF, but that the IFB provided such poor heat transfer to human skin that it seemed safer without a metal cladding. 

     

    I am aware that knifemakers in general tend to cluster towards the obsessive end of the scale and that the unfinished look would be completely unacceptable to some (many, most?).

  9. What size are the gas jets in your "Venturi" burners? Can you change them fairly readily?

     

    If you are using mig tips as gas jets, try a smaller size.  It may seem counter-intuitive at first, but there is a very high probability it will increase your temperature and dramatically reduce the Dragons Breath. I would aim for about half the area at first; 0.707 times the diameter and see where you need to go from there.

     

    Ideally, you want to get to, or just above, the maximum (welding) temperature you'll ever want with the choke slides fully open. Then you can reduce airflow with the chokes to get cooler, more reducing, temperatures for forging. 

     

    Note that mig tips are sized by the wire diameter they are intended for welding with and the hole is typically about .006" or .015mm bigger than the nominal size. 

     

     

    I'd certainly spend a couple of hours playing with the burners you have before embarking on a ribbon burner build. Even if you don't get them working to your satisfaction, you'll become familiar with the effect that varying the mixture has on the flame temperature.

  10. 11 hours ago, Bob Ouellette said:

    Thanks Alan. I plan on having individually adjustable burner valves, so I'll put the inlet in after the pipe turns down towards the burner.

     

    I wouldn't.

     

    I'd put the fuel inlet out to the right of your picture and I'd definitely only have the one.

     

    Having a gas inlet per burner means you are going to have to adjust both to the same mixture to get the same air:fuel ratio and flame temperature from each burner. That simply ain't gonna happen, at least not regularly. If you inject the fuel and mix thoroughly before the burner feeds separate, both burners will always get the same mixture without any faffing about on your part. 

     

    I might also look at redesigning the manifold. I think the left-hand burner will tend to have a bigger flame than the right-hand one simply because flow is past the right-hand burner, which I think would tend to reduce the static pressure to that burner, and hits the end of the line at the elbow into the left-hand burner, which I think will tend to increase the static pressure. This may suit your application, but in general, I'd aim for symmetry in the system.

     

    Most folk seem to have success with a relatively large "gas jet" on blown burners, using a needle valve to control gas flow: effectively a fixed-pressure, variable-area adjustment mechanism. The "gas jet" can be as simple an open-ended tube. It is also possible (and equally effective) to use a gas jet which provides some restriction and gives a fixed-area, variable-pressure adjustment system.

     

     

    • Thanks 1
  11. What is the actual time from start to stop?

     

    Is it 105 minutes?

     

    It may be that you have put in too simple a program.

     

    If you have a system that will reach 1825 degrees from a starting temperature of 50 degrees in 90 minutes under full power, the first part of the Temperature/time curve will be very steep. It will tail off as the temperature gets higher. 937.5 degrees (the half-way temperature) might well be reached within 5-10 minutes, leaving 80-85 minutes to achieve the other 887.5 degrees. 

     

    If you slow down the time to 937 degrees with the first 45 minutes of the 90-minute ramp, you will only have 45 minutes of your 90-minute ramp left to get from 937 deg to 1825 deg, which takes 80-85 minutes at full power. You will not be able to reach the setpoint in the time you have allowed. You will probably still be way short of 1825 deg when the soak times out.

     

    I'd try 

    C01    50

    T01     5

    C02    1825

    T02     100

    C03    1825

    T03     -121

     

    It'll still give the 105 minute cycle, but should (if I'm not wildly off) get you a higher temperature, perhaps even 1825 deg.

     

    One thing I do with my ovens is give them full power and log the temperature every minute until it is at the highest temperature I will ever want. That way, I have a reference chart for setting achievable ramps.

     

     

  12. I'm not familiar with that controller.

     

    Looking at the manual online, it appears times are in minutes?

     

    From 50 to 1825 degrees is 1775 degrees of temperature change.

     

    90 minutes is 5400 seconds.

     

    1775/5400 is 0.33 degrees/second

     

    1.65 degrees every 5 seconds, so your 1-2 deg per 5 sec seems about right to me?

     

    I'll try to look at it again when there's a higher concentration of blood in my alcohol stream, and see if I can work out why it's going to "stop".

