Claude on Question 7, Sep 15

A refrigerator doesn't actually create cold; it moves heat from the inside of the insulated cabinet to the room around it. This seems to run against the ordinary tendency of heat to flow from hot places to cold ones, so the machine needs to do work, which is why your refrigerator has to be plugged in. The trick it uses relies on a substance called a refrigerant, a fluid chosen because it boils and condenses at convenient temperatures, and on the simple fact that a liquid absorbs a great deal of heat when it evaporates and releases that heat again when it condenses.

The cycle begins at the compressor, the humming black canister usually tucked at the back or bottom of the appliance. The compressor takes in refrigerant as a cool low-pressure gas and squeezes it into a hot, high-pressure gas. Compressing a gas raises its temperature, so the refrigerant leaves the compressor considerably warmer than the kitchen air. It then travels through the condenser, a network of thin tubes, often with fins attached, mounted on the outside of the cabinet where air can circulate past them. Because the refrigerant is hotter than the room, heat flows outward into the air, and as the gas gives up that heat it condenses into a warm liquid, still under high pressure.

Next the liquid is forced through a narrow constriction, either a long thin capillary tube or an expansion valve. On the far side of this restriction the pressure is much lower, and when a liquid finds itself at low pressure it begins to evaporate vigorously. Evaporation demands energy, and the refrigerant takes that energy from its immediate surroundings, so its temperature plunges, typically to well below the freezing point of water. This very cold mixture of liquid and vapor now passes through the evaporator coils, which line the freezer compartment or sit behind a panel inside the cabinet. Here the situation of the condenser is reversed: the refrigerant is colder than the food and air around it, so heat flows inward from the compartment into the tubing, completing the evaporation and warming the refrigerant back into a gas. That heat, drawn out of your milk and vegetables, is what leaves them cold. The gas then returns to the compressor and the whole sequence repeats indefinitely, the same refrigerant circulating in a sealed loop rather than being consumed.

Several supporting features make the arrangement practical. Thick foam insulation and magnetic door gaskets slow the leakage of heat back into the cabinet, so the compressor need only run intermittently. A thermostat senses the interior temperature and switches the compressor on and off to hold it near the chosen setting. In most modern units a small fan blows air over the evaporator and distributes it through ducts to both the freezer and the fresh-food compartment, which is why one adjustable vent can shift the balance of cooling between the two. Because the evaporator runs below freezing, moisture from the air condenses and freezes on it, so frost-free models periodically energize a small heater to melt the accumulation, and the resulting water drains into a shallow pan near the compressor where the waste heat evaporates it away.

The consequence of all this is that a refrigerator is a net heat source for the room it occupies. It removes a certain amount of heat from its interior but dumps that heat plus the electrical energy consumed by the compressor into the surrounding air, which is why leaving the door open on a hot day makes a kitchen warmer rather than cooler, and why an air conditioner, which works on exactly the same principle, must have its condenser placed outside the building.