Cooling an Environmental Chamber with Dry Ice. What's the coldest you can get the air temp down to? 3

[Shockdesigner331]

I've got an insulated 2 Part ev chamber. One part of the chamber holds many blocks of dry ice, the other part holds the test specimen to be cooled. 2 insulated ducts with fans push and pull air through the chambers circulating the cold air.

After a day of circulating the cold air the AIR temperature flat lines at -45°F. I know dry ice is -100°F. What is the coldest temp that I can get the air to?

The EV chamber is quite well insulated. Outside of the chamber is not cold to the touch, no condensation visible, fans running with no problems. I need to get the air to a lower temp.


Any thoughts? thanks.

 

[ornerynorsk]

Can you make use of liquid nitrogen instead? Much colder.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[btrueblood]

Try turning off the fans (which are dumping some amount of heat into the chamber), and putting blocks in the first chamber in addition (more blocks = more surface area = more cooling). Also try pelletizing/crushing the blocks (creating more surface area) and placing it in wire baskets.

Dry ice sublimes at about -75F at std. atmospheric pressure. You have enough heat leakage to drive a 30F delta-T.

 

[Shockdesigner331]

Thanks guys. We've got a solenoid operated valve for a liquid nitrogen tank somewhere.... Definitely thinking I'm going to have to go that route.

btrueblood - The first chamber is loaded with dry ice, we tried maximizing the surface area, leaving airflow space around each stack of blocks. We've got 10 of the 10" x 10" blocks in there. We'll try adding some blocks to the other chamber.

So -75° would be the coldest that we'd ever even hope to get to with all things being perfect.

 

[racookpe1978]

I use pelletized dry ice, not blocks. Much greater heat transfer air per mass of CO2.

Look again at your "well-insulated" outer walls. What actual temperature is each part of each wall? Use a IR/non-touch thermometer - or better a heat camera - then correct the corners, joints, and hinge areas that are coolest. Try again, then insulate the next cool area.

 

[Shockdesigner331]

racoope1978 - What temperature are you able to get the air to in your setup with the pelletized ice?

 

[ctopher]

We use liquid helium to test space/rocket units.
-270°C, -452°F

Chris, CSWA
SolidWorks '15
SolidWorks Legion

 

[MintJulep]

To cool a system you need to remove heat from the system.

Your setup doesn't do that.

 

[arbreen]

"To cool a system you need to remove heat from the system."

The system in question is basically the mass of air in the chambers. If it wasn't working at all, the chamber would be at room temperature not 45 below.

The flatline air temperature is just the equilibrium point between heat transfer in from outside the chamber and heat transfer into the dry ice. Plus added heat from the fans, which may or may not be significant compared to the other two values. You can lower the temperature by reducing heat transfer into the air or increasing heat transfer out of the air. More insulation is probably the hard way unless you have a big heat leak (at the very least we know it's not a sealed environment, might be worth a look). Increasing surface area of the dry ice or placing it so that either the fan or natural convection get it more airflow is one way to increase heat transfer out of the air. So is using something colder, like liquid nitrogen.

If your dry ice chamber is deep and the vents are at the top, you might not be getting circulation of the cold air at the bottom which would make a big difference.

 

[Compositepro]

1. If your systems is working efficiently there will be no air in the chambers.
2. Seal the chambers tightly, except for one vent at the top. Cold CO2 is much denser than ambient air and will leak out of any low openings will drawing-in warm humid air. Your chamber should be able to hold water like a bathtub.
3. Your fan is adding heat to the system and mixing cold CO2 with warmed CO2 from the walls. Place the blocks at the top of the chamber. The dry ice that sublimates will fall rapidly over your sample. CO2 that warms at the walls will rise to the top of the chamber where it can vent out. Your insulated ducts are basically just heaters.
4. Use an aluminum foil liner inside to reduce radiated heat from the walls.

 

[tbuelna]

What happens to the composition of the circulating atmosphere in your test cell as the frozen CO2 sublimates and mixes with the atmospheric air that you started the test with? Obviously you must be continually venting some of the gas within the chamber, otherwise sublimation of the frozen CO2 would create dangerous pressure levels. Atmospheric air contains very little CO2, but after several hours of operation I'd imagine the atmosphere in your test cell would contain a significant percentage of CO2, and would no longer resemble "air".

