This paper will describe a case study of a non-chemical treatment method for control of microbes, scale and corrosion in cooling towers. This product is marketed by Clearwater Systems under the trade name of Dolphin and is a pulsed-power system (PPS) electromagnetic device. Much of the scientific data on PPS applications in cooling towers was developed by the University of Connecticut Environmental Research Institute (ERI) under a grant from the Electric Power Research Institute in 1995.
In January 1999, two PPS devices were installed on a commercial cooling tower at Engelhard Corporation in East Windsor, CT. This Engelhard facility specializes in the fabrication and repair of aero-engine parts. Because of unusual restrictions imposed by the FAA and Engelhard's internal environmental concerns, this tower was operated without any blowdown for a period of over four months. The details of this period of extreme conditions are informative in understanding how the PPS performs in atypical operating conditions.
Non-chemical Water Treatment
Non-chemical water treatment has been around for many years, generally in the form of "permanent magnets". In the U. S. there is a strong sentiment, backed up by plenty of examples, that the products are "gadgets" and don't work. The occasional success story is viewed as an aberration due to some particular and unusual condition of the water or tower or test method. In Europe there is a more pragmatic view, with the success and failures given equal weight. Cranfield University's School of Water Science in Cranfield, England is a center for these investigations having hosted three international symposiums over the past few years. They have published both summary review technical papers and independent investigations.
Their view is that most of the successful case studies involve recirculating-water systems rather than once through systems. The successful laboratory studies generally require a "dynamic magnetic treatment", i. e., the solution must be moving rapidly through the magnetic field. In these laboratory studies changes are observed in colloidal systems (such as solutions of calcium carbonate/bicarbonate) under the influence of these devices. The changes take the form of enhanced aggregation and modified crystal growth. These studies generally focus on scale amelioration rather than microbial control. Since movement of charged particles (the solution) through a magnetic field is thought to be required for scale amelioration, most of the theoretical mechanisms for this effect focus on the induced electrical field not the magnetic field.
Use of PPS in recirculating water systems was pioneered in Italy in the early 1990's. An imported device was brought into the US in 1994. The original imports were an example of how "by building a better mousetrap the world does not beat a path to your door". Although the device was successful in all of the installations, a lack of understanding of how the system worked hampered its marketing. The system was brought to the University of Connecticut's Environmental Research Institute where a detailed multi-departmental investigation was made. That study was ended in late 1997 when the US importer and co-sponsor of the project filed for bankruptcy for unrelated issues. Because of the bankruptcy a final paper was never issued; however, the bulk of the research was complete.
In this era two reports were published on PPS case studies. The first was a Cooling Tower Institute Paper describing an installation at Schick Warner-Lambert in 1995, the second was a report by the Toxic Use Reduction Institute (TURI) at the University of Massachusetts describing an installation in 1997.
The TURI installation was done just as the importing company went bankrupt. With the demise of the importing company, the support for this application disappeared and within a few months the PPS devices were removed. However, the Schick application was installed and stabilized while the importing company was still viable. The Schick application continues not only on the original tower but has expanded to all of the other cooling towers in the facility with similar results to those reported in the original paper.
of a Pulsed-Power non-Invasive Device
PPS devices consist of two main components: the transformer panel and the coil and pipe assembly. The transformer panel brings line voltage down to the operating voltage of the system and contributes to system capacitance. The coil and pipe assembly consists of an unobstructed replacement spool with the coils and circuitry completely outside of the fluid. The product is not sized by pressure, velocity or flow rate, only by pipe diameter. There is no direct contact of the recirculating water with any electrodes. The fields, which are generated by exterior coils, are predominately located on the interior of the pipe.
A. Electromagnetic Signal
PPS devices impart pulsed, high frequency electromagnetic energy into flowing water by inducing varying electromagnetic fields 60 times per second. During each cycle, the field strength varies from 0 to a maximum value then back to 0. Half way through each cycle the field is pulsed causing a ringing effect. This ringing has a natural frequency based on the geometry of the coil and the capacitance of the circuitry. Over a time period of about 3 milliseconds, this field decays to a few percent of its original intensity. The transient decay causes harmonics of the natural frequency resulting in measurable frequencies up to the megahertz range. This time varying magnetic field induces a rapidly changing electric field in the water system of the same frequency as the magnetic field but in a direction around the circumference of the pipe.
B. Interaction of the Induced Electric Field with Colloidal Particles
PPS work by changing how calcium carbonate and other dissolved minerals precipitate from solution. Pulsed-power fields activate colloidal nucleation sites in the bulk solution. These activated sites become the preferential nucleation sites for precipitation.
Calcium carbonate is the mineral commonly present in water with the lowest solubility, i.e.; it is the first precipitate to form in a cooling tower. Precipitation of calcium carbonate and other dissolved minerals occurs when the minerals in solution reach supersaturation levels. Supersaturation is primarily reached by the evaporation of water. The evaporated water contains no minerals and the decreased volume of fluid increases the concentration of minerals.
