Claude on Question 7, Sep 19

A refrigerator works by moving heat from the inside of the insulated cabinet to the room around it, which is possible because a fluid can be made to absorb heat when it evaporates and release heat when it condenses. The machine doesn't create cold so much as it relentlessly removes warmth, and if you put your hand near the back or underneath a running fridge you can feel the heat it has taken out of your food being dumped into the kitchen.

The working fluid, called a refrigerant, circulates in a sealed loop through four main parts. It begins as a cool low-pressure liquid entering a set of tubes inside or just behind the walls of the food compartment, known as the evaporator. Because the pressure there is low, the refrigerant boils at a very cold temperature, and as it evaporates it soaks up heat from the air and food inside the cabinet, much the way rubbing alcohol on your skin feels cold as it evaporates. The refrigerant leaves as a cold vapor carrying that absorbed heat with it.

That vapor is drawn into the compressor, the humming pump usually mounted at the bottom rear of the appliance and driven by an electric motor. The compressor squeezes the gas into a much smaller volume, which raises both its pressure and its temperature, so the refrigerant emerges considerably hotter than the room. This is the step that requires electricity, and it is what makes the whole one-way transfer of heat possible; heat will not flow from cold food to a warm kitchen on its own, so work has to be put in to push it that direction.

The hot, high-pressure gas then travels through the condenser, a coil of tubing on the back or in the sides of the cabinet, often helped along by a fan. Since the refrigerant is now hotter than the surrounding air, heat flows out of it into the kitchen, and as it cools it condenses back into a liquid, releasing still more heat in the process of changing state. This liquid, still under high pressure, next passes through a narrow restriction, either a long thin capillary tube or an expansion valve, which throttles the flow and lets the pressure drop sharply. The sudden drop chills the refrigerant and makes it partly flash into vapor, returning it to the cold low-pressure condition it needs to enter the evaporator again, and the cycle repeats.

A thermostat inside the compartment switches the compressor on and off to hold the temperature near a set point, which is why a fridge hums intermittently rather than constantly. Most modern units also route some of the cooling to a freezer section, where a fan circulates air between compartments, and they periodically run a small heater near the evaporator to melt accumulated frost, draining the meltwater into a shallow pan where the compressor's waste heat evaporates it away. Thick foam insulation and magnetic door gaskets reduce how much heat leaks back in, which is why leaving the door open or packing the shelves so tightly that air cannot circulate makes the machine work harder and run longer.