Enhancing Performance of Cooling Tower
The dearth of water in power generation is a major concern. This has forced to design the recent power generation units with closed cycle cooling water system. Further, the quality of available water at different sites largely varies and leads to deterioration in the cooling system performance. The various causes of water quality parameters and the remedial measures to restore the water quality for optimized cooling tower performance is described in this paper…
In an evaporative cooling water system during the process of cooling, water evaporates leaving the dissolved solids originally present in the water. The dissolved solid remains will increase, is not controlled to lead to corrosion, scale, deposition or biological fouling. This will affect the heat transfer and durability of system components.
Corrosion: Corrosion is an electrochemical oxidation process. This will destruct the metals of the cooling system equipment such as copper, etc. If this is not controlled, this will lead to equipment failure, plugging of corroded passages in condenser resulting in decreased heat transfer and energy efficiency. Corrosion products deposit on heat transfer area acts a layer of insulator and decreases the thermal conductivity.
Scale: Scaling is a chemical process resulting of increase in dissolved salts concentration in cooling water. This increased concentration of dissolved salts exceeds its solubility limit and precipitates in the forms of salts of calcium or magnesium. The insolubility becomes more with higher water temperature and prone to form deposits at cooling tower heat transfer surface such as fills. The scale formation reduces and blocks the water and air flow path and thus reduces the air water interaction inside the cooling tower and reduces the performance. Further, due to the scale formation on the fill material increases the weight of the fill substantially over a period of time and thus leading to collapsing of the fill packs leaving void spaces inside the cooling tower.
Deposition: Deposition in any cooling system is a cause of carryover of air borne suspended matter from the surroundings of the site. This may be due to process such as leakage of oil from coolers and suspended matters in the makeup water. The effect of this deposition will be similar to scaling, reduces the thermal insulation leading to poor heat transfer and less efficiency.
Biological Fouling: Slime and algae formations in heat transfer area such as fill and other structures of cooling tower may reduce heat transfer, promote corrosion, and harbor pathogens such as Legionella. This too will act similar to the above discussed scale and deposition.
Some of the water quality parameters and its impact on cooling system are given in the Table 1:
Water Quality Improvement
Corrosion and Scale Control
The specific measures required for corrosion and scale control vary from system to system and are largely dependent on the following:
i. Chemistry of the makeup water
ii. Metallurgy of the heat transfer devices in contact with the recirculating water and piping
iii. Operating temperature of the cooling water system.
Scale can be controlled or eliminated by application of one or more proven techniques.
Typical measures taken to control scale are:
i. Controlling cycles at a set level.
ii. Chemical scale inhibitor treatment.
iii. pH adjustment by acid addition.
iv. Softening of cooling water system makeup.
Controlling cycles at a set level
In normal conditions, the blow down or bleed which continues flow of a small portion of the recirculating water to drain is sufficient enough to control the concentration of dissolved solids and in turn to control scale and corrosion. Further installation of a high quality system for automatic blow down based on conductivity or metered makeup will control the cycles.
Chemical Scale Inhibitors
Most of the system requires chemical scale and corrosion inhibitors which raise the allowable level of dissolved solids without the risk of scale and corrosion. Chemical scale inhibitors function by either selective adsorption on growing scale crystals, converting the crystal structure into a non-scaling type which does not form a hard scale, or through chemical reactions with the scale forming ions, converting them into non-scale forming materials.
Control of scale with pH adjustment by acid addition functions via chemical conversion of the scale forming materials to more soluble forms. Thus, calcium carbonate is converted to calcium sulphate (using sulphuric acid for pH adjustment), a material several times more soluble. Normally, it is not desirable to add sufficient acid to convert all of the scale forming materials due to a substantial increase in the corrosivity of the cooling water if this is accomplished. Addition of excessive acid to the cooling water results in depressed pH values and extremely rapid corrosion of all system metals.
Scale can be completely eliminated by softening all cooling system makeup water. Using softened makeup water for scale control is the safest, most cost effective method available for obtaining high cycles, or zero blowdown, with hard makeup water. Normally, the added cost of softened makeup water is balanced by the decreased chemical and water usage resultant from the increased cooling system cycles made possible by the soft water.
