Coolant for -25¦C 6

[drodrig]

Hello everyone,

We are working in a project where we need to cool a volume to -25ºC, Let's say the minimum temperature in the system is -40ºC (-40ºF). A special cooling plant (with pump, heat exchanger...) is being prepared; it will work underpressure to avoid leakage inside the system. Insulation, calculations... are being carried out.

But the coolant to be used has not been chosen yet. There are mainly 4 ideas:

- Methanol (in principle water methanol mixture, 60-40%)
- Ethanol (in principle pure, but it could be also combined with water to increase the specific heat)
- Glycol (also mixed with water)
- Perfluorohexane, C6F14 (pure)

Each one has its own advantages and disadvantages.

Ethanol and Methanol are cheap and have pretty good properties, but they corrode the metal and are flammable.

Glycol is non-flamable but very viscous; on the other hand the Perfluorohexane has low viscosity but it's expensive. Both are non flammable.

We don't want to use silicone oils because they are very dirty (difficult to clean in case of leakage).

Does anyone have experience with cooling for a system in this temperature range?

Attached a comparison with the properties

http://files.engineering.com/getfil...f1-484b-89d7-89b949f3ce19&file=Comparison.pdf

Thank you in advance
Regards,

 

[IRstuff]

Standard 60:40 EG should work down to -40ºC, but it might a bit sluggish

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

 

[Longinthetooth]

EG is the way to go, but you should specify the coolant before you do the circulation rate calculation, heat transfer calculations, and all that stuf, since the coolants have very different physical properties.

 

[macmet]

If I recall correctly, propylene glycol is considered non-toxic, which may be beneficial to you, but standard propylene glycol is more viscous than EG, so you would have to take that into consideration.

I believe there was a Dowtherm product that had a similar viscosity to EG though. But, it has been years since I've looked at this and I hope I am not leading you off in the wrong direction.

 

[IRstuff]

PG, even at 60:40 is supposedly pretty gooey down around -30ºC

My one comparison chart shows 50:50 EG having ~1/4 the viscosity of 50:50 PG at -34ºC. Obviously, one has to weigh the toxicity and handling constraints imposed by EG vs PG.

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

 

[EdStainless]

There are some very low viscosity PAO synthetic oils available that would be suitable.
They have little viscosity change with temperature.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[moltenmetal]

Forget about the perfluorinated material- I have no idea why you would even consider it, unless this system is under a litre in total volume.

If you're worried about silicone oils leaking, forget about ethanol or methanol- either will leak much worse, with consequences far worse than merely being a mess to clean up. Both have flash points below room temperature.

I'd build a tight, leak-free system and use a thin silicone oil designed for low temperature, such as Syltherm XLT.

 

[DmitryKalganov]

Hi all, may be to use a vortex tube? Ranque-Hilsch Effect? Then no need to contact with refrigerant.
PS Sorry for my english ))

 

[IRstuff]

Sure, if the OP has sufficient shop air or something similar, then a vortex tube is doable, assuming the heat load is within the capacity of the vortex tube.

So the OP needs to stipulate how much heat needs to be rejected.

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

 

[DmitryKalganov]

I agree. It is necessary to consider the heat balance.

 

[drodrig]

Hi there,

First of all, thank you for all the answers.

We don't have any problem about toxicity so we can forget about the PG.

I haven't mentioned we are going to use a leakless system where we work under atmospheric pressure, so if there is a leak inside the important parts, air will come in and not the coolant out.

The problem of working below atmospheric pressure is we must check carefully the pressure drop along the system, since there is not much room low viscosity is very important; that's why perfluorinated is being considered.

Cheers,

 

[racookpe1978]

We are working in a project where we need to cool a volume to -25ºC, Let's say the minimum temperature in the system is -40ºC (-40ºF). A special cooling plant (with pump, heat exchanger...) is being prepared; it will work underpressure to avoid leakage inside the system. Insulation, calculations... are being carried out.

I agree completely with being prudent, and with your caution of "expanding" your operating temperature zone past the expected minimum of -25 deg C .... But the difference in brittle fracture and coolant behavior between -25 deg C and -40 degrees C is very, very significant.

Are you really, really sure you want to go that far below the expected -25 degrees? Even using -30 deg C will let you use several different options that are less expensive in the long run costs of operation and fabrication. Very few places on earth get below -30 C for long periods of time, so you should be able to control the environmental temperature to safely maintain the "outside" operating temperature above -30 deg C, but only you - after all- actually know your client's conditions. (Said he who has had to use frozen outdoor portable-toilets in -10 deg F weather ...)