ASHRAE Journal - June 2024 - 26

COLUMN ENGINEER'S NOTEBOOK
Elevation Relative to the Pump
Suction
The static head
associated with the
elevation difference
between the water surface
level in the tower cold
basin and the centerline
of the pump suction is a
significant contributor.
If the cooling tower is on
the roof of a high-rise
and the pumps are in the
basement, NPSHA issues
are far less likely than if
the pumps are on the roof
next to the cooling tower.
FIGURE 4 Visualizing what we have to work with.
16
14
12
10
8
6
4
2
Pressure Drop Due to Flow in the
Piping from the Cold Basin to the Pump Suction
The friction loss due to the flow of water through the
pipe and fittings between the tower and pump will
always subtract from the NPSHA. This is one of the
reasons a lot of folks try to avoid strainers on the suction
side of condenser water pumps, especially if not much
elevation difference exists between the tower basin and
the pump suction.
This component can snowball because it generally
varies with the square of the flow. So, a condenser water
pump that is just fine if it is operating at its design flow
may cavitate if it runs out its curve, partially because the
NPSHR will go up, but also because the NPSHA will drop
due to the higher pressure drop in the piping circuit.
Saturation Temperature and Pressure of the Fluid
Online Figure 2 is a portion of a steam table; i.e., a table
documenting the properties of water at saturation.
Of particular interest for our discussion are the
temperature and pressure columns, which correlate the
temperature at which water will boil at a given pressure.
Most of us are very familiar with the parameters for
sea level standard pressure: water boils at 212°F (100°C)
at 0 psig/14.7 psia (0 kPa gauge/101 kPa absolute).
And those who live at higher elevations like Mexico
City or Johannesburg or Denver, which are all at
elevations of 5,000 ft-5,600 ft (1500 m-1700 m) above
sea level with atmospheric pressure in the range of
26
ASHRAE JOURNAL ashrae.org J U N E 2024
Saturation Pressure, psia, vs. Temperature, ° F
Ambient Pressure, 14.700
Saturation Temperature of
Circulated Water, 85
Saturation Pressure vs.
Temperature
Pressure Drop That will Cause
Cavitation = 14.103 psi
or 32.6 ft w.c.
Saturation Pressure of
Circulated Water, 0.597
Saturation Temperature at
Ambient Pressure, 212.00
20
40
60
80
100
120
140
Temperature, ° F
12.23 psia-11.96 psia (84.32 kPa-82.46 kPa absolute),
may be accustomed to water boiling at 201°F-204°F
(93.9°C-95.6°C) due to the lower atmospheric pressure
(yellow highlight in Online Figure 2).
The same thing is true for water as it passes through
the impeller. If the static pressure drops below the
saturation pressure associated with the water's
temperature (due to the water velocity increasing due
to flow through a smaller cross-sectional area of the
impeller, see Figure 2), then the water will boil and
cavitation will ensue.
This means NPSHA will vary with the temperature of
the water being pumped. If the water temperature goes
up, the NPSHA goes down, all other things being equal.
Suction Pressure and NPSHA are Not the Same Thing
Online Figure 3 is a picture of a compound gage, which
also includes a " maximum indication " feature. It can
read pressures below 0. Typically, the negative pressures
will be scaled in terms of inches of mercury, even though
the positive pressures are in pounds per square inch.
Providing this type of gage on the suction side of a
pump can be helpful, especially for a cooling tower
pump that is located in close proximity to the tower with
little static head, because the pressure drop due to flow
could cause the suction pressure to be subatmospheric.
It is important to realize that while the pressure at the
suction side of the flange is related to the NPSHA for the
pump, it is not a direct reading of NPSHA.
160
180
200
220
240
Pressure, psia
http://www.ashrae.org
ASHRAE Journal - June 2024

Table of Contents for the Digital Edition of ASHRAE Journal - June 2024
