Cooling towers system balances

Process Technology 2—Systems—study of common process systems found in the chemical process industry, including related scientific principles. Includes study of pump and compressor systems, heat exchangers and cooling tower systems, boilers and furnace systems, distillation systems, reaction systems, utility system, separation systems, plastics systems, instrument systems, water treatment, and extraction systems. Computer console operation is often included in systems training. Emphasizes scale-up from laboratory (glassware) bench to pilot unit. Describe unit operation concepts solve elementary chemical mass/energy balance problems interpret analytical data and apply distillation, reaction, and fluid flow principles. 

This section reviews some basic definitions and formulas in thermodynamics. These definitions will be used to develop energy balances to describe cooling tower operations. In our discussions we will use the following terms system, property, extensive and intensive properties, and... 

The authors briefly discuss some of the problems for air cooled heat exchangers and cooling towers using axial fans. The balance of the paper discusses ways to improve system efficiencies in three areas before the fan system design is finalized, improvements in the physical equipment as installed, and recognition of performance problems caused by adjacent equipment. Results of a full-scale test illustrating fan efficiency contributions of various components are given. 1 ref. cited. 

Prior to making the decision to discharge RO reject to the cooling tower, an analysis should be conducted to determine what impact the reject will have on tower operations. There is a need to balance conservation and recovery of RO reject water with the impact on the cooling system. 

I. Set out material-balance equations for the cooling tower. When the system is at equilibrium, the makeup must equal the losses, so, by definition,... 

Refinery wastewaters generally are susceptible to conventional biological treatment methods after adequate pretreatment. In addition, phosphorus supplementation may be required in some cases to provide a nutrient-balanced system for biological treatment. This supplemental nutrient requirement will depend on the phosphorus content of the cooling tower blowdown and its inclusion in the process wastewater. 

With each pass through the system, some water is vaporized, so that some makeup water is necessary. The amount of water evaporated can be determined by running an energy balance. As water is added the concentration of minerals and other substances in the water increases, since they are not removed by evaporation and every pound of makeup water adds some more. To counteract this build-up, some water must be continuously removed from the system. This is known as blowdown. As a rule of thumb, the blowdown is about 0.3% of the water being recirculated for each 10°F (5°C) of cooling that occurs within the tower. This assumes a solids concentration in the water of 4-5 times that in the makeup water. In places where water is scarce and hard, a deionization system may need to be installed.13... 

Optimum Reflux Ratio. The reflux ratio affects the cost of the tower, both in the number of trays and the diameter, as well as the cost of operation which consists of costs of heat and cooling supply and power for the reflux pump. Accordingly, the proper basis for choice of an optimum reflux ratio is an economic balance. The sizing and economic factors are considered in a later section, but reference may be made now to the results of such balances summarized in Table 13.3. The general conclusion may be drawn that the optimum reflux ratio is about 1.2 times the minimum, and also that the number of trays is about 2.0 times the minimum. Although these conclusions are based on studies of systems with nearly ideal vapor-liquid equilibria near atmospheric pressure, they often are applied more generally, sometimes as a starting basis for more detailed analysis of reflux and tray requirements. 

A complete Type R tower is shown in Figure 2.22. This drawing illustrates the basic process and its essential auxiliaries as well as the external heat and material balance quantities, in this type of installation, sidestream draw trays are total draws, i.e., the total liquid leaving the draw tray—sidestream product plus reflux to the tray below—is withdrawn from the lower. The reflux is pumped back to the tower after cooling rather than overflowing internally from the draw tray to the tray below as in the case of Type U and Type A systems. Figure 2.22 will be the basis for discussing the heat and material balance calculations in this section. 

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