Managing Cooling Water: Preventing Scale Deposits
This article, which explores methods for preventing scale deposits in cooling water systems, is the fourth in an occasional series on water management basics and technologies.
Scale deposits typically occur when the concentrations of dissolved salts in the cooling water exceed their solubility limits and accumulate on surfaces that are in contact with the water. The most common scale formers, calcium salts, exhibit reverse solubility in that they become less soluble as the temperature of the water increases. This property causes scale formation in the most sensitive area — the heat transfer surfaces of production equipment.
Because the thermal conductivity of scale is substantially lower than metal, scale deposits typically reduce the heat removal capability of the equipment, and production speeds must be lowered to compensate. In extreme cases, enough material precipitates to physically block the cooling water passages, and the affected equipment must be removed from production for chemical (acid) or mechanical cleaning. Scale formation on the condensers of Freon cycle chillers reduces the efficiency of these units in chilled water systems, therefore requiring an increase in the amount of power needed to obtain a given volume of chilled water. Various studies have shown a nonlinear electrical power cost increase with increased scale thickness. For instance, 0.5 mil of calcium scale results in a power cost increase of 3.5 percent while 1.5 mil increases the power cost by approximately 12.5 percent.
Fortunately, scale can be controlled or eliminated by controlling the cycles (the number of times that the replacement water, or makeup, is increased in concentration), using a chemical scale inhibitor, adjusting the pH with acid additions, or softening the cooling water system makeup.
Controlling the Cycles
The cycles are best controlled by installing a high-quality system for automatic blowdown (the intentional removal of water from the cooling tower) based on the cooling water conductivity or metered makeup.
However, blowdown results in potential environmental problems, increased water use and wastewater disposal costs, so it should be minimized as much as possible. Zero blowdown can be achieved through the use of softened makeup and bypass filtration but presents substantial challenges with regard to corrosion and biological control.
Using Chemical Scale Inhibitors
Chemical scale inhibitors function either through selective adsorption on growing scale crystals, in which the crystal structure is converted into a type that does not form a hard scale, or through chemical reactions with the scale-forming ions, in which the ions are converted into nonscale-forming materials.
Table 1 lists some of the chemical scale inhibitors commonly used. As with corrosion inhibitors, mixtures of scale control chemicals generally provide better performance than single-component products. Typical formulations usually contain one or more phosphonates combined with polyacrylate. As a general rule, these formulations can be used as long as the saturation index (SI) value of the cycled cooling water does not exceed 2.0. (The SI can be easily calculated with a hand calculator or computer program.) Using co- and ter-polymers combined with surfactants can control cycled cooling water SI values up to 3.5. Multiple water treatment firms have reported that the operation of cooling systems with newer treatment chemistries are scale-free at cycled SI values from 2.5 to 3.5 without pH adjustments.
Adjusting the pH
Adjusting the pH with acid additions converts the scale-forming materials to more soluble forms. For example, adding sulfuric acid converts calcium carbonate to calcium sulfate, a material several times more soluble. However, adding excessive acid to the cooling water results in depressed pH values and extremely rapid corrosion of all system metals. For this reason, it is not usually desirable to add enough acid to convert all of the scale-forming materials.
The SI and the Ryznar saturation index (RSI) are used for system setup when acid additions are used for scale control. Both indexes are merely convenient means of reducing the integrated parameters of calcium, alkalinity, pH, dissolved solids and temperature to a single value that indicates the tendency of water to form a calcium scale or promote corrosion. A positive SI number (RSI less than 5.0) indicates a scale-forming water while a negative SI number (RSI greater than 7.0) indicates a scale-dissolving, or corrosive, water.
The normal practice is to maintain a slightly positive SI number of 0.2 to 0.5 (RSI between 5.0 and 6.0) when using acid additions and add a chemical scale inhibitor to cope with the resulting slight tendency to scale. Instances have been reported where scale has been controlled with makeup calcium water hardness values up to 3,000 mg/l as CaCO3 with a combination of acid additions and chemical scale inhibitors.
To avoid serious corrosion damage, acid should only be added to cooling water with an automatic pH control system and well-trained operators. Additionally, only high-quality pH controllers equipped with acid pump lockout timers should be considered for this critical application, and daily plant control testing is required.
Softening Cooling System Makeup Water
While not yet a common practice, scale can be completely eliminated by softening all cooling system makeup water. The added cost of softened makeup water may be offset by the decreased chemical and water use resulting from the increased cooling system cycles made possible by the soft water. (Note that operation above six cycles requires the use of bypass filtration to prevent deposition problems. See the sidebar, “Preventing Deposition.”)
The increased general corrosiveness of the softened water is countered by the high pH values (8.5 to 9.5) developed when the cooling system is cycled with softened makeup. When softened water is used in conjunction with a good chemical corrosion-inhibitor program, lower corrosion rates can be achieved compared to using chemical scale inhibitors alone or controlling the pH through acid addition. However, specific inhibitor technology for controlling white rust and corrosion of copper must be used with softened makeup water.
Softened makeup water also can be used to obtain zero blowdown discharge from cooling systems to reduce fresh water use and environmental concerns. However, facilities using softened makeup water for this purpose should consult a water management firm that has experience in this area due to the increased potential for corrosion and white rust, as well as a possible increase in deposition from the high number of cycles required.
Editor's Note: In Part 5 of this series, Mr. Keister will look at the effects of biological fouling on a cooling system.
SIDEBAR: Preventing Deposition
Deposition is a general term for problems in a cooling water system that are not due to scale, corrosion or biological activity. Deposition can result from airborne material that is scrubbed from the ambient air by the cooling tower, contamination of the cooling water due to a process leak (e.g., leaking oil coolers), or suspended material in the makeup water. It affects process operations much like scale, in that the deposits act as a thermal insulator to decrease heat transfer efficiency in the production equipment. Deposition also can cause physical blockage of cooling water passages and can increase corrosion rates by blocking corrosion-inhibitor access to the base metal, i.e., under-deposit corrosion.
Measures taken to control deposition depend on the cause of the problem. Process contamination problems are best corrected by eliminating the process leakage, while adding dispersant/surfactant chemicals to the cooling water can control deposition caused by suspended solids. These materials function by charge-neutralizing the suspended particles and emulsifying the binding agents, thereby breaking up existing deposits and preventing the particles from agglomerating and forming new deposits. The table lists some of the dispersant/surfactant chemicals commonly used.
Severe suspended solids deposition should be treated with a combination of chemical dispersants/surfactants and an element filter, hydrocyclone or media filter in a side stream configuration. A hydrocyclone set up to discharge through system blowdown is often preferred over a media filter due to the hydrocyclone's lower cost and ability to achieve a zero excess water loss operation. However, element filters using reusable bags or cartridges are finding increasing applications in zero-discharge cooling systems where blowdown must be eliminated for either water use reduction or environmental reasons.
Note that if a cooling system is operated at more than six cycles, some form of bypass filtration is required to prevent deposition and the resulting corrosion problems underneath deposits. In dusty environments, such filtration might be required regardless of the chemistry or number of cycles operated.
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