NY wrote
Mr Pounder Esquire wrote

When I left the trade 18 years ago I remember there was a thing called a
duel differential thermostat. This somehow sensed the temperature in the
freezer.

The old days of twin compressors and a stat in the freezer compartment I
think are long gone.

Er hang on. Are you saying that modern fridge-freezers only have one
thermostat?

Yep, and its trivial to check that.

How do they maintain different temperatures (4 C and -18C) in the fridge
and freezer compartments?

By ensuring you get that result when the the fridge thermostat
maintains 4C etc, by how the flow of cold is done in.

Surely the freezer compartment will need more frequent cooling (ie a
heavier compressor duty cycle) because it is set to be at a colder
temperature and therefore for the same heat loss through the walls will
tend to warm it up more to room temperature.

Not when you design it so the coldest air with a frost free
goes into the freezer part first and then to the fridge part.

Or do modern fridge-freezers make some naive assumptions that if the rate
of heat loss in degrees per square metre is the same for both cabinets,

Nope.

then as long as the freezer gets a greater proportion of the refrigerant,
a single thermostat in the fridge compartment can control both compressors

There arent two compressors.

(indeed, they are the same compressor) and as long as the fridge is
regulated to temperature, the freezer will be also?

Yep.

I can see how a single thermostat and carefully-proportioned refrigerant
flow

It isnt done like that. The freezer gets the coldest air
first with a frost free and then the rest of the fridge.

could achieve two different temperatures, but not why it would
*inherently* impose a very high lower limit on the room temperature.

That limit with the room temp is due to something
else entirely, the refrigerant used in the compressor.

I can believe that some refrigerants won't work as well at lower
temperatures because they may solidify instead of remaining liquid.

Not just that, their latent heat varys too.

But I can't believe that a single thermostat in one cabinet can correctly
regulate the temperature in the other without making some very naive
assumptions.

Its not a naïve assumption, its careful design.

You say "[the dual differential thermostat] somehow sensed the temperature
in the freezer" as if it was difficult or required ingenuity, whereas
surely each cabinet has its own bimetallic strip thermostat, set to a
different cutoff temperature, and then you either have two separate
compressors and separate refrigerant circuits or else if you want to cut
costs you have a single compressor, controlled by both thermostats using
"OR" logic, with one valve per cabinet, controlled by its thermostat,
which directs refrigerant where it is needed at that time - in the same
way that a central heating boiler that also heats a cylinder of water
usually has a single boiler and pump but valves to direct the heated water
where it is needed (CH / HW / CH and HW).

That obscenity of a sentence should be taken out the back and beaten
to death with the largest waddy you can find, before it multipys }-(
http://www.wordnik.com/words/waddy

I'm sure I'm about to be educated... :-)

Remains to be seen if that is even possible now }-(