Pressure Loss Calculations

  • All calculations use the D'Arcy-Weisbach equation for flow of fluids in pipes. These equations are those reproduced in CIBSE, ASHRAE and similar publications.

  • Friction Factor

  • The Colebrook and White equation is used to solve the D'Arcy friction factor in the turbulent flow regime and the Poiseuille equation is used in the laminar flow regime.

  • Pipework Heat Transfer

  • For uninsulated pipes, equations using the Nusselt, Prandtl and Grashof numbers are used, which assume that the outside temperature of the pipe is the same as the fluid; for insulated pipes standard heat transfer equations derived from Q = U x A x dT are used. A discontinuity occurs between the heat transfer given by the two equations, ie between the heat transfer from an uninsulated pipe and one with a very thin layer of insulation. This is an unlikely situation to occur, however, if required, it should be modeled as a bare pipe.

  • For plastic pipe materials, the calculation treats the plastic as a layer of insulation.

  • If the pipe length is greater than 5m (16ft), the pipe is sub-divided into 5m (16ft) lengths and each section is calculated separately, using the final temperature of each section as the initial temperature of the next.