I'll keep this blueprint page for "archival" and reference purposes though
About this design:
This is a TILE of 4x fractionators where each one has its own loop.
It works best when the incoming hydrogen is stacked 4 high, and that can be easily achieved either through the endgame research as output from an ILS, or by using 4 single-stacked belts with pilers to stack the input hydrogen belt to 4 units "tall". Each loop's piler re-stacks the hydrogen to maintain the maximum flow rate.
You can optionally add a sprayer to the inbound hydrogen as well, and a Mk. 3 proliferator spray can further increase the deuterium output chance from 1% to 2%, but it also increases each fractionator's power usage to 9.9 MW (from 3.96 MW [With 4-stacked hydrogen input]), but that may not matter if you have a lot of power to spare in the mid-game to late-game.
The input hydrogen belt is in the center, raised one level so it enters the splitters at the top entrance.
I think this is an optimal, compact and efficient setup for getting a lot of deuterium very quickly and efficiently.
Note that my understanding is that this design may not be optimal UPS for the "endgame"; for that, please see my other design (linked above). This design should be fine for "midgame" though.
(Updated 2024-02-25, made obsolete by newer designs linked above)