ASHRAE Journal - September 2024 - 20

COLUMN ENGINEER'S NOTEBOOK
leak-free until the pump(s) started. Then, it would
need to ramp open in a way that allowed the pumps to
establish flow and fill the spray headers and any of the
elevated pipe on the discharge side of the pumps that
had drained at shutdown, without having water flow in
reverse in the riser, resulting in water overflowing the
cooling tower cold basin. Similarly, on shutdown, the
valve would need to close immediately (or really, really
fast, to put it in technical terms) to trap the water in the
header.
Unfortunately, the 18 to 30 second stroke time for
the electric actuator made it ineffective in terms of
addressing the problem, especially at shutdown.†
Thus,
an event like a power outage-even a momentary one-
will likely be sufficient to drain the elevated piping
and knock out the condenser water system, requiring
operator intervention to bring the plant back online.
Basin Overflow
When dealing with an open system with piping at
elevations above the cold basin-on the suction side or
elsewhere-it is important to realize that some of the
water in the elevated lines will drain back to the cold
basin when the pump(s) shut down. At a minimum this
will include:
* Water in the elevated suction line in situations like
the one we are considering.
* Water in the hot basins for gravity distribution-type
towers or in the distribution manifolds for spray distribution-type
towers.
* Water in piping at an elevation above the hot basin.
For the case study towers, you can get a sense of the
volume available for draindown by watching the video
associated with the hot spot over the eliminators on the
cooling tower in View H.
If the check valve(s) on the pump(s) do not hold (and
for some cooling tower bypass valve configurations),
most, if not all, the water in the system above the level of
the cold basin will drain back to the cold basin.
If the cold basin has not been appropriately sized and/
or if the operating levels have not been properly set to
accommodate the draindown without overflow, this
water will overflow the basins every time the pump
cycles, wasting water and related treatment chemicals.
A friend of mine recently encountered this in
the facility he was working with, and it was costing
thousands of dollars a year for the water and in the
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ASHRAE JOURNAL ashrae.org S E PTEM B ER 2024
related booster pump energy to deliver the water to the
roof because in normal operation, the lag chiller and its
pump would cycle multiple times a day.
At first blush, it might seem that you could solve this
problem by properly adjusting the makeup system so
that the volume available in the basin with the makeup
valve closed was sufficient to capture the draindown
without overflowing. But this can be more challenging
than it sounds because:
* Factory cooling tower basins are not particularly
deep and thus represent a limited volume compared to
field-erected towers with concrete, cast-in-place basins.
* At start-up, water that has drained back needs to
be pumped back to fill up places it drained from. This
means there will be a lag between when water is withdrawn
from the basin and when it makes its way back.
If the flow rate produced by the condenser pump is
high enough‡
and/or the level that opens the makeup
valve low enough, then at start-up the basin level might
drop to the point where a vortex forms,§
entraining air
into the pump, which can cause a number of problems
including tripping the chiller flow safety.
To counter this, the operator may increase the cold
basin water level. But as a result, they may eliminate the
volume required to catch the draindown volume and
trigger the overflowing basin problem.
Air Coming Out of Solution
Most of us are familiar with the fact that air is less
soluble in warm water than cold (Figure 2); it turns out
solubility of air in water is also a function of pressure.
One inherent characteristic of an open condenser
water system is that the water is spread out into a very
thin film as it flows over the fill in the cooling tower to
† I am aware of one installation with a similar problem where
a check valve was retrofitted into the system to successfully
address it. The retrofit also included adding a makeup connection
downstream of the check valve and a vent at the high point to give
the operating team a way to fill and vent the elevated piping if it
was drained for service and at initial start-up.
‡ There are a number of reasons the flow rate might be higher than
anticipated, including the fact that until the elevated pumping is
filled, the head on the pump is reduced due to less lift, and it runs
out its curve. It could also occur because multiple pumps sharing
a common header will interact. This is because the pressure drop
due to flow for the header will vary with number of pumps running.
Thus, when the number of pumps online are less than the design
requirement, the online pumps will tend to run out their curve.
§ Visit this link for a video illustrating this: https://tinyurl.com/
CoolingTowerOutletVortex
https://tinyurl.com/CoolingTowerOutletVortex https://tinyurl.com/CoolingTowerOutletVortex http://www.ashrae.org
ASHRAE Journal - September 2024

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