An Overview of Water Tower Cooling using the GGP Silver Bullet Water Treatment System System
The cooling process involves the removal of heat, expressed in British Thermal Units or BTUs, from water. This study involves the removal of heat from the cooling tower.
The removal of heat from a building can be accomplished in a number of ways. The most common way is to use water as a transfer media because it is a more efficient carrier than any other media such as air.
The cooling tower water, also called condenser water, goes to a cooling tower where it eliminates the heat contained within it through a process called evaporation. When exposed to air, water releases its heat to air through evaporation and converts it to water vapor. We have a similar process occur when we sweat. The liquid sweat evaporates from our skin, pulling heat from our body surface it lowers the surface temperature and we feel cooler. The cooling tower lowers its water temperature through the same process.
When the tower evaporates and cools the remaining water, the minerals (dissolved salts) contained in that water stay behind. In other words, when water evaporates, it does not also carry off the minerals dissolved in it. We have the same thing occur when we sweat. The salts contained within our sweat do not evaporate and our skin feels “sticky” as these salt layers build up. So, one of the problems involved in the process of using cooling towers is how to deal with the minerals that are left behind.
In order to understand the role that minerals play in cooling towers, we must first understand the nature of salts and water. Salts have two names with the first name referring to the part of the salt that when dissolved in water has a positive charge and the last name to the part that has a negative charge. So when we dissolve ordinary table salt, sodium chloride, in water, the sodium has a positive charge and the chloride, the negative charge. When the sodium and chlorides dissolve in water, we now call them ions because they have either a positive or negative charge.
In cooling tower water we will be dealing with a different salt called calcium carbonate. This salt is the primary ingredient in limestone. As a dissolved salt, calcium carbonate has one important characteristic. It changes from its dissolved ion state back to its solid state when in a very warm environment like a large chiller. We see examples of calcium forming deposits in other warm environments like home hot water heaters, heated tea pots and on the spray nozzles of shower heads. Just as the calcium deposits in our home hot water heater make them less efficient as it builds up calcium layers at the bottom of the hot water heater, so too do the calcium deposits make chillers less efficient because they produce an insulating layer that limits the heat being transferred to the outside cooling tower.
How does the amount of calcium increase in a cooling tower? It will build up in the same way that occurs on our skin, that is by the continuous evaporation of water. We have two ways of measuring this increase in the amount of calcium in tower water. One way is by comparing the amount of calcium in the makeup water that goes into the tower to the amount of calcium in the tower water itself. The ratio of calcium in the tower water to calcium in the makeup water is called cycles of concentration (*COC*). If the calcium in the makeup water is 100 parts per million and in the tower water, 500 parts per million, then we have five cycles of concentration.
We also have another method of calculating cycles of concentration. If we compare the gallons of makeup water going into the tower to the amount of water going down the drain (called bleed-off or blow-down water), we will also get cycles of concentration. The formula for this calculation is:
Makeup Water divided by Bleed-Off Water = Cycles of Concentration
For example, if we have 15 gallons going into the tower and 3 gallons going to the drain, we also have 5 cycles of concentration. If 15 gallons went into the tower and 3 gallons went down the drain, where did the 12 gallons of makeup water go? Those 12 gallons evaporated taking with them lots of heat and cooling the tower in the process.
Why send those 3 gallons down the drain? If we just let all the water evaporate, then the concentration of calcium ions would get so high that we would have massive calcium deposits in the chillers. By sending the amount of water going down the drain, we control the amount of calcium we have in the tower water. For example, if we let the evaporation rate stay constant at 12 gallons per minute but increased (doubled) the bleed-off rate from 3 to 6 gallons, we would increase the makeup water rate from 15 gallons to 18 gallons because the makeup water rate is simple the addition of evaporated water plus bleed-off water. We have, however, dropped the cycles of concentration (COC) by increasing the bleed-off rate from 5 COC [15/3 = 5] to 3 COC [18/6 = 3].
One of the primary problems with cooling towers is how to use the least amount of water for cooling purposes and thus conserve water usage. Simply stated, we need to reduce water consumption by reducing tower water bleed-off. We will discuss this process later when we discuss SCALE CONTROL.
The second biggest item after SCALE CONTROL is controlling bacteria in cooling towers. We call this process MICROBIOLOGICAL CONTROL. Cooling towers are great breeding grounds for bacteria. Inside the tower you have a warm, moist, sunlit environment that bacteria of all sorts love to thrive it. You also have a great environment inside chillers for other types of bacteria to grow. These bacteria can produce a material that can act as an insulating slime barrier to reduce the efficiency of the chillers in much the same way that calcium provides a solid barrier. In addition, certain types of bacteria called anaerobic bacteria because they thrive in the absence of oxygen, can grow and produce acids as part of their metabolism that can attack the steel components of the chillers.
