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physics
thermodynamics

Thermodynamics An Engineering Approach 8th edition Yunus A. Cengel, Michael A. Boles - Solutions

An escalator in a shopping center is designed to move 50 people, 75 kg each, at a constant speed of 0.6 m/s at 458 slope. Determine the minimum power input needed to drive this escalator. What would your answer be if the escalator velocity were to be doubled?
Consider a 1400-kg car cruising at constant speed of 70 km/s. Now the car starts to pass another car, by accelerating to 110 km/h in 5 s. Determine the additional power needed to achieve this acceleration. What would your answer be if the total mass of the car were only 700 kg?
How is the combined pump-motor efficiency of a pump and motor system defined? Can the combined pump- motor efficiency be greater than either the pump or the motor efficiency?
Define turbine efficiency, generator efficiency, and combined turbine - generator efficiency.
Can the combined turbine-generator efficiency be greater than either the turbine efficiency or the generator efficiency? Explain.
Consider a 24-kW hooded electric open burner in an area where the unit costs of electricity and natural gas are $0.10/kWh and $1.20/therm (1 therm 5 105,500 kJ), respectively. The efficiency of open burners can be taken to be 73 percent for electric burners and 38 percent for gas burners. Determine
A 75-hp (shaft output) motor that has an efficiency of 91.0 percent is worn out and is to be replaced by a highefficiency motor that has an efficiency of 95.4 percent. The motor operates 4368 hours a year at a load factor of 0.75. Taking the cost of electricity to be $0.12/kWh, determine the amount
Consider an electric motor with a shaft power output of 20 kW and an efficiency of 88 percent. Determine the rate at which the motor dissipates heat to the room it is in when the motor operates at full load. In winter, this room is normally heated by a 2-kW resistance heater. Determine if it is
The steam requirements of a manufacturing facility are being met by a boiler whose rated heat input is 5.5 3 106 Btu/h. The combustion efficiency of the boiler is measured to be 0.7 by a hand-held flue gas analyzer. After tuning up the boiler, the combustion efficiency rises to 0.8. The boiler
Reconsider Prob. 2-59E. Using EES (or other) software, study the effects of the unit cost of energy, the new combustion efficiency on the annual energy, and cost savings. Let the efficiency vary from 0.7 to 0.9, and the unit cost to vary from $4 to $6 per million Btu. Plot the annual energy and
A geothermal pump is used to pump brine whose density is 1050 kg/m3 at a rate of 0.3 m3/s from a depth of 200 m. For a pump efficiency of 74 percent, determine the required power input to the pump. Disregard frictional losses in the pipes, and assume the geo ther mal water at 200 m depth to be
An exercise room has 6 weight-lifting machines that have no motors and 7 treadmills each equipped with a 2.5-hp (shaft output) motor. The motors operate at an average load factor of 0.7, at which their efficiency is 0.77. During peak evening hours, all 12 pieces of exercising equipment are used
A room is cooled by circulating chilled water through a heat exchanger located in a room. The air is circulated through the heat exchanger by a 0.25-hp (shaft output) fan. Typical efficiency of small electric motors driving 0.25-hp equipment is 54 percent. Determine the rate of heat supply by the
The water in a large lake is to be used to generate electricity by the installation of a hydraulic turbine-generator at a location where the depth of the water is 50 m. Water is to be supplied at a rate of 5000 kg/s. If the electric power generated is measured to be 1862 kW and the generator
A 7-hp (shaft) pump is used to raise water to an elevation of 15 m. If the mechanical efficiency of the pump is 82 percent, determine the maximum volume flow rate of water.
At a certain location, wind is blowing steadily at 7 m/s. Determine the mechanical energy of air per unit mass and the power generation potential of a wind turbine with 80-m-diameter blades at that location. Also determine the actual electric power generation assuming an overall efficiency of 30
Reconsider Prob. 2-66. Using EES (or other) software, investigate the effect of wind velocity and the blade span diameter on wind power generation. Let the velocity vary from 5 to 20 m/s in increments of 5 m/s, and the diameter vary from 20 to 120 m in increments of 20 m. Tabulate the results, and
Water is pumped from a lake to a storage tank 15 m above at a rate of 70 L/s while consuming 15.4 kW of electric power. Disregarding any frictional losses in the pipes and any changes in kinetic energy, determine (a) the overall efficiency of the pump-motor unit and (b) the pressure difference
Large wind turbines with blade span diameters of over 100 m are available for electric power generation. Consider a wind turbine with a blade span diameter of 100 m installed at a site subjected to steady winds at 8 m/s. Taking the overall efficiency of the wind turbine to be 32 percent and the air
A hydraulic turbine has 85 m of elevation difference available at a flow rate of 0.25 m3/s, and its overall turbine-generator efficiency is 91 percent. Determine the electric power output of this turbine.
