NiChrome Furnace/kiln build - question on wiring

Discussion in 'General Electronics Chat' started by RogueRose, Jul 30, 2016.

  1. RogueRose

    Thread Starter Member

    Oct 10, 2014
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    I am considering building a small furnace/kiln that will be heated with nichrome wire. I have 2 old hair dryers (1500w each) with very similar coils. I was planning on straightening the coils and place it on the bottom of the kiln. I would keep each element wired seperately, so 2 plugs, and keep the resistors and switches from the original dryer but remove the thermistor as that will make it blow most likely.

    Is there any reason that this setup wouldn't work? I would like to be able to control the temp so I was thinking of using something like a high wattage dimmer or a router speed controller but if there is another way to control the temp I'm interested in hearing.
     
  2. crutschow

    Expert

    Mar 14, 2008
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    That should work but you may need two separate main's circuits, depending upon the power rating of your outputs (it's typically about 1500W per circuit in the USA).
    A dimmer circuit should indeed allow you to manually control the power.
     
  3. SLK001

    Well-Known Member

    Nov 29, 2011
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    The wire is just one part of a kiln. What are you going to use as insulating material? Without good insulating materials (fire, or refractory brick), it will take a while to get to any useful temperature.

    What do you plan on using the kiln for?
     
  4. #12

    Expert

    Nov 30, 2010
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    A nichrome element that is designed to work in an air flow will melt with no air flow. You must discover the power limit for your nichrome by starting at a low power setting and stopping at (or below) cherry red.
     
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  5. tcmtech

    Well-Known Member

    Nov 4, 2013
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    Being stretched out they may get enough convection air flow to work. I've done it before and it works but they will likely run bright orange at full power and their service life will be quite a bit shorter too but if you have a high-powered dimmer on them that shouldn't be a problem being you can just turn them down.

    Option two if you don't need the full 3000 watts would be to put diodes on each heating element so one is running on the forward half of each cycle and the other is running on the reverse half thus splitting the 1500 watts between the two of them.
     
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  6. wayneh

    Expert

    Sep 9, 2010
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    @crutschow is right. I have nowhere in the house where I could plug in 2 hair dryers without tripping a circuit breaker.

    @#12 is also right. The hair dryer wire will be sized on the assumption of some air flow. Take away the air flow and it may over heat.

    You may want a different temperature thermal fuse, but it's a good idea to leave in a high temperature cut-out. In fact, you might add a few more. Safety first.
     
  7. #12

    Expert

    Nov 30, 2010
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    It's called a, "thermostat".
    In a kiln, you need a lot higher temperature range than a home heater or a kitchen baking oven. Probably best to buy a thermostat for that range of temperature.
    There isn't much convection in a kiln. Sure, tcm might have done this, but he's a natural born tinkerer. The majority of people will melt their nichrome several times before they figure out how hard to push the power and how long it takes for a kiln to arrive at the final temperature goal. Kilns have been invented. People have figured out the heat loss through the fire bricks and the right temperature for the product you are making, but, as usual, Rogue Rose did not describe any of the particulars needed to calculate anything.
     
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  8. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
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    The "thing" inside the hair dryer is a thermal fuse. Greater than X temperature, the insides melt.
     
  9. Robin Mitchell

    Well-Known Member

    Oct 25, 2009
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    What temperature do you require from your kiln? And what do you expect to put in there?
    If a high temperautre is what you want you could investigate into an induction furance where the heat generated is transfered to the item that is to be cooked.

    Otherwise charcoal may be a better choice with a hairdyer on a cold setting.

    Really? Only 1500W per circuit?

    Across the water here you have light circuits on a 6A fuse (1.5kW). My workshop circuit is on a 32A circuit giving me 7.36kW to play with. Why are American systems so low rated?
     
  10. #12

    Expert

    Nov 30, 2010
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    We have 240 volts available in residential, but it's center tapped to provide a pair of 120 volt supplies.
    A "standard" electric furnace for a home might use size 6 AWG wire on a dual 60 amp circuit breaker to drive a 10KW heater.

    Now to the kiln.
    Based on: http://www.bryantrefractory.com/uploads/products/20_9.pdf
    3000 watts can raise 2/3 of a cubic foot of interior space to 2600F.
    That's assuming I did the math correctly, you can get 3KW out of the 3KW worth of nichrome without melting it, and 3 inch thick firebrick.
    Interior dimensions are an 8 inch cube.

