Hi Bandito,
With regards to your power limitations, another thing you can consider is timesharing of power by the different heating elements. I ran into this same issue with my RIMS system. Naturally I have a heating element in my RIMS system which happens to be 2400 W which is the max for a typical 240V house circuit. I also want to control my HLT element with the same unit, however, this two is 2400 W which means if both are running then I'm looking at 4800 W which is too much.
The way around this I think is to consider how you limit the power to these demands (thanks LethalCorpse for the theory). You don't control these elements via a potentiometer, you use burst fire control which is like PWM (pulse width modulation) for high power AC current. Effectively it means you turn full power on to the burner for a set amout of time, and off for a set amount of time to limit the average power to the element. So, knowing that you have 50 cycles per second of mains power, you have 100 half cycles per second as well (each half cycle being 1000ms/100 = 10ms, or 10 milliseconds). If I need to run my element at 24% then, during every second, I have the element at full power for 24 half cycles (240 ms), and off for 76 half cycles (760 ms), using a zero crossing solid state relay (SSR).
During the time that the RIMS element is off (760ms), then I can send 100% of the unused mains power run my HLT element, meaning that I never draw more than 10 amps.
Now the question becomes, "is there a time when I need both elements at more than 50% concurrently?", I don't know, but I don't really think so. I'll get the HLT up to temp before mashing, and then it's just maintaining the temp in the HLT somewhere around 77C, meaning I can do it to it with the RIMS element. Your system might have different requirements.
I'll be doing all this with and embedded PIC microcontroller, however, I'm sure it can be done many ways, ie PLC or computer.
Having said that, I agree with a few of these folks when it comes to brewing experience. Although I don't know your level of experience, and obviously you have a good bit of process experience, it's crucial that you have the
brewing process down pact(sp) before you begin building the thing. It's fine to begin the design now, but as you gain more experience in brewing you'll want to change things as you become more familiar with the process, and wish you had incorporated a change before building it, ie before it's "too late". This is why it took me ages to design my RIMS system. As I found problems in the process, or pains, etc. i incorporated solutions into my RIMS system (ie sampling vavles, and bleed valves for priming) which sometimes meant almost starting back from scratch wasting heaps of time. In your case you might decide power timesharing is necessary and have to change the whole process, who knows.
We'll be watching this space with interest, good luck.
Rob
