1.Easy
A refrigerator removes heat from its cold space at a rate of 2.4 kW while its compressor uses 1 kW of power. Find its coefficient of performance COPR.
Q̇L = 2.4 kWẆ = 1 kW
COPR =
Heat engines & refrigerators worksheet with answers
2.Easy
A heat engine takes in 4200 kJ of heat from a hot reservoir and rejects 2310 kJ to a cold reservoir in each cycle. Find the net work W it produces per cycle.
QH = 4200 kJQL = 2310 kJ
W = kJ
3.Medium
A reversible refrigerator keeps a cold room at 0 °C by removing 1.5 kW of heat, and rejects heat to the surroundings at 30 °C. Find the rate Q̇H at which it rejects heat.
TL = 0 °CTH = 30 °CQ̇L = 1.5 kW
Q̇H = kW
4.Medium
A heat engine works between a hot reservoir at 300 °C and a cold reservoir at 15 °C. Find the highest possible thermal efficiency ηmax.
TH = 300 °CTL = 15 °C
ηmax = %
5.Hard
A refrigerator keeps its cold space at -18 °C in a kitchen at 35 °C, removing heat from the cold space at 5 kW. Find the minimum power input Ẇmin.
TH = 35 °CTL = -18 °CQ̇L = 5 kW
Ẇmin = kW
6.Hard
A heat engine takes in 4100 kJ of heat from a reservoir at 500 °C, produces 1271 kJ of work and rejects the rest to a reservoir at 15 °C, per cycle. Find the total entropy generated Sgen per cycle.
TH = 500 °CTL = 15 °CQH = 4100 kJW = 1271 kJ
Sgen = kJ/K
Answer key
- 1. COPR = 2.4
- 2. W = 1890 kJ
- 3. Q̇H = 1.665 kW
- 4. ηmax = 49.73 %
- 5. Ẇmin = 1.039 kW
- 6. Sgen = 4.515 kJ/K
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