Pressure Loss Calculations

  • For UK/Europe and USA Current duct sizing options, calculations use the D'Arcy-Weisbach equation for flow of fluids in ducts, with friction factors determined by the Colebrook and White equation. These equations are those reproduced in CIBSE, ASHRAE and similar publications.

  • For the USA Traditional duct sizing option, calculations use empirical formulae. These formulae reproduce traditional sizing methods, including 'ductulators'.

  • In accordance with standard duct pressure loss calculation methods, pressure losses within Fanfare are total pressures rather than static pressures.

  • Velocity Calculations

  • Unless noted otherwise, all velocities are the velocity in the equivalent circular duct, based on equal volume and pressure drop.

  • The difference in results obtained using UK and USA calculations result from the different equations used to determine the circular equivalent of rectangular ducts.

  • Velocity Pressure

  • In accordance with current methods for determining pressure loss in duct fittings, the velocity pressure is based on the mean velocity in the duct rather than the velocity in the equivalent circular duct.

  • Equivalent Diameter

  • Throughout, equivalent diameters of ducts are those which give the same pressure drop for a given air flow.

  • Fan Motor Sizing

  • Centrifugal cabinet fans: The motor sizes predicted for cabinet fans become increasingly inaccurate the smaller the fan duty, as the efficiency falls rapidly. Fanfare adds an additional pressure drop of 100 Pa (0.4 in H2O) for fans with a duty of less than 100 l/s (200 cfm) to allow for the resistance through the cabinet. However, it may be necessary to reduce the drive efficiency to approx. 10% for the smallest of fans.

  • Propeller fans: Although propeller fans are commonly considered to operate against zero external pressure, this will give zero absorbed power. Fanfare adds a fixed resistance of 50 Pa (0.2 in H2O) to entered values.

  • The losses through an air handling unit should be included in the system total loss, if the components have not been allowed for in the ductwork system.