Claude on Question 7, Sep 30

A refrigerator does not create cold; it moves heat from the inside of the cabinet to the room around it. Since heat will not flow on its own from a cooler place to a warmer one, the machine has to do work to force it in that direction, which is why a refrigerator needs to be plugged in and why the back or sides of one feel warm when it has been running.

The trick is accomplished with a fluid called a refrigerant, which is chosen because it boils and condenses at convenient temperatures and pressures. The refrigerant circulates continuously through a closed loop of tubing that passes both inside and outside the insulated box. Its usefulness comes from the fact that a liquid absorbs a great deal of heat when it evaporates into a gas, and releases that same heat when it condenses back into a liquid. By arranging for the evaporation to happen inside the refrigerator and the condensation to happen outside, the appliance effectively carries heat out of the food compartment and dumps it into the kitchen.

Following the loop around, the cycle begins at the compressor, the humming pump usually tucked at the bottom rear of the cabinet. It takes in low pressure refrigerant vapor and squeezes it to a much higher pressure, which also makes it hot. This hot, pressurized gas then travels through the condenser, the coils or flat panel on the outside of the appliance. Here the gas is hotter than the surrounding room air, so heat flows out of it and into the kitchen, and as it loses heat the refrigerant condenses into a warm liquid, still under high pressure.

Next the liquid reaches a deliberate restriction in the tubing, either a narrow capillary tube or an expansion valve. Passing through this bottleneck drops its pressure sharply, and because the boiling point of a fluid falls as pressure falls, the refrigerant emerges on the far side as a very cold mixture of liquid and vapor. This cold mixture flows through the evaporator coils, which sit inside the cabinet or in a compartment behind its inner wall. Air and food inside the box are now warmer than the refrigerant, so heat flows out of them and into the coils, boiling the remaining liquid into vapor. That vapor returns to the compressor and the whole sequence repeats.

Several supporting parts make the system practical. Thick foam insulation and magnetic door gaskets slow the leakage of heat back in, so the compressor only has to run intermittently. A thermostat or electronic sensor monitors the interior temperature and switches the compressor on and off to hold it near the chosen setting. Most modern units also have a small fan blowing air across the evaporator to distribute the chill evenly and to feed cold air to the freezer and fresh food sections, and another fan to help the condenser shed heat. Because the coils operate below freezing, moisture from the air condenses and frosts onto them, so a heater periodically melts that frost and the resulting water drips into a shallow pan where the warmth of the machine evaporates it away.

The net effect is a heat pump running in one direction, and it explains a common misconception: leaving a refrigerator door open will not cool a room. Everything the appliance removes from inside gets released out the back, along with the extra energy consumed by the compressor, so the room ends up slightly warmer overall.