By Neal Walsh & Robert Downey
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. Although evaporative cooling is great at saving energy, it does consume some water; the benefits on energy savings outweigh the water usage, which in some places is a scarce resource.
Water conservation is therefore a high priority in designing and operating water-cooled equipment and plays an important role in USGBC’s Leadership in Energy and Environmental Design certification and other sustainability programs. LEED assigns credit points to reduce water usage.
Two paths for reducing potable water consumption
The LEED options for reducing potable water consumption in cooling towers can be separated into two strategies: Get more use from the potable water and substitution of non-potable water. Maximizing the use and value of each gallon of potable water used in evaporative cooling can be accomplished by optimizing the cooling tower’s cycles of concentration. The success of this strategy and the ease or difficulty of implementing it is highly dependent on the quality of the available potable water.
This strategy can be combined with the first — Substitution of non-potable water, such as HVAC condensate or rainwater, which can be done for some portion of the total water consumed. Analogous to the use of solar and wind as alternative energy sources to replace or supplement fossil fuel consumption, the use of non-potable water acts as an alternative water source, replacing potentially scarce drinking water. The viability of this approach depends on the types of available non-potable water sources.
Maximizing cycles of concentration
A fundamental measure of cooling tower efficiency is cycles of concentration, also known as concentration ratio. Cycles of concentration is defined as the ratio of the dissolved solids (conductivity) in the tower water to the dissolved solids (conductivity) in the makeup. This is easily and commonly determined by taking the specific conductance of the cooling water and dividing it by the conductance of the makeup. The calculation can also be done using minerals not affected by the chemical treatment regime, such as chloride or silica.
An alternate, equally accurate method of calculating COC is to take the make-up water volume and divide it by the bleed volume. This is easily done if the tower is equipped with water meters on the make-up and bleed lines, a practice highly recommended.
The better the quality of the make-up water — in other words, the lower the total dissolved solids, suspended solids and corrosiveness — the higher the COC that can be achieved. TDS, and specifically water hardness, is by far the most significant factor affecting strategies to conserve water by increasing the COC. Increasing COC is much easier to implement in locations where water is naturally soft, such as portions of the Northeast, Southeast and Northwest.
Non-potable water as an alternative source
Where available, non-potable water sources can be a great way to conserve potable water, and a path to earning LEED credits. The four common categories of non-potable water are HVAC condensate, rainwater and stormwater, recycled municipal water and gray water.
HVAC condensate is an ideal source of make-up water for cooling towers for two reasons. First, the timing of the generation of condensate from air conditioning systems aligns well with the timing of need for make-up water for the cooling towers. This alignment means that a storage tank may not be necessary. Second, condensate water is pure with a very low dissolved mineral content. However, a potential downside of HVAC condensate is it sometimes contains heavy metals, which may require treatment prior to use as make-up water.
Rainwater and stormwater are commonly harvested from roofs and hard surfaces, such as roadbeds or parking lots. Regulations vary by state. Rainwater pH frequently is around or below 6 and needs to be mitigated before use in a cooling tower to minimize the risk of corrosion and contamination. Also, control of microbiological growth must be included in any water treatment plan where harvested rainwater is being used. The level of treatment required for harvested rainwater depends on the source. Two common issues are bird droppings if the rain is harvested from a roof and oil if harvested from roadbeds and parking lots.
Recycled municipal water: Local municipalities are increasingly developing the capability to reclaim and sell treated wastewater rather than discharging it into a lake or river. Purple pipe, along with appropriate signage, is used to distinguish such distribution systems from potable water lines. This water is often good quality, although the concentration of minerals is usually higher than potable water. An advantage is the increased silica, alkalinity, hardness and phosphate content in reclaimed water are often less corrosive than tap water. When using recycled municipal water, water quality management teams need to evaluate how corrosion inhibitors from the municipal process may impact water treatment strategies for cooling tower make-up water.
Gray water: Unfortunately, typical commercial sources of gray water — e.g., urinals and laundry — are not appropriate for use as a direct non-potable water source without significant further treatment. Soaps found in laundry can be problematic because they act as a food source for microbiological growth.
Addressing water quality challenges
Depending on the quality of the water available, and based upon testing and recommendations of water treatment professionals, the strategy for conserving water may require the implementation of one or more mitigation methods. These methods divide into two basic categories: (a) improving the water with chemical treatment and filtration and (b) protecting the system with materials of construction that offer high protection from corrosion.
Another mitigation strategy is to select a material of construction for the cooling tower that offers some protection from corrosion, such as stainless steel. For the highest level of corrosion protection, polyurethane basin coatings can be applied. Those that address scale, bacteria and corrosion will maintain peak system efficiency and extend the life of the evaporative cooling equipment.
These various strategies afford many options for earning LEED points in existing buildings and new construction. 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 is business manager at Baltimore Aircoil Company.
Robert Downey is global sales development manager and water treatment specialist at Baltimore Aircoil Company.