The precipitation of calcium carbonate can occur as either a surface-nucleating scale or a colloidal-nucleating powder in the bulk solution. Growth on existing solids is thermodynamically favored. The unique spectrum of electromagnetic fields produced by pulsed power is theorized to shift the CaCO3 equilibrium chemistry to favor formation of stable crystal nuclei in the bulk solution. Thus crystal growth and precipitation will occur in solution and accumulate as a loose powder instead of on a surface as a scale.
C. Control of Microbial Populations
PPS is a bacteriostatic device rather than a true bactericide. Although the bacteria are not killed, they are controlled. A chemically controlled tower trying to balance the cost of biocide chemicals and the cost of corrosion inhibitors that are needed to protect the tower from the biocides will usually aim for a Total Bacteria Count as measured in a standard 48-hour, 35 0C heterotrophic plate count (method SMEWW 9215) of about 20,000 to 50,000 CFU/ml. Lower levels are achievable but at a cost. In addition, many of the bactericides are species specific, and they must be periodically changed to prevent a different species from flourishing. Mutation of species such that previously effective biocides become ineffective on the mutated species is a problem for chemically controlled towers.
With a PPS, although the bacteria are not truly killed, they are controlled. Total Bacteria Count using the standard 48-hour test will typically show 1,000 to 2,000 CFU/ml in a cooling tower being operated under a normal blowdown regime. Figures 1 and 2 show the typical result achieved with pulsed-power. Figure 1 is from a metals company corporate headquarters in Pennsylvania. This is a 600-ton tower being operated at 6 cycles of concentration. Figure 2 is from a manufacturing facility in Connecticut. This is a 175-ton tower being run at 8 cycles of concentration.
|There are two separate mechanisms that cause these low levels of biological activity. The first relates to the well-established effect in water treatment that coagulation and calcium carbonate precipitation will result in a microbial reduction. Operating a tower with coagulation and precipitation occurring in the bulk|
|solution as happens in water treatment plants, will incorporate microbes in the water into the growing crystal precipitates. This effect is not as pronounced in scale formation since only the bacteria close to surfaces can be incorporated into the precipitate. The encased bacteria can be revived, but they are physically constrained in the precipitate and unable to reproduce. With microorganisms controlled by physical incorporation, changes in the make-up of the microbe population over time will have no effect on the efficacy of the system. Mutation or resistant strains of bacteria do not flourish in a pulsed-power controlled tower.|
The second mechanism involves sub-lethal injury that controls bacteria even when there is no precipitation occurring. This mechanism is based on the interaction of the low frequency radiation generated by the pulsing with the bacteria.
Maxwell Laboratories in California was first to commercially develop pulsed-power. Pulsed-power is the basis of cold pasteurization, a FDA approved technique to pasteurize pumpable fluids such as fruit juices. The original Maxwell patent details a process in which microbes are exposed to pulsed magnetic fields of equivalent frequencies, duration and decay rate as those in a cooling tower but at 100 times higher fluxes. At this flux and frequency, a single pulse in a low-conductivity fluid, such as cooling-tower recirculating water, will reduce the population of microorganisms by two or more orders of magnitude.
Pulsing a magnetic field this way generates very low frequency, non-ionizing electromagnetic radiation. This radiation has a much lower frequency than microwave radiation, even lower than radio waves, but has a demonstrable effect on microorganisms. Frequency is a measure of how much energy each individual photon of the radiation contains. The square of the flux is a measure of the quantity of photons. Both the frequency and the flux are related to the total energy input from the device.
Since the frequency of the PPS cooling-tower devices are the same as the Maxwell devices the energy of each individual photon is the same with both devices. The flux with the Maxwell devices is 100 times higher than the flux with PPS devices therefore the number of photons per pulse is 10,000 times higher. A cooling-tower PPS device thus uses a fraction of the energy that the Maxwell devices use on a per pulse basis. However, because cooling towers involve recirculating water, a PPS device on a cooling tower will "see" each bacterium many times before the bacterium exits the system. In a typical cooling tower set-up, each bacterium will see over 5000 pulses, exposing the bacteria to over 50% of the total amount of radiation as with the Maxwell device.
Spreading this exposure over a few hours does not have the same biological effect as a single dose; however, there is damage to the microorganisms. This damage is sufficient to inhibit reproduction but not sterilize the system. The bacteria can recover in a few days, but while they are recirculating through the PPS they are inactive.
D. Summary of Operation
A PPS device controls two major problems of recirculating water in cooling towers, scale formation and biological activity, solely by electronic means with no chemical additions of any kind. Without the additions of corrosive chemicals, the recirculating water in a cooling tower is naturally benign water. The recirculating water is devoid of active biocidal agents, virtually free of bacteria and bio-film, and has a non-acid pH in the range of 8.0 to 9.0. A cooling tower and heat exchanger made of common construction materials operating in this water chemistry will exhibit no aggressive corrosion attack on any of the wetted surfaces
The Engelhard Corporation in East Windsor, Connecticut, is an aerospace turbine engine fabrication and repair shop. Engelhard manufactures, repairs and refurbishes aero-engine parts under strict FAA, primary engine fabricator and airline specifications. The strict and multiple approvals required for this sensitive operation make changes to qualified procedures difficult.
