Cycles of concentration

Alkalinity Reduction. Treatment by lime precipitation reduces alkalinity. However, if the raw water alkalinity exceeds the total hardness, sodium bicarbonate alkalinity is present. In such cases, it is usually necessary to reduce treated water alkalinity in order to reduce condensate system corrosion or permit increased cycles of concentration. 

Scale control can be achieved through operation of the cooling system at subsaturated conditions or through the use of chemical additives. The most direct method of inhibiting formation of scale deposits is operation at subsaturation conditions, where scale-forming salts are soluble. For some salts, it is sufficient to operate at low cycles of concentration and/or control pH. However, in most cases, high blowdown rates and low pH are required so that solubihties are not exceeded at the heat transfer surface. In addition, it is necessary to maintain precise control of pH and concentration cycles. Minor variations in water chemistry or heat load can result in scaling (Fig. 12). 

Removal of Particulate Matter. The amount of particulate entering a cooling system with the makeup water can be reduced by filtration and/or sedimentation processes. Particulate removal can also be accompHshed by filtration of recirculating cooling water. These methods do not remove all of the suspended matter from the cooling water. The level of fouling experienced is influenced by the effectiveness of the particular removal scheme employed, the water velocities in the process equipment, and the cycles of concentration maintained in the cooling tower. 

Blowdown discards a portion of the concentrated circulating water due to the evaporation process in order to lower the system solids concentration. The amount of blowdown can be calculated according to the number of cycles of concentration required to limit scale formation. Cycles of concentration are the ratio of dissolved sohds in the recirculating water to dissolved solids in the makeup water. Since chlorides remain soluble on concentration, cycles of concentration are best expressed as the ratio of the chloride content of the circulating and makeup waters. Thus, the blowdown quantities required are determined from... 

Cycles of concentration involved with cooling-tower operation normally range from three to five cycles. Below three cycles of concentration, excessive blowdown quantities are required and the addition of acid to limit scale formation should be considered. 

Removing suspended solids, decreasing cycles of concentration, and clarification all may be beneficial in reducing deposits. Biodispersants and biocides should be used in biofouled systems. Simple pH adjustment may lessen precipitation of certain chemical species. The judicious use of chemical corrosion inhibitors has reduced virtually all forms of aqueous corrosion, including underdeposit corrosion. Of course, the cleaner the metal surface, the more effective most chemical inhibition will be. Process leaks must be identified and eliminated. 

Water treatment to be used/cycles of concentration Process... 

Cycles of concentration Blowdown/purge rate Power cost analysis Drift loss requirement Local authority requirements Discharge qualities Stmctural specifications Pack specifications... 

Lack of dealkalization with high alkalinity MU water NOTE Anion dealkalizers do not use acid, but also do not reduce MU water TDS and introduce additional chlorides (risk of pitting). Excessive CR line corrosion BW carryover Low cycles of concentration higher operating costs... 

Where acid process contaminant or acidic water ingress occurs, the bulk BW pH may quickly drop as low as 5.0. This may occur, for example, where there is only limited alkaline buffering of BW because of low cycles of concentration and/or high-purity FW. Where acidic incursions occur, a general thinning of the boiler tubes, drums, and shell rapidly takes place, giving rise to a clearly visible irregular surface. 

Where the alkaline constituents of the FW are low, or where simply raising BW cycles of concentration fails to provide adequate BW alkalinity without incurring risks of foaming, carryover, or similar problems, the use of an alkalinity booster (alkalinity builder) is recommended. 

FW values for marine propulsion, WT, oil-fired, drum-type boilers assume 100 cycles of concentration (COC) to BW and are not restricted to any specific MU water pretreatment. 

NOTE This guideline refers to 100% chelant present in the boiler. Where, for example, 38% EDTA solution is used, the maximum permitted chelant product concentration is 26.3 ppm (mg/kg). If the cycles of concentration in the boiler is 20x, the product is injected, proportional to FW demand, at a feed rate of approximately 1.3 ppm, direct to the boiler. 

Very high cycles of concentration (high BW conductivity)... 

NOTE This uses a chelant cleaner for phosphate-based programs. Although the feed rate is theoretically based on meeting the chelant demand, in practice, however, much of the demand is satisfied by the existing phosphate precipitant. The bulk of the EDTA is available to strip off old calcium deposits. Cycles of concentration should be limited. Use this formulation at 300 to 400 ppm in the boiler. At 400 ppm product, 30 ppm chelant is provided. 

