Cooling Tower Best Practices: Water Conservation (Pt. 1)
In order to understand how to optimize cooling tower efficiency, one must understand the basics of cooling tower operation. The equation for the water balance on a cooling tower is below, as well as definitions for the basic terms utilized in cooling tower discussions:
Makeup = Evaporation + Blowdown + Process Leaks + Drift
Makeup: In order to maintain the cooling system in proper working order, all water leaving the system must be replaced. This incoming water is referred to as makeup.
Evaporation: Water that evaporates in the cooling tower and is rejected to atmosphere as vapor. Evaporation provides the necessary cooling for the system.
Blowdown: When water evaporates from the system, the dissolved solids and minerals are left behind. Blowdown is water that is purposefully drained from the system to remove dissolved or suspended solids and prevent scale and corrosion problems in the cooling system.
Drift: A very small amount of water will escape the cooling tower as mist or water droplets in the air, and is referred to as drift. Compared to evaporation or blowdown, water loss due to drift is insignificant, but can be a factor at the highest levels of water conservation efforts.
Process Leaks: Water leaking from the cooling tower basin overflows, cooling system piping, pumps, heat exchangers, etc. is essentially another form of blowdown.
Cooling Tower Water Conservation
In order to maximize the water efficiency of a cooling tower, there are several key techniques that should be considered.
- Maximize “Cycles of Concentration”: The ratio of the concentration of dissolved solids (conductivity) in the blowdown to the makeup water is recognized as cycles of concentration, or cycles. This can also be expressed as the ratio of the makeup water flow to the blowdown water flow, as shown below.
- Maximizing cycles of concentration will minimize blowdown water quantity and provide the largest gain in water efficiency of the tower. Increasing cycles of concentration from 3 to 6 will decrease blowdown water by 50%. Cycles will be limited, though, by the scaling potential of makeup water and possible corrosion in the system. Consult a water treatment professional to determine the maximum cycles for your system and carefully control cycles at that level to minimize water consumption.
- Optimize Conductivity Control: Install a conductivity controller to continuously monitor conductivity and automate blowdown at a specific conductivity set point. Ensure the conductivity meter is functioning properly, and verify calibration weekly. Most conductivity controllers provide an option for enabling conductivity alarms via email, text, or page, and can quickly alert you of cooling system malfunctions or water losses before they become a major problem.
- Use a Quality Blowdown Valve: A blowdown valve that sticks open or fails to fully close can waste a lot of water in a short period of time. Cheap diaphragm valves might be attractive, but are prone to failure and require constant maintenance. Install a quality quarter-turn ball valve with a “fail-closed” or spring-return actuator to ensure proper blowdown control and prevent unnecessary water losses.
- Consider Alternatives to Chemical Treatments: Be sure that your water treatment provider is aware that water conservation is a high priority. Chemical water treatment providers may not initiate water efficiency efforts on their own, because increasing water efficiency also reduces the amount of chemicals the facility will purchase. Many times, savings on chemical costs are much larger than water cost savings. Alternative water treatment technologies are available, including electrolysis, ozonation, ionization, etc. but be sure that the specific technology is appropriate for your makeup water quality. Insist that both you and your water treatment vendor share a common goal—minimizing the total cost of cooling water treatment, inclusive of chemical costs, water costs, and cooling system longevity.