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

Table 1. Makeup Water Analysis

Calcium (Ca)                                26.8 mg/L
Magnesium (Mg)                              8.80 mg/L
Chloride ([Cl.sup.-])                        7.5 mg/L
Sulfate ([SO.sub.4])                        23.9 mg/L
PH                                          7.29 S.U.
Silica ([SiO.sub.2])                        2.38 mg/L
Total alkalinity (as [CaCO.sub.3])            72 mg/L
Phosphorous (P)                             0.29 mg/L
Conductivity                            214 micro Siemens

Chemical feeds to the cooling water system consist of an oxidizing disinfectant (12.5% sodium hypochlorite solution), a non-oxidizing disinfectant (1.5% isothiazolinone), and a proprietary scale inhibitor from a specialty chemical supplier. Each chemical feed is independently controlled by an automatic controller to deliver timed dosing of chemicals via chemical metering pumps in a side-stream loop. Sodium hypochlorite dosage averaged 0.09 gal/day (3.9 [micro]L/s) to maintain an average concentration of 0.32 mg/L free residual chlorine; isothiazolinone dosage averaged 0.12 gal/day (5.3 [micro]L/s) (concentration not measured); and corrosion inhibitor dosage averaged 0.14 gal/day (6.1 [micro]L/s) to maintain an average concentration of 0.69 mg/L. Conductivity, pH, temperature, product concentrations, and other system parameters are measured weekly to ensure consistent performance. Bacterial concentrations are manually monitored by adenosine-triphosphate (ATP) bioluminescence, and chemical dosing control timer setpoints are adjusted based on these readings.

Cold water is supplied from a 5,000 gal storage tank through strainers to two 896 [ft.sup.2] (83 [m.sup.2]) plate heat exchangers connected in parallel.

The HCD Hydrodynamic Cavitation Unit

The HCD continuously treated water at 60 gpm (3.8 L/s) via a 5 hp (3.7 kW) centrifugal pump. It was connected as a side stream treatment on the cold well tank. Water was pumped through the HCD and returned to the cold well. A separate bag filtration system was used to remove precipitated [CaCO.sub.3] and other suspended solids. It was operated off a side stream of the cold well using a 5 hp (3.7 kW), 140 gpm (8.8 L/s) submersible pump through a 200 [micro]m bag filter. Excluding the two pumps, there are no moving parts in the system.

The cavitation system consists of a pressure equalizing chamber and a cavitation chamber. Water is pumped into the pressure equalizing chamber at 94 psig (648 kPa) and then channeled into two pairs of conical nozzles positioned opposite each other in the cavitation chamber. As water is forced into the nozzles, it follows a spiral path around the axis of the cones before exiting through the nozzles. The rotation creates a vacuum of 13.5 psia (93 kPa), which leads to the formation of micro-sized gas bubbles in the water streams. Upon exiting each nozzle, the opposing streams collide at the midpoint of the cavitation chamber, causing a dramatic pressure increase that, in turn, leads to spontaneous bubble collapse.