Cengel & Boles -- Chapter 4 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).

4-33 Steam enters an adiabatic turbine at 10MPa and 400 C and leaves at 20kPa with a quality of 90 percent. Neglecting the changes in kinetic and potential energies, determine the mass flow rate required for a power output of 5 MW.

How does the required mass flow change if we alter the dryness at the outlet? Why? (hint: do a sensitivity plot)

4-34 Steam flows steadily through a turbine at a rate of 25,000 kg/h, entering at 8MPa and 450 C and leaving at 30 kPa as saturated vapor.

If the power generated by the turbine is 4 MW, determine the rate of heat loss from the steam.

4-38 Refrigerant-12 enters an adiabatic compressor as saturated vapor at -20 C and leaves at 0.7 MPa and 70 C. The mass flow rate of the refrigerant is 1.2 kg/s. Determine (a) the power input to the compressor and (b) the volume flow rate of the refrigerant at the compressor inlet.

Does this system work for all fluids? Try some other fluids (at least 3). For which does it work? Why (or why not)?

For another fluid which works, is the power input the same? Describe.

4-41 Helium is to be compressed from 120 kPa and 310K to 700kPa and 430K. A hat loss of 20kJ/kg occurs during the compression process. Neglecting kinetic energy changes, determine the power input required for a mass flow rate of 90 kg/min.

Does the outlet pressure effect compressor work? Why or why not? (test this out in CyclePad)

4-45C Would you expect the temperature of air to drop as it undergoes a steady flow throttling process? Write your initial assumption here.

In CyclePad design a simple system with a throttling device. Compare air and water going from a pressure of 4 MPa to 100kPa with a starting temperature of 200 C. What is the final temperature for each? What is the significance of your results?

4-47 Refrigerant-12 is throttled from the saturated liquid state at 800 kPa to a pressure of 140kPa. Determine the temperature drop during this process and the final specific volume of the refrigerant.

4-51C When two fluid streams are mixed in a mixing chamber can the mixture temperature be lower than the temperature of both streams? How?

MJ #1 CyclePad assumes mixers are adiabatic.

How would you model a non-adiabatic mixer in CyclePad? (try it and draw a diagram here)

4-54 A hot water stream at 80 C enters a mixing chamber with a mass flow rate of 0.5kg/s where it is mixed with a stream of cold water at 20 C. If it is desired that the mixture leave the chamber at 42 C, determine the mass flow rate of the cold-water stream. Assume all the streams are at a pressure of 250kPa.

Does increasing pressure matter? (try in Cyclepad. Hint: equate P(s1) = P (s2) then use the sensitivity tool)

MJ#2 Saturated vapor at 100 C enters a mixing chamber with a mass flow rate of 1kg/s where it is mixed with a stream of cold water at 20 C. If it is desired that the mixture leave the chamber as saturated liquid, determine the ratio of the cold-water stream to the vapor. Assume all the streams are at the same pressure.

4-59 (modified) Refrigerant-12 at 1 MPa and 80 C with a mass flow rate of 3kg/s is to be cooled to 1 MPa and 30 C in a condenser by air. The air enters at 100kPa and 27 C and leaves at 95 kPa and 60 C. Determine the volume flow rate of the air.

What is the volume flow rate if water is used instead of air? Explain the reason for the difference.