Just as we measure calcium minerals in parts per million, we measure quantities of bacteria in Colony Forming Units per milliliter, abbreviated CFU/ml. The higher the count, the more bacteria you have. It is generally accepted that the maximum acceptable count in a cooling tower is 1,000,000 CFU/ml.
The simplest way to control bacteria is to rupture the cell walls that surround their cellular bodies. We use what are called oxidizing biocides to do this job. Examples of oxidizing biocides are chlorine and bromine which are frequently found in drinking water to control bacteria. We will discuss MICROBIOLOGICAL CONTROL and the GGP Silver Bullet Water Treatment System system later in this report.
The third item we will discuss is CORROSION CONTROL. CORROSION CONTROL is keeping corrosion rates in a cooling tower or chiller to the lowest rate possible. We measure corrosion rates in mils per year, abbreviated mpy. A mil is 1/1000 of an inch so a pipe that is 1 inch thick with a corrosion rate of 2 mpy will last 500 years before it finally is destroyed. Since the 2 primary metals found in chillers or towers are steel and copper, we use a method that will protect these metal surfaces by coating them with a protective film of calcium. We will discuss this mechanism later in our report.
Mechanisms of the GGP Silver Bullet Water Treatment System system
The GGP Silver Bullet Water Treatment System system has 2 key components, production of monatomic oxygen and filtration using special glass media.
The production of the monatomic oxygen is accomplished by modifying the oxygen molecule found in air. By exposing the oxygen to an ultraviolet light source, the oxygen structure is changed, giving it a negative charge. This modified oxygen is then continuously drawn into the tower water by means of a vacuum. The oxygen stays dissolved in the tower water where it continues to circulate. Once in the water, some of the monatomic oxygen combines with the water to form hydrogen peroxide. The remaining monatomic oxygen stays dissolved in the water in much the same way that regular oxygen can stay dissolved but it carries with it a negative charge.
The monatomic oxygen has an ability to stay dissolved in water to a much greater degree, however, than ordinary oxygen because it is attracted to the positively charged part of the water molecule.
The second part of the process involves filtration. Filtration plays a key role in the removal of dirt particles. Dirt particles generally enter in a cooling tower because it is suspended in the air, gets washed out as air flows through a cooling tower and then becomes suspended in the tower water itself. We call this dirt a suspended solid because it can be removed by filtration. Dirt can play a harmful role in cooling tower water by not only acting as a harbor for bacteria to grow, but also by allowing anaerobic bacteria to grow in cooling towers where those bacteria produce acids that destroy the tower metallurgy. Dirt particles also produce deposits in chillers to rob them of their efficiency. Dirt can also affect the efficiency of plate and frame heat exchangers that are used in arid climates to significantly reduce energy costs in colder weather. If dirt is allowed to build up in the narrow spaces between the stainless steel plates, water flow is reduced and energy costs go up.
Comparison of the GGP Silver Bullet Water Treatment System system with conventional chemical treatment
Conventional chemical treatment depends upon the continuous feed of assorted chemicals to prevent the formation of scaling crystals by altering their scaling tendencies. Chemicals are fed continuously because a portion of the tower water is regularly sent to the drain (bleedoff) to keep the calcium levels in a certain range. The bleedoff process brings in fresh makeup water with lower calcium levels which dilutes the concentration of calcium in the tower water. The bleedoff process was previously described on page 2 with the main point being that reducing the bleedoff rate saves water but also increases the level of calcium in the water.
The GGP Silver Bullet Water Treatment System system uses the strong negative charge of the monatomic oxygen to bond with the positive charge of the calcium ion. The net result is that the calcium ions stay dissolved in water in much the same way as with conventional chemical treatment.
Keeping the calcium in a dissolved state prevents calcium from forming harmful deposits in chillers. It is important that we have a means of confirming that the calcium in the tower water is dissolved. We use field or laboratory tests as our means of verifying the existence of the dissolved calcium by comparing the ratio of dissolved calcium in the tower water to the amount of calcium in the makeup water. We are back again to measuring Cycles of Concentration (COC). We need to have another dissolved ion that remains in a dissolved regardless of temperature to determine what the real cycles of concentration are. That is the chloride ion and our field test also included testing makeup and tower water for chlorides as well.
