Thermodynamics

WARMWATER60 kg/s40°CAIRINLET1 atmT db = 22°CT wb = 16°C26°CCOOLWATERMakeupwaterAIREXITFIGURE P14–11234°C90%14–113 A wet cooling tower is to cool 25 kg/s of coolingwater from 40 to 30°C at a location where the atmosphericpressure is 96 kPa. Atmospheric air enters the tower at 20°Cand 70 percent relative humidity and leaves saturated at35°C. Neglecting the power input to the fan, determine(a) the volume flow rate of air into the cooling tower and(b) the mass flow rate of the required makeup water.Answers: (a) 11.2 m 3 /s, (b) 0.35 kg/s14–114 A natural-draft cooling tower is to remove waste heatfrom the cooling water flowing through the condenser of asteam power plant. The turbine in the steam power plantreceives 42 kg/s of steam from the steam generator. Eighteenpercent of the steam entering the turbine is extracted for variousfeedwater heaters. The condensate of the higher pressurefeedwater heaters is trapped to the next lowest pressure feedwaterheater. The last feedwater heater operates at 0.2 MPa andall of the steam extracted for the feedwater heaters is throttledfrom the last feedwater heater exit to the condenser operatingat a pressure of 10 kPa. The remainder of the steam produceswork in the turbine and leaves the lowest pressure stage of theturbine at 10 kPa with an entropy of 7.962 kJ/kg K. The coolingtower supplies the cooling water at 26°C to the condenser,and cooling water returns from the condenser to the coolingtower at 40°C. Atmospheric air enters the tower at 1 atm withdry- and wet-bulb temperatures of 23 and 18°C, respectively,and leaves saturated at 37°C. Determine (a) the mass flow rateof the cooling water, (b) the volume flow rate of air into thecooling tower, and (c) the mass flow rate of the requiredmakeup water.Chapter 14 | 747Review Problems14–115 The condensation of the water vapor in compressedairlines is a major concern in industrial facilities, and thecompressed air is often dehumidified to avoid the problemsassociated with condensation. Consider a compressor thatcompresses ambient air from the local atmospheric pressureof 92 kPa to a pressure of 800 kPa (absolute). The compressedair is then cooled to the ambient temperature as itflows through the compressed-air lines. Disregarding anypressure losses, determine if there will be any condensationin the compressed-air lines on a day when the ambient air isat 20°C and 50 percent relative humidity.14–116E The relative humidity of air at 80°F and 14.7 psiais increased from 25 to 75 percent during a humidificationprocess at constant temperature and pressure. Determine thepercent error involved in assuming the density of air to haveremained constant.14–117 Dry air whose molar analysis is 78.1 percent N 2 ,20.9 percent O 2 , and 1 percent Ar flows over a water bodyuntil it is saturated. If the pressure and temperature of airremain constant at 1 atm and 25°C during the process, determine(a) the molar analysis of the saturated air and (b) thedensity of air before and after the process. What do you concludefrom your results?14–118E Determine the mole fraction of the water vapor atthe surface of a lake whose surface temperature is 60°F, andcompare it to the mole fraction of water in the lake, which isvery nearly 1.0. The air at the lake surface is saturated, and theatmospheric pressure at lake level can be taken to be 13.8 psia.14–119 Determine the mole fraction of dry air at the surfaceof a lake whose temperature is 18°C. The air at the lakesurface is saturated, and the atmospheric pressure at lakelevel can be taken to be 100 kPa.14–120E Consider a room that is cooled adequately by an airconditioner whose cooling capacity is 7500 Btu/h. If the roomis to be cooled by an evaporative cooler that removes heat at thesame rate by evaporation, determine how much water needs tobe supplied to the cooler per hour at design conditions.14–121E The capacity of evaporative coolers is usuallyexpressed in terms of the flow rate of air in ft 3 /min (or cfm),and a practical way of determining the required size of anevaporative cooler for an 8-ft-high house is to multiply thefloor area of the house by 4 (by 3 in dry climates and by 5 inhumid climates). For example, the capacity of an evaporativecooler for a 30-ft-long, 40-ft-wide house is 1200 4 4800cfm. Develop an equivalent rule of thumb for the selection ofan evaporative cooler in SI units for 2.4-m-high houseswhose floor areas are given in m 2 .14–122 A cooling tower with a cooling capacity of 100 tons(440 kW) is claimed to evaporate 15,800 kg of water per day.Is this a reasonable claim?