I'm sorry, but it actually doesn't! I'm more confused than ever! It's not obvious to me how this works.
OK. There's a pump on a timer. It pumps nute solution up the fill tube (the shorter tube rising from the reservoir.) The nutrient solution rises to the high point of the overflow at the end of the return side of the loop--so the height of that overflow determines the water level in the trays/cups.
I use a telescoping 1/2" PVC riser so I can adjust it on the fly.
Then the water falls down the other side of the overflow back into the res. During the flood cycle it is recirculated back through to maintain the water pressure and so the water level (although the nute solution pumped into the trays will stay there for the cycle and not be replaced until the next one once the lines are pressurized.)
When the pump turns off, the system drains fully through the return leg of the loop, as it is slightly lower.
Does that make sense?
I'm sure there's some physical complexity preventing you from doing so, but a straight shot to the tray from the pump means that whatever drains out of the tray will go back to the tank and not remain in the plumbing.
Yes, I have to go up and over an obstacle from the reservoir and then back down to below plant level, which isn't shown in the diagram. This would trap stale nutrient solution at the top of the system.
The immediate response might be--so fill through the leg your draining with now, and don't use a loop. I considered that, but it doesn't really work for me--there are still bends in the return after the overflow that aren't in the diagram. Some amount of solution would still be trapped (less, but at least a few cups.) With this setup, it doesn't matter if some solution remains in the return leg, it will be cycled through the res before being pumped back up again.
Plus, I still liked the idea of keeping the solution circulating during the flood cycle. Finally, and maybe most importantly, using a loop with an overflow not only allows you to adjust the water level on the fly but makes precise timing of the flood cycle to achieve that water level unnecessary. I don't have to worry about a pump running too long and causing a tray or cup to overflow. This is important as while my res in on the floor, the loop and the plants are in a cabinet above a row of appliances! Otherwise one would have to add an overflow to the trays, which would fix the water-level (and isn't really feasible with cups.)
If you want the benefit of the overflow without the hassle of the loop, you can just add one with a tee, in-line between the res and the trays in a standard setup. It works the same way, the trays will fill to the level of the overflow and then the overflow will return excess to the res. To change fill level you adjust the height of the overflow, and stop mucking around with pumping times.
Also for really large setups you could use something more efficient and reliable than an aquarium pump. With this kind of setup, scale up the PVC size and you could use a sump pump with giant trays.
But I'm not knocking your tray system, it's much more straightforward. I'd be happily using it if I had the room. And I'll probably replace my cups with smallish trays, because these cups are stupid. I'm not sure why I did that.
Your point about elegance in design is well taken. I feel like what I'm describing is indeed something of an inelegant hack, but it does let you adapt to tricky physical layouts, or be more flexible about the pumping setup. It's not all that prone to failure but it is somewhat ugly and improvised.
