If the sheet of water falling down the stackpasses a stack fitting through which the discharge from a fixture is entering the stack, thewater from the branch mixes with or deflects therapidly moving sheet of water. An excess pressure in the drain from which the water is enteringthe stack is required to deflect the sheet of waterflowing downward or mix the branch water withit. The result is that a back pressure is createdin the branch, which increases with the flow rateand flow velocity down the stack and with therate of flow out of the drain
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