This might work for building a HERM's
http://www.solarcoil.co.uk/products.html
It might, but 10 metres of 10mm pipe is less than €20, and you can get a bigger radius to maximise heat transfer in your HLT? Seems an expensive way to do it, if you can do a bit of diy.
Efficient herms are low runs max 2 metres and higher pump speed according to expert posts I've read elsewhere, any experts in fluid dynamics care to comment?
https://store.brewpi.com/mashing/herms-coils/weldless-herms-coil-kit-30cm-bsp
These shop bought ones are 15m in length! And by the way - be very careful if you google 'herms tubes'. The results that came up were 'interesting' to say the least!
My (limited) understanding was that the longer the coil, the quicker/more easily the temperature stabilises between the HLT and the mash tun? I'm about to make one so any advice appreciated!