Static Regain Sizing: Method

The Static Regain Method

Along a supply trunk the airflow falls at each takeoff. If the duct size stays the same the velocity falls too and part of the velocity pressure converts back into static pressure, a regain; if the duct is sized by equal friction the static pressure falls steadily and the far takeoffs see less pressure than the near ones, so the branches need dampers to balance. The static regain method chooses each section's size so that the regain at a takeoff equals the friction and fitting loss of the section that follows: the static pressure is then the same at every takeoff, and the branches balance by themselves.

The regain is not complete; turbulence at the takeoff and transition loses part of it. The regain efficiency R, typically 0.75 and rarely above 0.9, scales the ideal regain p_v,i−1 − p_v,i. The tool sizes the first section by the diameter entered (chosen for velocity or by equal friction) and solves each following diameter by bisection between 30 and 100 percent of the previous one.

The trade: regain sizes are larger than equal-friction sizes toward the end of a long trunk, so the method costs sheet metal and space and is used on long trunks of medium and high pressure where balancing by dampers would be noisy and wasteful. The fan pressure is the loss of the first section plus the branch, terminal and equipment losses, not the sum along the trunk.

Round each solved diameter to a stock size and rerun; the residual column then shows how far each takeoff departs from perfect balance.