How to cool a small amount of air?

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I have a project that requires me to cool about a cubic foot of air from room temp down to around 40 degrees F.  There are several products on the market that might be able to do this based on variations of melting ice or evaporative cooling ("swamp coolers"), but they add a lot of moisture to the air and I need it to be dry.  The only things I've been able to think of are portable air conditioners (which are overkill and bigger than I need) and something hacked together from several Peltier coolers.  I did a test with one with an airbrush compressor blowing air through a water block attached to the cold side and and a CPU cooler attached to the hot side, and it doesn't feel like it would scale.  If push comes to shove I'll just go with an air conditioner but thought I'd check here first for any suggestions.  Thanks for any tips.

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lautman wrote:
I did a test with one with an airbrush compressor blowing air through a water block attached to the cold side and and a CPU cooler attached to the hot side, ...
Might consider the same test with dry ice (solid CO2) from a local supermarket.

Continental Carbonic | Suppliers of Dry Ice & Dry Ice Blasting Machines

 

"Dare to be naïve." - Buckminster Fuller

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How quickly do you need to cool that volume of air ?
Is that volume of air (semi-) contained, or does air flow through it ?


Peltiers use a lot of power and their heat-transfer capacity isn't fantastic.
You will need to put the hot side of the peltiers totally outside of the metallic part of the chamber and also catch (or drain) the condensed water from the cold side.

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I could probably rig something up based on dry ice, except I'd have to keep replacing the ice.  I'm not sure how long my tests will last, so I need something that just runs on AC.

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mikech wrote:
How quickly do you need to cool that volume of air ? Is that volume of air (semi-) contained, or does air flow through it ?
Peltiers use a lot of power and their heat-transfer capacity isn't fantastic. You will need to put the hot side of the peltiers totally outside of the metallic part of the chamber and also catch (or drain) the condensed water from the cold side.

 

I could take 30 minutes or so to bring the temp down, and the volume can be constrained or open as required by the cooling.  I've been assuming constrained would work better and plan on making a box.  I don't think the power requirement of the Peltiers would be an issue.  I'd just need so many of them that THEIR cooling would become a problem.

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lautman wrote:

I'd just need so many [Peltiers] that THEIR cooling would become a problem.

 

Adafruit sells this:

 

Adafruit 1335 Peltier Module

 

--Mike

 

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Peltiers work best when the hot side is cooled a lot.

You could mount it between 2 CPU coolers. Heat pipe based or even liquid cooled with a small radiator.

With liquid, add anti-freeze.

 

Sometimes Peltiers are stacked in 2 layers to increase the temperature differential.

The hottest side often has 4 peltiers because it must also transfer the heat generated by the single peltier on the other side.

More than 2 layers seem to be useless. Efficiency is so low that you're just generating more heat.

 

Doing magic with a USD 7 Logic Analyser: https://www.avrfreaks.net/comment/2421756#comment-2421756

Bunch of old projects with AVR's: http://www.hoevendesign.com

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lautman wrote:
I have a project that requires me to cool about a cubic foot of air from room temp down to around 40 degrees F.
That's what refrigerators do.  Just turn the thermostat up a bit.   Unlike the old days, the air in the refrigerator is fairly dry.   

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steve17 wrote:

lautman wrote:
I have a project that requires me to cool about a cubic foot of air from room temp down to around 40 degrees F.
That's what refrigerators do.  Just turn the thermostat up a bit.   Unlike the old days, the air in the refrigerator is fairly dry.   

I was thinking the same thing.  Cheap office fridge?

 

https://www.amazon.com/s/s/ref=s...

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I don't know if a cheap office fridge will produce dry air.  Maybe one that has a freezer.

 

I'm no expert but it seems to me that the refrigerators I've looked at cool the air to around 0F.  I guess the moisture that condenses evaporates from a pan underneath.  The air is blown into the freezer.  The air then makes its way down to the refrigerator where it warms up to somewhere around 36F.   

 

According to this calculator, if the dew point is 0F and the temperature is 40F, the relative humidity is 18% which is very dry.

http://andrew.rsmas.miami.edu/bm...

Last Edited: Mon. Feb 11, 2019 - 11:50 PM
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kk6gm wrote:
I was thinking the same thing. Cheap office fridge?
You'll find that many of them are Peltier-based, e.g.:

https://www.amazon.com/SunRuck-refrigerator-Peltier-vibration-free-SR-R2001K/dp/B01EAG5D3I/ref=sr_1_1?keywords=refrigerator+peltier&qid=1549930744&s=home-garden&sr=1-1

"Experience is what enables you to recognise a mistake the second time you make it."

