I am building a 650 liters HERMS-brew-system. I would like to use 3/4" piping/hose. Does anyone know if it's realistic to use 15m of 3/4" stainless steel pipe as HERMS coil? Or would less do? The HLT will be powered by 3x8500W heat-elements.
It depends on how fast you want to move wort through the coil. I don't think you'll get very much heat exchanged in such a short coil, but it'll depend on the heat differential between wort and water, plus the speed of the wort.
The main issue is whether you're going to collapse the mash bed. I would strongly recommend a manometer, so you can maintain equilibrium between the mash and under screen.
I can't seem to find any information about dimension of HERMS-coils, in larger systems.
That's because HERMS is a more common homebrew technique. Normally, profile mashes are accomplished with more direct heating procedures. Often the mash vessel is heated directly or the wort is recirculated through an external vessel (like the kettle). Professional brewers don't usually like to lose heat from the HLT because it costs too much time.