One operation involves the cleaning of parts in one of two TCE vapor degreasers prior to a coating process. Due to the environmental concerns with handling TCE, Engelhard maintains very strict controls of any discharges from this area.
The vapor degreasers are connected to a 50-ton PROTEC model PCT-40 fiberglass tower with PVC fill. All piping to and from the tower is in 3" PVC. The cooling tower water loop runs through stainless steel tubing in the condensing sections of the vapor degreasers and a stainless steel holding tank. The vapor degreasers and the cooling tower operate 24 hours per day.
A new cooling tower was installed in late January 1999 using two PPS devices to control recirculating water chemistry. Previously, once-through city water was used to cool the degreasers. One 4" diameter PPS unit was located on the recirculating loop on the discharge side of the recirculating pump, a 3/4" PPS unit was located on the make-up water line. From the beginning, Engelhard Corporation was very concerned with discharge water quality. Engelhard's internal requirements, designed to completely assure that no accidental contamination of the non-contact cooling water with TCE occurred, required storage of all blowdown discharges from the cooling tower until it could be analyzed. The effect of this C. Water Chemistry
C. Water Chemistry
The incoming water quality into Engelhard is shown in Table 1. The water is from wells located in central Connecticut and has a moderate hardness. The tower was run for four and one half months for twenty-four hours per day with no blowdown and no filter system to remove precipitates.
The pH of the recirculating water settled in a narrow range about 8.5. This value represents an equilibrium between dissolved CO2 and total alkalinity. The total alkalinity stabilized at a value around 250 ppm as CaCO3. The total dissolved solids, total hardness and conductivity steadily increased over the entire period ending with a conductivity of about 5,000 micro-Siemens/cm. Precipitation, as a powder, began to occur after about a week of operation. The powder would settle in quiescent and low flow areas of the tower. Figure 3 plots both the conductivity and pH over the course of four and 1/2 months.
The Engelhard cooling system is made entirely of stainless steel, fiberglass and plastic; there is no copper, steel or galvanized metal in the entire loop. With a stainless steel system, no added chemicals, and a pH of 8.5, Engelhard personnel were not concerned with general corrosion and no coupon tests were performed. There was a concern that the high cycles of concentration might make the stainless prone to pitting attack. Detailed visual inspection of the stainless steel in the tower and supporting equipment was routinely made during the fourteen weeks and a more extensive visual examination was made when the tower was cleaned. There was no evidence of any pitting or other corrosive attack.
F. Engelhard Discussion
A typical chemically controlled tower operating with Engelhard's incoming water would run at 3 cycles of concentration. At that level there would be very little precipitation. At 3 cycles of concentration, the blowdown would be 500 gallons per day.
Under normal circumstances, a pulsed-power controlled tower runs at about 8 cycles of concentration using 140 gallons per day for blowdown. At this level precipitation will occur, but by locating the blowdown in a physically low point with high hydraulic turbulence, most of this precipitate will exit in the blowdown.
The tower at Engelhard was run without any blowdown until it appeared that it was no longer able to keep temperature. (Originally it was thought that the precipitate was causing the entire problem, but during the maintenance it was discovered that an electronic relay was malfunctioning and the cooling fan was not turning on as it was designed to. It is not clear how much of the problem was due to a lack of airflow). At this point it was shut down and cleaned. Approximately 100 pounds of precipitate powder was found throughout the system, particularly on the plastic fill in the tower. It was reasonably easy to remove the precipitate with a power spray from the outside area of the fill, but the center area was difficult to reach with the spray and difficult to clean.
|Table 2 shows a chemical analysis of the bulk solution precipitate removed during the clean out. Even after four months of concentration, the precipitate was predominately calcium carbonate,though some gypsum and silicates were present. X-ray diffraction was done on the sample by two separate laboratories. Both laboratories showed that the crystal structure of the bulk solution precipitate consisted|
|of about 60% calcite and 40% vaterite. The presence of vaterite was unexpected and further investigations are underway. In a pure calcium carbonate system vaterite is unstable at cooling tower operating temperatures and will in a matter of minutes transform into calcite. Some organic compounds will stabilize vaterite and that is believed what has happened in the Engelhard sample. Vaterite has a lower solubility than calcite or aragonite (calcite, aragonite and vaterite are three polymorphs of calcium carbonate) and its presence could indicate a mechanistic step in the activation of bulk solution nucleation.|
When the tower was finally cleaned of calcium carbonate precipitate, a visual inspection of the piping heat transfer regions was performed. There was no evidence of deposition occurring except in the cooling tower itself and no evidence anywhere of corrosion attack.
This installation has shown that scaling, biological activity and corrosion can be controlled using only an electronic PPS device under a very extreme condition. Running a tower without blowdown and without any method to remove precipitation is clearly not appropriate; however, even in this extreme condition the system performed acceptably. Engelhard continues to use PPS on this tower, on a second tower they have recently installed in Connecticut and on a new installation in South Carolina. Engelhard is setting up a procedure on this tower to periodically flush the system to relieve accumulated solid precipitates before they become unmanageable.