1.Easy
A heat engine takes in 2700 kJ of heat from a hot reservoir and rejects 2025 kJ to a cold reservoir in each cycle. Find its thermal efficiency η.
QH = 2700 kJQL = 2025 kJ
η = %
Heat engines & refrigerators worksheet with answers
2.Easy
A refrigerator removes heat from its cold space at a rate of 1.72 kW while its compressor uses 0.4 kW of power. Find the rate Q̇H at which it rejects heat to the kitchen.
Q̇L = 1.72 kWẆ = 0.4 kW
Q̇H = kW
3.Medium
A heat engine works between a hot reservoir at 425 °C and a cold reservoir at 25 °C. Find the highest possible thermal efficiency ηmax.
TH = 425 °CTL = 25 °C
ηmax = %
4.Medium
A reversible (Carnot) heat engine takes in 4300 kJ of heat from a reservoir at 800 °C and rejects 1094 kJ per cycle. Find the temperature TL of the cold reservoir in °C.
TH = 800 °CQH = 4300 kJQL = 1094 kJ
TL = °C
5.Hard
A Carnot heat engine receives 140 kW of heat from a source at 400 °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 -20 °C and also rejects heat to the surroundings at 20 °C. Find the rate Q̇L at which heat is removed from the freezer.
TH = 400 °CT0 = 20 °CTL = -20 °CQ̇H = 140 kW
Q̇L = kW
6.Hard
A refrigerator keeps its cold space at -25 °C in a kitchen at 28 °C, removing heat from the cold space at 3 kW. Find the minimum power input Ẇmin.
TH = 28 °CTL = -25 °CQ̇L = 3 kW
Ẇmin = kW
Answer key
- 1. η = 25 %
- 2. Q̇H = 2.12 kW
- 3. ηmax = 57.29 %
- 4. TL = −0.1207 °C
- 5. Q̇L = 500.2 kW
- 6. Ẇmin = 0.6407 kW
Full step-by-step solutions: practise this topic on examproblems.com.