Well, I am not familiar with a Z;duct persay, but I could imagine that this is analogous to a duct with two bends with no straight section in between them. THe reason this is different than a duct with two bends with a straight section inbetween is that the losses are much, ,much greater. After a bend, the the flow recovers some of the pressure loss, if there is no straight length, the separation is high off of all the corners and bends, and the recovery is low. Lokk up the SAE (soicety of aerospace engineers) losses for a duct with two bends.
There is an air-to-air heat exchanger used for heat recovery in ventilation systems called Z-duct. Do an internet search on "z-duct heat exchanger" and see if that is what you are looking for.
A Z transfer duct is often used between full height walls above the ceiling to transfer air from one plenum or area to another. The Z bend is to eliminate noise - sound gets absorbed when having to bounce through two 90-degree angles. You will often see Z-transfers over conference rooms, calling booths, private offices, etc. You can also ask this question in the HVAC forum for a greater response.
