Page 52 - HVAC Pump Handbook

Rishel_CH03.qxd 20/4/06 5:35 PM Page 49

Piping System Friction
Piping System Friction 49

The friction factor f is usually derived from the Colebrook equation:

1 2.51
2 log 10 (3.4)
f 3.7D R f
where R Reynolds number
absolute roughness parameter (typically 0.00015 for steel
pipe)
For practical purposes, the friction factor f is calculated from the
Moody diagram described later.

Williams and Hazen formula
1.85 gal/min 1.85
100
Hf 0.002083 L (3.5)
d
4.8655
C
where C a design factor determined for various types of pipe
d inside diameter of pipe, in
There are a number of sources for securing the data for the afore-
mentioned equations in either tabular or software form. Before any
data on pipe friction are used, either in tabular or computer software
form, be sure that the pipe under consideration has the same inside
diameter as that in the tables or computer software! The following
pages demonstrate some of the sources for pipe friction data in tabu-
lar form. A principal source is the Hydraulic Institute’s Engineering
Data Book. This book is based on the Darcy Weisbach formula, and
Table 3.5 has been developed from Hydraulic Institute data for steel
pipe. It is strongly recommended that this data book be acquired by
anyone who is involved in piping design.

3.3.2 Reynolds number and the Moody diagram
The Hydraulic Institute’s Engineering Data Book contains some
very practical information on the generation and use of Reynolds
number. Reynolds number is a dimensionless number that simpli-
fies the calculation of pipe friction under varying velocities and
viscosities.

V D
Reynolds number R (3.6)

where V velocity, ft/s
D pipe diameter, ft

kinematic viscosity, ft /s
2

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