HYDRONICS for PLUMBING ENGINEERS
BY ROY C.E. AHLGREN
Solving Cooling Tower Pumping Problems
Now that spring is finally approaching, it is time to shift
our attention from heating to cooling systems. The cooling
tower and condenser piping system play a crucial role in
many large chilled water systems. As always in hydronics,
proper selection and installation of the equipment goes a
long way toward minimizing operational problems, but
sometimes the towers and pumps are not installed exactly
right, leading to problems due to component wear and failure. In this article, I’ll review some important design, installation, and maintenance issues related to the condenser
loop.
Most cooling tower pumping systems are open systems,
where
• system water is exposed to the atmosphere at more
than one point, and
• elevation differences may cause flow.
These system characteristics are often directly related to
tower pumping problems.
POOR DESIGN
Vortexing sometimes can be cured by throttling at the
pump discharge, trimming the impeller, maintaining
proper level, or balancing the system. Poor system design
is another issue.
The system in Figure 1 has several flaws. NPSHA (net positive suction head available) and pump cavitation are a serious possibility in the system as shown. (See “NPSHA and
NPSHR: How Much Is Enough?” on p. 27 of the May/June
2003 issue and “NPSHR—Again” on p. 35 of the November/
December 2003 issue of PS&D for details on pump cavitation.) The condenser is higher than the pump, and there is
no check valve at the pump discharge to prevent draining
of the condenser and piping when the pump is off. When
the pump starts, it runs out on its curve, trying to fill the
empty condenser and the rest of the piping. The makeup
valve capacity is small compared to the pump’s capacity, so
it cannot keep the basin from emptying. Large slugs of air
will be introduced into the condenser loop with all these
problems.
AIR IN THE CONDENSER LOOP
Large quantities of air in the condenser
water loop can stop the chiller dead in its
Figure 1 No check valve allows condenser to drain
tracks by reducing heat transfer and causing excessive condenser pressure. Air
may even damage the pump. Alternating
large volumes of air and water entering
the pump cause high torsional stress as
the impeller speeds up in air, then suddenly slows down as a slug of water hits
it. A broken shaft is sometimes the result.
If too much air enters the pump, it may
“lose its prime” and fail to pump anything
at all, unless it is a self-priming pump.
Air can be introduced into the pump
several ways.
A vortex can form in the tower basin,
drawing large amounts of air through the suction piping
If it is necessary to locate the condenser above the pump
into the pump. High velocity in the suction piping is a major and basin level, then the discharge check valve and prescause of vortexing. This could be caused by an oversized sure-reducing valve shown in Figure 2 are required to keep
pump, resulting in excess flow; undersized suction piping the condenser full of water during system shutdown. Now
at the tower basin outlet; or, in multiple tower systems, poor when the pump starts, significant friction loss occurs to
balance between the towers and the pump suction. Towers keep the pump from running out on its curve. Avoiding the
near the pump have less suction pipe head loss, therefore excessive flow on startup means that the basin level is less
higher flow rates, than more distant towers.
likely to drop below the minimum required submergence
A vortex also may result if the water level in the tower level.
basin is too low. A minimum “submergence” is required to
A triple-duty valve is often used to act as a check valve,
avoid vortex formation at a given suction velocity. You can isolation service valve, and throttling valve. The pressurethink of submergence as the minimum required water level reducing valve is the same component used in closed loop,
above the basin outlet. This level is usually maintained by pressurized systems to establish the initial, or cold fill, presa small float-controlled valve that adds water to the basin sure. (See “What’s the Pressure?” on p. 28 of the January/
to make up for evaporation and drift losses. Many towers February 2004 issue of PS&D for details.) It is connected to
have anti-vortexing baffles above the basin outlet to elimi- the city water supply along with a backflow preventer. The
nate the problem. Vortexing may be more likely to occur in valve setting is determined by the height of the system. The
towers that are mounted near the ground, where the basin purpose of the pressure-reducing valve is to keep the conoutlet must be located on the side rather than the bottom.
denser and all the piping full of water in the event that the
48 Plumbing Systems & Design
MARCH/APRIL 2007
PSDMAGAZINE.ORG
Figure 2 Check valve and PRV keep the condenser flooded
check valve leaks back into the basin while the pump is off. If
this were to happen, the static pressure at the valve would drop
below set point, the valve would open, and the level would be
reestablished. If the check valve doesn’t leak, then the pressurereducing valve stays closed because its pressure setting is satisfied by the hydrostatic head of the water as measured from the
valve vertically to the top of the system. What happens when the
pump starts? The pressure at the pressure-reducing valve rises,
and the valve still stays shut.
ROY AHLGREN is director of the ITT Bell &
Gossett Little Red Schoolhouse (8200 N. Austin
Ave., Morton Grove, IL 60053). To comment
on this article or for more information, e-mail
[email protected].
MARCH/APRIL 2007
Plumbing Systems & Design 49