Pipe Sizing for Heating Systems
All pipe sizing in building services is based on the D'Arcy equation, where:
H = 4 . f . l . v2 / 2 . g . d
where H = head loss due to friction in a straight pipe (m)
f = friction coefficient
l = length of pipe (m)
d = diameter (m)
v = velocity of fluid (m/s)
g = acceleration due to gravity (m/s2)
The object of pipe sizing is to obtain the smallest diameter of pipe without too high a water velocity or too high a pressure drop and therefore large pumps.
It is more convenient to use pipe sizing tables when sizing pipes rather than the D'Arcy equation.
This is because the water velocity and head loss (or pressure loss) are unknown at the time of pipe sizing, and the friction coefficient (f) varies with Reynolds number which in turn varies with velocity and diameter.
A Flow of Water in Pipes Table is provided in these notes.
The CIBSE Guide C (2001) provides pipe sizing tables in sections;
4.9 Minibore copper ……..Water at 75oC
4.10 Heavy Grade Steel
4.11 Medium Grade Steel
4.12 Large Size Steel
4.13 Copper, Table X
4.14 Copper, Table Z
4.15 Galvanised Steel
4.25 Polybutylene (PB) BS2871
4.27 Polyethylene (PE-X) BS2871
4.29 Polybutylene (PB) BS5556
4.30 Polyethylene (PE-X) BS5556
4.33 Polyvinyl Chloride (PVC-C) BS5556
The CIBSE guide C (2007) provides pipe sizing tables in a CD.