Page 38 - HVAC Pump Handbook

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Source: HVAC Pump Handbook

Chapter
3

Piping System Friction

A comprehensive chapter on pipe friction has been included in this
Handbook for HVAC pumps because the sizing of pumps is deter-
mined principally on pump capacity and head. A poor computation of
system friction will have a disastrous effect on pump selection and
operation. There is not a more critical subject facing HVAC water
system designers than the development of better procedures for cal-
culating pump head for these systems.
As pointed out in the introduction to this book, pipe friction analysis
is, at best, an inexact science. Much needs to be done to achieve better
information on pipe and fitting friction. Research work on reducing
pipe friction through the use of additives to water is being carried out;
surfactants are one class of chemicals that are being studied to reduce
piping friction. The increase in cost of energy will provide the driving
force to achieve better piping friction data and better piping design.
Good piping design always balances first cost against operating
cost, taking into consideration all factors that exist on each installa-
tion. These are the two basic parameters that influence pipe sizing in
the HVAC industry, since excessive corrosion or fouling should not
exist in these water systems.
Obviously, piping costs increase and power costs decrease with
increases in pipe diameter. The American Society of Heating,
Refrigerating, and Air-Conditioning Engineers (ASHRAE) has infor-
mation that indicates velocities in the range of 10 to 17 ft/s in HVAC
systems do not create erosion or noise in the larger sizes of pipe. The
overall controlling factor is friction, which increases exponentially
with velocity. Friction in piping is the principal source of increased
operating costs for these water systems.

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