He probably means sequencers, but let him clarify. How Do Sequencers Work in Electric Furnaces? | eHow.com
I'll comment more later, but I will add:
It does depend on element size and the elements are usually sized differently. e.g. You could have a 15 KW element and a 5 KW element. The 15's could be made up of two 5's.
It could take some effort and expense to "make" at 20 KW element behave as any element between 0 and 15 KW. The part to do this isn't cheap.
An example: http://instrumentation-central.com/S...pci_series.pdf
The manufacturer would have to be questioned to what happens on start-up, power loss/power up, loss of setput signal/restoration
Something to think about.
Putting elements in series MIGHT be a possibility,but it might not be the power level you desire. I doubt it would work.
Eurotherm makes some of the best stuff out there.
So, it's entirely possible to run a system with such a controller and with a little magic so it so it runs at 100% and X% based using a very small switch.
You could run it at say 5 KW and 1 KW based on a simple switch. Giving you a total of 15 KW in one one and 6 KW in the other.
I'd have to brush up before committing to a design.
One way is to ignore the sequencer's all together, use a small PLC and duct temperature and roll your own.
6 KW and maybe 5, 10 and 15 KW based on temperature rise.