    • Haha 4
  13. The 37 mbar delivery pressure may well be sufficient on its own to explain the poor performance. Things could be looking up.

     

    There may be some deviation from normal Gas-Safe practice needed: I don't think LPG necessarily falls within the same regs as mains gas (for anyone reading this outside the UK, "Gas-Safe" is the registration body with which anyone working with mains gas in the UK must be registered).

     

    Naturally Aspirated burners use the momentum of gas emerging from a jet to entrain air and mix it with the gas. This means we need a high speed through the gas jet. The gas speed through a jet varies as the square root of the pressure loss across it. Downstream of the jet will be at, or pretty close to, atmospheric pressure. Upstream of the jet will be whatever pressure your regulator supplies. 

     

    37 mbar is about 0.53 PSI. At 2 bar, or 30 PSI, the amount of gas being burned (or at least fed to the forge) will be between 7 and 8 times as great as at 37 mbar.

     

    Varying the pressure controls how much gas is being fed to the forge: effectively how big your flame is.

     

    Varying the ratio of air to fuel controls the temperature of the flame: how hot it is.

     

    Many (most?) Naturally Aspirated burners operate at a single Air:Fuel ratio. Some have an adjustable choke, allowing the ratio to be varied. The most Fuel-Lean mixture is obtained with the choke fully open. Closing down the choke then allows the mixture to be made richer and, in the case of gas forges, the flame to be made cooler.

     

    Probably the biggest change from normal gas-safe practice is that we generally run fuel-rich. This is pretty much unheard-of in the world of heating. The reason for it is that it helps to prevent Oxidation of the workpiece by maintaining a reducing forge atmosphere. Your father may need coffee and aspirin.

     

    Tuning a NA burner is mainly a case of finding a gas jet size that gives the richest mixture ratio consistent with achieving the flame temperature required for the highest-temperature job it needs to do. 

     

    If you are buying a regulator, I recommend a plugged welding regulator for Propane. These are around 25 quid if you shop around, against around 15 quid for the cheapest thing that'll do the job. The welding regs are designed for use by hairy-ar5ed guys wearing welding gloves and are pretty rugged. They have large, grippy adjusting knobs and have the setting scale marked on the body. The scale isn't particularly accurate, but it's probably not much less accurate than the gauges that you'll find on cheap regulators and is plenty good enough for "us". We just need adequate repeatability: I'm forging today, so need X PSI. Welding needs Y PSI, etc. You'll find the values for X and Y for your setup by using it. Gauges have a tendency to get damaged. I use them when they are useful, but feel things are usually safer without them.

     

     

     

     

     

    • Like 2
  14. I'd say that looks just enough like a forge to fool the beginner into hitting the buy button: Prepare for a precipitously steep learning curve.

     

    First off, can you get some photos of the whole lot? Something is obviously wrong. We can see that from the photos of the flame, but we have no information with which to diagnose the cause.

     

    The flame looks horribly rich and therefore cool. There's either too much gas for the amount of air, or not enough air for the amount of gas. Same thing, but not everyone sees this.

     

    My first impression is that the mixture is pants and that there's also not enough of it.

     

    There could be a number of reasons. We need to start somewhere, so we'll start with the stuff that's easy to check

     

    The first thing to check is your cylinder. Propane or Butane? Propane is good. Butane is not. Screw-in regulator, or clip-on? Screw-in is good. Clip-on is not.

     

    If it's a screw-in regulator, what is its outlet pressure range? 0- 2 bar (0-30 PSI) is good. 0-4 (0-60 PSI) bar is perhaps better, though not by much. Some regs are 0.5-4 bar (8-60 PSI) and, whilst they do work, I really loathe them for the lack of control at the bottom end. They also make lighting the forge unnecessarily exciting.

     

    The clip-on regs are fixed low pressure over here, 35 mbar for Propane and 28 mbar for Butane. No good for Naturally Aspirated burners. 

     

    If you are on Propane, it is regulated to a sensible pressure, and are still getting that sort of flame, are there chokes on the burners? If so, are they open? If they are closed, open them fully and try again.