    Those numbers resemble this: http://www.bigceramicstore.com/paragon-sc3-kilns.html
    costing about $1000 as a commercially available kiln which heats about the same amount of space to 2000F with 1800 watts, so I think my math is pretty close.
     
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  11. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
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    In the US, we typically have 240 split phase or 120-0-120 for residential. 15 A at 120 V circuits were commonplace. There is also a requirement now to have 20 A circuits in the kitchen and I think the bathroom for such things as hairdryers, microwaves and toasters.

    The NEC say that the circuits should not be loaded more than 80% continuously. There version of continuous is not 24/7.

    The 240 circuits are dedicated and might go to the outside air conditioner, electric clothes driers, electric water heaters, electric stoves or maybe a window AC unit (rare). So, the house I live in, only has one 240 V circuit for the outside AC unit. At one point in time, it had a 240 V circuit for a commercial window AC/heating unit that was used in a hotel.

    If one was actually putting in a "workshop", they might run a sub-panel for like 60 A at 240 VAC.

    We also don't have whole house GFCI's (RCD's across the pond), nor do we have power cords with fuses in them. The requirements for GFCI's and AFCI's have changed, but you don't have to retro-fit. AFCI's are Arc Fault Circuit Interrupters. They turn off the power when it sees a "spark" signature and they make combo AFCI/GFCI units.

    In general, 3 phase is not available for residential, but some power companies allow it.
     
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  12. boatsman

    Senior Member

    Jan 17, 2008
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    We were told that the US has 110VAC because they can afford the cost of the much thicker copper wire needed for the higher amperage. Europe and most of the Middle East has 220VAC.
     
  13. #12

    Expert

    Nov 30, 2010
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    And I was told that there is 240 VAC in the breaker box, any time I need it.
    I don't see a problem.
     
  14. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I would have thought a better solution would have been stove element as the element is embedded in ceramic and insulated, another alternative to the hair dryer approach is obtain a spare heating element for an electric furnace, the problem with this and the hair dryer solution is the element is exposed.

    N.A. has both 120v & 240vac available in a residential supply.
    Max.
     
    Last edited: Jul 31, 2016
  15. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
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    Over time, "they" bumped it up on us from 110/220 to 120/240. I remember when the standard test voltage was 117 V. Old times wuill still hay i have 110 in the house.

    The fin part is with commercial three phase it's usually 240, high leg delta and/or 208 V 3 phase Y. The unusual building I was in had both.
    They also has German power in parts of it. They were running some German furnaces.

    Most things didn't matter, but our diffusion pumps had to have 208 V heaters when we moved from a 240 VAC high leg delta location.
     
  16. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Nichrome melts at 1400C. The wire gets to 1400C (2500C) long before the kiln - possible in a few seconds unless it is in close contact with something else to conduct the heat away or good airflow exists (hair drier fan).
     
  17. tcmtech

    Well-Known Member

    Nov 4, 2013
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    Fully exposed heating elements in a kiln is normal.

    I've been looking into building one for a while now (~3 - 4 Cu foot interior) and gradually collecting materials for it. I have the Vermiculite and silica sand plus several cans of sodium Hydroxide for making the refractory liner and related stuff. My initial plans were to run open elements for heating it but as of lately I have been leaning more towards building a blast furnace instead and use an old fuel oil burner set up to run on used oil instead.

    Since I no longer have the Ex here I don't need a kiln for curing ceramics but a good high capacity blast furnace that could do 50 - 100+ pounds of aluminum or iron in a single batch would be nice to have.
     
  18. SLK001

    Well-Known Member

    Nov 29, 2011
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    I've been wanting to smelt iron for quite a while. I can't do it where I now live (the neighbors would freak), so that project will have to wait until I move to NC and have plenty of room for fun projects like that. Just remember that melts like these contain serious thermal energy.
     
  19. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    In the area I live in they did this for over a century with no electricity. In some thing called a cupola furnace. There are still the remnants of them around if you know where to look and what to look for. They used wood and charcoal for the heating. There are lots of guys still doing it to make castings at home, the now defunct Lindsay books had several books on how to build one at home. The internet has plans too. https://www.google.com/search?q=cupola&ie=utf-8&oe=utf-8#q=cupola+furnace
     
  20. SLK001

    Well-Known Member

    Nov 29, 2011
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    Yep... I have most of them. That's the first thing I though of when tcmtech told us about his waste oil score - a blast furnace that uses waste oil for fuel.
     
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