Duct 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.

  • Fitting Loss Coefficients

  • Fitting loss coefficients determined by different researchers vary considerably and should be considered to be approximate only. Consideration should also be given to manufacturing differences and installation practices.
  • There is a real possibility that data presented currently is becoming more difficult to interpret and apply to real-world systems and is likely to give a false sense of accuracy. The fitting losses can form the major proportion of the total losses and it is recommended that a calculation margin of at least 25% be applied to the calculated system pressure loss.
  • Koch(5) has compared in detail the available loss coefficient methods available in 2000, highlighting the discrepancies and errors in measurement and interpretation of data in presented information.
  • Within Fanfare UV, coefficients are generally calculated using data determined by Idelchik(1), Millar(2,3) and CETIAT(4). The data is variously reproduced by CIBSE(6), ASHRAE(7) and SMACNA(8).
  • Coefficients for dampers are based on representative manufacturers data.

  • 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.

  • References

    1 Handbook of Hydraulic Resistance, Idelchik I E, 3rd edition, USA (1994)
    2 Internal flow: A guide to losses in pipe duct systemsk, Millar DS, British Hydromechanics Research Association (1979)
    3 Internal flow systems, Millar DS, USA (1990)
    4 Catalogue des coefficients de perte d’énergie mécanique, CETIAT, France (1990)
    5 Comparisons and choice of pressure loss coefficients for ductwork components, Koch P, BSER&T (2001)
    6 CIBSE Guide C Reference Data, Chartered Institute of Building Services Engineers (2001)
    7 Fundamentals Handbook, ASHRAE (2001)
    8 HVAC Systems Duct Design, SMACNA (1990)