     

    No chokes, or chokes fully open already, we'll need to see some detail of the air intake end of a burner. We also need to know what the gas jets are and what size they are. Mig welder tips are often used. otherwise it's usually drilled holes. The flame photos suggest the gas jet holes need to be smaller: easy if you can swap out mig tips for smaller ones (they are sized for the welding wire diameter. We'll need to know what size is marked on them and also the thread diameter and pitch). Considerably less easy if they are drillings.

     

    Check the above and get some photos. We can probably make it workable.

     

    Where in Somerset are you? What do you want to make initially?

     

     

  15. I usually use 2.5 mm 3-core from the 13A plug to the VFD and 1.5 mm 4-core from the VFD to the motor.  I usually use H07RN-F cable if I can (it has a tough rubber sheath and is nice and flexible). I don't know whether it's in accordance with the current wiring regs though.

     

    When I build a VFD into an IP66 enclosure, I do like to fit a socket for the 3-phase out, mainly so that I can use the VFD for running different machines, just by plugging in the one I want to run.

     

    A few years back, it was relatively cheap and easy to use the blue 230V 3-phase and earth IEC60309 plugs and sockets. They've become hard to find and expensive over the last few years. Unless you have a particularly strong reason for using a plug and socket, it's probably best to just wire direct initially and keep an eye out for affordable plug(s) and a socket.

     

     

    • Like 1
  16. Something that is quite likely to happen at some point is that you'll get the flame running back up the burner tube. It can be pretty worrying if you are not expecting it.

     

    The air/fuel mixture needs to be moving towards the forge faster than the flame-front can move through the mixture in the opposite direction. 

     

    The flame-front speed is not a constant. It varies with temperature, pressure and air:fuel ratio. 

     

    With your burner (which is a Naturally Aspirated design without an adjustable choke), the air: fuel ratio is effectively fixed. The mixture pressure (as distinct from the gas pressure upstream of the gas jet, which is what your regulator controls) is reasonably constant: it will vary with atmospheric pressure and you may see another inch or so of Water Column in the forge if you are giving it the beans and have the opening restricted.

     

    When you start with a cold forge, it can be difficult to get the flamefront speed high enough (or the mixture speed low enough) to keep the flame stable. As the forge gets hotter, the flamefront speed increases and the flame stabilizes. 

     

    At some point, things can get hot enough that the flamefront moves faster than the mixture and runs back along the burner tube until it runs out of mixture to burn and goes out. Gas will keep flowing and will draw more air and mix with it. The mixture will flow along the burner tube until it reaches the hot forge, ignites and repeats the cycle. As the flamefront moves along the burner tube, it will heat it up a little. This will make the mixture a little warmer the next time, increasing the speed of the flamefront. 

     

    The noise is something like a pulsejet. If you catch it quickly enough, you can often turn up the gas pressure to increase the mixture flow and break the cycle. Don't pussy-foot about if it happens. Double the gas pressure for starters, which will give a 40% increase in mixture flow (Gas flow through a jet varies as the square root of the pressure doubling the pressure will increase flow by a factor of root2; 1.41). 

     

    Once you know what you are looking for, you can establish the minimum pressure your forge will run without burning back without unnecessary anxiety.

     

     

  17. Kast-O-Lite 30: The number needs specifying.

     

    Kast-O-Lite is the trade name of a range of insulating castables. The number that follows is the rated temperature in hundreds of degF. Kast-O-Lite 30 is rated for 3000 degF. 

     

    As a general rule, the higher the temperature rating, the higher the Alumina content (Alumina content is a big factor in flux-resistance) and the higher the thermal conductivity (higher conductivity = poorer insulator). 

     

    There are certainly a Kast-o-lite 23 and a 25, probably others too. I'd hate for someone to search for Kast-O-Lite, buy 23 because they don't know the difference, and end up with a dribbly mess instead of a forge lining.

    • Like 1
  18. The length of the burner tube matters. There is a pressure loss (resistance to flow)  associated with the length of the pipe and the flowrate. The maths gets pretty complex pretty quickly. 

