[chris (OP)]

How do the variety of freezers and chillers that remain resolutely ice-free for years actually achieve this feat?

Chris

[another_someone]

Frost free freezers have large gaps around all the drawer, and simply ensure good circulation of air, and allow the cold of the freezer to dry out the air.  Any ice in the freezer (even where you might want it, in an ice tray), will evaporate in the dry air within the freezer, and then because of the good air circulation within a frost free freezer, that air will efficiently be drawn away.

[Bored chemist]

" that air will efficiently be drawn away."
Where to? It's a sealed box.

[chris (OP)]

Sorry George, I'm not buying it - you'll have to do better than that. I have a feeling the freezers have some sort of defrost cycle which quickly and regularly melts ice to water and collects it.

Chris

[another_someone]

Sorry George, I'm not buying it - you'll have to do better than that. I have a feeling the freezers have some sort of defrost cycle which quickly and regularly melts ice to water and collects it.

Chris

Sorry, you are right - about every 6 to 24 hours, it does an internal defrost - but I still think internal circulation is critical, since all the frost free freezers I have seen seem to be designed with open trays, and generaly more space for internal circulation.

[another_someone]

" that air will efficiently be drawn away."
Where to? It's a sealed box.

Whatever the answer was, the water has to go somewhere, so the box cannot be totally sealed (although clearly you don't want too much matter leaving or entering the box).

[lyner]

The secret is in the air flow. There is a separate chamber with the cooling matrix in it and a fan blows cold air over the food compartment. Any water vapor  ends up on the matrix by the end of the cycle. To defrost, they disable the circulation fan and heat up the matrix- leaving the frozen food frozen. The water drains off  through a pipe to an external tray which is over the external heat exchanger. When the compressor starts, the outside heat exchanger evaporates the defrost water and this actually improves the overall efficiency of the system.
They need a bit more of a 'brain' than  conventional freezers and mine has let me down. I needed to replace two thermistors and also needed to loosen up a flap which allows cold air from the freezer half to get to the fridge part.
The rear chilling compartment does get clogged in some cheap freezers and it requires a lot of effort to restore the proper function ; you get a block of solid ice - I've been there with my Mother in Law's fr freezer.
You have to use sealed, non-cuboidal, 'ice cube' bags.

[drewsk404]

frost-free freezers use a device called a defrost timer. it shuts down the compressor every so often and also warms the coils to melt any ice frozen on them. its a very simple round device that clamps on the coils and does its job either based on an internal timer or a thermostat, depending on the freezer model.

[lyner]

Mine uses a thermistor to tell when the ice has melted.

[qazibasit]

there is basically a heater place in the side wall of ur freezer that glows when the wall get cooled and hence doesnt allow the walls to get the frost on it. ask any refrigerator repairmen he will show u the lamp. its not that there is a window or any ventilation in the freezer as another_someone said .

[oobflyer]

If you pull out the bottom drawer of the freezer and look at the 'floor' of the freezer you will see a drain.  The ice is melted during defrost and drains through this hole at the bottom.
The drain on my freezer recently became clogged and now each time it defrosts the water pours out onto the floor of the kitchen. 
The drain is only about 2 cm in diameter (1 inch) and it's in the very back of the bottom of the freezer, so I can't even look down into it to see why it's plugged.
My question is - where does the water go after it goes down this drain?  The freezer is not hooked up to any sewer pipes in the house.  Does it just evaporate? 
Anyone know how to unclog this small drain?  My freezer was working fine for 10 years and only recently started having problems.

[akostadinov]

Liebherr have a nice description how NoFrost works. Not allowed to post links.

You can look that up in internet search but here's the relevant part:

With Liebherr’s NoFrost technology a different story is being written… not only is there no need to defrost, but the appliances provide professional, reliable, long-term freshness. How it works: water vapour condenses at the coldest point, and so we see condensation forming on the window panes at home because they are the coldest surface. In the same way, the moisture in a freezer condenses at the coldest point, on the evaporator – the pipe system with the coolant flowing through it. On older style freezers, the evaporator pipes run through the aluminium panels that support the freezer shelves, and this is where the ice forms.

But on NoFrost appliances the evaporator pipes are compactly arranged in a sealed unit at the top of the appliance. A fan distributes the cold air from the evaporator unit throughout the appliance. Although this compact evaporator is prone to icing over, a sensor regulates the level of ice and triggers a defrost cycle whenever necessary: the fan is switched off and an electric heater starts the defrost process by warming the evaporator pipes and melting the ice. The melt water is collected in a tray and fed out of the freezer compartment to the compressor in the back of the appliance. Because the compressor is always slightly warm when operating, the condensate gradually evaporates. This technology means that the freezer is always free of ice and that stored food never frosts over. So, there is no need to defrost anymore!

A further benefit of NoFrost: as integral evaporator panels are no longer part of the structure, you can adjust the drawer heights as you require so that it is easy to make space for larger items. Liebherr’s VarioSpace offers a convenient way to create a particularly large storage space to accommodate, for example, a multi-tiered cake for mum’s ‘big’ birthday or other such celebrations.

Now, that leaves us with a question… Does NoFrost use more energy due to the fan? In one respect, yes it does, as the fan needs power. However, conventional appliances consume more energy when frosted over and since NoFrost appliances remain frost free, the energy consumption of the two appliance types is about the same.
A truly ingenious solution has been implemented for defrosting the evaporator in our commercial appliances: Liebherr engineers have worked out how to use the hot-gas that is created during the refrigeration cycle for defrosting so that almost no additional energy is used for pipe warming. In addition, the defrost cycle is shortened by about 10 minutes and there is barely any increase in the interior temperature. On the other hand, an electric heater would use additional electricity and the defrost cycle would be longer.