Page 72 - HVAC Pump Handbook

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Piping System Friction
Piping System Friction 69

Figure 3.9 Comparison of 4-in PVC and steel elbow fitting loss
coefficients. (Copyright © 1999. American Society of Heating,
Refrigerating and Air-Conditioning Engineers, Inc.
www.ashrae.org Reprinted by permission from ASHRAE
Transactions, Vol. 105, Part 1.)

pipe size. This is its principal drawback for larger cooling tower
installations.
Thermoplastic pipe offers a lower resistance to water flow than steel
pipe. It is the plastic pipe industry’s standard to use the Williams-
Hazen formula for calculating pipe friction with a C factor of 150. This
is acceptable, since most applications of plastic pipe are for water near
60°F. On warmer water, the manufacturer of the plastic pipe under
consideration should provide friction loss data comparable with those
secured from the use of Reynolds numbers and the Darcy Weisbach
equation.
There are a number of types of plastic pipe now available to the
HVAC industry, from PVC, schedule 40, to Fiberglas. ASHRAE’s 1992
issue of the System and Equipment Handbook provides information
on the various types of plastic pipe. Table 10, “Properties of Plastic
Pipe,” page 42.13, provides a summary of this important type of
piping. In assessing plastic material for a particular application,
the following factors should be evaluated:
1. Temperature. The temperature and pressure ratings of some
types of plastic pipe and fittings decrease appreciably above 100°F.
2. Expansion. Most plastic pipe has a coefficient of expansion
greater than steel pipe; expansion must be taken into considera-
tion on HVAC systems where variations occur in both ambient air
temperature and the liquid being transported.

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