     

    The gas issuing from the jet generates a low-pressure zone around the gas stream that draws air in. The gas and air mix with the gas slowing down and the overall gas/air mixture retaining the momentum of the original gas flow. This effectively results in a (very) small overpressure that drives the turbulent mixture along the burner tube towards the forge. "Some" length of burner tube is necessary to get adequate mixing, but the longer the tube is, the greater the pressure loss along the tube. "About" 8 pipe diameters seems to be the length that most of the homebuilt NA burner designers have found works best. Elbows and other fittings offer a restriction to flow that can be expressed (for "simplicity", though that is a relative term) as an equivalent length of pipe.

     

    The combination of pipe lengths and the elbow looks, to my strictly inexpert eye, to be equivalent to about 5 or 6 times the "works best" value.

     

    The (fuel) gas pressure is so high that the gas will flow regardless. The extra resistance to flow will therefore affect the airflow almost exclusively, reducing the amount of air relative to fuel.

     

    Going to a smaller gas jet should help to get the mixture back towards a "good" mixture.

     

    If the resistance to flow in your long pipe is high, the mixture speed will be relatively slow. The mixture needs to move towards the forge faster than the flame-front can move through the mixture in the opposite direction. If the flamefront moves faster through the mixture, the flame will run back down the burner tube until it runs out of mixture to burn (somewhere near the gas jet, where the gas and air have not had the time/distance to form a flammable mixture). The flame will go out, mixture will start to flow. After a time, it will reach the hot forge chamber and ignite, whereupon the process will repeat. I am guessing this may be the pulse jet effect you mention. 

     

     

    • Thanks 1
  19. It would be nice to see the position of the gas jet relative to the end of the long threaded nipple with everything assembled.

     

    Looking at the photo, it seems that with the brass sleeve-thing inserted into the threaded nipple with enough sticking out at the back end to get the hose on, the jet will be somewhere close to flush with the end of the threaded nipple? I suspect that'll cause all kinds of turbulence right where you don't want it. 

     

    I think you probably want the full length of the mig tip exposed to allow air to be drawn relatively smoothly into the low-pressure zone created by the gas coming out of the jet.

     

    Once you've got it something like, there are plenty of adjustments available with that design.

     

    If the installed burner is similar to the right-hand one in the first pic, there's a very long length (in total) of pipe on it plus an elbow. A screwed 90-degree elbow is usually considered to be equivalent to "about" 30 pipe diameters in terms of pressure loss IIRC. I think the left-hand one in the top pic is probably pretty close to convention at "about" 8 pipe diameters. The one on the right looks to be equivalent to perhaps 40-50 diameters. 

    • Thanks 1
  20. There was a group buy of 2" x 36" Multitool attachments on the now-defunct British Blades forum a few years ago. I think it was an importers stock clearance, but the price was a fraction of list (I think they were about 60 quid each). I bought 3 of them, and a 4" x 36.

     

    One of the guys on the forum came up with a kit to fit the MT to an IEC metric 80-frame foot-and-face-mount motor. It was a pretty simple kit: basically a few bits of plasma-cut 6mm plate, a few fasteners and a bit of 1mm shim stock to wrap round the 19mm motor shaft and adapt it to the 3/4" bore of the wheel. I bought 2 of them. They worked brilliantly, though the shim thing was a lot of faffing about and I just used Loctite on the second one I built.

     

    I used 0.75 kW (1 HP), 3-phase, 4-pole motors because I had a number of different VFDs available. If I was doing it again now, I would use a 1.1 kW (1.5 HP) 2-pole motor and a Sensorless Vector VFD.

     

    I mounted the third attachment to a really cheap, nasty 1/2 HP bench grinder. It beats the wotsits out of having no belt grinder, but is definitely underpowered. I had a try of one that someone had put on a 900-Watt bench grinder and that was a properly useful tool in a fab shop.

     

    The MT kits we had were for a 3/4" shaft with reducing bushes supplied to fit smaller sizes. I gather there have been other kits that will only go to a maximum 5/8" shaft, so cannot be readily used on an 80-frame motor. It's worth checking maximum shaft size on the one you are considering.

     

    In your shoes, I'd get the Multitool, mount it to a cheap/available bench grinder, then save for a motor and VFD. Have a think about making an adaptor plate in the meantime. That way, the only thing that you'd need to buy that wasn't on the direct upgrade path would be the cheap bench grinder.

