How is the combined pump-motor efficiency

Assignment Help Mechanical Engineering

Reference no: EM13924013

1. What is mechanical efficiency? What does a mechanical efficiency of 100 percent mean for a hydraulic turbine?

2 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?

3. Define turbine efficiency, generator efficiency, and combined turbine-generator efficiency.

4. Can the combined turbine-generator efficiency be greater than either the turbine efficiency or the generator efficiency? Explain.

5. 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 = 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 the rate of energy consumption and the unit cost of utilized energy for both electric and gas burners.

7. A 75-hp (shaft output) motor that has an efficiency of 91.0 percent is worn out and is replaced by a high-efficiency 75-hp motor that has an efficiency of 95.4 percent. Determine the reduction in the heat gain of the room due to higher efficiency under full-load conditions.

8. A 90-hp (shaft output) electric car is powered by an electric motor mounted in the engine compartment. If the motor has an average efficiency of 91 percent, determine the rate of heat supply by the motor to the engine compartment at full load.

10. A 75-hp (shaft output) motor that has an efficiency of 91.0 percent is worn out and is to be replaced by a high-effi-ciency 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 $0.08/kWh, determine the amount of energy and money saved as a result of installing the high-efficiency motor instead of the standard motor. Also, determine the simple payback period if the purchase prices of the standard and high-efficiency motors are $5449 and $5520, respectively.

11. The steam requirements of a manufacturing facility are being met by a boiler whose rated heat input is 5.5 X 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 operates 4200 hours a year intermittently. Taking the unit cost of energy to be $4.35/106 Btu, determine the annual energy and cost savings as a result of tuning up the boiler.

12. In a hydroelectric power plant, 65 m3/s of water flows from an elevation of 90 m to a turbine, where electric power is generated. The overall efficiency of the turbine-generator is 84 percent. Disregarding frictional losses in piping, estimate the electric power output of this plant.

13. A 0.3-m³ rigid vessel initially contains saturated liq¬uid-vapor mixture of water at 150°C. The water is now heated until it reaches the critical state. Determine the mass of the liquid water and the volume occupied by the liquid at the initial state.

14. What is the difference between mass and molar mass? How are these two related?

15. Under what conditions is the ideal-gas assumption suitable for real gases?

16. What is the difference between R and Ru? How are these two related?

17. Propane and methane are commonly used for heat¬ing in winter, and the leakage of these fuels, even for short periods, poses a fire danger for homes. Which gas leakage do you think poses a greater risk for fire? Explain.

18. What is the specific volume of nitrogen at 300 kPa and 227°C?

19. A 3-ft3 container is filled with 2-ibm of oxygen at a pressure of 80 psia. What is the temperature of the oxygen?

20. A 2-kg mass of helium is maintained at 300 kPa and 27°C in a rigid container. How large is the container, in m3?

21 A spherical balloon with a diameter of 9 m is filled with helium at 27°C and 200 kPa. Determine the mole num-ber and the mass of the helium in the balloon.

22. Reconsider Prob. 21. Using EES (or other)

software, investigate the effect of the balloon diameter on the mass of helium contained in the balloon for the pressures of (a) 100 kPa and (b) 200 kPa. Let the diame¬ter vary from 5 m to 15 m. Plot the mass of helium against the diameter for both cases.

23. A 1-m3 tank containing air at 25°C and 500 kPa is connected through a valve to another tank containing 5 kg of air at 35°C and 200 kPa. Now the valve is opened, and the entire system is allowed to reach thermal equilibrium with the surroundings, which are at 20°C. Determine the volume of the second tank and the final equilibrium pressure of air.

24. 2.3 ibmol of air at 65°F and 32 psia is contained in an elastic tank. What is the volume of the tank? If the volume is doubled at the same pressure, determine the final temperature.

25. A rigid tank contains an ideal gas at 1227°C and 200 kPa gage. The gas is cooled until the gage pressure reads 50 kPa. If the atmospheric pressure is 100 kPa, determine the final temperature of the gas.

Reference no: EM13924013

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