Panache
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'Cold sinks'
This seemed to best place to ask the question. I have an ultra-low freezer i bought for a song (actually i bought two for a song but one is without
ethane so its's a big desiccator).
Unfortunately the one that was working had no shelves. I queried Sanyo on the cost and they were joke prices, like several hundred dollars. Instead i
made some of my own shelves by freezing slabs of water in my conventional freezer and using them as shelves in the ultra-low, propped with upright
slabs of ice, kind of has the 'student living' kitsch thing happening.
This turned out to have the unexpected side benefit of making the freezer much more thermally stable after opening or closing it, due to the amount of
material in the freezer at -85C.
Two questions,
1) Does having all this ice in there (about 100litres i used i think) mean its using anymore energy holding the thing down at that temperature.
Intuitively i think not, once i have spent the energy getting the ice to -85.
2) Does anyone know of an aqueous solution of a benign substance that when taken to -85 will solidify but will take far more energy to heat up, ie
whose specific heat capacity is far greater than that of water? This way i boost the ability for the freezer to stay cold on those hot 45C melbourne
days, as well as having an excellent 'coolant' block for ready dispensation into coolant baths.
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garage chemist
chemical wizard
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I think having open ice in such a low-temperature freezer is a bad idea. It will sublimate onto the evaporator coils, causing thick frost buildup on
them which acts as thermal insulation, making efficiency and cooling speed drop and power consumption skyrocket.
There's a reason why freezers have to be defrosted periodically because of aerial moisture causing frost buildup. Open ice will constantly provide
aerial moisture inside the freezer.
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497
National Hazard
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I doubt you're going to find something with a heat of fusion much better than water.. But having an added substance to lower the freezing point might
be helpful. If you use straight water, it wouldn't start actually helping cool much until above 0*C, but with something like glycol-water it would
kick in down at -40*C... and I'm sure there are better additives than that. I'm not sure how the structural integrity of the ice slabs would be
effected by the additive..
Since solid CO2 would keep at those temperatures, having a large amount of it (maybe make the shelves out if somehow?) would provide a huge "buffer"
effect as soon as the temp got up to -78.5*C... It would also absorb twice as much energy pound for pound compared to ice. And it would conveniently
sublimate to gas instead of forming a puddle...
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Panache
International Hazard
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Quote: Originally posted by garage chemist  | I think having open ice in such a low-temperature freezer is a bad idea. It will sublimate onto the evaporator coils, causing thick frost buildup on
them which acts as thermal insulation, making efficiency and cooling speed drop and power consumption skyrocket.
There's a reason why freezers have to be defrosted periodically because of aerial moisture causing frost buildup. Open ice will constantly provide
aerial moisture inside the freezer. |
It doesn't have any coils, just walls which it is recommended you scrap the CO2 off from time to time. The main ice build up comes around the second
door seal but as its some polymer construction the ice does not adhere to it at all so can be snapped off. I only place ready frozen blocks in their
so i imagine water's vapour pressure above ice at around -85C would be quite low, but i don't know
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Panache
International Hazard
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| Quote: Originally posted by 497 | I doubt you're going to find something with a heat of fusion much better than water.. But having an added substance to lower the freezing point might
be helpful. If you use straight water, it wouldn't start actually helping cool much until above 0*C, but with something like glycol-water it would
kick in down at -40*C... and I'm sure there are better additives than that. I'm not sure how the structural integrity of the ice slabs would be
effected by the additive..
Since solid CO2 would keep at those temperatures, having a large amount of it (maybe make the shelves out if somehow?) would provide a huge "buffer"
effect as soon as the temp got up to -78.5*C... It would also absorb twice as much energy pound for pound compared to ice. And it would conveniently
sublimate to gas instead of forming a puddle... |
I don't agree necessarily. If i open the empty freezer and place a jar of nuts inside, there is heat flow from the nuts to the air inside the freezer.
There is a rise in temperature that would equal the mass of the air and multiplied by its specific heat capacity. If this rise was large enough the
compressor would kick in and remove that heat to bring the temperature back down tot he set point.
If you had 100kg of ice in there also, averaged over all this mass the temperature rise is far more negligible and hence the conditions remain more
stable.
That's my take on it.
The dry ice is a good idea however it still must have appreciable vapour pressure above the solid at -85 because if i leave an open container of dry
ice in there it quickly disappears and reappears over the walls of the freezer. Covered it keeps for a very long time.
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Panache
International Hazard
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Could someone perhaps check a table in the CRC and confirm my interpretation of it. The Table in my 56th edition is at D-103 'Heat Capacity of Aqueous
solutions of Various Acids'.
Assuming a degree of linearity (as the figures quoted are all for 25C), the table suggests that Propionic acid solutions require the greatest amount
of heat input to raise their temperature. I assume the reverse applies to lower their temperatures.
Also what are the first two columns (n and m, (both in italics)) measures of, it is not specified, i assume they are some measures of concentration.
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kosec
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blocks of solid isopropyl alcohol might do the trick if the fridger goes down to -89°C, otherwise blocks of water saturated with CaCl2
Jonny was a chemist's sun,...
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Sedit
International Hazard
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Why exactly can't you just use a sheet of metal again? Why the need for a frozen liquid?
Knowledge is useless to useless people...
"I see a lot of patterns in our behavior as a nation that parallel a lot of other historical processes. The fall of Rome, the fall of Germany — the
fall of the ruling country, the people who think they can do whatever they want without anybody else's consent. I've seen this story
before."~Maynard James Keenan
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BromicAcid
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Mixing saw dust with water and freezing provides a material that is more workable. It can be sawed and such easily and is not brittle (you can hit it
with a sledge hammer). Additionally it is safe from melting, if left out in the summer heat it can take weeks to melt entirely (a reasonably sized
piece). There had been a plan in WWII to make a ship out of the material and it has been re-evaluated recently for a number of purposes, you should
be able to google it.
As other posters have said though, what about other materials of construction? What about just wood? Easy enough to build from.
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Texium
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Thread Moved
19-11-2023 at 14:59 |
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