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

INTRODUCTION

In general, water in cooling towers needs to be treated to control microbial growth, scale formation, and metal corrosion. For any control process, the heat transfer performance of the cooling tower must also be maintained.

MICROBIAL GROWTH CONTROL

The conventional method for control of bacteria, algae, and fungi in cooling towers is by addition of antimicrobial pesticides (biocides). Various oxidizing and non-oxidizing chemical disinfectants are commercially available and have been successfully employed (Kim et al. 2002). These biocides are continually added to the recirculating bulk water to maintain a desired level of residual and make up for the loss through chemical reactions and blowdown. Methods of disinfectant addition include manual slug feed, automatic dosing based on the volume of makeup water, and timer-controlled automatic intermittent feed. Even with automatic addition methods, chemical treatment systems must be monitored and adjusted frequently to maintain the desired level of residual and verify control of microbial growth. In addition, some microorganisms may become resistant when subjected to continuous use of a single disinfectant, and periodic alternation of disinfectant type is often required to mitigate this effect.

In order to avoid use of chemical disinfectants, several nonchemical alternatives have been developed, including UV irradiation, ultrasonic cavitation, and hydrodynamic cavitation. A hydrodynamic cavitation device (HCD), the subject of this study, uses hydrodynamic cavitation in combination with scale filtration to provide a mechanical means of disinfection. As such, it eliminates the hazards and costs associated with chemical disinfectants and may prevent bacteria from becoming resistant to the disinfection technique.

Cavitation is the formation, growth, and implosion of vapor bubbles in a liquid. They can be created by sound waves (ultrasonic or acoustic cavitation), lasers, or by fluctuations in fluid pressure (hydrodynamic cavitation). Early cavitation research focused on the damaging effects of uncontrolled cavitation on pumps and rotating propeller blades. During the collapse of vapor bubbles, extremely high fluid velocities, pressures, and temperatures occur that can cause pitting and erosion of surfaces. More recent efforts have focused on controlling cavitation to produce beneficial effects.

Cavitation has been reported to kill bacteria through chemical reactions by free radicals formed from cavitation, cell disruption from pressure pulses, micro-jets, and heat associated with localized high temperatures. In aqueous liquids, cavitation leads to the formation of hydrogen and hydroxyl radicals and hydrogen peroxide (Kalumuck et al. 2003). These short-lived reactive radicals are capable of effecting secondary oxidation and reduction reactions (Suslick et al. 1997) including disinfection, but only in the immediate vicinity of the bubble.

During bubble collapse, a pressure pulse up to 1,450 psia (10 MPa) is produced in the immediate vicinity of the bubble. In addition to the pressure pulse, shear forces and shock waves may impinge on a bacterial cell wall within a few radii and cause cell lysis (Brennen 1995). Asymmetric bubble collapse can also lead to emission of micro-jets of fluid (seen as a downward conical structure in Figure 1). Lohse (2003) has postulated that jet formation may be responsible for cell-wall disruption based on electron micrographs of leukemia cells exposed to low-intensity ultrasound having conspicuous holes in their walls.

[FIGURE 1 OMITTED]

Thermal effects become very pronounced in the final stage of collapse when the bubble contents are highly compressed by the inertia of the inrushing liquid. Locally, temperatures can reach as high as 8,540[degrees]F (5,000[degrees]K) at the liquid/vapor interface. These regions of extreme temperature only exist for a fraction of a microsecond, e.g., 2 [micro]s (Brennen 1995). Nonetheless, the heat associated with the high temperatures could be sufficient to kill microbes located within a few bubble radii.

Cavitation appears to be effective against numerous strains of planktonic bacteria. Of particular note, Stout (2002) has demonstrated the efficacy of HCD against Legionella pneumophila serogroup 1 in a controlled laboratory setting under batch conditions (Figure 2). Along with the rapid kill of the bacteria, Stout also detected some oxidants expressed as free chlorine during the HCD experiment, which is consistent with the above discussion on the generation of free radicals and oxidants by hydrodynamic cavitation.

[FIGURE 2 OMITTED]

Scale Control

Scaling is the formation of hard deposits on piping, heat exchangers, and other process equipment surfaces. Calcium carbonate ([CaCO.sub.3]) scales are the principal cause of cooling-tower scaling problems and adversely and significantly affect heat-transfer efficiency. This scale formation is controlled by temperature, rate of heat transfer, pH, dissolved solids, and alkalinity. Cavitation has been reported to provide a favorable condition for [CaCO.sub.3] formation by (1) vacuum stripping [CO.sub.2] gas from water, thus increasing pH, and (2) locally increasing temperature. The latter mechanism likely predominates since [HCO.sub.3.sup.-] is the prevalent species at typical cooling tower pH (Hutchinson 1957).