1.Easy
A heat pump with a coefficient of performance of 4.1 delivers 11.5 kW of heat to a house. Find the rate Q̇L at which it takes heat from the outside air.
COPHP = 4.1Q̇H = 11.5 kW
Q̇L = kW
Heat engines & refrigerators worksheet with answers
2.Easy
A heat engine takes in 3400 kJ of heat from a hot reservoir and rejects 1870 kJ to a cold reservoir in each cycle. Find its thermal efficiency η.
QH = 3400 kJQL = 1870 kJ
η = %
3.Medium
A reversible refrigerator keeps a cold room at -20 °C by removing 1.5 kW of heat, and rejects heat to the surroundings at 25 °C. Find the rate Q̇H at which it rejects heat.
TL = -20 °CTH = 25 °CQ̇L = 1.5 kW
Q̇H = kW
4.Medium
A reversible refrigerator keeps a cold room at -25 °C by removing 8 kW of heat, and rejects heat to the surroundings at 28 °C. Find the rate Q̇H at which it rejects heat.
TL = -25 °CTH = 28 °CQ̇L = 8 kW
Q̇H = kW
5.Hard
A refrigerator keeps its cold space at 4 °C in a kitchen at 25 °C, removing heat from the cold space at 4.5 kW. Find the highest possible coefficient of performance COPmax.
TH = 25 °CTL = 4 °CQ̇L = 4.5 kW
COPmax =
6.Hard
A heat pump delivers 16.5 kW of heat to a heating system at 22 °C, taking heat from a source at 5 °C. Its actual COP is 40% of the Carnot COP between these temperatures. Find the electrical power Ẇ it uses.
TH = 22 °CTL = 5 °CηII = 40%Q̇H = 16.5 kW
Ẇ = kW
Answer key
- 1. Q̇L = 8.695 kW
- 2. η = 45 %
- 3. Q̇H = 1.767 kW
- 4. Q̇H = 9.709 kW
- 5. COPmax = 13.2
- 6. Ẇ = 2.376 kW
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