ASHRAE Journal - June 2024 - 29

COLUMN ENGINEER'S NOTEBOOK
(88 L/s), the NPSHR is 12 ft w.c. or 5.195 psia (36 kPa).
To prevent cavitation at the pump suction, the lowest
pressure we could tolerate at the suction fl ange of the
pump with 85°F (29°C) water circulating in the system
would be 5.791 psia (39.93 kPa), as illustrated in Figure 5.
The saturation temperature of the 85°F (29°C) water is
added to the pump manufacturer's requirement.
That means as a designer, we would need to size and
confi gure our piping run so the maximum pressure
drop was 8.9 psi (61 kPa) to avoid cavitation.
Higher Elevations and Warmer Water Work Against Us
If the system was in Denver rather than at sea level,
we would have even less allowable pressure drop to
work with because of the lower reference pressure. And
if there was a problem with the cooling tower and our
water temperature had drifted up to 105°F (40.6°C),
that would make things even worse since the saturation
pressure for the warmer water would be higher. All of
this is illustrated in Online Figure 4.
Bottom line, the sizing and confi guration of the piping
run between the cooling tower and the pump suction
can be crucial in determining NPSHA. As designers,
unlike the local atmospheric pressure, getting the sizing
and confi guration right is not out of our control.
Assessing the Pipe Run Pressure Drop
The confi guration of a real-world piping network will
result in pressure drops due to fl ow, and this works
against us as regarding preventing cavitation in terms of
the pressure drop on the suction side of the pump.
Tower Elevation Above the Pump Has a Positive Impact
The elevation of the cooling tower relative to the
pump suction can have a positive impact on this and
offset some of pressure losses due to fl ow. Consider
a cooling tower where the pressure drop due to fl ow
from the tower basin to the pump suction represents
a loss of 4 ft w.c. or 1.732 psi (12 kPa). If the water level
in the cold basin is 2 ft (0.6 m) above the centerline of
the pump suction, that represents a positive static head
of 2 ft w.c. or a positive 0.866 psi (6 kPa). As a result,
the pressure you would read at a gage on the pump
suction at design fl ow would be -0.866 psi (-6 kPa). This
is a -1.72 psi (-12 kPa) pressure drop due to fl ow, plus
0.866 psi (6 kPa) of positive pressure associated with the
cooling tower being higher than the pump suction.
Doing the Math
If you " do the math " 6 for the system in our case study,
you conclude that there is about 21.8 ft w.c. or 9.4 psi
(65 kPa) of loss in the suction line from the most remote
tower cell to the condenser water pumps and that a
compound gage on the pump suction would read about
-19.22 in. of Mercury (-488 mm of Mercury) at the design
fl ow rate (Online Figure 5, which includes the math).
This translates to 5.260 psia (36.26 kPa). That's the
atmospheric pressure of 14.7 psia (101.4 kPa) minus the
pressure drop of 9.44 psi (65.1 kPa). This is absolute
pressure-the pressure that would exist at the pump
suction fl ange relative to an absolute vacuum.
However, water at 85°F (29°C) will boil at a
pressure that is above an absolute vacuum. Thus,
to understand what the NPSHA is, we will need to
subtract the saturation pressure for 85°F (29°C) from
the pressure that we calculated will exist at the pump
suction fl ange.
If we subtract the saturation pressure of the 85°F
(29°C) water from this (0.597 psia [4.116 kPa]), we can
estimate that the NPSHA (the difference in the pressure
that will exist at the suction fl ange and the pressure
at which the water would start to boil) is 4.663 psi or
10.8 ft w.c. (32.3 kPa) Since the pump manufacturer
indicates that the NPSHR is 12 ft w.c. (36 kPa) (Figure 3 ),
we can conclude that the pump will cavitate given the
piping confi guration that exists.
If we plot the calculated pressure drop on the chart we
have been using, we can get a visual representation of
this (Figure 6). Notice how the blue NPSHA line is below
the purple NPSHR line.
Resolving the Problem and Conclusion
It is much easier and less costly to resolve a problem
like this proactively rather than reactively. My next
column will explore options for mitigating the problem.
NPSH can be a challenging topic to understand and
apply. Hopefully, this column has provided some useful
insights gained from my personal experience as I
worked to understand it over the course of my career.
Acknowledgements
Thanks to Jess Rebick for sharing her site with us for
a fi eld class, which exposed us to the example I worked
with and to my fellow authors and editors for their
images and insightful review comments.
J U N E 2 0 2 4 ashrae.org ASHRAE JOURNAL
29
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ASHRAE Journal - June 2024

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