Controlling cooling tower water quality by hydrodynamic cavitation

ASHRAE Transactions, July, 2007 by W.A. Gaines, B.R. Kim, A.R. Drews, C. Bailey, T. Loch, S. Frenette

Analytical Methods

Cooling tower water was analyzed for Legionella in accordance with ISO 11731 (ISO 1998; CDC 1992), while general bacterial enumeration was performed using heterotrophic plate counts (Method 9215D, Standard Methods 1998) and dip slides. Dip slides for bacterial enumeration were placed in an incubator after inoculation with tower water, held at 86[degrees]F [ or -] 3.6[degrees]F (30[degrees]C [ or -] 2[degrees]C) for 24[ or -]2 hours, and compared against a calibration chart for preliminary bacterial enumeration. The slides were then placed back into the incubator for an additional 24 hours for final bacterial enumeration and reporting (mimicking the Standard Methods requirements for heterotrophic plate counts at 95[degrees]F [35[degrees]C]). Finally, the slides were returned to the incubator at 86[degrees]F [ or -] 3.6[degrees]F (30[degrees]C [ or -] 2[degrees]C) for an additional three days to obtain a five-day fungus, yeast, and mold count and were checked against a calibration chart for fungal enumeration.

Additional water parameters monitored included total suspended solids (TSS) (Method 160.2, US EPA 1983), volatile suspended solids (VSS) (Method 160.4, US EPA 1983), total dissolved solids (TDS) (Method 160.1, US EPA 1983), and alkalinity (total and free [carbonate]) (Method 310.1, US EPA 1983).

Temperature (Method 2550, Standard Methods 1998) and pH (Method 150.1, US EPA 1983) were measured using a portable pH meter having an accuracy of [ or -]0.01 pH and [ or -]0.1[degrees]C. Temperature measurements were taken by direct thermometer readings of the four heat exchanger ports (cooling tower in and out and process cooling loop in and out).

Conductivity was measured in accordance with Method 120.1 (US EPA 1983). Concentrations of chlorine residuals (total and free) were measured using N,N-diethyl-p-phenylenediamine (DPD) agents. Ca and Mg were measured by ICP/MS (EPA Method 200.8, US EPA 1994). Hardness was calculated from Ca and Mg concentrations.

Particulate samples were analyzed using X-ray fluorescence (XRF) and X-ray diffraction (XRD). For both analyses, specimens were prepared by wet grinding in ethyl alcohol and drying. XRF specimens were first heated to 1200[degrees]F (650[degrees]C) for two hours in air, then cooled and spread as a loose powder in a thin window analysis cup. XRF intensities were measured using a wavelength dispersive spectrometer. XRD specimens were prepared by dusting the surface of an off-axis cut quartz single crystal prepared with a thin layer of petroleum jelly. Data were collected in focusing (Bragg-Brentano) geometry using Cu-K radiation on a diffractometer in steps of 0.03[degrees] from 5[degrees] to 90[degrees]. The observed diffraction profiles were compared to peak positions and relative intensities found in the Powder Diffraction File (ICCD 2003) to determine the crystalline phase assemblage.

Corrosion was monitored using coupons made of mild steel, galvanized steel, copper, and stainless steel in a "coupon rack" or zigzag layout of piping external to the main circulating loop. The coupons were placed in the rack following their galvanic series: galvanized steel (the most active) followed by mild steel, stainless steel, and finally copper in direction of flow. This prevented the more noble metal from cathodically depositing on downstream coupons and creating a galvanic cell. Each coupon was oriented so that water flowed from the back of the coupon, where it was attached to a nonconducting coupon holder. Individual coupons were orientated with their broad face in a vertical position to minimize the possibility of debris accumulating on the faces. The coupons were exposed to the system water for 65 days and then given metallographic analysis.