If the goal of your environmental testing is to establish the effect of extremely cold air on your test article, then you need to isolate/control the atmosphere within the test cell from any uncontrolled outside influences. This would mean using some type of heat exchanger to chill the circulated air flow within the test cell. As MintJulep noted, your existing arrangement is not a good approach for this application.

 

[dvd]

Is the system sealed and only recirculating the air, or is there air leakage/exhaust and infiltration/make-up? I think that you may be getting condensation on the surface of the dry ice that freezes and thickens to form an insulating layer. There would be a thermal gradient across the frozen H2O. If you could use dry air or dry nitrogen, you may get cooler temperatures.

 

[dvd]

If it is a sealed system, what is the pressure? Can you get to that point on the sublimation line?

 

[LittleInch]

Can you draw a section through your system? If the flow isn't through the blocks you won't get cold flow. Sounds like your blocks are too big. Remember that to become gas requires heat. If none is coming in from the air then the block will become colder and stop vapourising into gas.

What is the ratio of solid co2 volume to overall volume?. I suspect until this is 1 to 3 or 4 you won't get much below your current temp.

You have a lot of options expressed here, let us know how it goes and there's no such thing a too much insulation

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[IRstuff]

Are you using air or nitrogen for chamber? -45F sounds a lot like what you might expect from typical "dry air" supply dewpoint.

TTFN
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[georgeverghese]

Could the recirc air temp sensor be covered in water-ice?

 

[chicopee]

Looking at the pressure/temperature graph presented above and from your description of the dry ice being at -100dC, and assuming pressure at 1 bar, the surface layer of the dry ice block would first has to heat up to -78dC before sublimating to a vapor state at which point this cold vapor would enter the second chamber to cool it down. At steady state the second chamber is at -42.78 dC(-45df) from which this cold air would return to the first chamber enabling the sublimation process to continue. In essence heat transfer to the outside from the first and second chambers as well as the operation of the fan adding energy to the system are the factors limiting the temperature to -45dC, otherwise, the second chamber would get a lot colder but never to below the level of the sublimation temperature at 1 bar. My thought.

 

[tbuelna]

Shockdesigner331-

Don't know what financial resources you have available for this test effort, or how important the accuracy of the test results are to you. But I have been involved in a few test programs during my career, and one thing I have learned is that the worst mistake you can make is attempting to conduct a test using unproven/unvalidated test equipment. Your test results are only as reliable as the procedures and equipment used to produce them.

If you need to perform environmental testing at temps down to -100degF, you might consider using a commercially available conditioning unit that uses liquid nitrogen. It should not be difficult to integrate one of these conditioning units with your existing test chamber.

http://www.russells-tech.com/images/pcu-cutsheet.pdf

 

[Shockdesigner331]

Thanks for all the responses guys.

I guess the system is not "air tight", I mean every opening is sealed but I'm sure that it isn't hermetic. We are not building up any pressure...

The chambers start off with ambient air. The ratio of dry ice to volume is 1:3. I think I may have found a culprit. After running it for 48 hours we took the setup apart. Noticed that one of the fans had a pretty good buildup of ice, it was still working, but the ice meant that there was an air leak nearby. There was a gash in the insulation at that point.
If this allowed humid air into the CO2 chamber then the blocks could have had a layer of insulating ice on them too. perhaps.

We've got 5 thermocouples arranged throughout the chamber to map the temperatures. All read within a degree of each other, none had ice build up.

I like the idea of a top mount vent to vent off air, going to have to incorporate something like that on the next project.

We are just going to switch over to the liquid nitrogen setup. From all your input I know that I should be able to get colder with dry ice, but the amount of redesign and effort makes it a no brainer to just use the liquid nitrogen.

Thanks for all your input and advice, I truly appreciate it. I think my rule of thumb from now on will be -45°F min for dry ice chambers. Any colder and just go with the liquid nitrogen.

 

[Compositepro]

You can also do direct CO2 injection for cooling (make your own dry ice). The advantage is that CO2 can be stored as a high pressure liquid and there are no evaporation losses during storage as there is for LN2. Also the latent heat of vaporization is is 570 J/g for CO2 while it is 200 for LN2.