Best Management Practice #10: Cooling Tower Management
Cooling
towers dissipate heat from recirculating water used to cool chillers,
air conditioners, or other process equipment to the ambient air. Heat is
rejected to the environment from cooling towers through the process of
evaporation. Therefore, by design, cooling towers use significant
amounts of water.
From a water efficiency standpoint, you want to maximize cycles of concentration. This will minimize blowdown water quantity and reduce make-up water demand. However, this can only be done within the constraints of your make-up water and cooling tower water chemistry. Dissolved solids increase as cycles of concentration increase, which can cause scale and corrosion problems unless carefully controlled.
In addition to carefully controlling blowdown, other water efficiency opportunities arise from using alternate sources of make-up water. Water from other facility equipment can sometimes be recycled and reused for cooling tower make-up with little or no pre-treatment, including:
Overview
The thermal efficiency and longevity of the cooling tower and equipment depend on the proper management of recirculated water. Water leaves a cooling tower system in one of four ways.-
Evaporation: The primary function of the tower and the method that transfers heat from the cooling tower system to the environment.
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Drift: A small quantity of water may be carried from
the tower as mist or small droplets. Drift loss is small compared to
evaporation and blowdown and is controlled with baffles and drift
eliminators.
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Blowdown: When water evaporates from the tower,
dissolved solids (such as calcium, magnesium, chloride, and silica)
remain in the recirculating water. As more water evaporates, the
concentration of dissolved solids increases. If the concentration gets
too high, the solids can cause scale to form within the system. The
dissolved solids can also lead to corrosion problems. The concentration
of dissolved solids is controlled by removing a portion of the highly
concentrated water and replacing it with fresh make-up water. Carefully
monitoring and controlling the quantity of blowdown provides the most
significant opportunity to conserve water in cooling tower operations.
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Basin leaks or overflows: Properly operated towers
should not have leaks or overflows. Check float control equipment to
ensure the basin level is being maintained properly, and check system
valves to make sure there are no unaccounted for losses.
Make-Up = Evaporation + Blowdown + Drift
A key parameter used to evaluate cooling tower operation is "cycle of
concentration" (sometimes referred to as cycle or concentration ratio).
This is determined by calculating the ratio of the concentration of
dissolved solids in the blowdown water compared to the make-up water.
Because dissolved solids enter the system in the make-up water and exit
the system in the blowdown water, the cycles of concentration are also
approximately equal to the ratio of volume of make-up to blowdown water.From a water efficiency standpoint, you want to maximize cycles of concentration. This will minimize blowdown water quantity and reduce make-up water demand. However, this can only be done within the constraints of your make-up water and cooling tower water chemistry. Dissolved solids increase as cycles of concentration increase, which can cause scale and corrosion problems unless carefully controlled.
In addition to carefully controlling blowdown, other water efficiency opportunities arise from using alternate sources of make-up water. Water from other facility equipment can sometimes be recycled and reused for cooling tower make-up with little or no pre-treatment, including:
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Air handler condensate (water that collects when warm, moist air
passes over the cooling coils in air handler units). This reuse is
particularly appropriate because the condensate has a low mineral
content and is typically generated in greatest quantities when cooling
tower loads are the highest
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Water used once through a cooling system
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Pretreated effluent from other processes provided that any chemicals used are compatible with the cooling tower system
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High-quality municipal wastewater effluent or recycled water (where available).
Operation and Maintenance
To maintain water efficiency in operations and maintenance, federal agencies should:-
Calculate and understand "cycles of concentration." Check the ratio
of conductivity of blowdown and make-up water. Work with your cooling
tower water treatment specialist to maximize the cycles of
concentration. Many systems operate at two to four cycles of
concentration, while six cycles or more may be possible. Increasing
cycles from three to six reduces cooling tower make-up water by 20% and
cooling tower blowdown by 50%.
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The actual number of cycles of concentration the cooling tower system
can handle depends on the make-up water quality and cooling tower water
treatment regimen. Typical treatment programs include corrosion and
scaling inhibitors along with biological fouling inhibitors.
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Install a conductivity controller to automatically control blowdown.
