re: advice on deep freeze
29 apr 1997
the tree by the river wrote:
>email@example.com (ben and theresa mesa) write:
>>just keep in mind that a chest freezer is more cost effective in the
>>long run, because when you open an upright freezer, the cold air goes
>>pouring out the bottom onto the floor (hot air rises, cold air sinks).
>now, to simplify things, i figured that we were perhaps talking about
>fifteen cubic feet of interior freezer space and that the entire
>volume of air was replaced every time the freezer door is opened.
conservative, as you say, although the fresh air will also condense
water vapor on the sides and shelves and contents of the freezer.
>i don't know what the efficiency of the refrigeration mechanism is,
anyone know this? it could be more than 100%, like a heat pump with a cop
of 3, moving 3 btu of heat for every 1 btu of electrical energy consumed.
>...the back-of-the-envelope calculation came up with about 0.006 kilowatt-
>hours of energy involved in cooling that fifteen cubic feet of air.
one btu heats about 55 ft^3 of air 1 f, so heating 15 ft^3 from 30 to 70 f
takes about 15/55x(70-30) = 10.9 btu, or 10.9/3410 = 0.0032 kwh of electrical
energy, at 100% efficiency.
>my electric bill shows a cost of about seven cents per kilowatt-hour,
>so that's about 0.0004 cents per door opening
0.0224 cents, by my calculation.
>if we open the freezer three times per day (since it's not our main
>freezer, that seems high if anything), that would require a bit more
>than two years to make up one dollar in energy savings.
100 cents = 0.0224x365x3xn ==> n = 4.1 years, figured this way.
>so, even if we're off by a fair bit, the air spilling out is probably
>not a major difference.
unless it spills out all the time, through a leaky door gasket? a crack the
width of a dollar bill (0.003"?) around a 3'x6' vertical gasket has an area
of 0.0045 square feet, like 2av in the empirical formula for chimney airflow,
cfm = 16.6 av sqrt(h dt), where h is the height in feet and dt the farenheit
temperature difference, so cfm = 0.58, so that gasket might (arguably) leak
833 ft^3/day, ie 833/55(70-30) = 606 btu of sensible heat or 0.178 kwh or
1.24 cents of electrical energy, about 20 times the daily door opening loss,
losing $1 in 100/(639/3410x7x365) = 0.21 years, ie 11 weeks.
and 70 f air at 50% humidity contains about 0.0012 pound of water vapor per
cubic foot, and 30 f air at 100% r. h. contains 0.00028 lb/ft^3, and a pound
of water vapor loses about 1000 btu in condensing and 144 btu in freezing, so
each cubic foot of room air puts about (0.0012-0.00028)1144 = 1 btu of latent
heat into the freezer, ie another 833 btu/day or 1.7 cents of electrical energy,
more than the sensible air heat loss from the gasket, making the time to lose
$1 roughly 100/(1472/3410x7x365) = 0.091 years or 5 weeks...
although we have to heat up the contents of the freezer to defrost it...
>offhand, i'd guess that the more cubic shape (meaning a lower surface area
>to volume ratio) of the chest freezer would have more to do with the
>generally lower operating cost.
my whirlpool refrigerator with freezer on top is about 27" deep x 66" high
x 29" wide, outside dimensions, with about 60 ft^2 of exterior surface, with
a gross interior volume of about 23x57x25/12^3 = 19 ft^3, with 2.25" freezer
walls and 1.75" refrigerator walls, including the outside metal and inside
plastic skins. this might make average r10 walls, with foamboard insulation,
so its thermal conductance, not counting air leaks, frost-free feature, or
"exterior moisture control" (leaving this door heater switch off), would be
about 60ft^2/r10 = 6 btu/h-f, so it needs about 24x6x(70-30) = 5760 btu or
1.7 kwh or 12 cents a day to stay cool owing to wall insulation, about 4 times
more than it needs for air leaks. dare i glue another 2" of foamboard all over
the outside to reduce this heat loss by half? seems like it needs a perfect
vapor barrier on the outside, to prevent condensation and rust on the metal
a 3' cubical chest version would have about the same interior volume, with a
surface of 54 ft^2, about 10% less. not a big difference, especially if the
condensing coils are underneath. and it takes up more floorspace, and food
becomes less accessible ("where is that pint of ice cream i made in 1982?")