Cooling tower: how to save!

November 16, 2016 0 Comments

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.

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.

1. Evaporation: The primary function of the tower and the method that transfers heat from the cooling tower system to the environment. 
2. 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.
3. 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.
4. 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.

The sum of water that is lost from the tower must be replaced by make-up water:

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:

• 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
• Water used once through a cooling system
• Pretreated effluent from other processes provided that any chemicals used are compatible with the cooling tower system
• High-quality municipal wastewater effluent or recycled water (where available).

U.S. Environmental Protection Agency (EPA) WaterSense at Work cooling towers best management practice.

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%.
• 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.
• 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.
• 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.
• 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.
• 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. 
• 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.
• 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.
• 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.

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