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.

  • Fitting Loss Coefficients

  • Fitting loss coefficients determined by different researchers vary considerably and should be considered to be approximate only. However, for a piping system, the losses form only a minor proportion of the total losses, with coil, control valve and measuring valve losses dominating. Consideration should also be given to manufacturing differences and installation practices. It is recommended that a calculation margin of at least 15% be applied to the calculated headloss.
  • Koch(4) has compared in detail the available loss coefficient methods available in 2000.
  • Within Blowpipe UV, coefficients can be calculated using 1) the Darby(3) 3K method for elbows, tees and valves, or 2) Hydraulic Institute(1) data for elbows (as reproduced by CIBSE(5) & ASHRAE(6)), Idelchik (2) data for tees (as reproduced by CIBSE) and Darby 3K data for valves.
  • Coefficients for expanders and reducers are calculated using standard equations based on the assumed geometry of the fitting.

  • 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.

  • References

    1 Handbook of Hydraulic Resistance, Idelchik I E, 3rd edition, USA (1994)
    2 Engineering Data Book, Hydraulic Institute, USA (1979)
    3 Chemical Engineering Fluid Mechanics, 2nd Ed, Darby R, USA (2001)
    4 A survey of available data for pressure loss coefficients for elbows and tees of pipework, Koch P, BSER&T (2000)
    5 CIBSE Guide C Reference Data, Chartered Institute of Building Services Engineers (2001)
    6 Fundamentals Handbook, ASHRAE (2017)