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

Corrosion Control

Corrosion is typically controlled by maintaining water at alkaline pH. Except during the purging period prior to the HCD trial, pH did not change appreciably during the course of this study. The 8.8 pH of the (alkaline sodium hypochlorite) chemical program decreased to about 8 during purging but increased to 8.7 within five days of HCD start-up and remained 8.7 [ or -] 0.1 pH units for the duration of the trial. If significant [CO.sub.2] stripping were occurring during operation, then equilibrium might be expected to shift away from carbonic acid and the system pH would increase. Since dissolved gases were not monitored in the bulk solution, insufficient data exist to provide a definitive explanation for this effect at this time.

The results of nonpassivated corrosion-coupon tests are shown in Table 4 as well as the data from two prior tests obtained during the chemical program. The cleaning procedures are described in GCC (2003). Other than the galvanized steel coupon, no appreciable corrosion was observed, and the other three coupons appeared brand-new after cleaning. The observed corrosion rates of copper and mild steel were either equivalent or better than those obtained during the chemical program. Corrosion rates observed under both the chemical and HCD programs fall into the "Negligible or Excellent" category set by the National Association of Corrosion Engineers guidelines shown in Table 5 (Boffardi 2000) and are also within acceptable limits established at the beginning of the trial, as shown in Table 4. With regard to galvanized steel, the observed corrosion rate (4.3 mil/year) during the study period was much higher than the acceptable rate set at the trial beginning (1 mil/year). However, it was discovered that an unpassivated galvanized steel coupon had been used and, hence, would not have properly modeled actual tower conditions if the tower remained properly passivated. Zinc concentration in the recirculating water remained relatively stable during the trial and was 0.2 [ or -] 0.1 mg/L. Since no data exist for either the bulk water zinc concentration or the corrosion rate of galvanized steel prior to the start of the HCD study, it is unknown whether or not the corrosion was worse during the HCD program. No "white rust" was observed on galvanized tower surfaces, and the cause for the elevated zinc in the precipitate sample remains indeterminate at this time.

Table 4. Measured Corrosion Rates (mil/year) of Test Coupons

Date      Days Exposed  316L SS    Copper

Historic        23                 <0.1
Pretrial        61                 <0.1
HCD             65       <0.1      <0.1

Date      Galvanized Steel  Untreated Mild  Treated Mild
                                Steel           Steel

Historic                       1.3
Pretrial                                       0.5
HCD              4.3           0.3

Table 5. Guidelines for Corrosion Rates (mil/year)

Rating                       Copper               Mild Carbon Steel

Excellent          [less than or equal to]0.1  [less than or equal to]

Very good                    0.1-0.25                    1-3

Good                        0.25-0.35                    3-5

Moderate                    0.35-0.5                     5-8

Poor                         0.5-1                       8-10

Very poor to                  >1                         >10
severe

Heat Transfer Efficiency

Calculated values of the overall heat-transfer coefficient for both heat exchangers and their regression lines (Figure 7) appeared to show a marginal improvement during the study period. However, the number of readings collected was not sufficient for a meaningful statistical analysis.

[FIGURE 7 OMITTED]

The testing operations, by their inherent nature, vary greatly in heat load due to the fluctuating number of tests being conducted and the duration of the tests. Therefore, the variation of the heat transfer coefficient was plotted against observed heat load as shown in Figure 8. Heat load was calculated using the measured flow rate of the tower water supply pumps and the temperatures entering and exiting the cooling tower. As is evident from correlation in Figure 8, some of the variability in the heat transfer coefficient might be due to the variation in heat load, and it is, therefore, difficult to quantify the extent to which the use of the HCD unit impacted the heat transfer efficiency.