1.Easy
A refrigerator removes heat from its cold space at a rate of 6 kW while its compressor uses 1.5 kW of power. Find the rate Q̇H at which it rejects heat to the kitchen.
Q̇L = 6 kWẆ = 1.5 kW
Q̇H = kW
Heat engines & refrigerators worksheet with answers
2.Easy
A heat pump with a coefficient of performance of 4.9 delivers 8 kW of heat to a house. Find the electrical power Ẇ it uses.
COPHP = 4.9Q̇H = 8 kW
Ẇ = kW
3.Medium
A heat engine works between a hot reservoir at 250 °C and a cold reservoir at 20 °C. Find the highest possible thermal efficiency ηmax.
TH = 250 °CTL = 20 °C
ηmax = %
4.Medium
A reversible (Carnot) heat engine takes in 3800 kJ of heat from a reservoir at 375 °C and rejects 1895 kJ per cycle. Find the temperature TL of the cold reservoir in °C.
TH = 375 °CQH = 3800 kJQL = 1895 kJ
TL = °C
5.Hard
A refrigerator keeps its cold space at -10 °C in a kitchen at 35 °C, removing heat from the cold space at 4.5 kW. Find the minimum power input Ẇmin.
TH = 35 °CTL = -10 °CQ̇L = 4.5 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 15 °C. Both engines produce the same work. Find Tm in °C.
TH = 550 °CTL = 15 °C
Tm = °C
Answer key
- 1. Q̇H = 7.5 kW
- 2. Ẇ = 1.633 kW
- 3. ηmax = 43.96 %
- 4. TL = 50.07 °C
- 5. Ẇmin = 0.7695 kW
- 6. Tm = 282.5 °C
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