The following properties are used to determine airflow rates and duct pressure losses

Density

  • Density is corrected for air temperature, humidity and altitude.


  • Specific Heat Capacity

  • Specific heat capacity (Cp) is corrected for air temperature only within Fanfare. Cp reduces with reduced pressure, but within the range 0 to 2500m / 8000ft permissible within Fanfare may be considered to be independent of altitude.

  • The value of specific heat capacity depends on calculation/test method and the air condition used as the basis. Within the UK (UK/Europe duct sizing options within Fanfare), a value of Cp is used which varies with temperature from approx 1.006 at 0C to approx 1.05 at 40C . At 20C / 68F, Cp is approx 1.018 KJ/kg K. (0.243 Btu/lb F)

    Within the US (USA traditional and USA current duct sizing options within Fanfare, a value of Cp is used which is almost constant value over normal range of temperatures. At 20C / 68F, Cp is approx 1.006 KJ/kg K (0.240 Btu/lb F).

    This leads to a 1% difference in calculated volume flowrates at 20C / 68F and 3 % difference at 40C/104F.


  • Dynamic Viscosity

  • Dynamic viscosity is corrected for air temperature. It is usually considered to be independent of altitude, varying by no more than 2% in 5000m.