here we go, I think ive cracked it.
The system works kinda like this, i have a reserviour outside the grow room (helps keep temps in check, connected to a chiller, via a 1000l/hr pump. this pumps nutrient from the res to the chiller and back.
Also in the res i have a 6000l/min water pump, connected to 21.5mm plumbing overflow pipe and fittings, these go to the spraybars, which are located inside the top of the totes.
The water sprays from the spray bar into the totes, and the nutrient re-circulates from the bottom of the totes , via 40mm pipework and tank connectors, back to the res.
This is the basic description, but the pics should make it clear. The returns are balanced, with one on each side of the tote, to prevent the roots migrating down the return pipes.