"Good judgement comes from experience.  Experience comes from bad judgement."

"Wisdom is always wont to arrive late, and to be a little approximate on first possession."

"When you hear hoofbeats, think horses, not unicorns."

"Fast.  Cheap.  Good.  Pick two."

"We see a lot of arses on handlebars around here." - [J Ekdahl]

 

Last Edited: Tue. Feb 12, 2019 - 12:20 AM
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You haven't said how long this cooling is to take or how long  it should stay cooled. The initial air volume will hold some moisture which will condense out when cooled. Is that important?

 

The simplest way to cool a cubic foot of air would be to place a cold block of steel in your container. Circulate the air over it. Place some silica gel crystals to absorb the condensation.

Ross McKenzie ValuSoft Melbourne Australia

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avr-mike wrote:

lautman wrote:

I'd just need so many [Peltiers] that THEIR cooling would become a problem.

 

Adafruit sells this:

 

Adafruit 1335 Peltier Module

 

--Mike

 

That's essentially what I rigged up, with a water block on top to "absorb" the cold.  It gets plenty cold, but blowing cold air through it doesn't chill the air that much, I assume because the air's not inside it long enough.  I've even tried using an extra long water block, to extend the amount of time the air's exposed to the block, but the results aren't much better.  That's why I figured it would take a lot of Peltiers, heat sinks, fans etc. to get what I want, and thought to come here for other ideas.

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steve17 wrote:

lautman wrote:
I have a project that requires me to cool about a cubic foot of air from room temp down to around 40 degrees F.
That's what refrigerators do.  Just turn the thermostat up a bit.   Unlike the old days, the air in the refrigerator is fairly dry.   

That;s an interesting idea.  Unfortunately, I need to put my hands into the space and conduct some experiments in the cold environment.  Once the door is opened, I think the temperature is going to start rising pretty fast.  I might be able to turn it into a glovebox, though, by replacing the door with a piece of clear acrylic with a couple of holes and a pair of gloves attached.  Need to think about it.  It'd be nice if I could get it to work since it would take up less space than an air conditioner and wouldn't need to be vented to the outside, a problem with my space..

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steve17 wrote:

I don't know if a cheap office fridge will produce dry air. 

I've got a hygrometer I'm going to put in my fridge right now.

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valusoft wrote:

You haven't said how long this cooling is to take or how long  it should stay cooled. The initial air volume will hold some moisture which will condense out when cooled. Is that important?

 

The simplest way to cool a cubic foot of air would be to place a cold block of steel in your container. Circulate the air over it. Place some silica gel crystals to absorb the condensation.

I'll only need it to be cold for about 10 minutes at a time, but I may need to perform several 10 minute sessions over the course of an hour or two.  I'm not sure yet if condensation will be a problem or not.  I can't have condensation on the apparatus I'll be placing in the space, but if I let the air condense first, it might give up its moisture that way, leaving none to condense on my stuff.  Silca gel's a great idea, regardless.  Thanks.

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Peltiers are what I first thought of, but you'd have to check how cold the ones you purchase can get.

Thermo is definitely an area I know very little about, but IIRC the rate of heat flow is directly proportional to the surface area and directly proportional to the Delta T.

That means you need a very large heat sink on the cold side to increase the cooling rate of the air.

If your Peltier only goes into the 30'F range, then your Delta T is also very low, so the rate of heat transfer will drop off the closer you get to your 40'F target.

 

I think I'd go with a small refrigerator.

A quick check shows that Walmart has a full lineup of small refrigerators, 1.7, 2.7, 3.2, 3.5, 4.3 Cu Ft, etc.

The 1.7 Cu Ft'er is $80, and has a real compressor.

That's dirt cheap for an environmental cooling chamber.

This one has a reversible door, so that means it is easy to take the door off, and there won't be any hidden thermostats, lights, wiring, etc., within the door.

 

As you suggested, replace the door with a hinged sheet of thick plexiglass, or make your own thermopane with two sheets and a partial vacuum between them.

It all depends on haw fancy you wish to get, and how many tests you need to run.

 

 Given that it has a mechanical thermostat, it should be easily hackable for a micro to turn it on and off, and give you a nice display of the current temperature.

 

Sounds like a cool project...

 

JC

 

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DocJC wrote:

Sounds like a cool project...

 

Heh heh

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Compress wanted volume of air then expand it. The temperature should drop markedly.

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lautman wrote:

steve17 wrote:

lautman wrote:
I have a project that requires me to cool about a cubic foot of air from room temp down to around 40 degrees F.
That's what refrigerators do.  Just turn the thermostat up a bit.   Unlike the old days, the air in the refrigerator is fairly dry.   

