Adding an extra set of heating coils

[MarkTilles]

Hi,

I live in Sweden and have an old 60 liter toploader Rohde.  When I bought it, it had difficulty reaching top temperature of 1260 C, although it would eventually get there after many many many hours. I decided to change the heating coils after checking all the other variables I could come up with that were applicable. 

I measured the thickness of the Kanthal wire in the system, calculated the diameter and number of coils, and wound new coils with a homemade coil winding machine which worked very well. I took the measurements of the original coils when cold, which were 22 ohms. I used that calculation for the new coils and the calculations nearly matched perfectly with the specifications for the new Kanthal  wire. Check.

Unfortunately, even with the new heating elements in place, it has difficulty reaching top temperature of 1260 C, and takes 6 to 8 hours from 950C. In my mind unacceptable.

So, my question is, since this is a two phase 240V/phase kiln, can I route a chase into the round bottom of the oven, and put coils there for a third element and run this circuit as well? Any ideas are welcome. Thanks so much!

[Dick White]

I am not familiar with the electrical systems in Sweden, but it is an immutable law of physics (Ohm's Law) applicable everywhere in the world that adding heating power (watts) will draw more amperage from the circuit. If the mains to that outlet are not large enough to carry the additional amperage, and if the internal wiring of the kiln are not sufficient to carry the additional amperage, you risk overloading the wires and starting a fire.  Yes, you can put more heating power in the floor of the kiln, but you will need to address all the wiring considerations. Finally, it is common for glass slumping kilns to have heating coils in the base, not so much for additional heat to go to a higher temperature, but to provide even heat horizontally across the kiln as well as vertically along the sides. Another aspect of glass work is the temperatures tend to not to go much above 1000C. Higher heat would melt the glass to a puddle, not retain a shape. At that temperature, heat is still moving by convection. At red heat and above, i.e., for ceramic work, heat moves by radiation. The ware needs to be exposed to the heating coils in order to gain heat. With bottom coils, the bottom shelf will become hotter, and in turn will radiate to the ware on it. But that's not very efficient. Consider that if bottom coils were superior for ceramic kilns, every manufacturer would be doing it. But they only do it for glass kilns.

Skipping back to general electrical theory, when you wound new coils that exactly matched the resistance (22 ohms) of the old worn coils, you did not actually add any more heat. As coils age and wear out, the resistance increases. Generally speaking, when the resistance of a worn coil is 10% higher than the resistance when new, it is considered too worn out to finish the job of reaching the highest temperatures. If you can't get the original coil resistance from a Rohde representative, you can reverse-calculate it using Ohm's Law based on the stated voltage and amperage on the electrical rating plate on the kiln.

[neilestrick]

First, what is the max temp rating of the kiln? If it could be that you're maxing out your kiln, and unless the elements are in perfect condition, it won't get to temp. What Rockhopper said is also good advice. 

As for adding in another element, there are a lot of 'it depends' situations at play which make it unsafe for us to say you can or can't. Basically, if it's a digital kiln, that means another relay, wiring, etc, which may or may not fit in the control box. The increased amperage draw may require a new power cord, and would most likely require a larger breaker, which would also require new wiring from the breaker to the outlet. If it's a manual kiln, you can't necessarily hook it into an existing switch, as it may be too much amperage for that switch. So that would mean putting in another switch, and all the increased amperage issues as before. Routing a proper channel for the element is also not very easy. Again, too many variables to even attempt it unless you know what you're doing with electricity. The better option is to not fire so hot. Most things you can do at cone 9 you can do at cone 6 (1222C). You'll get longer element life, and the kiln won't struggle.

[neilestrick]

2 minutes ago, Dick White said:

But they only do it for glass kilns.

I get what you're saying, but they are available in ceramic kilns. However they're not necessary unless you're consistently firing very dense loads like tiles or plates where some extra oomph from the bottom is needed. They're not the main source of heat, though, as the wall elements can provide enough heat for most situations. I think I've only ever sold one floor element kiln in the last 15 years.

[Bill Kielb]

On 3/24/2021 at 5:46 PM, MarkTilles said:

So, my question is, since this is a two phase 240V/phase kiln, can I route a chase into the round bottom of the oven, and put coils there for a third element and run this circuit as well? Any ideas are welcome. Thanks so much!

1260 is cone 9, which is sort of odd. Anyway as everyone has said above but let me simplify this. Get the original wattage of the kiln and see if you are consuming that much wattage now by measuring the current draw and voltage. Watts = Volts x Amps. You must measure both as accurately as practical.  As stated above 10% reduction in wattage generally means your kiln cannot make its design temperature  in a reasonable time frame. If either the voltage is low by 10% or the elements resistance has risen such that the amperage has been reduced by 10% your kiln will not make top temperature acceptably.

Watts are everything, it is simply the amount of energy designed to make that kiln fire to its rated cone. They designed the kiln new with enough energy (Watts)  to be able to fire to rated cone offsetting the shell losses, the brick mass, your ware mass,  shelf mass, and some amount of  minimal air leakage.

Bottom elements offset the heat loss from the bottom of the kiln and for ceramics the floor element is usually sized smaller. Cone art, Bailey they use floor elements and it does even the heat out but really is not necessary for most kilns and takes some design work to do safely and effectively.

I would start with measuring the wattage you have, likely it is less than a new kiln which means you don’t have enough power, from there getting the right amount of power will be your first step. If this kiln is only rated to cone 9 then it will not fire often to that temp before it needs new elements. Cone six is definitely more reasonable for an electric kiln.

Tell us the rated wattage, the measured voltage and measured amperage and we likely can suggest how to solve  this in a practical way.