Work with a water treatment specialist to determine the maximum cycles
of concentration the cooling tower system can safely achieve and the
resulting conductivity (typically measured as micro Siemens per
centimeter, µS/cm). A conductivity controller can continuously measure
the conductivity of the cooling tower water and discharge water only
when the conductivity set point is exceeded.
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Install flow meters on make-up and blowdown lines. Check the ratio of
make-up flow to blowdown flow. Then check the ratio of conductivity of
blowdown water and the make-up water (handheld conductivity meters can
be used to determine the relative mineral concentration of the
recirculating and make-up water). These ratios should match the target
cycles of concentration. If both ratios are not about the same, check
the tower for leaks or other unauthorized draw-off. If the system is not
operating at, or near, the target cycles of concentration, check system
components including conductivity controller, make-up water fill valve,
and blowdown valve.
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Read conductivity and flow meters regularly to quickly identify
problems. Keep a log of make-up and blowdown quantities, conductivity,
and cycles of concentration. Monitor trends to spot deterioration in
performance.
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Consider using acid treatment such as sulfuric, hydrochloric, or
ascorbic acid where appropriate. When added to recirculating water, acid
can reduce the scale buildup potential from mineral deposits and allow
the system to run at higher cycles of concentration. Acid treatment
lowers the pH of the water and is effective in converting a portion of
the alkalinity (bicarbonate and carbonate), a primary constituent of
scale formation, into more readily soluble forms. Make sure workers are
fully trained in the proper handling of acids. Also note that acid
overdoses can severely damage a cooling system. The use of a timer or
continuous pH monitoring via instrumentation should be employed. It is
important to add acid at a point where the flow of water promotes rapid
mixing and distribution.
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Select a water treatment vendor with care. Tell vendors that water
efficiency is a high priority and ask them to estimate the quantities
and costs of treatment chemicals, volumes of blowdown water, and the
expected cycles of concentration ratio. Keep in mind that some vendors
may be reluctant to improve water efficiency because it means the
facility will purchase fewer chemicals. In some cases, saving on
chemicals can outweigh the savings on water costs. Vendors should be
selected based on "cost to treat 1,000 gallons of make-up water" and
“highest recommended system water cycle of concentration." Treatment
programs should include routine checks of cooling system chemistry
accompanied by regular service reports that provide insight into the
system’s performance.
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Ask the water utility if it provides sewer credits for evaporative
losses, which can be calculated as the difference between metered
make-up water minus metered blowdown water.
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Implement a comprehensive air handler coil maintenance program. As
coils become dirty or fouled, there is increased load on the chilled
water system to maintain conditioned air set point temperatures.
Increased load on the chilled water system not only has an associated
increase in electrical consumption, it also increases the load on the
evaporative cooling process, which uses more water.
Retrofit Options
The following retrofit options help federal agencies maintain water efficiency across facilities:-
Consider installing a side-stream filtration system. These systems
filter silt and suspended solids and return the filtered water to the
recirculating water. This limits the fouling potential for the tower
system, which is particularly helpful if the cooling tower is located
in a dusty environment.
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Install a make-up water or side-stream softening system when hardness
(calcium and magnesium) is the limiting factor on cycles of
concentration. Water softening removes hardness using an ion exchange
resin and can allow you to operate at higher cycles of concentration.
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Install covers on open distribution decks on top of the tower.
Reducing the amount of sunlight on tower surfaces can significantly
reduce biological growth such as algae.
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Consider alternative water treatment options, such as ozonation or
ionization and chemical use. Be careful to consider the life cycle cost
impact of such systems.
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Install automated chemical feed systems on large cooling tower
systems (more than 100 tons). The automated feed system should control
chemical feed based on make-up water flow or real-time chemical
monitoring. These systems minimize chemical use while optimizing control
against scale, corrosion, and biological growth.
Replacement Options
The following replacement options help federal agencies maintain water efficiency across facilities.-
Get expert advice to help determine if a cooling tower replacement is
appropriate. New cooling tower designs and improved materials can
significantly reduce water and energy requirements for cooling.
Replacing a cooling tower involves significant capital costs, so be sure
to investigate every retrofit and operations and maintenance option
available, and compare the costs and benefits to a new tower.
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For specifics, consult with experts in the field. The first resource
should be local or headquarters engineers, but do not overlook input
from experienced contractors or other government agencies.
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