Concentration ratio when a tower is unmetered or meter readings are lacking. Cooling towers commonly lack metering or robust monitoring that otherwise would permit direct calculation of concentration ratio. Fortunately, the degradation in water quality endemic to cooling towers provides a fine proxy for calculating concentration ratio, or the ratio of analytes in blowdown water (CB) to analytes in makeup water (CM), or:

CR = CB ÷ CM

TDS and, by proxy, conductivity are common parameters for CB and CM.

Concentration ratio, water efficiency, and improvements. Concentration ratio is used to describe the relative (nonconsumptive) water efficiency of cooling towers. Cooling towers with high concentration ratios are more water-efficient because the dumping of water as blowdown is minimized, and, thus, less makeup is required. On a per-100-ton basis, water use is much higher at low concentration ratios than it is at high ones (Figure 1).

Water savings associated with elevating concentration ratio can be calculated. Where CR1 is pre-improvement concentration ratio, and CR2 is post-improvement concentration ratio, percent savings can be calculated as follows:

100{(CR2 − CR1) ÷ (CR1[CR2 − 1])}

Water savings associated with a variety of improvements in concentration ratio are presented in Table 1. Clearly, towers with low concentration ratios are the most economical candidates for improvements.

Drift-reduction-improvement calculations. Whereas drift is reported as a percentage of tower flow rate, drift-reduction-improvement savings are calculated as follows:

STFR = TFR(D2 – D1)

where:
STFR = water savings
TFR = tower flow rate
D1 = initial drift-loss percentage
D2 = post-improvement percentage

With the direct measurement of drift rates rare, nominal rates from manufacturers must be relied on for post-improvement percentages. For initial drift-loss percentages, the conservative approach is to use original specifications. Modifications of tower flow rate must be considered separately, as follows:

TFR2D2 – TFR1D1

Final scaling of calculations. Whether you are calculating savings related to concentration-ratio improvements or drift reduction, use correct tonnages, and have a reasonably good estimate of duty factor (DF) or average plant load over the timeframe of interest. Final savings can be estimated as follows:

S = DF(STFR)

where: S = final savings estimate

In the case of multiple towers, one must appropriately weigh the operating time of each for an accurate total estimate of water savings.

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