during cooling tower discharge, when the water flows down a stack past the branch that is discharging at the same time, the out flow from the branch has to deflect the vertical stream in order to enter the stack and as the result a back pressure is created. This has the effect of retarding the branch flow and if the back pressure greatly exceeds the head causing flow, water may reenter the appliance (i.e. back flow out from the floor trap)

back pressure greatly exceeds the head causing flow, water may reenter the appliance

during cooling tower discharge