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re: cost to run freezer
9 nov 2001
karen rathmann wrote:
>the available surface area is not a limiting factor in determining how
>much water condenses from the air that enters when the freezer door is
>momentarily opened and closed. what is a limiting factor is how much
>warm, moisture-laden air enters the freezer each time the door is opened.
or maybe the total amount of air that flows through (vs enters and stays),
eg in at the top part of the opening and out the bottom part, during the
time when the door is open, in an upright. but that isn't everything. if
a large amount of room air flows through, but only a small proportion of
that air touches cold surfaces, the freezer may not gain much heat.
>a freezer stacked to capacity will only have a small amount of
>air enter and therefore the total moisture content will also be small.
this could be true for a chest freezer, but in that case, most of the
cold air stays in the box when we open the lid at the top, since it is
denser than room air, no? and an upright "stacked to capacity" may have
lots of 1" airspaces between containers, which makes it a much more
efficient heat sink than an empty box, if air moves through the spaces.
>otoh, a freezer that is mainly empty can have a large exchange of warm,
>moist air to replace the cold, relatively dry air.
sure. the cold dense air initially slides out the bottom part of the
opening, but what causes the entering air to move after that? the air that
enters the top needs to lose moisture and cool in bulk to become denser
and move down. that requires heat transfer and heat transfer surface.
the more surface, the faster the flow, and the more condensation.
>then after the door is closed, the warm air cools and the water vapor
>condenses out and freezes - thereby consuming electrical energy both
>for cooling the air and for freezing the water vapor.
i suspect the main energy loss involves the amount of air that flows
through the freezer when it is open, vs the amount trapped inside when
the door closes. i suspect most of the condensation occurs when the door
is open, vs after it closes.
>the available surface area doesn't play a role since the water vapor
>will still find a place to condense and freeze - if there's not much
>area it will just form a thicker layer on the area there is.
the air has to move to do that. air only contains about 1% water vapor,
which has a hard time moving through the 99% non-condensable layer at
the cold surface, and thick layers of condensation have more thermal
resistance than thin ones.
>the full freezer will take more energy to initially cool down when first
>installed,
sure, but who cares?
>but will consume less on an ongoing basis due to smaller exchanges
>of warm and cold air on each door opening.
i doubt that.
>the least total energy would be consumed by a freezer that's largely
>filled with empty but sealed containers - these don't take much energy
>to cool initially
again, that seems unimportant, unless you turn off your freezer and
let it come up to room temp more than once a week :-)
>but still keep warm air from entering when the door is opened.
if you are determined to fill a freezer with containers, empty ones may
reduce the door-opening energy loss vs water-filled ones, but maybe not
much. i suspect that the walls of empty containers still have enough heat
capacity to condense almost as much water out of the room air that flows
through the freezer during the relatively short time that the door is open,
and i still imagine that an empty freezer uses less energy than a full one.
but you've raised some doubts in a way that makes more sense to me than
someone who writes "i believe this because the government tells me so."
thanks for your thoughts.
nick
ps: i estimate that starting a fridge compressor consumes about 1 btu...
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