That;s an interesting idea.  Unfortunately, I need to put my hands into the space and conduct some experiments in the cold environment.  Once the door is opened, I think the temperature is going to start rising pretty fast.  I might be able to turn it into a glovebox, though, by replacing the door with a piece of clear acrylic with a couple of holes and a pair of gloves attached.  Need to think about it.  It'd be nice if I could get it to work since it would take up less space than an air conditioner and wouldn't need to be vented to the outside, a problem with my space..

 

Heres a possible solution to this.

 

As already noted just buy a small refrigerator used or new(they are cheap enough).

 

Now go get a sawzall (google it) and cut a window opening in the door and silicone a clear plastic 'window' where the hole now is.  Next, grab the sawzall again and cut two openings for your hands.  get some large vinyl gloves and silicone glue those to the openings.  now you can put your test gear in the fridge(it should have no issue with 40 degree temps) and add a small led light.  Then stick your hands in the gloves and do your tests.

 

THis is the closest example of the window and gloves.  Its a media blaster for cleaning rust of car parts and such:

 

Jim

I would rather attempt something great and fail, than attempt nothing and succeed - Fortune Cookie

 

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Atmel Studio6.2/AS7, DipTrace, Quartus, MPLAB, RSLogix user

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One of these babies:

 

"Experience is what enables you to recognise a mistake the second time you make it."

"Good judgement comes from experience.  Experience comes from bad judgement."

"Wisdom is always wont to arrive late, and to be a little approximate on first possession."

"When you hear hoofbeats, think horses, not unicorns."

"Fast.  Cheap.  Good.  Pick two."

"We see a lot of arses on handlebars around here." - [J Ekdahl]

 

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Image result for neonatal incubator

 

We use similar housing in medicine, but we usually heat the contents to keep it warm!

 

JC

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I have a project that requires me to cool about a cubic foot of air from room temp down to around 40 degrees F.

I think you left out a most important parameter...how much cooling (capacity) do you need?   Are you cooling down a wristwatch, a cell phone charger, a 2KW motor driver, or even a can of burning thermite?

 

The cheapest office fridge is prob sized to cool down very low heat producing things like food (in a not too hot room)....it probably has enough capacity for something that is not too energetic.

 

====

You could blow in some dry nitrogen, to 99% eliminate any moisture/condensation.  A small tank could last a long time.  We had a CO2 thermal chamber that would take blast of CO2

 and drop 100 degrees very rapidly, for thermal shock tests (CO2 got used up fairly quick, since our gear consumed a lot of watts).

    

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Tue. Feb 12, 2019 - 04:59 AM
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Adafruit sure knows how to ask money for stuff. USD 35 for such a small TEC?

https://en.wikipedia.org/wiki/Thermoelectric_cooling

These things sure can make something cold (although efficiency is low)

A single-stage TEC will typically produce a maximal temperature
difference of 70 °C between its hot and cold sides.

Directly from China these things are about the price (and size) of a few postage stamps.

https://www.aliexpress.com/wholesale?SearchText=tec

But it's not entirely fair, the Fan + aluminimum probably cost more than the TEC itself.

But still it's tempting to buy a few just to have fun with.

 

Elektor once had a fun project where they used one of these things with a candle and some LED's.

With the heat of the candle they were able to generate about 6x more light than the light from the candle itself

 

A few notes:

You don't want to PWM these things because heat generated increases with the square of the current.

You don't want to use on/off control because it adds a lot of thermal stress to the TEC.

A simple SMPS circuit with a big inductor and beefy MOSfet seems a logical choice.

 

Doing magic with a USD 7 Logic Analyser: https://www.avrfreaks.net/comment/2421756#comment-2421756

Bunch of old projects with AVR's: http://www.hoevendesign.com

Last Edited: Tue. Feb 12, 2019 - 09:06 AM
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Paulvdh wrote:
You don't want to use on/off control because it adds a lot of thermal stress to the TEC.

Analog Technologies - Best Thermoelectric Cooler Module | Low Cost TEC Modules

...

 

2. Q: What is the life expectancy of the TEC modules and how is it determined? 
...

For regular TEC modules, they can apply the full maximum voltage back and forth (switch the polarity) for up to 500 times. For thermal cycling TEC module, the number of times can be up to 3x10^7 times, the calculated MTBF was 125,000 hours.

Analog Technologies has a 15A TEC controller priced at about an order of magnitude more than a TEC module (current sourced from one or two SLA batteries in 1S or 2S)

A test chamber to hold room temperature when ambient is approximate to room temperature : 

constant_temperature_chamber - Analog Technologies, Inc.

otherwise Analog Technologies' temperature control products are mostly for control of spot temperature.

