Due to the amount of water used in evaporative cooling, water consumption should be a “high priority” for the designers and operators of water-cooled HVAC&R systems – such as water-cooled ammonia (R717)-based chillers – according to Neal Walsh, Business Manager at Baltimore Aircoil Company (BAC), and Robert Downey, the company’s Global Sales Development Manager and Water Treatment Specialist.
“Although evaporative cooling is great at saving energy, it does consume some water, but the benefits on energy savings outweigh the water usage, which in some places is a scarce resource,” they said in a statement. “Water conservation is therefore a high priority in designing and operating water-cooled equipment.”
“Water conservation is therefore a high priority in designing and operating water-cooled equipment.”Neal Walsh and Robert Downey, BAC
“Evaporative cooling towers play an important role in green buildings by significantly reducing energy consumption when they supplement or replace traditional air conditioning systems, thereby reducing carbon footprint and operating costs,” said Walsh and Downey.
In 2018, Wako Chemicals opted for a low-charge ammonia chiller from Evapco that used a cooling tower at its plant in Richmond, Virginia. The new system’s COP was 17.8% higher than the alternative R404a option.
However, the amount of water used in cooling towers must also be considered, with water consumption being a key focus of sustainable building programs like the U.S. Green Building Council (USGBC)’s Leadership in Energy and Environmental Design (LEED) certification, which assigns credit points to reduce the amount of water used – particularly potable water – they explained.
“By reducing the consumption of energy and potable water, well-designed and well-maintained cooling tower systems conserve scarce natural resources and save money,” they added.
“By reducing the consumption of energy and potable water, well-designed and well-maintained cooling tower systems conserve scarce natural resources and save money.”Neal Walsh and Robert Downey, BAC
In LEED, there are two ways to reduce potable water consumption: getting more use from the potable water consumed and substituting potable water with non-potable water.
According to Walsh and Downey, there are multiple ways to earn LEED points in both existing buildings and new construction through better system design.
“For some of these design paths, achieving the desired reduction of potable water use comes with some potential side effects and new challenges that must be evaluated and addressed,” they explained.
HVAC&R owners and operators that maintain “peak system efficiency” can also “extend the life of [their] evaporative cooling equipment,” they added.
Improving use of potable water
By optimizing the cooling tower’s cycle of concentration (or concentration ratio) and reducing the amount of water lost during the cooling process, the use of potable water in evaporative cooling is maximized, explained Walsh and Downey.
While operators can expect to lose around 1.8gal (6.8l) of water for every ton-hour (3.5kWh) of cooling through the evaporative process, other means of water loss “must be carefully controlled,” they said.
Including a drift eliminator in a cooling tower can reduce the amount of water lost through drift, which is water mist or droplets blown off the tower “with little or no beneficial cooling effect.”
Leaks and overflow can be avoided through proper operation and regular maintenance.
For blowdown – i.e., “the water that is bled from the system to keep the concentration of dissolved solids in the circulating cooling water within acceptable limits” – ensuring that the system’s water is of high quality can minimize the amount of blowdown required.
According to Walsh and Downey, water hardness is what affects water conservation in water-cooled systems the most.
Substituting non-potable water
To reduce the amount of potable water used in a water-cooled system, a portion of the water consumed can be substituted with non-potable water, such as HVAC condensate, rainwater, recycled municipal water and greywater.
According to Walsh and Downey, HVAC condensate and rainwater are often the best non-potable water sources for cooling towers, noting that they can, however, “introduce the potential problems of airborne particulates that foster biological growth and of corrosiveness to the cooling equipment.”
This approach depends on the types of non-potable water sources available. Due to the water quality of these sources, additional processes, like filtration or chemical treatment, or applying protective materials, like corrosion protection coatings or stainless steel, are generally required, they said.