Cooling-Tower Design Tips _ Air Conditioning Content From HPAC Engineering

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Cooling-Tower Design Tips

By KEITH RINALDI, CEM; Dome-Tech Field Engineering Services; Edison,

N.J. | HPAC Engineering

A cooling tower is only as good as the details of its design

and the specification of its supporting components.

Details such as pump net positive suction head (NPSH),

strainers, and pipe size can have a major impact on

performance and energy bills. Make sure the impact is

positive by keeping the following in mind.

RELATED

A STRAINER AT THE TOWER INLET CAN PREVENT HEADACHES AFTER START-UP

When cooling towers are replaced, the system and piping often are not. When the

system is filled and started, water flows from the tower, through the pump, through the

chiller, and to the tower spray nozzles. Eventually, pump-discharge pressure increases

and flow decreases, indicating a problem. Building personnel blow down or pull and

clean/replace the pump inlet strainers, which are loaded with dirt/scale. When the

system is started again, pump-discharge pressure still is high, and low flow persists.

What is the problem?

The problem is that the pump inlet strainers catch only dirt and debris between the

tower outlet and the pump inlet. They do nothing to remove dirt and scale originating

between the pump outlet/chiller and the tower. There is a good chance that some, if

not all, of the tower nozzles will be partially blocked. Removing and cleaning each

nozzle will be expensive and time-consuming. Making matters worse is that the nozzles

likely will plug again as soon as the system is restarted.

For this reason, in systems with questionable water-treatment practices and systems

that have been drained or dormant, consider installing a new strainer, block valves,

and inlet- and outlet-pressure gauges on the common header just prior to the tower

inlet. Utilize a strainer that can be opened, cleaned, and placed back into service

quickly. In some systems, this new “tower strainer” will have to be cleaned several

times a day for the first week or two of service to remove all of the dirt and scale in the

system. However, cleaning a strainer like this usually can be done quickly with

in-house personnel.

DO NOT UNDERSIZE THE EQUALIZING LINE

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Water drains out of a cooling tower's cold-water basin via gravity. Typically, between

the cold-water basin and the point of connection for the common return header is only

a few feet of vertical drop. As a result, the pressure differential between the tower and

the common header may be only a few feet (or even inches) of water column.

History has shown that even minor differences in piping geometry between cell outlets

and the point of common connection on return headers can have a profound effect on

how water drains from tower cells. In some instances, differences in cell-outlet pipe

length of only 5 to 10 ft have resulted in one cell basin running nearly empty while the

other overflows. A common field “fix” is to balance water flow between the two cells.

Tower performance will be affected, however, as one cell will be receiving extra water,

while the other will be receiving less and not performing at its rated conditions. This

situation easily can be avoided by paying attention to the design and sizing of the

equalizing line. Follow the tower manufacturer's recommended guidelines, and make

the tower-outlet piping from each cell identical. If it is not identical, slightly oversize it

from the cells to the header.

WATCH THE PUMP NPSH

When an existing roof structure will not support the weight of a new cooling tower —

as is the case in many retrofit situations — the cooling tower may be installed on a

different support platform, perhaps one adjacent to the building. Rarely does the

support platform reach the height of the building's roof. Often, the tower cold-water

basin is located only 10 to 12 ft above ground.

If pumps are to be installed on a foundation and base, only a few feet below a tower

basin, it is imperative that low-NPSH-required (NPSHR) pumps be utilized and that

there be a safety factor in the NPSH available (NPSHA)/NPSHR. A pump selected at

NPSHA is a problem waiting to happen. No one wants to tell an owner that the reason

his or her brand-new pumps sound as if they are pumping marbles is that they were

selected improperly and are going to have to be replaced.

In situations in which a tower is located on an elevated structure, but only 5 or 10 ft

above grade, avoid up and down runs of tower-discharge piping, especially to a level

higher than that of the cold-water basin. Though unusual, this has been witnessed by

the author on two occasions, causing the following problems:

LOCATE THE COOLING TOWER AWAY FROM ACTIVE AREAS

Cooling towers launch water vapor and a small amount of liquid into the air, a

phenomenon known as drift loss. Depending on the amount of drift loss and the

weather, a mist may be felt in the area surrounding the cooling tower. Normally, this is

not a problem. But if a cooling tower is located on a roof directly above or adjacent to a

pedestrian thoroughfare or parking lot, mist detectable by pedestrians quickly can turninto an air-quality issue. Is the water safe? What chemicals are used? What

microorganisms are in the water? Mist landing and then drying on parked cars also

can pose problems. Try explaining to the CEO that the mist is no big deal and that he

simply needs to get his car washed every other day.

The installation of extra or high-efficiency drift eliminators may solve the problem of

drift loss. However, it wil l not stop water vapor in the air from condensing and then

blowing into a parking lot. In short, do yourself a favor, and locate cooling towers as far

from active areas as possible.

Keith Rinaldi, CEM, is vice president of Dome-Tech Field Engineering Services.

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When pumps were shut off, water in upward-sloped pipe ran backward into thetower, causing it to overflow.

Elevated discharge pipe acted as an air trap. Eventually, air filled the pipe and

“slugged” through the system, damaging the pumps and causing the chillers to trip

off line because of lost flow. Because the suction piping often ran at a vacuum

condition, the air could not be vented by conventional means.


4/21/2014



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Cooling-Tower Design Tips _ Air Conditioning Content From HPAC Engineering

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