Interesting process Wood....especially you getting in and actually tackling this ....
.....but I'm not sure a parallel HX will do the job.
What's your thinking here?

For some reason I cant view photos 14 onwards...so forgive me for possibly asking stupid questions that may have been answered by the photos....
Water may stagnate to the rear of the HX. Choosing to preferentially short circuit through the front "chamber" effectively reducing the overall working length.
Hopefully you've ground out all the flanging/chamber webs smooth for flow to get up the back.

Will look at this in more detail later.....
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at 2.6Gal/min (10Lpm)...50% glycol cp 3.36kJ/kg
water in 77F/25C
water out 122F/50C...= 48,000BTU/14kW
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