1.Easy
A refrigerator removes heat from its cold space at a rate of 5.46 kW while its compressor uses 2.6 kW of power. Find the rate Q̇H at which it rejects heat to the kitchen.
Q̇L = 5.46 kWẆ = 2.6 kW
Q̇H = kW
Heat engines & refrigerators worksheet with answers
2.Easy
A heat engine takes in 1300 kJ of heat from a hot reservoir and rejects 975 kJ to a cold reservoir in each cycle. Find its thermal efficiency η.
QH = 1300 kJQL = 975 kJ
η = %
3.Medium
A reversible refrigerator keeps a cold room at -10 °C by removing 3 kW of heat, and rejects heat to the surroundings at 25 °C. Find the rate Q̇H at which it rejects heat.
TL = -10 °CTH = 25 °CQ̇L = 3 kW
Q̇H = kW
4.Medium
A reversible refrigerator keeps a cold room at 5 °C by removing 6.5 kW of heat, and rejects heat to the surroundings at 30 °C. Find the rate Q̇H at which it rejects heat.
TL = 5 °CTH = 30 °CQ̇L = 6.5 kW
Q̇H = kW
5.Hard
A heat engine takes in 1900 kJ of heat from a reservoir at 400 °C, produces 779 kJ of work and rejects the rest to a reservoir at 10 °C, per cycle. Find the total entropy generated Sgen per cycle.
TH = 400 °CTL = 10 °CQH = 1900 kJW = 779 kJ
Sgen = kJ/K
6.Hard
A refrigerator keeps its cold space at 2 °C in a kitchen at 28 °C, removing heat from the cold space at 2 kW. Find the minimum power input Ẇmin.
TH = 28 °CTL = 2 °CQ̇L = 2 kW
Ẇmin = kW
Answer key
- 1. Q̇H = 8.06 kW
- 2. η = 25 %
- 3. Q̇H = 3.399 kW
- 4. Q̇H = 7.084 kW
- 5. Sgen = 1.136 kJ/K
- 6. Ẇmin = 0.189 kW
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