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Static Regain Sizing

Sizing of a supply trunk by the static-regain method: after each takeoff the trunk velocity is reduced so that the regain of static pressure from the lower velocity pressure, less the regain efficiency, pays for the friction and fitting loss of the next section. The static pressure then stays nearly constant at every takeoff, which is what balances the branches of a long trunk. Enter the first section's size and flow and the flow and length of each following section; the tool solves each section's diameter, reports the velocities, pressures and the residual, and compares the result with equal-friction sizing.

  • Section-by-section solution of the diameter that makes regain equal the section loss

  • Velocity, velocity pressure, friction loss and static pressure at every takeoff; residual imbalance

  • Comparison with the equal-friction sizes at the first section's friction rate

  • Chart of static pressure along the trunk

Static Regain Sizing

Unit System: /

Air and First Section





Trunk Sections (first section first; each ends at a takeoff)
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The Static Regain Method | Method page
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Static Regain Sizing

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 pv,i−1 − pv,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.



References

American Society of Heating, Refrigerating and Air-Conditioning Engineers (2021). ASHRAE Handbook: Fundamentals. ASHRAE, Peachtree Corners, GA.

Sheet Metal and Air Conditioning Contractors' National Association (2006). HVAC Systems Duct Design, 4th ed. SMACNA, Chantilly, VA.

Tsal, R. J., Behls, H. F., and Mangel, R. (1988). "T-Method Duct Design, Part I: Optimization Theory." ASHRAE Transactions, Vol. 94, No. 2, pp. 90–111.

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