Can someone correct me if I am wrong and add some calculations/experience if possible.

My flow rate looks like it will be 7.5LPM through 15m of coil in the HLT based on some initial tests.

say I have 30 Litres in the HLT covering the coil and 8KW of elements.

to heat 30L of water 1 degree with 8KW takes .26 minutes. To include the mash that's 130L and would take 1.2 minutes.

Say 1 degree in 1.5 minutes heating capacity to be conservative -

http://processheatingservices.com/water-heating-time-calculator/
For all 100L of mash to pass through the HEX coil will take 13 minutes (at 7.5LPM)

If I set my HLT to desired temperature it should theoretically get there after ~15-20 minutes regardless of the increase (I don't know how to work out if the elements will keep up so am assuming the limiting factor is flow).

Now this means that the wort is being raised way faster than 1 degree per minute, is this a problem? Is 7.5LPM going to compact the grain bed? I've seen 5 thrown around as the magic number on a lot of systems...

Sorry about all the questions