Claude on Question 7, Sep 28

A refrigerator does not create cold; it moves heat from the inside of the cabinet to the room outside. Because heat will not flow on its own from a cooler place to a warmer one, the appliance has to spend energy to push it in that uphill direction, which is why a refrigerator needs to be plugged in and why the kitchen gets slightly warmer rather than cooler when one is running.

The work is done by a fluid called a refrigerant, which circulates endlessly through a closed loop of tubing and changes back and forth between liquid and gas. The trick that makes the whole thing possible is that a liquid absorbs a large amount of heat when it boils into vapor, and gives that heat back up when it condenses into liquid again. By arranging for the refrigerant to boil inside the cold compartment and condense outside it, the machine ferries heat across the insulated wall.

Following the loop around, the cycle begins at the compressor, the humming pump usually tucked at the back or bottom of the cabinet. It takes in low-pressure refrigerant vapor and squeezes it, which raises both its pressure and its temperature so that the gas becomes hotter than the surrounding room air. This hot, high-pressure gas then travels through the condenser, the coil of black tubing on the outside of the appliance, where the room air carries heat away. Losing that heat makes the vapor condense into a warm liquid, still under high pressure.

The liquid next reaches a deliberate constriction, either a narrow capillary tube or an expansion valve, which lets it trickle slowly into a region of much lower pressure. Dropping in pressure makes the refrigerant's boiling point plunge, so it becomes very cold and begins to vaporize. In this chilled state it passes through the evaporator coils inside the freezer or the back wall of the fridge, where it is now colder than the food and air around it. Heat therefore flows naturally from the contents into the refrigerant, boiling it fully into vapor and leaving the compartment cooler. The vapor returns to the compressor and the sequence repeats.

Several supporting features make the arrangement practical. Thick foam insulation and magnetic door gaskets slow the leakage of heat back in, so the compressor does not have to run constantly. A thermostat senses the interior temperature and switches the compressor on and off to hold a set range, which is why a fridge cycles audibly rather than humming without pause. Many models use a fan to blow air over the evaporator and distribute chilled air through both compartments, and this circulating air also dries the interior, depositing moisture as frost on the coldest coils. Frost-free designs periodically warm those coils with a small heater and drain the meltwater to a shallow pan near the condenser, where the warmth of the machine evaporates it away. An air conditioner and a heat pump work on exactly the same principle, differing only in which side of the wall the cold coil and the hot coil happen to sit.