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re: refrigerator -keep it full?
2 may 1997
mark a. yedinak  wrote:
>polar wrote:

>>         i'd always heard that refrigerators should be kept full to
>> work at maximum efficiency.

if the door is closed, full or empty won't make much difference. if the door
is opened once in a while, empty seems better, because the greater the surface
area inside the refrigerator (eg the more stuff inside) the more heat will
transfer to those cold surfaces from warm room air that circulates around them
when the door is open. this is known as "newton's law of cooling," invented by
isaac newton, who also brought us gravity and calculus :-) 

>>         an engineer friend has just debunked this theory as follows:
>>         he says the basis for the "theory" was that keeping refrig.
>> full decreased the volume of air which might escape when door opened.
>> the warm air entering, would then have to be cooled.

it seems to me that as long as the door is open, room air circulates through
the fridge, all the while warming the contents, if any. it's not like a single
slug of room air replaces a single slug of refrigerator air, and then airflow
stops. a 6' tall fridge might have 16.6x2ft^2sqrt(6'(70f-40f)) = 500 cubic
feet per minute of air flowing through it when the door is open. if the door
is open for 10 seconds, 80 ft^3 of air might flow through the fridge.

suppose an empty 3' cubical fridge contains 27 ft^3 of space and 54 ft^2 of
wall area, and it's 30 f inside and the door is open 10 seconds, allowing
70 f room air to circulate through the fridge. suppose when the door closes,
all the air in the fridge is 70 f and it needs to be cooled to 30 f again.
since 1 btu can cool about 55 ft^3 of air 1 f, cooling that mass of air
requires 27(70f-30f)/55 = 19.6 btu (0.006 kwh.) but while the door was open,
the circulating air heated the 54 ft^2 of interior surface (slightly) with a
thermal conductance of about 1.5 btu/h-f-ft^2, by about 10/3600(70-30)54x1.5
= 9 btu. so opening the door added about 29 btu to the empty fridge.

now suppose we fill the fridge with 216 2 liter soda bottles (each about 12"
tall x 4" diameter, with about 1.2 ft^2 of surface area) in 3 layers of 72
bottles, in a nice regular array, with air free to circulate around them, and
open the door for 10 seconds. when the door closes, we now only have to cool
27 ft^3 - 216x2/28.32 = 11.75 ft^3 of air, which requires only 8.5 btu, but
while the door was open, we heated 54 + 216x1.2 = 313 ft^2 of surface by
10/3600(70-30)313x1.5 = 52 btu, adding a total of about 60 btu to the fridge,
twice as much heat. (and perhaps more importantly, we also condensed a lot
more water vapor onto that greater surface, at 1000 btu/lb. the vegetables
inside may also have cooled themselves by evaporating more water, which
dripped out through the bottom of the fridge and evaporated into the room.) 

>> ...the energy expended to cool the water bottles, or whatever, is much
>>greater than than which would be expended to cool the air entering a
>>near-empty frig.

true, but that's like comparing apples and oranges, or non-recurring and
recurring costs, and it's related to that old myth that setback thermostats
don't save money. we only need to spend the energy to cool the thermal mass 
of the water once, not every time the door is opened.

>your engineer friend is incorrect. the reason a refridgerator runs more
>efficiently when full than when empty...

does it? it might, if the compressor cycles less. but years ago (ca 1930),
when people cared more about energy efficiency (and today, in sunfrosts),
refrigerators used to have "heat accumulators," thermal mass on the outside of
the fridge, attached to the hot coils on the back, to allow them to stay warm
even when the compressor was not running, so they could more efficiently
transfer heat to the room on a full time vs a part time basis, keeping the
hot coils cooler (while making hot water?) i guess empty fridges tend to
cycle more often, ie work more in this way. 

>is because the refridgerator itself is not required to work as much.

a full fridge has to work harder at first to cool the contents, but after
that, things are about the same. that initial cost is a lot less than the
recurring cost, over a few days or more. 

>when an empty refridgerator is opened and then shut again, the warmer air
>which entered needs to be cooled again. the only thing which will be able
>to do this is the fridge itself.

agreed, but the objects the warm air warmed while the fridge was open also
need to be cooled (and dried) again. the fewer things inside, the less of
that cooling is required. 

>...when the fridge is full, those items take longer to warm, therefore they
>stay cool even after the fridge has been opened and closed. these items now
>help in cooling the air around them.

agreed, but that doesn't seem relevant to this kind of energy consumption (vs.
"thermal mass refrigerators" powered by outdoor coolth, eg in earthships.)

>so, try to keep the fridge full. it will run more efficiently because it
>will need to run less.

so, try to keep the fridge empty. it will run more efficiently because it
will need to run less. 


it's a snap to save energy in this country. as soon as more people become
involved in the basic math of heat transfer and get a gut-level, as well as
intellectual, grasp on how a house works, solution after solution will appear.

                                          tom smith 

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