Heat engines & refrigerators worksheet with answers

Free printable Heat engines & refrigerators worksheet: 6 problems from easy to exam level, with an answer key. Print it or save it as a PDF.

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1.Easy

A refrigerator removes heat from its cold space at a rate of 5.25 kW while its compressor uses 2.1 kW of power. Find its coefficient of performance COP_R.

Q̇_L = 5.25 kWẆ = 2.1 kW

COP_R =

2.Easy

A heat engine takes in 2000 kJ of heat from a hot reservoir and rejects 1400 kJ to a cold reservoir in each cycle. Find its thermal efficiency η.

Q_H = 2000 kJQ_L = 1400 kJ

η = %

3.Medium

A heat engine works between a hot reservoir at 575 °C and a cold reservoir at 25 °C. Find the highest possible thermal efficiency η_max.

T_H = 575 °CT_L = 25 °C

η_max = %

4.Medium

A heat engine receives heat from a source at 600 °C and rejects heat to the surroundings at 25 °C. Its actual thermal efficiency is 33%. Find its second-law efficiency η_II, the ratio of the actual efficiency to the Carnot efficiency.

T_H = 600 °CT_L = 25 °Cη = 33%

η_II = %

5.Hard

Two Carnot engines work in series. Engine A receives heat from a reservoir at 1150 °C and rejects heat to an intermediate reservoir at temperature T_m. Engine B takes all that heat and rejects heat to a reservoir at 40 °C. Both engines produce the same work. Find T_m in °C.

T_H = 1150 °CT_L = 40 °C

T_m = °C

6.Hard

A heat pump keeps a house at 22 °C while it is -10 °C outside, delivering 6 kW of heat to the house. Find the minimum power input Ẇ_min.

T_H = 22 °CT_L = -10 °CQ̇_H = 6 kW

Ẇ_min = kW

Answer key

1. COP_R = 2.5
2. η = 30 %
3. η_max = 64.85 %
4. η_II = 50.11 %
5. T_m = 595 °C
6. Ẇ_min = 0.6505 kW

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