Page 160 - Mechanical Engineer's Data Handbook

FLUID MECHANICS 149

L v2
4.2.4 Friction in pipes Pressure loss in a pipe ~~=4f-p-(Nm-~)
D 2
The formula is given for the pressure loss in a pipe due
to friction on the wall for turbulent flow. The friction Friction factor f This depends on the Reynold's number
factor f depends on both Reynold's number and the P VD
surface roughness k, values of which are given for Re=- P
different materials. In the laminar-flow region, the and the relative roughness k/D (for values of k, see
friction factor is given by f= 16/Re, which is derived table).
from the formula for laminar flow in a circular pipe. For non-circular pipes, use the equivalent diameter
This is independant of the surface roughness.
-_
-
For non-circular pipes and ducts an equivalent D, = 4xArea 4A
diameter (equal to 4 times the area divided by the Perimeter P
perimeter) is used.
Example
Let :
L=length (m) For a water velocity of 0.5 m s- ' in a 50 mm bore pipe
D 5 diameter (m) of roughness k=0.1 mm, find the pressure loss per
and
V-5 velocity (m s- I) metre (viscosity=0.001 N-S~-~ p= lo00 kgm-3
p=density (kgm-3) for water).

Critical zone
FTurbulent region
Laminar region ~~

x

-
C
0
.-
--
0
ti

\ Reynolds number, Re
Recr,,
Smooth pipe