A water pump delivers 6 hp of shaft power when operating. If the pressure differential between the outlet and the inlet of the pump is measured to be 1.2 psi when the flow rate is 15 ft3/s and the changes in velocity and elevation are negligible, determine the mechanical efficiency of this pump.
Water is pumped from a lower reservoir to a higher reservoir by a pump that provides 20 kW of shaft power. The free surface of the upper reservoir is 45 m higher than that of the lower reservoir. If the flow rate of water is measured to be 0.03 m3/s, determine mechanical power that is converted to
The water behind Hoover Dam in Nevada is 206 m higher than the Colorado River below it. At what rate must water pass through the hydraulic turbines of this dam to produce 100 MW of power if the turbines are 100 percent efficient?
An oil pump is drawing 44 kW of electric power while pumping oil with r 5 860 kg/m3 at a rate of 0.1 m3/s. The inlet and outlet diameters of the pipe are 8 cm and 12 cm, respectively. If the pressure rise of oil in the pump is measured to be 500 kPa and the motor efficiency is 90 percent, determine
How does energy conversion affect the environment? What are the primary chemicals that pollute the air? What is the primary source of these pollutants?
What is acid rain? Why is it called a "rain"? How do the acids form in the atmosphere? What are the adverse effects of acid rain on the environment?
Why is carbon monoxide a dangerous air pollutant? How does it affect human health at low and at high levels?
What is the greenhouse effect? How does the excess CO2 gas in the atmosphere cause the greenhouse effect? What are the potential long-term consequences of greenhouse effect? How can we combat this problem?
What is smog? What does it consist of? How does ground-level ozone form? What are the adverse effects of ozone on human health?
Calculate the total kinetic energy, in Btu, of an object with a mass of 10 lbm when its velocity is 50 ft/s.
A Ford Taurus driven 12,000 miles a year will use about 650 gallons of gasoline compared to a Ford Explorer that would use 850 gallons. About 19.7 lbm of CO2, which causes global warming, is released to the atmosphere when a gallon of gasoline is burned. Determine the extra amount of CO2 production
Consider a household that uses 14,000 kWh of electricity per year and 900 gallons of fuel oil during a heating season. The average amount of CO2 produced is 26.4 lbm/ gallon of fuel oil and 1.54 lbm/kWh of electricity. If this household reduces its oil and electricity usage by 15 percent as a
When a hydrocarbon fuel is burned, almost all of the carbon in the fuel burns completely to form CO2 (carbon dioxide), which is the principal gas causing the greenhouse effect and thus global climate change. On average, 0.59 kg of CO2 is produced for each kWh of electricity generated from a power
Repeat Prob. 2 - 82 assuming the electricity is produced by a power plant that burns coal. The average production of CO2 in this case is 1.1 kg per kWh.
A typical car driven 20,000 km a year emits to the atmosphere about 11 kg per year of NOx (nitrogen oxides), which cause smog in major population areas. Natural gas burned in the furnace emits about 4.3 g of NOx per therm (1 therm 5 105,500 kJ), and the electric power plants emit about 7.1 g of NOx
What is a blackbody? How do real bodies differ from a blackbody?
Define emissivity and absorptivity. What is Kirchhoff's law of radiation?
Calculate the total potential energy, in Btu, of an object with a mass of 200 lbm when it is 10 ft above a datum level at a location where standard gravitational acceleration exists.
The inner and outer surfaces of a 5-m 3 6-m brick wall of thickness 30 cm and thermal conductivity 0.69 W/m·8C are maintained at temperatures of 208C and 58C, respectively. Determine the rate of heat transfer through the wall, in W.
