Thermodynamics

748 | Thermodynamics14–123E The U.S. Department of Energy estimates that190,000 barrels of oil would be saved per day if every householdin the United States raised the thermostat setting in summerby 6°F (3.3°C). Assuming the average cooling season tobe 120 days and the cost of oil to be $20/barrel, determinehow much money would be saved per year.14–124E The thermostat setting of a house can be loweredby 2°F by wearing a light long-sleeved sweater, or by 4°F bywearing a heavy long-sleeved sweater for the same level ofcomfort. If each °F reduction in thermostat setting reducesthe heating cost of a house by 4 percent at a particular location,determine how much the heating costs of a house can bereduced by wearing heavy sweaters if the annual heating costof the house is $600.14–125 The air-conditioning costs of a house can bereduced by up to 10 percent by installing the outdoor unit(the condenser) of the air conditioner at a location shaded bytrees and shrubs. If the air-conditioning costs of a house are$500 a year, determine how much the trees will save thehome owner in the 20-year life of the system.14–126 A 3-m 3 tank contains saturated air at 25°C and97 kPa. Determine (a) the mass of the dry air, (b) the specifichumidity, and (c) the enthalpy of the air per unit mass of thedry air. Answers: (a) 3.29 kg, (b) 0.0210 kg H 2 O/kg dry air,(c) 78.6 kJ/kg dry air14–127 Reconsider Prob. 14–126. Using EES (orother) software, determine the properties ofthe air at the initial state. Study the effect of heating the air atconstant volume until the pressure is 110 kPa. Plot therequired heat transfer, in kJ, as a function of pressure.14–128E Air at 15 psia, 60°F, and 50 percent relative humidityflows in an 8-in.-diameter duct at a velocity of 50 ft/s.Determine (a) the dew-point temperature, (b) the volume flowrate of air, and (c) the mass flow rate of dry air.14–129 Air enters a cooling section at 97 kPa, 35°C, and30 percent relative humidity at a rate of 6 m 3 /min, where it iscooled until the moisture in the air starts condensing. Determine(a) the temperature of the air at the exit and (b) the rateof heat transfer in the cooling section.14–130 Outdoor air enters an air-conditioning system at10°C and 40 percent relative humidity at a steady rate of22 m 3 /min, and it leaves at 25°C and 55 percent relativehumidity. The outdoor air is first heated to 22°C in the heatingsection and then humidified by the injection of hot steamin the humidifying section. Assuming the entire process takesplace at a pressure of 1 atm, determine (a) the rate of heatsupply in the heating section and (b) the mass flow rate ofsteam required in the humidifying section.14–131 Air enters an air-conditioning system that usesrefrigerant-134a at 30°C and 70 percent relative humidity at arate of 4 m 3 /min. The refrigerant enters the cooling section at700 kPa with a quality of 20 percent and leaves as saturatedvapor. The air is cooled to 20°C at a pressure of 1 atm. Determine(a) the rate of dehumidification, (b) the rate of heattransfer, and (c) the mass flow rate of the refrigerant.14–132 Repeat Prob. 14–131 for a total pressure of 95 kPafor air.14–133 An air-conditioning system operates at a total pressureof 1 atm and consists of a heating section and an evaporativecooler. Air enters the heating section at 10°C and70 percent relative humidity at a rate of 30 m 3 /min, and itleaves the evaporative cooler at 20°C and 60 percent relativelyhumidity. Determine (a) the temperature and relativehumidity of the air when it leaves the heating section, (b) therate of heat transfer in the heating section, and (c) the rateof water added to air in the evaporative cooler. Answers:(a) 28.3°C, 22.3 percent, (b) 696 kJ/min, (c) 0.13 kg/min14–134 Reconsider Prob. 14–133. Using EES (orother) software, study the effect of total pressurein the range 94 to 100 kPa on the results required in theproblem. Plot the results as functions of total pressure.14–135 Repeat Prob. 14–133 for a total pressure of 96 kPa.14–136 Conditioned air at 13°C and 90 percent relativehumidity is to be mixed with outside air at 34°C and 40 percentrelative humidity at 1 atm. If it is desired that the mixture havea relative humidity of 60 percent, determine (a) the ratio of thedry air mass flow rates of the conditioned air to the outside airand (b) the temperature of the mixture.14–137 Reconsider Prob. 14–136. Determine thedesired quantities using EES (or other) softwareinstead of the psychrometric chart. What would theanswers be at a location at an atmospheric pressure of 80 kPa?14–138 A natural-draft cooling tower is to remove50 MW of waste heat from the cooling waterthat enters the tower at 42°C and leaves at 27°C. Atmosphericair enters the tower at 1 atm with dry- and wet-bulbtemperatures of 23 and 18°C, respectively, and leaves saturatedat 37°C. Determine (a) the mass flow rate of the coolingwater, (b) the volume flow rate of air into the coolingtower, and (c) the mass flow rate of the required makeupwater.14–139 Reconsider Prob. 14–138. Using EES (orother) software, investigate the effect of airinlet wet-bulb temperature on the required air volume flowrate and the makeup water flow rate when the other inputdata are the stated values. Plot the results as functions of wetbulbtemperature.14–140 Atmospheric air enters an air-conditioning system at30°C and 70 percent relative humidity with a volume flow rateof 4 m 3 /min and is cooled to 20°C and 20 percent relativehumidity at a pressure of 1 atm. The system uses refrigerant-134a as the cooling fluid that enters the cooling section at