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I want build a furnace using firebricks and coiled nichrome wire. The dimensions of the furnace are 1×1×1 m. I need temperature inside the furnace to be 650°c. I have 220 VAC mains.

The furnace will be insulated with ceramic insulation plates and closed very well using a metal door. The heat can build up slowly.

It will used for drying Biomass and woods. The material will be placed into the cold furnace, then brought up to temperature for a time, then allowed to cool.

Now my questions are:

  1. What is the suitable type and thickness for the nichrome wire?
  2. What is the proper length of the coiled nichrome wire to get 650°c ?
  3. If I will use electricity generator to supply the furnace with electricity , how many kilo watt the generator should generate to supply this furnace? Thank you
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  • \$\begingroup\$ Do you already construct the furnace? Do you will use a fan? First determine thermal resistance of the enclousure. You can use a well known heat source and thermometers. Then with the W/oC you can estimate the power required etc. \$\endgroup\$
    – GR Tech
    Commented Feb 28, 2015 at 1:32
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    \$\begingroup\$ Sweet suffering Jesus. Do you know what the ignition temperature of wood is? \$\endgroup\$
    – WhatRoughBeast
    Commented Feb 28, 2015 at 2:14
  • \$\begingroup\$ Yes, volatile flammable gasses will be released from the biomass. If you have a small fire going, they will ignite and help heat the biomass. But if they are kept inside the kiln, an explosion is very likely. \$\endgroup\$
    – user57037
    Commented Feb 28, 2015 at 4:47
  • \$\begingroup\$ GR Tech, mkeith. thank you for your comments I didn't construct it yet.and will not use fan, it will be small fire going for expulsion the flammable gases from the inside of the furnace \$\endgroup\$
    – ayman
    Commented Feb 28, 2015 at 8:27

3 Answers 3

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This is may not the answer that you expect, but your construction becomes very complicate. I will not tuch the the practices that using for wood drying, the safety matters etc, because I have no idea. I hope you are aware about this issues.

The kiln surface, the wall material and the temperature you want to rise, after a rough calculation requires an electric power of around 20kW. So first check if your electric company service can supply this 90A in a single phase.

Second, because of this current, you can not use a single nichr wire. The key factor here is the surface loading that you have to optimize. In the temperature range of 600~700 degr Celc. use a maximum factor of 4.5W/cm2 for a reasonable element life. This value is also affectd by the bonding method of the wire.

To obtain this value of surface load, needs a heating strip follow by a complicate calculations. However, you need also a slow temperature increase, that means a higher resistance element and consiquently a thin wire, or more precisely many thin wires connected in parallel.

As you can understand, all the above factors needs a very carefull design and this is the job of a specialist.

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  • \$\begingroup\$ GR tech, thank you very much for your helpful answer that made me sure that making a big furnace will be very difficult for non professional specialist \$\endgroup\$
    – ayman
    Commented Feb 28, 2015 at 22:05
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You should start by determining how much heat you need, and then work backwards.

  • You have an enclosed space of 1 cubic meter. How is it insulated? How much heat will you lose when the furnace is at full temperature?

  • How quickly do you want the space to heat up? If you can handle a slow increase in heat, then you can pump less power into your furnace.

  • How much can you allow the internal temperature to fall when you put something into the furnace? Are you intending to heat anything with a large thermal mass? This will effect the previous answer.

Etc...

Once these types of questions are answered, then you can start specifying the nichrome.

If you update your question, we'll try to help further!

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  • \$\begingroup\$ 1- the required heat is 650 c 2- insulation using ceramic plat so the heat lose will be very small 3- heat up will be slow increase 4- I will add the things inside the furnace and at the beginning then close the furnace door and at the end after finishing the drying process will open the door 5- it will be for drying biomass \$\endgroup\$
    – ayman
    Commented Feb 28, 2015 at 0:45
  • \$\begingroup\$ @ayman Thanks for your update! I'll have to wait before I can work on this. I expect you'll get some helpful answers soon. \$\endgroup\$
    – bitsmack
    Commented Feb 28, 2015 at 1:03
  • \$\begingroup\$ if the heat loss is "very small" then the power you need to reach 650C with a surface area of 6m^2 is "quite a lot". If that's not accurate enough you'll need better specifications, starting with the specific heat conductivity of that ceramic, and its thickness. \$\endgroup\$
    – user16324
    Commented Feb 28, 2015 at 1:48
  • \$\begingroup\$ I might start by looking at the power of similar sized kilns. I'm guessing ~5kW... maybe more? \$\endgroup\$
    – George Herold
    Commented Feb 28, 2015 at 2:13
  • \$\begingroup\$ ceramic thickness 12 mm and its thermal conductivity 0.059/0.034 \$\endgroup\$
    – ayman
    Commented Feb 28, 2015 at 9:10
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No idea how much heat you need but a good place to purchase manufactured nichrome heating elements is an appliance repair shop. The heating element in many electric clothes dryers is a coil of nichrome wire strung through ceramic insulating bushings.

These heating elements come tightly coiled but you stretch them out so that there is space between the individual coils. They are available in a variety of voltage and wattage ratings.

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