     

    Even on a 1/2 HP bench grinder (a little under 3000 RPM, assuming you are on 50 Hz mains), it's able to stock-remove for knifemaking faster than most beginners can do with files. It does mean that you get to develop grinding skills, rather than filing skills.

  21. As mentioned, some systems have a built-in flow-limiting valve. It is broadly similar to the "air fuse" sometimes fitted to pneumatic systems before a hose. The idea being that if the hose gets cut, the air fuse will close and limit the airflow, so you are not having to deal with several yards of angry hose thrashing about and trying to beat you to death. It will limit the flow to some (fairly small) fraction of the maximum design flow and will automatically reset when the flow drops to zero (or near-zero). With a gas system, the potential is for several yards of angry hose with several feet of even angrier flame issuing from the end, so it's a pretty good safety feature really. 

     

    If you have such a system in your cylinder, it will be designed to limit the flow once it gets over the maximum design flow for the cylinder. The supplier should be able to tell you what the maximum design flow is. You can then calculate what your burner takes at different pressures using the BTU calculator on Ron Reils site (other calculators are available) and see whether or not it'll do what you need. It is always a good idea to back off the regulator before closing the cylinder valve and to open the cylinder valve before increasing the pressure with the regulator. Opening the cylinder valve with the regulator turned up can cause an initial flow surge that may be enough to trigger such a safety system.

     

     

  22. The "Inverter duty and TENV motors" curve in Cody's second graph will almost certainly be for Non-Ventilated motors and for fan-cooled motors with a separately-powered cooling fan: effectively a mains-powered, fixed-speed fan which maintains the cooling airflow regardless of the motor speed.

  23. The VFD will effectively vary the Voltage and the Frequency together from minimum speed up to the rated speed of the motor. The maximum Current (in Amps) is whatever the motor is rated at, regardless of speed. Power can be thought of (in slighly oversimplified terms) as Volts times Amps. As the Volts go up, and the Amps stay the same, the power goes up. This is the "constant torque" part of the torque/speed/power diagram: Power is also Torque times RPM.

     

    Once the rated motor speed is reached, the rated Voltage is also reached. The VFD cannot increase Voltage beyond this, but can still increase the frequency. Power is again Volts times Amps. This is the "constant power" part of the torque/speed/power diagram. Power is Torque times RPM, so as the RPM increases, the torque must reduce.

     

    In real terms, the 1750 RPM 2 HP motor and the 3500 RPM 2 HP motor will both be giving 2 HP at 3500 RPM. Both will be giving the same torque because power is torque times RPM.

     

    Most of the heat generated within a motor is down to the Current (Amps) being drawn. At full Current (Amps), the heat generated will be broadly similar, whether the motor is running at maximum speed or at minimum speed.

     

    For TEFC motor, the cooling airflow moved by the cooling fan will vary with the speed of the shaft.

     

    At full rated speed, there is plenty of airflow and the motor can run at full-load continuously. As the speed reduces, the airflow reduces and the ability of the motor to cool itself reduces, such that the motor will need to be derated at lower speeds.

     

    Grinders tend not to be run at constant maximum torque, even at high speed, but particularly not at low speeds for a number of reasons: usually the operator has some degree of mechanical sympathy, workpieces get hot and need to be cooled down (during which the motor is under minimal load), belts get changed, etc, etc.  This means that in "our" application, the derating usually happens without anyone needing to think about it.

     

     

  24. I think the consensus view on the internet (insofar as such a thing is possible) is 7 times the burner port area as the minimum for the combined openings. In most cases, the need to have an opening big enough to get the workpiece in and out will mean that the area of the openings comfortably exceeds this.

     

    Taking a worst-case scenario with a 3/4" burner: the thinnest wall, largest inside-diameter pipe anyone is likely to use is Schedule 5, which has 1.050" OD and .065" wall thickness for an ID of 0.92" and an area of 0.66 sq in. Multiply by 7 to get 4.7 sq in. A 2 1/2" x 2" opening will be enough, even with a 5/8" rebar handle in it. 

     

    From the photo, it looks like the front hole you already have may be big enough, even if you close up the back hole when doing short stuff, and I'm guessing it'll need to be enlarged for workpiece access anyway?

     

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