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.
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.
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)