The inner and outer surfaces of a 0.5-cm-thick 2-m 3 2-m window glass in winter are 158C and 68C, respectively. If the thermal conductivity of the glass is 0.78 W/m·8C, determine the amount of heat loss, in kJ, through the glass over a period of 10 h. What would your answer be if the glass were
Reconsider Prob. 2-92. Using EES (or other) software, investigate the effect of glass thickness on heat loss for the specified glass surface temperatures. Let the glass thickness vary from 0.2 to 2 cm. Plot the heat loss versus the glass thickness, and discuss the results.
An aluminum pan whose thermal conductivity is 237 W/m·8C has a flat bottom whose diameter is 20 cm and thickness 0.6 cm. Heat is transferred steadily to boiling water in the pan through its bottom at a rate of 700 W. If the inner surface of the bottom of the pan is 1058C, determine the temperature
The inner and outer glasses of a 2-m 3 2-m double pane window are at 188C and 68C, respectively. If the 1-cm space between the two glasses is filled with still air, determine the rate of heat transfer through the air layer by conduction, in kW.
Two surfaces of a 2 - cm - thick plate are maintained at 08C and 1008C, respectively. If it is determined that heat is transferred through the plate at a rate of 500 W/m2, determine its thermal conductivity.
Hot air at 808C is blown over a 2-m 3 4-m flat surface at 308C. If the convection heat transfer coefficient is 55 W/m2·8C, determine the rate of heat transfer from the air to the plate, in kW.
For heat transfer purposes, a standing man can be modeled as a 30-cm diameter, 175-cm long vertical cylinder with both the top and bottom surfaces insulated and with the side surface at an average temperature of 348C. For a convection heat transfer coefficient of 10 W/m2·8C, determine the rate of
A 9-cm-diameter spherical ball whose surface is maintained at a temperature of 1108C is suspended in the middle of a room at 208C. If the convection heat transfer coefficient is 15 W/m2·C and the emissivity of the surface is 0.8, determine the total rate of heat transfer from the ball.
It is well known that warm air in a cooler environment rises. Now consider a warm mixture of air and gasoline on top of an open gasoline can. Do you think this gas mixture will rise in a cooler environment?
A 1-m3 tank contains 2.841 kg of steam at 0.6 MPa. Determine the temperature of the steam, using (a) The idealgas equation, (b) The van der Waals equation, and (c) The steam tables.
Reconsider Prob. 3-100. Solve the problem using EES (or other) software. Again using the EES, compare the temperature of water for the three cases at constant specific volume over the pressure range of 0.1 MPa to 1 MPa in 0.1 MPa increments. Plot the percent error involved in the ideal-gas
During a hot summer day at the beach when the air temperature is 30oC, someone claims the vapor pressure in the air to be 5.2 kPa. Is this claim reasonable?
Consider a glass of water in a room that is at 20oC and 40 percent relative humidity. If the water temperature is 15oC, determine the vapor pressure (a) At the free surface of the water and (b) At a location in the room far from the glass.
On a certain day, the temperature and relative humidity of air over a large swimming pool are measured to be 25oC and 60 percent, respectively. Determine the water temperature of the pool when phase equilibrium conditions are established between the water in the pool and the vapor in the air.
During a hot summer day when the air temperature is 35oC and the relative humidity is 70 percent, you buy a supposedly "cold" canned drink from a store. The store owner claims that the temperature of the drink is below 10oC. Yet the drink does not feel so cold and you are skeptical since you notice
Consider two rooms that are identical except that one is maintained at 25oC and 40 percent relative humidity while the other is maintained at 20oC and 55 percent relative humidity. Noting that the amount of moisture is proportional to the vapor pressure, determine which room contains more moisture.
A thermos bottle is half-filled with water and is left open to the atmospheric air at 608F and 35 percent relative humidity. If heat transfer to the water through the thermos walls and the free surface is negligible, determine the temperature of water when phase equilibrium is established.
Water in a pressure cooker is observed to boil at 260oF. What is the absolute pressure in the pressure cooker, in psia?
Carbon-dioxide gas at 3 MPa and 500 K flows steadily in a pipe at a rate of 0.4 kmol/s. Determine(a) the volume and mass flow rates and the density of carbon dioxide at this state. If CO2 is cooled at constant pressure as it flows in the pipe so that the temperature of CO2 drops to 450 K at the
A tank contains argon at 6008C and 200 kPa gage. The argon is cooled in a process by heat transfer to the surroundings such that the argon reaches a final equilibr state at 300oC. Determine the final gage pressure of the argon. Assume atmospheric pressure is 100 kPa.
