1.Easy
A refrigerator removes heat from its cold space at a rate of 3.33 kW while its compressor uses 0.9 kW of power. Find its coefficient of performance COPR.
Q̇L = 3.33 kWẆ = 0.9 kW
COPR =
Heat engines & refrigerators worksheet with answers
2.Easy
A heat engine takes in 4600 kJ of heat from a hot reservoir and rejects 3220 kJ to a cold reservoir in each cycle. Find its thermal efficiency η.
QH = 4600 kJQL = 3220 kJ
η = %
3.Medium
A heat engine works between a hot reservoir at 600 °C and a cold reservoir at 40 °C. Find the highest possible thermal efficiency ηmax.
TH = 600 °CTL = 40 °C
ηmax = %
4.Medium
A heat engine works between a hot reservoir at 425 °C and a cold reservoir at 15 °C. It takes in 2900 kJ of heat. Find the most work Wmax it could possibly produce.
TH = 425 °CTL = 15 °CQH = 2900 kJ
Wmax = kJ
5.Hard
A Carnot heat engine receives 200 kW of heat from a source at 850 °C and rejects heat to the surroundings at 20 °C. All of its work drives a Carnot refrigerator that removes heat from a freezer at -25 °C and also rejects heat to the surroundings at 20 °C. Find the rate Q̇L at which heat is removed from the freezer.
TH = 850 °CT0 = 20 °CTL = -25 °CQ̇H = 200 kW
Q̇L = kW
6.Hard
A refrigerator keeps its cold space at -25 °C in a kitchen at 35 °C, removing heat from the cold space at 4 kW. Find the minimum power input Ẇmin.
TH = 35 °CTL = -25 °CQ̇L = 4 kW
Ẇmin = kW
Answer key
- 1. COPR = 3.7
- 2. η = 30 %
- 3. ηmax = 64.14 %
- 4. Wmax = 1703 kJ
- 5. Q̇L = 815 kW
- 6. Ẇmin = 0.9672 kW
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