The cooling water flow rate to the condensers of a big coal-fired power plant is 8,460 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 6,700 kg/s of dry air, at a dry-bulb temperature of 30 C and a humidity of 0.014 kg of water/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. The pressure in the tower is assumed to be constant at 101.3 kPa.
(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 530 mg/L of dissolved solids. To avoid fouling of the condenser heat-transfer surfaces, the circulating water is to contain no more than1,590 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.