[time-nuts] Temp/Humidity control systems?
jimlux at earthlink.net
Thu Oct 27 10:06:47 EDT 2016
On 10/27/16 6:30 AM, Poul-Henning Kamp wrote:
> In message <C1109A57B22F4DAB86B0BB172981F264 at Alta>, "David J Taylor" writes:
>> You can buy the smallest "window" airconditioner and "plumb" it to your
>> chamber (I used dryer vent hose, cardboard, and lots of duct tape)
>> Attached is a plot temperature and RH of an insulated box about 1.2
>> meter wide, 2 meters tall and 60 cm deep, filled with 100 or so 750 ml
>> bottles of liquid.
> You can do *much* better with an old fridge and a small waterpump to
> circulate water in the cooling loop.
I tried that first...
Advantage is that the mass of the water (serving as a thermal transfer
medium) is much greater than that of air. You wind up with kilos of
water at a relatively constant temperature.
1) It leaks
2) It grows stuff (even with additives to prevent it)
3) you've increased the number of thermal transfers: refrigerator coils
to air to water to air to contents of box. Both of the air:water
transfers are not particularly efficient in a "cobbled together in the
garage" sort of scenario.
I think your suggestion of just adding (solid) mass to the system is
better - the advice to people with wine cellars is to fill empty slots
with bottles filled with wine (actually, the advice is to buy more wine,
so your cellar doesn't have any empty slots.. but if that's not
possible, fill the empties with water, recork and stow them)
The challenge with a refrigerator as chiller is that getting decent
coupling from the cooler coils to the water is tough: you're pretty much
restricted to air as the transfer medium. In a "real chiller" they put
the evaporator coils in the water so there's good thermal contact.
Refrigerators/freezers aren't made for this - I tried 3 different
approaches of varying complexity:
1) Put a 5 gallon plastic bucket of water in the refrigerator - the
bucket of water does get cold, but--- it also evaporates inside the
refrigerator, and the water condenses on the coils, freezes, and then
eventually is lost to the air when the unit is defrosted.
2) Put an array of copper tubing in close contact with the cold plate in
the freezer, weighted down by bricks to make close contact - well, let's
just say I found all sorts of interesting galvanic reactions can occur,
even at low temperatures - the other problem is that if the circulation
rate slows, the water in the loop can freeze, and once it starts to
freeze, it has positive feedback - the flow rate slows even more, and
pretty quickly, you have tubes full of frozen coolant. - it is a good
thing I was doing this in the garage.
3) trying to make a cold plate by using two sheets of aluminum, some
aluminum spacers, plenty of silicone, and some hose barbs is a lot of
work, and doesn't seem to work much better.
I think one problem is that the refrigerator/freezer control system is
designed to work off two sensors: one is a air temperature sensor and
the other is a sensor on the actual evaporator unit (basically, if it
gets too cold, it shuts off the compressor to prevent low pressure
damage). The "design point" for all of this is also probably not
optimum for moving heat out of your equipment closet.
And that's just the challenge on "improvised water chiller"
Then you have the other "water to air heat exchanger".. serpentine
tubing would seem to be the best way, but it turns out that this is
non-trivial to design so that you get even flow rates in multiple loops,
if you have multiple paths. And, arranging the tubing effectively is
hard. There's also all the fabrication/leakage/hose connection issues.
I tried making serpentines out of copper and aluminum tubing that would
be part of the shelf on which the bottles are piled. That cools the
bottom bottles nicely, but the thermal transfer among the bottles is slow.
The best approach to "get cold to all bottles" (or, more correctly, take
heat from warm bottles" is to have a fan to circulate air among the
stacked and racked bottles. Well, once you are rigging up a fan to push
air through cold tubing (a re-purposed car heating core - more
fabrication of adapters from one tubing size to another - I had a big
box of hose clamps, between size adapters, and pieces of hose of all
sizes). Remember that this is all wet at one point or another, either
from leaks or condensation, so stuff corrodes, rusts, etc.
Yep - a $99 window airconditioner bought on sale (about this time of
year is good, in the Northern Hemisphere) worked just fine. Plumbing
air is a lot easier than plumbing water or glycol coolant. You won't
get down to <10C because window air conditioners aren't refrigerators -
the choice of refrigerant and internal components and set point range
isn't compatible with that (and putting a small heater on the AC's temp
sensor, which is in the return air flow to the evaporator did not allow
me to "bias" the set point)
However, for a time-cave - I think it would work great - cheap
airconditioner, large thermal mass buffer, well stirred air. Concrete
blocks with holes and have the air blow through that would make a fine
low pass filter - keeping the humidity "reasonable" is probably
straightforward - most of time-nuts stuff isn't humidity sensitive - as
long as it's "non-condensing". You'd just want to watch and make sure
that you don't have cold air blowing on a metal surface intermittently -
I worked in a screen room (solid walls) that had the AC blowing on the
walls.. when the AC cycled off, the walls instantly started to sweat as
our respiratory moisture condensed on what was now the coldest thing in
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