Repeat Prob. 8–114 if heat were supplied to the pressure cooker from a heat source at 180°C instead of the electrical heating unit?
8–114 A 4-L pressure cooker has an operating pressure of 175 kPa. Initially, one-half of the volume is filled with liquid water and the other half by water vapor. The cooker is now placed on top of a 750-W electrical heating unit that is kept on for 20 min. Assuming the surroundings to be at 25°C and 100 kPa, determine (a) the amount of water that remained in the cooker and (b) the exergy destruction associated with the entire process.
FIGURE P8–114
(a)
The final mass of water that remained in the cooker.
Answer to Problem 115RP
The final mass of water that remained in the cooker is
Explanation of Solution
Express the mass balance for a pressure cooker which acts as a system.
Here, initial mass is
Write an energy balance for a system.
Here, internal energy at state 1 and 2 is
Calculate the initial mass in the tank
Here, saturated liquid specific volume is
Calculate the initial internal energy
Here, specific internal energy of saturated fluid is
Calculate the initial entropy
Here, specific entropy of saturated fluid is
Write the internal energy at state 2
Here, dryness fraction at state 2 is
Write the specific volume at state 2
Write the formula to calculate the mass of the water remained in tank
Here, volume of the cooker is
Conclusion:
From the Table A-5 of “Saturated water: Pressure”, obtain the saturated liquid specific volume
From the Table A-5 of “Saturated water: Pressure”, obtain the enthalpy
Since one half of the volume is filled with liquid water and other half by water vapor, calculate the volume of tank
Substitute
Substitute 1.893 kg for
Substitute 1.893 kg for
Substitute
Substitute
Re-write the Equation (II) using the Equation (I),
Substitute
Substitute
Substitute 0.001918 for
Substitute
Thus, the final mass of water that remained in the cooker is
(b)
The amount of exergy destructed during the heat supplied to pressure cooker from heat source.
Answer to Problem 115RP
The amount of exergy destructed during the heat supplied to pressure cooker from heat source is
Explanation of Solution
For an extended system, write the simplification rate form of the entropy balance.
Here, entropy generation is
Calculate the exergy destroyed during the process
Here, dead state temperature is
Substitute Equation (XI) in Equation (XII).
Calculate the entropy at state 2
Conclusion:
Substitute 0.001918 for
Substitute 1.8945 kg for
Substitute 298 K for
Thus, the amount of exergy destructed during the heat supplied to pressure cooker from heat source is
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Chapter 8 Solutions
Thermodynamics: An Engineering Approach
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