Assignment:
The cooling water flow rate to the condensers of a big coal-fired power plant is 8970 kg/s. The water enters the condensers at 29°C and leaves at 45°C. From the condensers, the water flows to a cooling tower, where it is cooled down back to 29°C by countercurrent contact with air. The air enters the cooling tower at the rate of 6500 kg/s of dry air, at a dry-bulb temperature of 30°C and a humidity of 0.016 kg of water/kg of dry air. It leaves the cooling tower saturated with water vapor at 38°C
(a) Calculate the water losses by evaporation in the cooling tower.
(b) To account for water losses in the cooling tower, part of the effluent from a nearby municipal wastewater treatment plant will be used as makeup water. The makeup water contains 500 mg/L of dissolved solids. To avoid fouling of the condenser heat-transfer surfaces, the circulating water is to contain no more than 2000 mg/L of dissolved solids. Therefore, a small amount of the circulating water must be deliberately discarded (blowdown). Windage losses from the tower are estimated at 0.2% of the recirculation rate. Estimate the makeup-water requirement.
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