In order to understand the function of the TowerKlean® cooling tower water treatment system it is helpful to have a basic knowledge of the chemistry involved. Although no treatment chemicals are added to the water in the system, there are significant chemical reactions that take place as a result of the water contacting the patented process media.
The fundamental substances that cannot be chemically modified, changed or divided are called atoms. An atom is the smallest particle into which an element can be divided and still retain its identity. An atom consists of protons and neutrons in the nucleus. The protons carry a positive charge where the neutrons carry no charge. Surrounding the nucleus are electrons, which have a negative charge. In an atom, the number of electrons and protons are equal and the charges are balanced.
These electrons orbit the nucleus in various layers. Although atoms are balanced electrically through their charges, another factor enters into the stability of the atom. Certain numbers of electrons (eight in most atoms) are desired in the outer most shell or orbit. To satisfy this need, electrons are either taken on, given up or shared by atoms. When these changes take place, an electron imbalance results and the atoms become ions. Ions either have a resulting positive or negative overall charge and readily combine with other ions to become compounds. The act of giving up electrons is called oxidation and taking on additional electrons to stabilize the outer orbit is called reduction.
Example: When an iron (ferrous) ion with a charge of +2 loses an electron, it becomes ferric with a charge of +3. This is the chemical reaction when rust is formed.
These reactions are called oxidation-reduction reactions and the ability of a water solution to effect these reactions can be measured by instrumentation. This measurement is called the oxidation-reduction potential or ORP.
In addition to ORP reactions, we have acid/base neutralization. Water consists of two atoms of hydrogen and one atom of oxygen. When combined, ionization of hydrogen takes place. The hydrogen donates one electron to the remaining hydrogen and oxygen. The chemical formula for water is H2O. To express the water molecule as it relates to ionization we can write it HOH.
When water is ionized we have hydrogen ions H+ and hydroxyl radicals (a radical is an ion that consists of more than one element) (OH)- in equal numbers. Any time we have an excess of hydrogen ions we have an acid and when we have an excess of hydroxyl radicals we have a base. The measurement scale that is used to determine this ratio is called pH, which is actually the measurement of free hydrogen ions. On the pH scale, which runs from 0-14, 0 represents the most acidic, 14 is the most basic and 7 represents neutral. At a pH of 7 there is an equal number of hydrogen ions (H+) and hydroxyl radicals [ (OH)-].
We can neutralize an acid with a base and the result is water and salt. For example: If we take hydrochloric acid (HCl) and sodium hydroxide (NaOH) combined in the proper ratios we will result in sodium chloride (common table salt) and water.
The chemical equation for this reaction is expressed as:
HCl + NaOH > NaCl + H2O
If expressing the equation showing the ions in the reaction is shown:
H+ Cl- + Na+ (OH)- > Na+ Cl- + H+ (OH)-
The oxidation/reduction and acid/base neutralization reactions are the basic reactions that take place with the TowerKlean® media.
As we discussed the oxidations and reductions take place as a result of the electron flow between the dissimilar metals in the media. This is the same phenomenon that takes place in a battery with two dissimilar metals and an electrolyte. The conductive water in the cooling tower serves as the electrolyte.
Metals are listed as to their positive or negative characteristics in the electromotive series of the elements. By knowing their placement on this chare we can accurately determine the actual voltage that will be generated. In the case of copper and zinc the potential difference is +1.1 volts. Since the copper is more noble (higher on the series) the zinc is sacrificed in the same manner as in zinc plating or galvanizing. The zinc is sacrificed in ionic form and is present in the low concentrations in the water.
The discovery of the reaction that eventually led to the TowerKlean system was made when it was found that free chlorine (Cl2) could be reduced to chloride (2Cl) when exposed to a copper/zinc alloy. Although the free chlorine appeared to disappear, it was eventually found to be present in an ionic form of chloride (Cl-). The observation of this reaction led to the discovery of other reactions that could benefit the treatment of cooling tower water and several other commercial applications. Heres how it works:
Scale in your cooling tower and related equipment such as heat exchangers is the result of hardness reacting with alkalinity. Hardness exists primarily as calcium and magnesium ions that are present in nearly all water supplies. As the water evaporates in a tower these ions are concentrated.
Alkalinity exists as a result of dissolving carbon dioxide from the air and creating carbonate radicals (CO3)-2. Once levels reach a saturation point, in relation with the hardness and temperature, hard lime scale will form as the compound calcium carbonate (CaCO3). This mineral can have more than one crystal structure, the most common of which is calcite. Calcite is a coarse crystalline structure that readily adheres to surfaces in the form of scale. In addition to the ability to continue to grow upon itself and plug openings, calcite scale is a heat insulator. One tenth of an inch of calcite can reduce the heat transfer efficiency by 40% and after its formation it is insoluble in water.
The TowerKlean® system reacts with the dissolved calcium to cause the formation of a different mineral form of calcium carbonate called aragonite. In this form, the calcium carbonate precipitates from the water into a small particle that is rounded and does not adhere to itself. This aragonite particle is chemically identical to calcite even though the crystal has been modified. The actual mechanism of the modification involves the soluble zinc ions present in the water. The zinc attaches to the forming carbonate crystal and forces it to change shape. It also begins to modify the structure of the existing calcite scale, causing it to fracture and begin to slough off. The precipitated particles and old scale deposits are then removed by the filter and backwashed to the sewer.
The magnesium hardness does not react the same way as the calcium. It is controlled in the TowerKlean® by backwash interval and blow down rate. A ratio must be maintained between the calcium and magnesium in order to control potential scaling.
To regulate the pH, the reaction media performs an unusual function. Dissolved oxygen is combined with free hydrogen ions to generate hydroxyl radicals. This elevates the pH by converting the acid (excess H+) to a base [excess (OH)-]. It is interesting to note that the pH will only rise to a certain level and then it stabilizes. This pH value is typically between 8.5 9.0. At this pH, corrosion is minimized.
Corrosion is also minimized as a result of zinc compounds that are formed and deposited on metallic surfaces. At the pH of 8.5 and 9.0 excess dissolved zinc forms hydroxide and sulfate compounds that coat the metal surfaces with an anodic anti-corrosion layer.
Finally, biological activity is controlled by several mechanisms:
First, the free cells that circulate through the media bed in the reaction column are exposed to a severe ORP shock at the surface film of the media. This alone is sufficient to kill most of these organisms.
Second, the copper in the media is toxic to algae and bacteria, on contact.
Third, the soluble zinc, released from the media is also toxic to algae and bacteria.
In summary, cooling tower water must be conditioned or controlled to address four areas. These are; scale formation, corrosion, pH control and biological fouling. The TowerKlean® system addresses all of these areas and is an effective alternative to chemical treatment without the costs, safety concerns and environmental hazards of chemicals.
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