Controlling of corrosion and scale can be achieved by maintaining the following water parameters given in Table 2:
Chemical treatment requirements
Chemical treatment needs the following:
i. Compatibility of the chemicals to be used with the materials of construction, pipes, heat exchanger, etc. of the cooling system.
ii. Automatic feeders must be used to introduce chemical scale and corrosion inhibitors into the circulating water system. The feeding point should be such a way that it ensures total mixing and dilution before reaching the cooling equipment. Widely preferred location in a cooling system is at the discharge side of the circulating pump. As these chemicals can severely damage areas directly contacted, these should not be batch fed directly into the cold water basin or water distribution system.
iii. If chlorine is being added to the system it must be ensured that it does not exceed 1 ppm as exceeding this limit may accelerate corrosion.
Measures taken to control deposition depend on the cause of the problem. Process contamination problems are best corrected by elimination of the process leakage, while most suspended solids deposition can be controlled by addition of dispersant/surfactant chemicals to the cooling water. These materials function by charge neutralization of the suspended particles and emulsifying binding agents, breaking up existing deposits and preventing agglomeration of the particles to form new deposits.
Heavier concentration of suspended solids deposition can be treated with a combination of chemical dispersants or surfactants and an element filter, hydrocyclone or media filter in a side stream configuration. 2 to 35 microns range of suspended solids in recirculating water is mot difficult to control. Below 2 microns dispersants are the effective control for deposits. Above 35 micron level, simple sedimentation of hydrocyclones is effective. Automatic backwashing filters are most effective for removal of suspended solids in the critical range of 2 to 35 microns. If a cooling system need to be operated at over six cycles of concentration, bypass filtration is required. In heavy dusty atmospheric conditions, such filtration is required regardless of the chemistry or cycles of concentration at which the cooling system is being operated.
Biological Fouling Control
Formation of anaerobic areas under the biological fouling layer results in a substantial corrosion rate increase. Widely used biocides classified in two major classes (i) oxidizing and (ii) non-oxidizing.
Cellular structure of the bio fouling organism is effectively destroyed and killed by chemical oxidation by oxidizing biocides. As this is a destructive form of action, it is impossible for any organism to show, or develop, significant immunity to an oxidizing biocide. Oxidizing biocides are usually quite cost effective due to their low unit cost, rapid effect on the target organism, and low effective dosage. Oxidizing biocides have the following disadvantages:
i. Some of them can decrease cooling water pH in an uncontrolled manner.
ii. Most of them increase the corrosive nature of the cooling water.
iii. Oxidizing biocides such as chlorine produce undesirable by-products from an environmental viewpoint.
iv. Some corrosion and scale control chemicals can be inactivated by contact with specific oxidizers.
v. None of the oxidizing biocides have any dispersant effect for removal of deadmicrobiological growth.
Non-oxidizing biocides are generally quite costly due to the high effective dosage, long contacttimes, and often high unit cost.
Non-oxidizing biocides do have advantages over the oxidizing biocides:
i. No effect on corrosivity is evident from their use.
ii. Do not interfere with corrosion and scale control chemicals.
iii. Specific type of organism can be targeted and treated.
iv. Definite dispersant effect for removal of dead microbiological growth.
Cooling water quality plays a vital role in maintaining the performance of the cooling tower and associated equipment. Poor cooling water treatment and control increases facility costs via destruction of expensive equipment, damage to the facility, increased costs for water and sewerage, and increased energy use and cost. A proper water treatment program, administered under the supervision of a competent water treatment specialist, is an essential part of routine maintenance to ensure the safe operation and longevity of evaporative cooling equipment, as well as other system components. Selection of a cooling water management program and knowledge of the chemistry and controls required will increase the probability of obtaining reliable equipment cooling with maximum heat transfer efficiency at the lowest total cost. Those facilities that devote sufficient time and resources to this vital area will be reaping the benefit of lowered costs and more reliable operation.
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