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re: most efficient refrigerator
27 jan 2004
daestrom  wrote:

>> >...a 'full' unit has less free air to 'spill'.  which is why a unit
>> >that is full or nearly so runs more economically than an empty.
>> otoh, it has more surface to instantly condense water vapor
>> from room air with a 50 f dewpoint when the door is open...
>but if you open the door to fetch something then close it, the total amount
>of water vapor let into the 'full' one is less...

that could be true of a chest freezer, but air flows continuously when
a fridge door is open, and the driving force is the temp diff...

>in both, the water vapor that gets in will eventually be condensed.

imo, "the" water vapor would be condensed very quickly, while the door
is still open and the air is still flowing. 

steves   wrote:

>...what's worse: cooling more incoming hot, humid air or replacing more 
>'coolth' lost through the insulation?

that isn't the choice we are talking about, but...

>suppose a 2'x3'x5' tall fridge has r10 insulation...

with g = 62ft^2/r10 = 6.2, so it gains 24h(70-36)g = 5059 btu/day of heat
through the outside surfaces... 

>and we open the door 1 minute 4x per day, and each time
>16.6'x3'x5'/2sqrt(2.5'(70-36)) cfm of 70 f air at 50% rh
>flows through the fridge,

totaling 4591 ft^3, about 344 pounds of air with w = 0.00787 pounds
of water vapor per pound of air...

>condensing water vapor until the outgoing air has 100% rh at 36 f...

air at 36 f and 100% rh has a vapor pressure of about e^(17.863-9621/(460+36))
= 0.171 "hg, so w = 0.62198/(29.921/w-1) = 0.00357. if 344(0.00787-0.00357)
= 1.48 pounds of water vapor condenses out of the room air, that adds
about 1480 btu/day of heat to the fridge, ie about 23% of the total,
ignoring the sensible heat gain from the airflow...


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