Once-through system

Traditionally, industry has tended to develop in areas with an adequate supply of cooling water. Originally, it was sufficient to pipe water through the plant and discharge it back to its natural source. Only nominal attention was paid to control of water chemistry, and it is in fact economically ridiculous to attempt the chemical treatment of large volumes of once-through water. 

However, even such minor additions as the threshold treatment with 1 to 2 ppm sodium hexametaphosphate, for example, would be unacceptable to modern environmental standards. Many coimtries, states, or provinces now forbid the return to source even when the concentrations of natural constituents were higher in the intake water than in the return water. In some areas, thermal pollution is forbidden since the discharge of the same water at a higher temperature than the inlet temperature may be harmful to certain marine species (e.g., oyster beds). 

The consensus today for once-through cooling systems seems to be that, where permitted, the materials of construction must be chosen to be resistant to the water, be it fresh water or seawater. All natural 

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Fig. 1. The energy cycle of a thermal electric generating station having two alternative cooling systems (—) the open-circuit or once-through system and (-------------------------------------) a representative closed-cycle, cooling-tower system. Reproduced by permission (3).

Inhibitors The use of various substances or inhibitors as additives to corrosive environments to decrease corrosion of metals in the environment is an important means of combating corrosion. This is generally most attractive in closed or recirculating systems in which the annual cost of inhibitor is low. However, it has also proved to be economicaUv attrac tive for many once-through systems, such as those encountered in petroleum-processing operations. Inhibitors are effective as the result of their controlling influence on the cathode- or anode-area reactions. 

FIGURE 4.41 Cooling water system (once-through system). 

No proposal for a laboratory ventilation system should be requested without a thorough study of the work to be performed. A once-through system may not always be required. The manager of a research laboratory, working closely with a designer, discovered that 50% recirculation would be permissible in their new building. While this is even less than in offices and stores, it proved to be adequate for their type of work. He would not recommend this ratio for other laboratories without a careful study. 

Figure 8-3 Three basic types of cooling water systems. Top the once-through system where the cooling water is used once and then discharged. Middle the open recirculation system where the water is cooled and recycled through a system in which it comes in direct contact with air. Bottom a closed recirculation system where the water is cooled and recycled without coming in direct contact with the atmosphere.

Treating Costs for Once-Through System Flow, GPM Daily Cost for Antifoulant, ... 

In a once-through system, the feedwater entering the unit absorbs heat until it is completely converted to steam. The total mass flow through the waterwall tubes equals the feedwater flow and, during normal operation, the total steam flow. As only steam leaves the boiler, there is no need for a steam drum. 

Noncontact cooling water is normally supplied to several processes from the utilities area. The system is either a loop that utilizes one or more evaporative cooling towers, or a once-through system with direct discharge. 

From this information determine how to maximize the fractional yield of R, or from a feed stream of 10 m /hr of C o = 350 mol/m. Cell or product separation and recycle are not practical in this system, so only consider a once-through system. Present your answer as a sketch showing reactor type, reactor volume, Cr in the exit stream, and the moles of R produced/hr. 

The use of various substances as additives to process streams to inhibit corrosion has found widespread use and is generally most economically attractive in recirculation systems, however, it has also been found to be attractive in some once-through systems such as those encountered in the petroleum industry. Typical inhibitors used to prevent corrosion of iron or steel in aqueous solutions are chromates, phosphates, and silicates. In acid solutions, organic sulphides and amides are effective. 

Results are shown graphically in Figure 4 for a brine temperature of 220°F., condenser tube velocity of 5 feet per second, blowdown temperature of 90°F., and brine concentration of twice sea water. As can be seen, a minimum water cost for these conditions is obtained with a 50-stage plant operating with a terminal temperature difference of about 4°F. Similar calculations were made for a blowdown concentration of 1.5 times sea water and for a once-through system. By cross plotting, it was then possible to determine the optimum blowdown salt concentration for the plant. It was about 1.7 times sea water. However, the curve is almost flat in the range of 1.5 to 2.0 times sea water. 

The kinetics of the microbiological kill reaction is more favorable for bromine. Thus bromine tends to kill microbes more quickly, and for any disinfection time period it will achieve a higher magnitude of kill. This can be important for cooling systems with short disinfection contact times, e.g., once-through systems. 

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