Mechanical energy

These devices (Figure 5) typically use pressure pumps to force water into plates, walls, or other water streams and produce cavitation. The cavitation forces soluble calcium carbonate (scale) to precipitate as small seed crystals that grow and are filtered from water. At the same time, dissolved carbon dioxide is removed, raising the pH of the water and inhibiting corrosion. Meanwhile, the effects of a high vacuum, cavitation, and high shear create an environment that destroys water-containing microorganisms without the possibility of resistance. Thus, these systems can be used for corrosion, scale, and microbiological control.

REDUCING WATER USE

One aspect of “greening” a cooling tower is improving its cycles of concentration (COC), or the number of times soluble mineral salts in water are concentrated. By increasing from two COC to four, savings in blowdown and makeup are approximately 67 percent and 33 percent, respectively (Figure 6). For a 300-ton cooling tower, this can translate to approximately 3.2 million gal. of water saved per year. If the current charge for fresh water were $1, and sewer charges were $2 per 1,000 gal., the total yearly savings would be $9,500.

In an actual cooling system, chemicals were eliminated, and the COC were increased from six to nine, resulting in annual savings of more than 5.4 million gal. of water. Moreover, use of the mechanical system has allowed more than 7 million gal. of non-potable water to be available for reuse on an annual basis.

RESPONSIBLE SPENDING

With some non-chemical treatment systems, a method of service comparable to a chemical treatment program is employed. This is effective because of the highly monitored, complex process of scale, corrosion, and bacteria treatment. The monthly service eliminates the capital expenditure of buying equipment.

Those non-chemical technologies can reduce cost by:

• Increasing COC and reducing the cost of fresh water.

• Decreasing the amount of water discharged to a POTW, reducing surcharges.

• Eliminating regulated chemicals, allowing water to be discharged into receiving streams.

• Allowing water reuse for non-potable applications.

• Providing a multiyear fixed treatment cost (no swings in chemical pricing).

• Providing rebates from utilities for water reuse.

REFERENCE

1. Washington State Department of Ecology. (2007). Cooling tower study: Facts & lessons learned. Retrieved from http://www.ecy.wa.gov/PROGRAMS/hwtr/TREE/exec_sum/ct.pdf

For past HPAC Engineering feature articles and columns, visit www.hpac.com.

As technical director for VRTX Technologies, Philip Vella, PhD, directs all experiments and laboratory work pertaining to potential applications for controlled hydrodynamic cavitation. He holds a doctorate in inorganic chemistry from University at Albany, State University of New York. He can be contacted at pvella@vrtxtech.com.