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Uncle Monster[_3_]

larry wrote:
wrote:
larry wrote:
wrote:
Uncle Monster wrote:

... The heat is just enough to trick the thermostat, not to warm the
refrigerator compartment.
How would that accomplish the goal?
The slight heat produced by the heater is enough to raise the
temperature
around the thermostat to trick it into running the compressor for a
longer
period of time. It's not necessary to increase the temperature of
the whole
refrigerated box, just the temperature around the thermostat.
That doesn't make sense to me...
It's like the timer with a 10K resistor clipped under a regular wall
thermostat to do an over-night "setback". Thermostat thinks it's
getting warmer and turns off the furnace when the room isn't warmer.

But that makes the room colder at night. How do we keep the fridge
a constant 40 F and the freezer frozen in a 40 F room?

If the freezer stays 0 F in a 60 F room while the 40 F fridge box
is gaining Q Btu/h from the room, cooling the room to 50 will reduce
the fridge gain to Q/2, so the compressor will run half the time.
But the required freezer run time will only go down by 50/60, so
we need to add enough heat to the fridge box (vs the thermostat box)
to raise the run time back up to 50/60 of the 60 F room run time, no?

The fridge box won't absorb any heat at all from a 40 F room, but
the freezer still needs 40/60 of the 60 F room run time to stay 0 F.

Nick

The way the closed loop control works on modern frig's is working
against you. You need a little redesign.

Given:
1. the freezer, goal=0F, always needs cool in your ambient
temp range, 40F to 80?F.
2. the coldbox (frig - i hate these terms), goal=36F, needs cool on hot
days and maybe none on cold days.

Plan:
1. freezer, meet the demand first, put the thermostat for the freezer IN
THE FREEZER, where the coils are to begin with. Closed the loop on the
freezer, it will take care of itself.

2. Coldbox. Block the existing air path between the freezer and the
coldbox below. Control that airflow with a moving vane and and fan
connected to a thermostat in the coldbox to cool the coldbox with cold
air from the freezer to the target temp 40F.

Cheap way of doing the coldbox control is a bimetalic strip in the
coldbox that pushes the vane open as the temperature exceeds 40F. A
small microswitch could switch on the fan as long as it detects the vane
is open.

Just about everything you need is already in the frig. Take up the
baseplate in the freezer, you will see the cooling coils, the air path,
with vane, to the coldbox, and a fan near the back, that circulates cool
in the freezer and coldbox. Plus a big heater to thaw the coils. There
is a schematic in an envelop near the compressor or defrost tray.

I don't know how this will affect defrost cycle.

second thought, close off the kitchen and roll the frig to the living
room... You can't be married... ;-)

-- larry/dallas

It's all about cost. The commercial refrigeration
systems I install/build/repair usually work well
regardless of the ambient temperature. I'm about
to move two walk in coolers and reinstall them
where the condensing units will be in a back room
rather than two stories up on a roof. The units
will work without a problem because of their more
sophisticated control system than a home refrigerator.
The control systems use not only thermostats but
pressure and fan controls to maintain the temperature.
I've worked on Sub-Zero brand refrigerators made
for home use that were built like commercial coolers
with semi-hermetic compressors and commercial controls
but GOLLY! those damn things are expensive. They will
work in just about any ambient temperature but the
cost is out of my range. I could always take a standard
home refrigerator and modify it for a lot less money.

[8~{} Uncle Monster