Building An Electric Kiln For Pottery

Thread Starter

Blue Crystal

Joined Mar 21, 2017
4
I intend to build an electric kiln for pottery, clay ( and other materials ) sculptures.
The internal dimensions would be 40cm(height) x 40cm(width) x 40cm(deep).
Therefore, its internal surface is 9600 cm2. I need a maximum temperature of 1300 Celsius.

1) I would like you to confirm or correct this information. I was informed that in a typical kiln the calculation of the total Electric Power necessary is given by this formula P ( Watts ) = 0.6 X Total Internal Area ( cm2). If that is true, then I would need 5760 Watts. However, I found in the internet values that are different from 0.6. So, if you are an expert please give me your informed opinion on that.

2) In order the temperature inside de kiln could be as much homogeneous as possible, its internal walls should be filled with as much heating element wires as possible. But there is a trade-off to be made. The more heating wires the bigger will be the resistance and the weaker will be the production of heat.Furthermore it may unnecessarily increase the cost of heating elements. Additionally, there will be the need to carve too many grooves on the refractory bricks to fit the elements inside which will weaken and possibly fracture them. By looking at the pictures of many different kilns it seems that the optimum distance between two parallel coiled wires is about 7cm ( axis to axis ). But if the grooves should be carved directly on the bricks and in the middle of their height then the distance between parallel grooves would be equal to their height or a multiple number of it.
Would you agree with my reasoning or do you have a different one ?

3) It seems to me that 3 resistors ( heating elements ) electrically connected in parallel is a good circuit because they would use only three relays and probably could be controlled by an electronic controller. What is you informed opinion on that ?

4) Another important aspect of the kiln design is the coiled wire ( heating element ) diameter. The bigger its diameter the bigger will be its durability. However bigger diameter reduces the resistance of the heating element and produces lesser heat. I intuitively think that a diameter of 1.30 mm would be OK. ( Of course, later on, I should also consider the coil diameter and the distance between its turns ).
Do you have a better informed proposition ?

( later on I may need further advice on the same project )
 

dl324

Joined Mar 30, 2015
8,852
High power, high temperatures. If I did a make versus buy analysis, I'd probably buy.
 

crutschow

Joined Mar 14, 2008
23,296
I'm not familiar with kilns but I can answer one of your questions.
The wire diameter indeed affects it's resistance and the power dissipated so you need to determine how long one heating wire will be in the grooves and from that you can determine the wire size.
Thus, for example, if you wanted one heater to dissipate 2kW from a 220V source then the wire total wire resistance would be 220² / 2kW = 24.2 ohms.
You then divide that by the wire length in the kiln grooves to get the required wire resistance per unit length.
You then look at resistance wire tables for the wire you want to use (nichrome I assume), and that will give you the wire gauge (diameter) you need.
 

LesJones

Joined Jan 8, 2017
2,316
Your formula leaves out the most important factor. This is the thermal insulation between the inside of the furnace and the outside. This is where all the power will go once the furnace is up to temperature. This website will give you an idea of what high temperature insulating materials are available. This is where I bought the insulation for my metal casting furnace.

Les
 

Thread Starter

Blue Crystal

Joined Mar 21, 2017
4
I'm not familiar with kilns but I can answer one of your questions.
The wire diameter indeed affects it's resistance and the power dissipated so you need to determine how long one heating wire will be in the grooves and from that you can determine the wire size.
Thus, for example, if you wanted one heater to dissipate 2kW from a 220V source then the wire total wire resistance would be 220² / 2kW = 24.2 ohms.
You then divide that by the wire length in the kiln grooves to get the required wire resistance per unit length.
You then look at resistance wire tables for the wire you want to use (nichrome I assume), and that will give you the wire gauge (diameter) you need.
Thank you, crutschow.
I am quite familiar with basic electricity formulas. My questions are related to more specific formulas like in the first question and the technical /theoretical details of the later questions.
 
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