Refrigerators.

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Presentation transcript:

Refrigerators

Reversing the Flow Heat flow through an engine was used to generate work. Work can also be used to create a net flow of heat. That flow can reverse the trend of the second law. An engine takes a temperature difference to create work. A refrigerator or heat pump takes work to push heat against the temperature difference.

Refrigerator Thermodynamically a refrigerator pumps heat from a small cold region into a large hot region. The work is supplied by a motor. Air conditioners are a type of refrigerator.

Refrigeration The motor forces gas at high pressure into a condenser The condenser allows the gas to cool exhaust heat to the outside. The cool gas goes into the unit and draws heat from the inside.

Performance The second law prohibits a perfect refrigerator. A coefficient of performance (Kr) compares the work to the low temperature side.

Heat Pump A heat pump is similar to a refrigerator. Work is used to move heat from a low temperature to a high temperature. A heat pump has the small volume at high temperature. Refrigerator is the opposite Heat pumps can become air conditioners in the summer.

Heat Pump Performance Heat pumps deliver heat to the high temperature Refrigerators took heat from the low temperature. A coefficient of performance (Kp) is different for heat pumps. Compare the high temperature side to the work.

Comfort Zone A heat pump has Kp = 3.0 and is rated to do work at 1500 W. How much heat can it add to a room per second? What is its Kr as an air conditioner in the summer? Find the heat directly from the definition. QH = Kp W QH = 3.0 (1500 W)(1 s) = 4500 J Use conservation to get QL. QL = QH - W QL = 3000 J Kr = QL / W = 2.0