Check out the right side of the still on the right.
It wasn't intuitive to me just how much heads could be run off. Initially the vapor is cooler than the preheating charge's boiling point (duh). But at some point that changes as the vapor weakens and heats up. Some rough, very rough guesses not accounting for realistic physics at all, so let's just assume a worm coil that captures all the latent heat and only check the sensible heat when the preheat charge would start boiling.
Energy to heat/preheat 10 gallons 10% wash from 70F to boiling: 3.6 kwh
Energy to condense the vapor (latent heat) when stripping 10g10% down to 20% distillate: 4.6 kwh
After 3.6 kwh (of stripping, so 7.2 kwh including initial charge heatup), the emerging vapor is at 208.5*F, so way hotter than the preheat charge initial bp (198.5) , confirms vapor can theoretically start boiling the next charge.
So the next 1kwh could produce 2.3 liters of 52% distillate from the preheat charge, that's 26% of the next charge's ethanol.
More than enough for those just wanting to do a fores/heads cut while stripping. Or even if it's saved as low wines, it represents not just a 100% savings on heatup energy and time for subsequent strips but a substantial reduction for the next strip itself since both charges will be stripping simultaneously roughly 20% of the time.
Another way to look at it is that you could strip as deep as you want without "wasting" any energy. The deeper you strip what's in the pot, the deeper you strip the preheat charge. Lets say you just kept going. If you strip 10 gallons of 10% for 8.2 kwh you will get 15.8 liters of 28.1% abv (down to 4% out the pipe!) AND the preheat charge will not just be preheated but also stripped down to the previous goal of 20% distillate, so an energy and time "free" additional low wines of 9.74 liters @ 39.1%. Combined that's a perfectly reasonable 25.5 liters @ 32.3% from 20 gallons of wash (yeah, I mix units, painted myself into a corner by starting with gallons!) IDK how realistic this scenario is, at some point the vapor and preheat temp will get close enough to severely reduce heat flow, but we also ignored the sensible heat from subcooling the distillate during preheating, so you might even be further along by this point?
Hell, a third scenario would be if you were ready to do a spirit run and also had a wash to strip. Strip in the pot, low wines in the preheater, and your entire spirit run is time/energy free compared to doing them separately. IDK, but the kwh are roughly feasible.
Personally I'm not chomping at the bit to build something like this right now. But if anybody has upgraded from a worm and flake stand this could be a cool project to repurpose them. I always thought worms are kinda dumb, but they are kinda neat if they make a 2nd still. Even if the above scenarios are beyond possible, they are a good first pass of considering ways to employ the trick. Maybe someone can make use of it.
(Paging NZChris, resident preheater evangelist who has probably been doing this for 30 years and telling us about it for 20. Is this a thing you have on yours? Is it typical on charentais even if it's not typical on drawings or highlighted in pictures? Does it have a name?)