The combustion in a gasoline engine may be approximated by a constant volume heat addition process. There exists the air-fuel mixture in the cylinder before the combustion and the combustion gases after it, and both may be approximated as air, an ideal gas. In a gasoline engine, the cylinder
One kilogram of R-134a fills a 0.090 m3 rigid container at an initial temperature of 240oC. The container is then heated until the pressure is 280 kPa. Determine the initial pressure and final temperature.
A rigid tank with a volume of 0.117 m3 contains 1 kg of refrigerant-134a vapor at 240 kPa. The refrigerant is now allowed to cool. Determine the pressure when the refrigerant first starts condensing. Also, show the process on a P-v diagram with respect to saturation lines.
One pound-mass of water fills a 2.649 ft3 weighted piston-cylinder device at a temperature of 400oF. The piston-cylinder device is now cooled until its temperature is 100oF. Determine the final pressure and volume of the water.
Ethane at 10 MPa and 100oC is heated at constant pressure until its volume has increased by 60 percent. Determine the final temperature using (a) The ideal gas equation of state and (b) The compressibility factor. Which of these two results is the more accurate?
A 13-m3 tank contains nitrogen at 17oC and 600 kPa. Some nitrogen is allowed to escape until the pressure in the tank drops to 400 kPa. If the temperature at this point is 15oC, determine the amount of nitrogen that has escaped.
A 10-kg mass of superheated refrigerant-134a at 1.2 MPa and 70oC is cooled at constant pressure until it exists as a compressed liquid at 20oC. (a) Show the process on a T-v diagram with respect to saturation lines. (b) Determine the change in volume. (c) Find the change in total internal energy.
4-L rigid tank contains 2 kg of saturated liquid- vapor mixture of water at 508C. The water is now slowly heated until it exists in a single phase. At the final state, will the water be in the liquid phase or the vapor phase? What would your answer be if the volume of the tank were 400 L instead of
The gage pressure of an automobile tire is measured to be 200 kPa before a trip and 220 kPa after the trip at a location where the atmospheric pressure is 90 kPa. Assuming the volume of the tire remains constant at 0.035 m3, determine the percent increase in the absolute temperature of the air in
A piston-cylinder device initially contains 0.2 kg of steam at 200 kPa and 300oC. Now, the steam is cooled at constant pressure until it is at 150oC. Determine the volume change of the cylinder during this process using the compressibility factor and compare the result to the actual value.
Steam at 400oC has a specific volume of 0.02 m3/kg. Determine the pressure of the steam based on (a) The idealgas equation, (b) The generalized compressibility chart, and (c) The steam tables.
A tank whose volume is unknown is divided into two parts by a partition. One side of the tank contains 0.03 m3 of refrigerant-134a that is a saturated liquid at 0.9 MPa, while the other side is evacuated. The partition is now removed, and the refrigerant fills the entire tank. If the final state of
Reconsider Prob. 3-122. Using EES (or other) software, investigate the effect of the initial pressure of refrigerant-134a on the volume of the tank. Let the initial pressure vary from 0.5 to 1.5 MPa. Plot the volume of the tank versus the initial pressure, and discuss the results.
Liquid propane is commonly used as a fuel for heating homes, powering vehicles such as forklifts, and filling portable picnic tanks. Consider a propane tank that initially contains 5 L of liquid propane at the environment temperature of 20oC. If a hole develops in the connecting tube of a propane
Repeat Prob. 3 - 124 for isobutene
A tank contains helium at 37oC and 140 kPa gage. The helium is heated in a process by heat transfer from the surroundings such that the helium reaches a final equilibrium state at 200oC. Determine the final gage pressure of the helium. Assume atmospheric pressure is 100 kPa.
If sufficient data are provided, complete the blank cells in the following table of properties of water. In the last column describe the condition of water as compressed liquid, saturated mixture, superheated vapor, or insufficient information; and, if applicable, give the quality.
Water initially at 300 kPa and 0.5 m3/kg is contained in a piston-cylinder device fitted with stops so that the water supports the weight of the piston and the force of the atmosphere. The water is heated until it reaches the saturated vapor state and the piston rests against the stops. With the
A tank contains helium at 378C and 140 kPa gage. The helium is heated in a process by heat transfer from the surroundings such that the helium reaches a final equilibrium state at 200oC. Determine the final gage pressure of the helium. Assume atmospheric pressure is 100 kPa.
