How does ice become thinner inside a no-frost fridge?

Question

How does ice become thinner in a no-frost fridge? I thought ice can't evaporate at all since, well, it's below freezing temperature and it's solid, too. In my other yes-frost fridge, the ice never becomes thinner, but thickens over time, so I have to de-frost it.

I don't believe the no-frost fridges have some patented magical mechanisms that make ice evaporate, but it has to somehow occur naturally, and they just speed up the process by drying the air. Still, I don't understand how the ice evaporates.

Answer 1

Let's start with the evaporation of water (or sublimation, in this case).

Carbon dioxide exists as a gas at normal temperature and pressure. If it is compressed and cooled, you make dry ice. When dry ice heats up, the solid becomes a gas directly (any liquid is from water condensing on the dry ice). This process is called sublimation.

Water (or ice) can also sublimate at temperatures below freezing. Because of the dry air in the freezer, ice cubes will sublimate and will disappear.

Also, any water vapor in the air can freeze out on cold surfaces. This is what happens in older fridges as the moist air from the room enters the freezer when the door is open (ice will build up).

In frost free fridges, a number of them go through a heating cycle to remove ice from the cooling surfaces. The heating cycle is not long enough to warm the contents of the freezer. Water (or ice) becomes water vapor faster when it is warmer then the air.