Thermodynamics

Chapter 14 | 737Analysis We take the mixing section of the streams as the system. Theschematic of the system and the psychrometric chart of the process areshown in Fig. 14–30. We note that this is a steady-flow mixing process.The mass flow rates of dry air in each stream arem # a 1 V# 1v 1m # a 2 V# 2v 2From the mass balance of dry air,·50 m 3 >min0.826 m 3 >kg dry air 60.5 kg>min20 m 3 >min 22.5 kg>min0.889 m 3 >kg dry airm # a 3 m # a 1 m # a 2 160.5 22.52 kg>min 83 kg>minSaturated airT 1 = 14°CV 1 = 50 m 3 /min12T 2 = 32°Cf 2 = 60%·V 2 = 20 m 3 /minMixingsectionP = 1 atm3·V 3v 3f 3T 3The specific humidity and the enthalpy of the mixture can be determinedfrom Eq. 14–24,which yieldm # a 1m # a 2 v 2 v 3v 3 v 1 h 2 h 3h 3 h 160.522.5 0.0182 v 3v 3 0.010 79.0 h 3h 3 39.4v 3 0.0122 kg H 2 O/kg dry airh 3 50.1 kJ>kg dry airThese two properties fix the state of the mixture. Other properties of the mixtureare determined from the psychrometric chart:T 3 19.0°Cf 3 89%v 3 0.844 m 3 >kg dry airFinally, the volume flow rate of the mixture is determined fromV # 3 m # a 3v 3 183 kg>min210.844 m 3 >kg2 70.1 m 3 /min1f 1 = 100%14°C 32°C32f 2 = 60%FIGURE 14–30Schematic and psychrometric chart forExample 14–8.Discussion Notice that the volume flow rate of the mixture is approximatelyequal to the sum of the volume flow rates of the two incoming streams. Thisis typical in air-conditioning applications.Wet Cooling TowersPower plants, large air-conditioning systems, and some industries generatelarge quantities of waste heat that is often rejected to cooling water fromnearby lakes or rivers. In some cases, however, the cooling water supply islimited or thermal pollution is a serious concern. In such cases, the wasteheat must be rejected to the atmosphere, with cooling water recirculatingand serving as a transport medium for heat transfer between the source andthe sink (the atmosphere). One way of achieving this is through the use ofwet cooling towers.A wet cooling tower is essentially a semienclosed evaporative cooler. Aninduced-draft counterflow wet cooling tower is shown schematically in