What is the physical significance of hfg? Can it be obtained from a knowledge of hf and hg? How?
Nitrogen is maintained at 400 psia and 2100oF. Compare the specific volume of this nitrogen as predicted by (a) The ideal gas equation of state, (b) The Benedict-Webb- Rubin equation of state, and (c) With the compressibility factor.
Although balloons have been around since 1783 when the first balloon took to the skies in France, a real breakthrough in ballooning occurred in 1960 with the design of the modern hot-air balloon fueled by inexpensive propane and constructed of lightweight nylon fabric. Over the years, ballooning
Reconsider Prob. 3-131. Using EES (or other) software, investigate the effect of the environment temperature on the average air temperature in the balloon when the balloon is suspended in the air. Assume the environment temperature varies from 210 to 30oC. Plot the average air temperature in the
Consider an 18-m-diameter hot-air balloon that, together with its cage, has a mass of 120 kg when empty. The air in the balloon, which is now carrying two 70 - kg people, is heated by propane burners at a location where the atmo spheric pressure and temperature are 93 kPa and 12oC, respectively.
A 300-m3 rigid tank is filled with saturated liquid- vapor mixture of water at 200 kPa. If 25 percent of the mass is liquid and 75 percent of the mass is vapor, the total mass in the tank is(a) 451 kg (b) 556 kg (c) 300 kg(d) 331 kg (e) 195 kg
Water is boiled at 1 atm pressure in a coffee maker equipped with an immersion-type electric heating element. The coffee maker initially contains 1 kg of water. Once boiling started, it is observed that half of the water in the coffee maker evaporated in 10 minutes. If the heat loss from the coffee
A 1-m3 rigid tank contains 10 kg of water (in any phase or phases) at 160oC. The pressure in the tank is (a) 738 kPa (b) 618 kPa (c) 370 kPa (d) 2000 kPa (e) 1618 kPa
Water is boiling at 1 atm pressure in a stainless steel pan on an electric range. It is observed that 2 kg of liquid water evaporates in 30 min. The rate of heat transfer to the water is (a) 2.51 kW (b) 2.32 kW (c) 2.97 kW (d) 0.47 kW (e) 3.12 kW
Water is boiled in a pan on a stove at sea level. During 10 min of boiling, it is observed that 200 g of water has evaporated. Then the rate of heat transfer to the water is (a) 0.84 kJ/min (b) 45.1 kJ/min (c) 41.8 kJ/min (d) 53.5 kJ/min (e) 225.7 kJ/min
A 3-m3 rigid vessel contains steam at 4 MPa and 500oC. The mass of the steam is (a) 3 kg (b) 9 kg (c) 26 kg (d) 35 kg (e) 52 kg
R-134a, whose specific volume is 0.6243 ft3/lbm, flows through a tube at 80 psia. What is the temperature in the tube?
A rigid tank contains 2 kg of an ideal gas at 4 atm and 40oC. Now a valve is opened, and half of mass of the gas is allowed to escape. If the final pressure in the tank is 2.2 atm, the final temperature in the tank is (a) 71oC (b) 44oC (c) 2100oC (d) 20oC (e) 172oC
A rigid tank contains 2 kg of an ideal gas at 4 atm and 40oC. Now a valve is opened, and half of mass of the gas is allowed to escape. If the final pressure in the tank is 2.2 atm, the final temperature in the tank is (a) 71oC (b) 44oC (c) 2100oC (d) 20oC (e) 172oC
The pressure of an automobile tire is measured to be 190 kPa (gage) before a trip and 215 kPa (gage) after the trip at a location where the atmospheric pressure is 95 kPa. If the temperature of air in the tire before the trip is 258C, the air temperature after the trip is (a) 51.1oC (b) 64.2oC
In 1775, Dr. William Cullen made ice in Scotland by evacuating the air in a water tank. Explain how that device works, and discuss how the process can be made more efficient.
Complete this table for H2O:
Complete this table for H2O
Reconsider Prob. 3-21. Using EES (or other) software, determine the missing properties of
Complete this table for H2O:
Complete this table for H2O:
Reconsider Prob. 3-24E. Using EES (or other) software, determine the missing properties of water. Repeat the solution for refrigerant-134a, refrigerant-22, and ammonia.

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