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re: refrigerator -keep it full?
13 may 1997
doug mounce wrote:
>...once the water gets frozen, the cycles return to a typical on-off pattern
>needed to keep the water frozen...
typical cycling: a 20 ft^3 freezer absorbing 500 btu/hr of room heat through
the walls, with a 1,000 btu/hr capacity cooler will run 12 hours per day,
consuming about the same energy, whether it's full or empty.
if it's empty, with say, 50 btu/f of inherent thermal mass, and the cooler
turns on at 2 f and off at 0 f, it will take 50x2/(1000-500) = 0.2 hours
to cool the inside from 2 to 0 f, and the room will warm it from 0 to 2 f
in 50x2/500 = 0.2 hours, so the cycle time will be 0.4 hours or 24 minutes.
if the freezer contains 20 ft^3 of ice with a heat capacity of 640 btu/f
(half that of water), the cooler will take 640x2/(1000-500) = 2.56 hours to
lower the ice temperature from 2 to 0 f, and the room will warm the ice from
0 to 2 f in 640x2/500 = 2.56 hours, so the cycle time will be 5.12 hours.
each time the freezer turns on, it consumes about 1 kw for about 1 second.
in the first case it turns on 60 times per day, in the second, about 5 times
per day, so the difference in energy used for starting is about 55 btu/day,
ie about 0.016 kwh/day, worth about 0.0016 cents at 10 cents/kwh, a penny
or so every 2 years.
nick
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