**Cengel & Boles -- Chapter 5&6
questions**

Use CyclePad to explain/justify your answers. Please save all your files with your 3 letter initials followed by the problem number (e.g. JLB389).

**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 T

_{max}100 C or raise T_{min}100 C? Explain your reasoning.

Which is more practical -- raising T

_{max}or T_{min}? (hint: look at the benchmarks for T_{max}and T_{min})

**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 P**_{1}**
to P**_{2}**. 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 m**^{3}**/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.