NOTE This is a chelant cleaner with added phosphate. It is necessary to attempt some precision in calculating the chelant demand to ensure adequate excess for online cleaning. Minimum level of300 to 400ppm is maintained in the boiler, with low cycles of concentration. The additional BD reduces the risk of simple transport and redeposition of old deposits. At 400 ppm, this product provides 30 ppm of EDTA and 13 ppm of SHMP. 

High levels of silica in the raw water supply can lead to serious risks of deposition in boilers, especially if cycles of concentration (COC) also are high. The incoming silica can be reduced by adsorption on magnesium hydroxide [Mg(OH)2] precipitate during lime-softening processes, or by the addition of magnesium hydroxide in a reaction tank, followed by filtration. 

AVT Barg BD BDHR BF BOF BOOM BOP BS W BSI BTA Btu/lb BW BWR BX CA CANDUR CDI CFH CFR CHA CHF CHZ Cl CIP CMC CMC CMC COC All-Volatile treatment bar (pressure), gravity blowdown blowdown and heat recovery system blast furnace basic oxygen furnace boiler build, own, operate, maintain balance of plant basic sediment and water British Standards Institution benzotriazole British thermal unit(s) per pound boiler water boiling water reactor base-exchange water softener cellulose acetate Canadian deuterium reactor continuous deionization critical heat flux Code of Federal Regulations cyclohexylamine critical heat-flux carbohydrazide cast iron boiler clean-in-place carboxymethylcellulose (sodium) carboxy-methylcellulose critical miscelle concentration cycle of concentration... 

As evaporated water is pure, solids are left behind in the recirculating water, making it more concentrated than the makeup water. The blowdown purges the solids from the system. Note that the blowdown has the same chemical composition as the recirculated water. Cycles of concentration is a comparison of the dissolved solids in the blowdown compared with that in the makeup water. For example, at three cycles of concentration, the blowdown has three times the solids concentration as the makeup water. For calculation purposes, blowdown is defined to be all nonevaporative water losses (drift, leaks and intentional blowdown). In principle, any soluble component in the makeup and blowdown can be used to define the concentration for the cycles, for example, chloride and sulfate being soluble at high concentrations can be used. The cycles of concentration are thus defined to be ... 

Figure 24.3 shows the relationship between the makeup, blowdown, evaporation and drift versus the cycles of concentration. For a given design of cooling tower, fixed heat duty and fixed conditions, the evaporation and drift will be constant as the cycles of concentration increase. However, as the cycles of concentration increase, the blowdown decreases and hence the makeup water decreases. 

The consequences of an increase in the cycles of concentration are that as the level of dissolved solids increases, corrosion and deposition tendencies also increase. The result is that, although increasing the cycles of concentration decreases the water requirements of the cooling system, the required amount of chemical dosing also increases. 

Improving control of cooling tower blowdown (see Chapter 24) for evaporative cooling water circuits to increase the cycles of concentration and reduce the cooling tower blowdown rate. 

Today, particle accelerators and computers are as much apart of astronomy as telescopes intent on spying out the visible and the invisible. In their accelerators, high-energy physicists are able to reproduce conditions in the Big Bang and in the stellar core. Then, taking over from them, numerical simulation by computer can write the story of matter through its various cycles of concentration, nucleosynthesis and dispersion. 

For example, a cooling tower with water containing four times as much total dissolved solids as its makeup supply would be operating at four cycles of concentration. The cycles of concentration are determined by the cooling tower design, water characteristics, operating conditions and the type of treatment system employed (cooling tower water treatment is discussed in detail in Chapter 8). 

Cycles of concentration compare the concentration of the dissolved salts in the circulating water with that of the makeup water. Usually, the circulating water salt content is limited to 3-7 times the makeup salt content (this is referred to the number of concentrations of 3-7). 

The chemical composition of the cooling water makeup supply used in the plant determines the choice of the cycles of concentration. Some of the important constituents that must be controlled in the tower are calcium, magnesium, silica, carbonate, bicarbonate and sulfate ions. Alkalinity levels are regulated by the addition of acid or alkali to achieve the desired pH. When adding H2S04 (sulfuric acid) for pH control, it should be assured that calcium sulfate solubility limits are not exceeded (see Chapter 8). 

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