Thermodynamics

opened, and one-half of the total mass is withdrawn from thetank in the liquid form. Heat is transferred to water from asource at 200°C so that the temperature in the tank remainsconstant. Determine (a) the amount of heat transfer and(b) the total entropy generation for this process.7–147E An iron block of unknown mass at 185°F isdropped into an insulated tank that contains 0.8 ft 3 of water at70°F. At the same time, a paddle wheel driven by a 200-Wmotor is activated to stir the water. Thermal equilibrium isestablished after 10 min with a final temperature of 75°F.Determine (a) the mass of the iron block and (b) the entropygenerated during this process.7–148E Air enters a compressor at ambient conditions of15 psia and 60°F with a low velocity and exits at 150 psia,620°F, and 350 ft/s. The compressor is cooled by the ambientair at 60°F at a rate of 1500 Btu/min. The power input to thecompressor is 400 hp. Determine (a) the mass flow rate of airand (b) the rate of entropy generation.7–149 Steam enters an adiabatic nozzle at 4 MPa and450°C with a velocity of 70 m/s and exits at 3 MPa and 320m/s. If the nozzle has an inlet area of 7 cm 2 , determine(a) the exit temperature and (b) the rate of entropy generationfor this process. Answers: (a) 422.3°C, (b) 0.0361 kW/KSpecial Topic: Reducing the Cost of Compressed Air7–150 Compressed air is one of the key utilities in manufacturingfacilities, and the total installed power of compressedairsystems in the United States is estimated to be about 20million horsepower. Assuming the compressors to operate atfull load during one-third of the time on average and the averagemotor efficiency to be 85 percent, determine how muchenergy and money will be saved per year if the energy consumedby compressors is reduced by 5 percent as a result ofimplementing some conservation measures. Take the unit costof electricity to be $0.07/kWh.7–151 The energy used to compress air in the United Statesis estimated to exceed one-half quadrillion (0.5 10 15 ) kJper year. It is also estimated that 10 to 40 percent of the compressedair is lost through leaks. Assuming, on average, 20percent of the compressed air is lost through air leaks and theunit cost of electricity is $0.07/kWh, determine the amountand cost of electricity wasted per year due to air leaks.7–152 The compressed-air requirements of a plant at sealevel are being met by a 125-hp compressor that takes in airat the local atmospheric pressure of 101.3 kPa and the averagetemperature of 15°C and compresses it to 900 kPa. Aninvestigation of the compressed-air system and the equipmentusing the compressed air reveals that compressing the air to750 kPa is sufficient for this plant. The compressor operates3500 h/yr at 75 percent of the rated load and is driven by anelectric motor that has an efficiency of 88 percent. Taking theprice of electricity to be $0.085/kWh, determine the amountof energy and money saved as a result of reducing the pressureof the compressed air.Chapter 7 | 4137–153 A 150-hp compressor in an industrial facility ishoused inside the production area where the average temperatureduring operating hours is 25°C. The average temperatureoutdoors during the same hours is 10°C. The compressoroperates 4500 h/yr at 85 percent of rated load and is drivenby an electric motor that has an efficiency of 90 percent. Takingthe price of electricity to be $0.07/kWh, determine theamount of energy and money saved as a result of drawingoutside air to the compressor instead of using the inside air.7–154 The compressed-air requirements of a plant are beingmet by a 100-hp screw compressor that runs at full load during40 percent of the time and idles the rest of the time duringoperating hours. The compressor consumes 35 percent of therated power when idling and 90 percent of the power whencompressing air. The annual operating hours of the facility are3800 h, and the unit cost of electricity is $0.075/kWh.It is determined that the compressed-air requirements ofthe facility during 60 percent of the time can be met by a 25-hp reciprocating compressor that consumes 95 percent of therated power when compressing air and no power when notcompressing air. It is estimated that the 25-hp compressorruns 85 percent of the time. The efficiencies of the motors ofthe large and the small compressors at or near full load are0.90 and 0.88, respectively. The efficiency of the large motorat 35 percent load is 0.82. Determine the amount of energyand money saved as a result of switching to the 25-hp compressorduring 60 percent of the time.7–155 The compressed-air requirements of a plant arebeing met by a 125-hp screw compressor. The facility stopsproduction for one hour every day, including weekends, forlunch break, but the compressor is kept operating. The compressorconsumes 35 percent of the rated power when idling,and the unit cost of electricity is $0.09/kWh. Determine theamount of energy and money saved per year as a result ofturning the compressor off during lunch break. Take the efficiencyof the motor at part load to be 84 percent.7–156 The compressed-air requirements of a plant are metby a 150-hp compressor equipped with an intercooler, anaftercooler, and a refrigerated dryer. The plant operates 4800h/yr, but the compressor is estimated to be compressing airduring only one-third of the operating hours, that is, 1600hours a year. The compressor is either idling or is shut off therest of the time. Temperature measurements and calculationsindicate that 40 percent of the energy input to the compressoris removed from the compressed air as heat in the aftercooler.The COP of the refrigeration unit is 3.5, and the cost of electricityis $0.06/kWh. Determine the amount of the energy andmoney saved per year as a result of cooling the compressedair before it enters the refrigerated dryer.7–157 The 1800-rpm, 150-hp motor of a compressor isburned out and is to be replaced by either a standard motorthat has a full-load efficiency of 93.0 percent and costs $9031or a high-efficiency motor that has an efficiency of 96.2 percentand costs $10,942. The compressor operates 4368 h/yr at