* If the drained (blow-off) water is reused instead of coming from the water supply, an additional 420,000 gallons of water could be saved by using pulse technology.
Water treatment with momentum and EPA/USGBC goals
Pulsed power physical water treatment has transformed the way evaporative cooling systems are treated. Pulsed power uses pulsed electric fields (a technology developed by the food industry for pasteurization) to control scale, biological growth and corrosion. This completely chemical-free approach to water treatment eliminates all of the environmental and health and safety issues associated with water treatment chemicals. With pulsed systems, there are no pumps to break and no chemical tanks to run dry. Additionally, they are more forgiving of operational disruptions and generally allow cooling towers to operate at higher concentration cycles (hence less blowdown and less water usage) than with traditional chemical treatment. Independent studies show that not only is the method effective for cooling towers, but the performance of pulsed systems is superior to traditional chemical treatment in terms of biological control and water usage. By eliminating the problems associated with chemical water treatment, water-cooled condensers' advantages make them an environmentally friendly, energy efficient and economical choice that is consistent with the goals of the EPA and USGBC initiatives.
Electronics are used to reduce the formation of limescale in piping systems caused by calcium or magnesium carbonate in the water. Electronic systems create a pulsed, time-dependent, induced electric field in a PVC pipe built directly into the cooling tower's recirculating water system. The electrical signal changes the way minerals precipitate in the water, completely avoiding hard limescale deposits by instead creating a non-stick mineral powder in the water. This powder is easily filterable and is mostly removed during normal blowdown, or it settles loosely in the cooling tower basin so it can be easily removed annually. Bacteria become incorporated into this mineral powder and therefore leave the system by blowdown, filtration, or settling. The encapsulated bacteria (some of which have had their membrane walls injured by electroporation, resulting in cell lysis) cannot reproduce, resulting in an extremely low bacterial population. Water softening is not required (nor recommended) with this type of system treatment, and high concentration cycles are usually achievable, resulting in significant water savings. This technology is based on the ability to eliminate biofilm (a slime layer with more than four times the insulating power of mineral deposits) using pulse power, and allows for energy savings of at least 5 percent compared to traditional chemical treatment processes.
Water-based cooling systems are typically treated with chemicals to prevent scale buildup, control biological activity, and prevent corrosion. In chemical water treatment, the chemicals or their toxic byproducts enter the environment through leaks, water runoff (blown-off), air emissions, spills, overspray, and drift. These chemical emissions are harmful to the environment. Chlorine or another biocide is often used in a cooling tower to control biological activity and reduce pathogens. Most of the chlorine added to the system is quickly released into the atmosphere as chlorine gas. Corrosion inhibitors such as zinc, silicates, molybdenates, and phosphates are released in drift from the tower and in overspray, and settle to the bottom with the blowdown and pass through the sewer system. Water softeners are often used to prevent scale buildup, and amounts of brine are discharged as part of the water softening process.
In a LEED-certified project with a relatively small cooling system (400 tons or 1,200 gpm, operating 12 hours per day, 5 days per week – 3,120 hours per year), it was demonstrated that the use of pulsed-rate water treatment avoided the discharge of 1,145,644 gallons of chemically contaminated water (20 ppm phosphates, 2 ppm zinc, and 1 ppm chlorine and disinfection byproducts) as blowdown and drift, 4 to 5 gallons of isothiazoline, and 749.1 gallons of industrial-strength chlorine bleach. Much larger cooling systems have been quantified in the same way in other LEED projects, with results understandably orders of magnitude higher.
With respect to the above project, when coupled with typical cooling tower characteristics as provided by the Cooling Technology Institute (CTI) in Houston, it can be calculated that 739.6 pounds or gallons of chlorine evaporate annually from this tower. Considering the hours of operation and the typical airflow from this tower, the chlorine concentration in the exiting air is 0.51 ppm. The level of chlorine release is significant: the OSHA short-term (15-minute) exposure limit is 1 ppm (3,000 µg/m3) and the OSHA TWA PEL (8-hour average) is 0.5 ppm (1,500 µg/m3).
As previously mentioned, this building's cooling tower system uses a 400 ton cooling tower system with an estimated circulation rate of 1,200 gpm. Using CTI's rules of thumb, it can be calculated that this system will evaporate 1,123,200 gallons of water annually, expel 22,464 gallons of water as drift, and blow down 1,123,200 gallons of water annually when chemically treated (at 2 concentration cycles) or 561,600 gallons of water when treated with pulsed power (at 3 concentration cycles). This gives a total annual water use of 2,268,864 gallons (at chemical treatment) or 1,707,264 gallons (at pulsed power). This analysis shows that using pulsed water treatment saves 561,100 gallons of water annually (that's how much less water is blown off), or approximately 50 percent of the total water blown off in a chemical treatment (or 25 percent of the total water used in a chemical treatment, minus the amount of water created by drift). While concentration cycles vary from project to project due to various factors (system size, loading, mineral content of the water, etc.), the water savings from pulsed technology over chemical treatment are typically 15 to 45 percent before considering the environmentally friendly and recyclable characteristics of the blowdown water discharged from a pulsed system, which can save significantly more water when reused.
In this project, the use of pulsed water treatment provides environmental benefits and water savings compared to traditional chemical water treatment. The amounts of chemicals that would be released annually during cooling tower operation using chemical treatment are:
- 739.6 pounds of chlorine gas.
- 4 to 5 gallons of isothiazoline by drift and blowdown.
- 1,145,644 gallons of contaminated blowdown and drift water containing 20 ppm phosphate, 2 ppm zinc, and 1 ppm chlorine (including disinfection byproducts).
The reduction in total water use calculates to an annual water savings of approximately 25 percent (or 561,600 gallons) compared to a typical open system with conventional chemical treatment. An additional 561,600 gallons of water discharged (blow-off) annually using pulse flow technology is environmentally friendly and has been recycled or reused at various sites (toilet flushing, lawn watering, etc.), saving even more gallons that do not need to be used via the water source (make-up water). The total annual “holistic level” water savings for this project could be 50 percent or 1,123,200 gallons.
Using the previously mentioned standard (2 gallons of water are used to generate one KWH), the annual energy consumption for this project using pulsed current technology is 280,800 KWH less than chemical treatment (not including the energy savings from biofilm removal). Using pulsed current technology, energy consumption could be further reduced by reusing blowdown water instead of using new makeup water.
Make a decision
The significant environmental benefits of pulse power technology, along with the significant reduction in greenhouse gas emissions from using water-cooled systems, contribute significantly to achieving EPA and USGBC goals. If you must choose between air-cooled or water-cooled HVAC systems, carefully weigh your options and think about the pros and cons of each system type.
Jerry Ackerman, former Marketing Director of Clearwater Systems (now part of EVAPCO), is an independent marketing consultant and freelance writer for various construction companies. You can reach him at [email protected].
This article was first written in 2008 and is still relevant in 2018.
