Page 57 - HVAC Pump Handbook

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Piping System Friction
54 The Basic Tools

3.3.3 Use of the Darcy Weisbach equation
For those who wish to study pipe friction further or use the Darcy
Weisbach equation for generating their own computer program for
pipe friction, the preceding example provides a guide for doing so.
Also, the two figures from the Hydraulic Institute’s Engineering Data
Book (Figs. 3.2 and 3.3) should clarify the use of Reynolds number for
the calculation of pipe friction.
Another source for Darcy Weisbach data is Cameron Hydraulic
Data, published by the Ingersoll Rand Company. Like the data book
of the Hydraulic Institute, it is an excellent source for pipe and water
data and is a necessary reference manual for any serious designer of
piping.
Both these sources do not include any allowance in their tables for
pipe aging, variation in pipe manufacture, or field assembly. The
Hydraulic Institute recommends that 15 percent allowance be made
for these factors; I consider this factor adequate for loop-type systems
such as hot or chilled water systems. It is not adequate for cooling
tower water, which is exposed to air in the water; it is recommended
that a factor of 20 percent be added for this service if steel pipe is
used. This possible increase in friction for steel pipe may demonstrate
the application for plastic pipe on cooling tower water; its use is limited
by size and structural capability.

3.3.4 Use of the Williams-Hazen formula
The Williams-Hazen formula is very popular in the civil engineering
field and can be used for HVAC piping design if it is understood
properly. This formula gives accurate values for liquids that have a
viscosity of around 1.1 cSt, such as water at 60°F. It is therefore
acceptable for chilled water and even condenser water. It will show an
error as much as 20 percent high if used for hot water over 200°F.
This formula is based on design factors that relate to the roughness
of the pipe involved. These design factors are called C factors that
appear in the preceding equation and range from 80 to 160, 80 being
for the roughest pipe and 160 for the smoothest pipe.
Table 3.6 is taken from an older edition of Cameron Hydraulic Data
and includes the C factors for various types of pipe. If a C factor of
140 is used with the Williams-Hazen formula, it will yield friction
data for steel pipe that is somewhat comparable with the Darcy
Weisbach formula with the 15 percent aging factor as recommended
previously. This is adequate for closed piping such as chilled water
but should not be used with cooling tower piping. Cooling tower pip-
ing should be calculated with a C factor of 130 for steel pipe when
using the Williams-Hazen formula.

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