We can then compare chloride to calcium COC and see if they are the same. If they are the same, then the calcium in the tower is remaining dissolved. For example, if makeup water had 25 parts per million of chlorides and tower water had 125 parts per million (hereafter abbreviated as ppm) of chlorides, we would have a ratio of 5 to 1 or 5 chloride cycles of concentration (COC). If that same makeup water had 100 ppm of calcium and the tower water had 500 ppm of calcium, then we would also have 5 calcium COC.
What if the field test showed, however, 300 ppm of calcium instead of 500 ppm of calcium on the same tower water described in the previous paragraph? Where did the missing 200 ppm of calcium go? Why did we have 5 chloride COCand only 3 calcium COC? The key to the answer is in the fact that we are only measuring DISSOLVED calcium with our field test. The missing 200 ppm of calcium are in an non-dissolved state, otherwise called a crystalline state. This crystalline state is the form calcium takes when it is forming the damaging deposits in the chiller condenser tubes.
The GGP Silver Bullet Water Treatment System system at this test site not only matched the best conventional chemical treatment program by having equal calcium and chloride COC, it exceeded the standard by having greater calcium COC than chloride COC. If we have greater calcium than chloride COC, we are actually DISSOLVING old calcium deposits making for a cleaner surface and better heat transfer. It would be like removing deposits from an old house water heater so it would take less energy to heat up the water. Only in this case we would be getting rid of more heat at the cooling tower so the chillers would operate more efficiently and use less energy.
Filtration also plays a key role in removing old calcium deposits as well as filtering out bacteria. Removing these physical contaminants through continuous filtration not only makes the water the most efficient heat transfer media possible but also extends the life of the cooling tower and chiller.
What is even more remarkable is that in the past, the chillers with conventional chemical treatment had to be acid cleaned to remove the calcium deposits. We eliminated that expense of chemicals, wasted energy and labor associated with acid cleaning.
As previously mentioned, microbiological control is generally achieved with conventional chemical treatment using oxidizing biocides like chlorine or bromine. With the GGP Silver Bullet Water Treatment System program, we are electrochemically producing another type of oxidizing agent, hydrogen peroxide, to control bacteria. Using commercially available dip slides to measure Colony Forming Units per milliliter, we consistently registered bacteria counts in the low 1,000 CFU/ml. which is even lower than the limit considered acceptable with conventional chemical treatment. Another contributing factor to low bacteria counts was the continuous use of a bypass filter that removed food sources for bacteria as well as taking out bacteria through filtration.
Corrosion control is achieved by using a small amount of calcium on the metal surfaces as a type of coating against the corrosive action of the water. The coating action is continuous with small amounts of calcium depositing on the metal surface only to be swept away and replaced by a new coat. This sweeping and depositing action is necessary to prevent excessive layers of calcium from building up and affecting water flow and heat transfer.
Corrosion control was monitored using a conventional corrosion coupon rack. This rack had weighed metal “coupons” that when exposed to water over a period of time had some minor weight loss that was translated into mils per year. The coupon rack had a measured flow rate of approximately 4 gallons per minute. It also had a strainer upstream of the coupons to remove particles that could interfere with the test like dislodged calcium deposits.
We were so effective in removing vast quantities of harmful calcium deposits that had built up over several years that the strainer frequently became clogged with these deposits and the flow rate dropped from 4 gallons per minute to zero. Since coupons must be continuously bathed with tower water, a drop in the flow rate resulted in stagnant water that gave false higher corrosion rates. We had corrosion rates that varied from 1 to 2 mils per year with mild steel and less than 0.2 mpy for copper when the flow rate was correct. These corrosion rates are considered very good by standard industry tests.
A more accurate measure of the success of the corrosion control was the appearance of the chiller metallurgy that showed virtually no evidence of corrosion.
We will continue to monitor corrosion rates bearing in mind that the coupon rack strainer has to be cleaned weekly to maintain consistent flow.
In summary, the data shows that the GGP Silver Bullet Water Treatment System system not only equaled parameters established over decades for quality water management for conventional chemical treatment but also exceeded those standards when it came to scale and microbiological control. These results were achieved without the addition of any chemical treatment. The cost of the program equaled that of conventional chemical treatment. The GGP Silver Bullet Water Treatment System system provided the additional benefit of removing airborne dirt that would reduce the efficiency of the plate and frame heat exchanger that saves so much energy in the winter months. The addition of the filter did not result in any increase in capital costs.