Claude on Question 7, Sep 27

A refrigerator doesn't actually create cold; it moves heat from the inside of the insulated cabinet to the room around it. This might seem to violate common sense, since heat naturally flows from hot things to cold things, but by doing mechanical work the appliance can force heat to travel in the opposite direction. The trick is accomplished with a fluid called a refrigerant, which is chosen because it boils and condenses at convenient temperatures and pressures, and which is pumped around a closed loop of tubing in a continuous cycle.

The cycle has four essential stages. It begins at the compressor, the humming motor usually tucked at the back or bottom of the unit, which takes in low-pressure refrigerant vapor and squeezes it to a much higher pressure. Compressing a gas raises its temperature, so the vapor leaves the compressor hot as well as pressurized. It then travels through the condenser, a set of coils or a flat panel on the outside of the cabinet where the refrigerant is hotter than the surrounding kitchen air. Heat therefore flows naturally out into the room, and as the refrigerant sheds that heat it condenses from a vapor into a warm liquid, still at high pressure.

Next the liquid passes through a restriction, either a narrow capillary tube or an expansion valve, which separates the high-pressure side of the system from the low-pressure side. As the liquid squeezes through and enters the much lower pressure region beyond, it expands and partially flashes into vapor, and this sudden expansion makes its temperature plummet to well below the temperature inside the food compartment. This frigid mixture now flows through the evaporator, the coils inside the freezer or behind the interior back wall. Because the refrigerant there is colder than the food and air inside the box, heat flows from the contents into the refrigerant, which absorbs that energy as it finishes boiling into a vapor. Air is usually blown across these coils by a small fan so that the chill is distributed evenly, and moisture from the air condenses and freezes on the coils, which is why there is a defrost heater and a drain pan that catches the meltwater.

The now cool, low-pressure vapor returns to the compressor, and the whole sequence repeats. The net effect is that heat is picked up inside and dumped outside, which is why the back of a refrigerator feels warm and why the room gets slightly hotter overall, since the electrical energy used by the compressor is also ultimately released as heat. A thermostat or temperature sensor monitors the interior and switches the compressor on and off, or varies its speed in newer models, to hold the chosen temperature. An air conditioner and a heat pump work on exactly the same principle, merely with different arrangements of what counts as inside and outside.