1.Easy
A refrigerator removes heat from its cold space at a rate of 0.9 kW while its compressor uses 0.2 kW of power. Find its coefficient of performance COPR.
Q̇L = 0.9 kWẆ = 0.2 kW
COPR =
Heat engines & refrigerators worksheet with answers
2.Easy
A heat engine takes in 1500 kJ of heat from a hot reservoir and rejects 1200 kJ to a cold reservoir in each cycle. Find the net work W it produces per cycle.
QH = 1500 kJQL = 1200 kJ
W = kJ
3.Medium
A heat engine works between a hot reservoir at 625 °C and a cold reservoir at 25 °C. Find the highest possible thermal efficiency ηmax.
TH = 625 °CTL = 25 °C
ηmax = %
4.Medium
A reversible refrigerator keeps a cold room at 4 °C by removing 7 kW of heat, and rejects heat to the surroundings at 28 °C. Find the rate Q̇H at which it rejects heat.
TL = 4 °CTH = 28 °CQ̇L = 7 kW
Q̇H = kW
5.Hard
A heat pump keeps a house at 20 °C while it is 7 °C outside, delivering 5 kW of heat to the house. Find the minimum power input Ẇmin.
TH = 20 °CTL = 7 °CQ̇H = 5 kW
Ẇmin = kW
6.Hard
Two Carnot engines work in series. Engine A receives heat from a reservoir at 550 °C and rejects heat to an intermediate reservoir at temperature Tm. Engine B takes all that heat and rejects heat to a reservoir at 35 °C. Both engines produce the same work. Find Tm in °C.
TH = 550 °CTL = 35 °C
Tm = °C
Answer key
- 1. COPR = 4.5
- 2. W = 300 kJ
- 3. ηmax = 66.8 %
- 4. Q̇H = 7.606 kW
- 5. Ẇmin = 0.2217 kW
- 6. Tm = 292.5 °C
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