The table below gives information on a closed system that undergoes a cycle comp
ID: 1818685 • Letter: T
Question
The table below gives information on a closed system that undergoes a cycle composed of four processes. Kinetic and potential energy changes can be neglected.
a) Fill in the blank spaces in the table. Use the sign convention from the text: heat added to the system is positive and work done by the system is positive. Values must have the correct sign.
Process Q (kJ) W(kJ) U (kJ)
1-2 -170 -120
2-3 -60 -60
3-4 +180 +100
4-1 0
b) Is the cycle a power cycle or a refrigeration cycle?
c) Determine the thermal efficiency if it is a power cycle or the coefficient of performance if it is a refrigeration cycle.
Explanation / Answer
Process Q (kJ) W(kJ) U (kJ)
1-2 -170 -50 -120
2-3 -60 0 -60
3-4 +180 +100 +80
4-1 +100 0 +100
Process 1-2:
By first law of thermodynamics
Q = U + W
w = Q - U = -170 -(-120) = -50 kJ
Process 2-3:
By first law of thermodynamics
Q = U + W
w = Q - U = -60 -(-60)= 0 kJ
Process 3-4:
By first law of thermodynamics
Q = U + W
U = Q - W = 180 - 100 = 80kJ
Process 4-1:
The net change in internal energy over a cycle is zero.
-120 - 60 + 80 + U4-1 = 0
=> U4-1 = +100 kJ
By first law of thermodynamics
Q = U + W
Q = +100 + 0 = +100 kJ
The net heat input is Qcycle= - 170 - 60 + 180 + 100 = +50 kJ
The net work output is Wcycle = -50 + 100 = +50kJ
Since the heat transfer is positive and work transfer is positive , the cycle is a power cycle.
Thermal efficiency, = Wcycle/Qcycle = +50/+50 = 1.0
Thermal efficiency, = 1.0
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