Drains and Double traps
Double traps are no good. End of tip. Ok, here is some detail.
Anytime your drain goes up and down more than once you have a double trap UNLESS you place an air vent between the two traps that vents ABOVE the drain inlet.
The double trap causes drainage issues because air becomes trapped between the two traps and air is lighter than water. This causes the air to want to travel up as the water flows down resulting in NO DRAINAGE.
A vent allows the air to move instead of becoming trapped. This is why you vent a drain after the first trap if there is another trap or the potential of another trap.
This is also why you vent a drain after the first trap and before a common drain if you are connecting more than one drain. It helps prevent the possibility of a double trap and thus prevents a nasty backup.
In general just pitch the drain properly, install only one trap and don’t interconnect (unless required) and you will have no problem.
That was easy!
Bryan Orr is a lifelong learner, proud technician and advocate for the HVAC/R Trade