1.Easy
A heat engine takes in 3700 kJ of heat from a hot reservoir and rejects 2035 kJ to a cold reservoir in each cycle. Find the net work W it produces per cycle.
QH = 3700 kJQL = 2035 kJ
W = kJ
Heat engines & refrigerators worksheet with answers
2.Easy
A heat engine takes in 4500 kJ of heat from a hot reservoir and rejects 3150 kJ to a cold reservoir in each cycle. Find its thermal efficiency η.
QH = 4500 kJQL = 3150 kJ
η = %
3.Medium
A heat engine receives heat from a source at 850 °C and rejects heat to the surroundings at 10 °C. Its actual thermal efficiency is 49%. Find its second-law efficiency ηII, the ratio of the actual efficiency to the Carnot efficiency.
TH = 850 °CTL = 10 °Cη = 49%
ηII = %
4.Medium
A reversible refrigerator keeps a cold room at -30 °C by removing 6 kW of heat, and rejects heat to the surroundings at 30 °C. Find the rate Q̇H at which it rejects heat.
TL = -30 °CTH = 30 °CQ̇L = 6 kW
Q̇H = kW
5.Hard
A Carnot heat engine receives 60 kW of heat from a source at 400 °C and rejects heat to the surroundings at 25 °C. All of its work drives a Carnot refrigerator that removes heat from a freezer at -15 °C and also rejects heat to the surroundings at 25 °C. Find the rate Q̇L at which heat is removed from the freezer.
TH = 400 °CT0 = 25 °CTL = -15 °CQ̇H = 60 kW
Q̇L = kW
6.Hard
A heat engine takes in 4300 kJ of heat from a reservoir at 750 °C, produces 1849 kJ of work and rejects the rest to a reservoir at 15 °C, per cycle. Find the total entropy generated Sgen per cycle.
TH = 750 °CTL = 15 °CQH = 4300 kJW = 1849 kJ
Sgen = kJ/K
Answer key
- 1. W = 1665 kJ
- 2. η = 30 %
- 3. ηII = 65.52 %
- 4. Q̇H = 7.481 kW
- 5. Q̇L = 215.7 kW
- 6. Sgen = 4.303 kJ/K
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