Cengel & Boles -- Chapter 5&6 questions

5-43 A household refrigerator runs one-fourth of the time and removes heat from food compartment at an average rate of 1200 kJ/h. If the COP of the refrigerator is 2.5, determine the power the refrigerator draws when running.

5-71 A Carnot heat engine operates between a source at 1000K and a sink at 300K. If the heat engine is supplied with heat at a rate of 800 kJ/min, determine (a) the thermal efficiency and (b) the power output of this heat engine. (hint: since the problem states a Carnot heat engine h -Carnot = h -thermal)

How do the high and low temperatures effect h ?

Is it more worthwhile to raise Tmax 100 C or raise Tmin 100 C? Explain your reasoning.

Which is more practical -- raising Tmax or Tmin? (hint: look at the benchmarks for Tmax and Tmin)

5-77 An inventor claims to have developed a heat engine which receives 800 kJ of heat from a source at 400K and produces 250kJ of net work while rejecting the waste heat to a sink at 300K. Is this a reasonable claim? Why?

5-92 An air conditioning system is used to maintain a house at 22 C when the temperature outside is 33 C. The house is gaining heat through the walls and the windows at a rate of 600 kJ/min, and the heat generation rate within the house from people, lights, and appliances amounts to 120 kJ/min. Determine the minimum power input required for this air conditioning system. (hint: in preferences: global choose "show T-environment")

6-42 Air is compressed by a 8-kW compressor from P1 to P2. The air temperature is maintained constant at 25 C during this process as a result of heat transfer to the surrounding medium at 10 C. Determine the rate of entropy change of (a) the air and (b) the surroundings. State the assumptions made in solving this problem. Does this process satisfy the second law of thermodynamics? (Assume a mass flow of 1)

6-51 The radiator of a steam heating system has a volume of 20L and is filled with superheated water vapor at 200 kPa and 200 C. At this moment both the inlet and the exit valves to the radiator are closed. After a while the temperature of the steam drops to 80 C as a result of heat transfer to the room air. Determine the entropy change of the steam during this process, in kJ/K.

6-53 A well-insulated rigid tank contains 2 kg of a saturated liquid - vapor mixture of water at 100 kPa. Initially, three-quarters of the mass is in the liquid phase. An electric resistance heater placed in the tank is now turned on and kept on until all the liquid in the tank is vaporized. Determine the entropy change of the steam during this process.

If this were a piston-cylinder tank instead of a rigid tank, would there be more or less entropy change in converting the fluid to vapor? Explain.

6-58 Refrigerant-12 enters an adiabatic compressor as saturated vapor at 140 kPa at a rage of 2 m3/min and is compressed to a pressure of 700 kPa. Determine the minimum power that must be supplied to the compressor.

How does entropy of gas change? (hint: check the required power at h = 90%. Minimum power is h - isentropic 100%. Use the sensitivity tool to plot shaft power v. isentropic efficiency and to plot entropy change v. isentropic efficiency) Describe the results you find.