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

Scaling

CoC is a key metric to gauge the efficiency of water usage of the cooling system. By operating at higher CoC and conductivity, the amount of blowdown released, thus, the consumption of makeup water, is reduced. Calculated CoC values plotted as a function of makeup water flow rate indicated that there is diminishing benefit of increasing CoC beyond ~5 to reduce makeup water usage.

Therefore, the cooling tower system was operated to achieve a CoC of ~5. Changes in CoC and makeup water flow rate before and during the study period are shown in Table 2 and Figure 5. CoC was initially calculated using both calcium and conductivity ratios, as well as on a volumetric basis. Chlorides were not measured during the study since sodium hypochlorite had been the primary disinfectant under the chemical program and chlorides had not been historically analyzed. The volumetric basis produced widely erratic results (probably due to poor estimates of the evaporation rate). Conductivity and calcium CoCs tracked very closely, and only calcium CoCs are presented here. Following system purging, the conductivity averaged 1,043 [micro]S/cm during the trial. During the first half of the study period, the average flow rate of makeup water decreased from approximately 4900 gal/ day (0.21 L/s) to 3600 gal/day (0.16 L/s). The flow rate decreased further to approximately 2000 gal/day (0.09 L/s) after the blowdown conductivity setpoint was increased to 1250 [micro]S/cm. The average CoC of 4.9 during the trial was comparable to the 150 day pre-trial CoC of 4.7 and did not adversely affect pH, LSI (Figure 6), or the system's ability to control corrosion (discussed below). Interestingly, while a well-managed chemical program is capable of sustaining 5 CoCs and greater, the three-year timeframe prior to the trial averaged only 3.5 CoC due to two unexplained periods when the tower operated at 2 CoCs and less.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

Table 2. Cooling Water Cycles of Concentration Before and During the
Study Period

                    Previous Three Years  150-Day Pretrial  Trial

Average                       3.5                4.7         4.9
Standard Deviation            1.4                0.4         0.3

Throughout the chemical program and during the study, the pH remained stable at 8.7-8.8 and the LSI remained positive (Figure 6), decreasing from 1.99 under the chemical program to 1.54 after switching to the HCD. The changes in the LSI suggest a decreased tendency to form scale during the study period, but that [CaCO.sub.3] coatings (if present) would remain and provide some continuing corrosion protection. Since it is generally desirable to operate in a range where the LSI is positive but small, the HCD program appeared to provide a more favorable chemical environment for scale control.

XRF analysis of the scale particles collected in the filter bag show that it is dominated by calcium compounds but with significant amounts of silicon, magnesium, aluminum, zinc, phosphorous, and iron (Table 3). For the purposes of estimating the amount of carbon and oxygen contributing to the matrix corrections, all alkali and alkaline earth metals were assumed to be in the form of (relatively) insoluble carbonates. All other elements were assumed to be common oxide forms other than the halogens. Based on pure element sensitivities, the sum of the concentrations of all compounds is ~94%, indicating that the estimated amounts of carbon and oxygen are consistent with the absorption required to account for the measured intensities. The presence of zinc in the scale is noteworthy, since it is not a significant contaminant of city water nor likely to be blown into the cooling tower (see further discussion in "Corrosion Control" below).