: Products : Cooling Water Treatment
Cooling Water Treatment
Cooling water treatment programs are designed to achieve control of corrosion and scale as well as microbiological fouling and contamination in open re-circulating system, closed re-circulating system and in once through cooling system.
Corrosion and scale lead to:
- Leakage in exchangers
- Unscheduled shutdown
- Reduction in water flow
- Loss of production
- Poor heat transfer
- Reduction in plant load
Microbiological growth also leads to microbiological induced corrosion, damage to wooden parts of cooling tower.
Our wide arrays of products are designed to achieve the following:
- Controlled corrosion
- Enhanced equipment and pipeline life
- No microbiological fouling
- No shutdown/production losses
- Greater water conservation
Sub divisions of Cooling Water Treatment:
1) Corrosion inhibitors
The corrosion process:
Corrosion is an electrochemical process in which a difference in electrical potential develops between two metals or two parts of a single metal. This difference in potential allows current to pass through the metals causing reactions at anodic and cathodic sites. Anode is the region of lower potential whereas cathode is the region of higher potential. At anode, metal ions go into solution. The lower the potential of anode, the greater the amount of metal dissolution and more serious the corrosion problem. Most common types of corrosion include the general corrosion, pitting type corrosion, galvanized corrosion, stress corrosion cracking, crevice corrosion and erosion corrosion. Corrosion in water is accelerated by chloride, dissolved oxygen in water, high temperature, suspended solids, bacteria and low water pH.
Corrosion inhibitors are substances which when added in small amounts to a corrosive medium, reduce the rate of corrosion. Inhibitors are classified as anodic, cathodic or both depending on which portion of the electrochemical corrosion cell they disrupt or polarize.
Types of corrosion inhibitors include oxide film types which include nitrite and molybdate based. They may also form insoluble salts with ions in water or with the protected metal ion e.g. polyphosphates, orthophosphates, phosphonates, zinc etc. or the aromatic azole types.
- Anodic inhibitors - nitrite, silicate, orthophosphate
- Cathodic inhibitors - Polyphosphates, Zinc, calcium carbonate
- Both - filming amines, phosphonates
Choice of suitable inhibitor depends on the cooling system design parameters, water composition, type of metals in the system, stress conditions, water velocity pH, dissolved oxygen and salt and suspended matter composition.
2) Scale/Deposit controllers
Deposits are a conglomeration of many constituents that can range from settled silt form airborne dust blown into the cooling tower to precipitated hardness salts which result from changes in temperature or in concentration characteristics of make-up water. Typical components of scale found in cooling water systems include calcium carbonate, calcium sulphate, calcium and zinc phosphate and silica and magnesium silicate. These scale components tend to deposit and adhere as scale on heat transfer surfaces with higher temperature because their solubilities decrease as pH and water temperature increase.
Since the thermal conductivity of the scale is extremely low in comparison with that of the tube material, the scale adhesion lowers the thermal efficiency of the heat exchangers.Cooling water deposit control agents function via Crystal growth distortion, Dispersion, Sequestration etc.
The present popular scale inhibitors include:
- Organic acids
These have been widely used as scale control agents and metallic control sequestrants. They provide excellent control of hydrated ferric oxide deposits at relatively low concentrations. They are also excellent for the control of hardness salts and are capable of better threshold inhibition than polyphosphates and phosphate esters. Phosphonates also serve as chelants or sequestrants for control of heavy metals (iron, copper and zinc) deposits. Most commonly used molecules are ATMP and HEDP.
Most commonly used phosphinocarboxylate is PBTC. This compound is an outstanding calcium carbonate scale inhibitor and shows excellent chemical stability in the presence of chlorine.
Gluconic acid is a particularly good sequestrant for iron in alkaline systems.
Low molecular weight polymers are used almost exclusively in cooling water treatment applications, especially when dispersion or crystal modification is the desired mechanism. Medium to high molecular weight polymers are generally the choice where flocculation of suspended solids is required. Commonly used polymers include polyacrylates, acrylic acid co-polymers, sulphonated polymers, polymaleic anhydride etc.
Polymeric products can be tailor- made to maximize dispersant on specific foulants.
Our common products in this category include DT-2625A, DT-1000, DT-2635B, KW-1002, CD DS HPA, CD DS 5004/6004 etc.
Micro-organisms which inhabit commercial or industrial cooling water systems can adversely affect the efficiency of the operation either by their sheer numbers, metabolic waste products generated or deposits created. The cooling tower is an excellent example of a contained water system that provides optimum conditions for microbial growth. Temperature and pH are usually within the ideal range and there is generally an abundance of nutrients in the form of organic matter, inorganic salts and sunlight required for their growth. Microorganisms enter a cooling system either through the make up water supply itself or from the air passing through the cooling tower. The microbial flora which normally inhabit the cooling waters include aerobic capsulated bacteria such as Aerobacter, Flavobacterium and Pseudomonas which are generally associated with slime, aerobic spore formers, aerobic sulfur bacteria notably Thiobacillus which reduces the pH due to oxidation of sulfur and sulfides to sulphate and sulfuric acid, sulfate reducing bacteria which reduce sulfates to hydrogen sulfide causing pitting type corrosion, plugging and reduced heat transfer.
Microorganisms usually contribute to under deposit corrosion. A slime-mass or a combination slime/inorganic salt mass produce a differential aeration cell. The area under the mass will therefore become actively anodic causing severe localized attack. Nitrifying bacteria oxidize nitrite to nitrates. This is most serious in closed re-circulating systems which use nitrite as a corrosion inhibitor.
Microbiocides inhibit microorganisms in a number of ways:
- Alteration of cell wall permeability
- Adsorption to cell membrane.
- Denauration of protein
- Inhibition of protein synthesis
- Oxidation of protein group
Microbiocides are classified as oxidizing and non-oxidizing:
These chemicals oxidize or accept electrons from other chemical compounds. The commonly accepted biocides in this category include chlorine, chlorine dioxide, chlorine donors and ozone.
Chlorine is widely used because of its efficacy and low cost. Chlorine gas dissociates instantaneously in water to form HOCl and HCl. pH of water determines the proportions of each. HOCl is about 80 times more effective in killing bacteria than OCl- . The mode of action of chlorine involves oxidation of microbial cell components.
Chlorine dioxide is emerging as a capable alternative to chlorine. Unlike chlorine, it does not dissociate in water, therefore its germicidal activity is relatively constant over a broad pH range. It reacts with organics by oxidation. It does not react with ammonia-nitrogen compounds as does chlorine nor does it form trihalomethane.
Bromine chemistry represents cost-effective oxidizing microbial control especially in alkaline waters and where nitrogen compounds may be present as a contaminant in the cooling water stream.
Compounds commonly used for slime control in cooling systems have two general modes of action against microbiocides. One mode of action results from chemicals with surface active properties. These biocide cause damage to the cell-membrane. Another general mode of action involves causing lesions in the metabolic production or use of energy.
CAWTL ranges of biocides include:
- KW-304 – QAC based
- KW-3010/KW-3010A – Dichlorophene based
- DT-280 – carbamate base
- DT-284 – methylenebisthiocyanate based
- DT-229 – gluteraldehyde based
- DT-250 – isothiazoline based