 

"Dare to be naïve." - Buckminster Fuller

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My circa mid 80s Coleman portable ice chest has a TEC module (Peltier, heatsink, fan) powered from 12V automotive that feels like 40F (4C, IIRC that's the maximum temperature for short duration food storage)

Its lid hinges might be secured by screws (been awhile)

It's way more portable than a mini fridge.

 

edit:

much improved (on sale?)

https://www.coleman.com/coolers-drinkware/40-quart-powerchill-thermoelectric-cooler/Thermo40.html?cgid=coleman-coolersandwaterjugs&dwvar_Thermo40_color=silver#prefn1=coolerType&prefv1=Thermoelectric&start=1

its web page :

40 Quart PowerChill™ Thermoelectric Cooler | Coleman

 

edit2 :

40 quarts are 38 liters

 

 

"Dare to be naïve." - Buckminster Fuller

Last Edited: Tue. Feb 12, 2019 - 01:33 PM
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gchapman wrote:
My circa mid 80s Coleman portable ice chest has a TEC module (Peltier, heatsink, fan) powered from 12V automotive that feels like 40F

We used the same setup for our "environmental" chamber with a polarity reversing switch so it could heat as well as cool (5C to 40C), this worked for several years until it was replaced with the real thing when a project came along that required an extended range not reachable by the cooler.  We drilled a 1.5 inch hole in one side to allow temperature and scope probes in for testing.

 

Jim

 

 

(Possum Lodge oath) Quando omni flunkus, moritati.

"I thought growing old would take longer"

 

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A single-stage TEC will typically produce a maximal temperature
difference of 70 °C between its hot and cold sides.

And just to be clear for those who may not have worked with one of these before, read that spec closely.

 

That's a Delta 70'F between the hot and the cold sides of the device, NOT cooling 70'F below ambient temperature.

The "hot side" heats up, while the "cold side" cools down.

So the hot side might be 40'F above ambient, while the cold side is 30'F below ambient, for a Delta of 70'F, and a 30'F drop below the ambient temperature.

 

JC 

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@doc: You've got the delta's right but mixing up C & F a bit devil

Doing magic with a USD 7 Logic Analyser: https://www.avrfreaks.net/comment/2421756#comment-2421756

Bunch of old projects with AVR's: http://www.hoevendesign.com

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A caution, here.

 

Lautman talks about "an experiment". If this has parts (in different places) are temperature sensitive, then you can have significant temperature gradients that can upset things substantially. I was called in on such a project by some students that I work with. They were trying to establish  the temperature sensitivity of a bridge-type load cell. Two arms of the bridge were on one side of the the springy part and the two other arms were on the opposite side. They were heating quite rapidly with a lightbulb in a styrofoam box. Their readings were crazy. It turned out that they were taking readings far too fast in a box with no air movement and opposite sides (of the otherwise temperature compensated) bridge were at different temperatures. 

 

So, provide sufficient internal air circulation and allow enough time to stabilize!

 

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

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I'm with gchapman on this.  Here's another idea for roughly the same price:

 

https://www.amazon.com/Wagan-621...

 

There are lots of similar ones on Amazon.  I had one in my old bus for awhile.  It was about of the quality one would expect for that price - it worked.

 

They have a couple advantages, running off nice friendly 12V (although you will need some serious amperage here...  That one's going to want 4A), and top-entry, thus most of the cold air will stay in the box when you open the lid.

 

Disadvantages definitely include lack of power.  That one's rated at 48W, and given that half of that (at least!) will be thrown away as waste heat, you'll only be removing at best 24 watts of heat.  If you put 'already-warm' stuff in, expect to wait awhile before it cools down.  S.

 

PS - If you really want a trace of silliness, LN2 (liquid nitrogen) isn't that expensive... 

 

https://procurement.uark.edu/e-p...

 

Edited to add price list from Univ. of Arkansas, not because I have anything to do with them but because it turned up first in a relevant Google search result.  S.

Last Edited: Tue. Feb 12, 2019 - 08:57 PM
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I just read:

How cool is a smart amount of air?

Me thinks it's time to go to bed now.

Doing magic with a USD 7 Logic Analyser: https://www.avrfreaks.net/comment/2421756#comment-2421756

Bunch of old projects with AVR's: http://www.hoevendesign.com

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How about an ice chest cooler with a heat exchanger. I've seen a few videos of DIY units. Here are commercial ones:

 

https://www.icybreeze.com

 

https://frankencooler.com/

 

As a quick hack, you might be able to use an ice chest cooler, but put all the ice in sealed containers (e.g. empty drink bottles)

 

- S