-70 Degrees C

[freezingcold]

Hi
I’m new to the forum but found it on my search for a cooling system.

I need a system that can supply a flow of air of 18 cfm (0.5 cubic meters per minuet) at -70 degrees C into a bench top thermal enclosure. Rapid cycle time down to this temperature is not critical but the system must be able to hold the low temperature for several hours at a time.

So far I have found Temperature forcing systems from FTS, Temptornics, Design Environmental and Firlabo. All there systems seem to cost around £20 K and also provide heating as well. We don’t really need the heating system and £20 K seems a bit excessive.

Has any one got any other alternate suggestions? Are there any suppliers of refrigeration components that could provide individual components so we cold build up our own system to meet our demands?

Many thanks

[US Iceman]

If your requirement is periodic (perhaps once a day or something similar) you might try using an expendable refrigerant like Nitrogen to cool the air stream.

The gas supplier will rent you the Nitrogen tank and keep it filled up, so all you would need to provide is the insulated enclosure and the air heat exchangers.

That is the quick answer, but there are some other issues to deal with such as frost formation on the heat exchanger or inside the enclosure.

This should certainly be less than £20 K.

[freezingcold]

Sorry I should have mentioned, we looked into using liquid nitrogen but the frost formation and general health and safety put us off using this.

[US Iceman]

If the air flow is supplied from a refrigerated drier that provides an air dew point temperature lower than -70C, then the frost from the air stream becomes a moot point.

The other project where I have investigated doing something similar to this (except we were using CO2 at slightly higher temp.'s) had the refrigerant vented to the outdoors. In our case, the CO2 was injected directly into the enclosure to cool a test apparatus after the liquid CO2 cooled a heat exchanger (for liquid fluid cooling). We used the latent heat for the initial cooling in the heat exchanger and then used the cold gas to pre-cool the test apparatus. The cold gas was then vented to external ambient.

So, essentially this was intended to be a closed control volume from a personnel standpoint.

The only other way I can think of is to use a low temperature refrigeration system. However, you still have the frost/dew point temperature to deal with.

[Brian_UK]

I haven'y worked on this type of system so this is a wild guess but...

Is it possible/feasible to use nitrogen fed through a pipe coil of some sort within an air duct to provide the cold air temperature required ?

The gas can be vented externally to reduce the danger element.

[Peter_1]

Brian, you're 'guess' is not that wild.This system was/is used on container boats.

Freezingcold, what are the inside dimensions you need in the cabinet? Perhaps this can be tailor made for you and this will be perhaps a lot cheaper.

I have some experience with this.
You can mail me also via a PM.

[freezingcold]

The enclosure measures 240mm x 250mm x 210mm. The problem is the project manager is pretty much dead set on not using liquid nitrogen unless absolutely necessary.

I was looking around yesterday for cascade systems and found one company Ernest West & Beynon Ltd, who could custom build a bespoke cooling system. Does any one know of any other companies besides MGi and Flowcool Ltd that may specialize in custom cold systems that may be able to solve the problem?

I have there must be other company’s similar out there but I just cant find them!

Thanks for the help people.

[EngFBN_BR]

freezingcold, I had the same problem designing a machine that could blow cold air (-35 deg. C) in a plastic mold in order to reduce cooling time of plastic products and raise the production of this process. The solution was to design an heaterless adsorption compressed air dryer to dehumidify air and keep its pressurized dewpoint (or freeze point, in this case ) below -60 deg. C Tdp to avoid water vapor sublimation (ice formation) inside an evaporator of a refrigeration cycle. The machine can cool 80 scfm steadily at -35 deg. C 24hr, 7 day/week. We have now several machines at field and happy customers!

I believe this principle is much a more elegant solution than the Joule-Thomson effect devices described in the previous posts. An adsorption air dryer can be designed to provide outlet air up to -100 deg. C Tdp.

[Peter_1]

engFbn, i can't follow you on this one: the original poster needs a flow of very cold air

What you describe is correct, you produce via an adsorption dry air with a dew point of -100°C - I personally thought that -85°C was the limit with this dryer but anyhow - this air isn't cold at all, it only has a very low dewpoint at ambient temperature.
And that's not what the poster needs.

And, perhaps I haven't read it corect but nobody spoke about Joule Thompson devices.

[EngFBN_BR]

Peter_1, maybe I did not explain my point clearly...

The air must be dehumidified. In my case, the air leaves the adsorption air dryer with -60 deg. C dew point. After the dryer, the air is pre-cooled in an air-water heat exchanger and them it is cooled to -35 deg. Celsius (dry bulb temperature, not dew point) by an evaporator of a refrigeration cycle.
That is how I solved my problem. freezingcold could use the same principle.

When you get a high pressure gas and expand it in an isoenthalpic process, the resulting cooling effect is called Joule-Thomson effect. This is what was suggested before, isn't it?

[TXiceman]

I think your simplest and least expensive alternate will be to use liquid nitrogen. As for a safety and environmental issue, the air we breath is 78.09% nitrogen. Just vent the N2 exhaust out of the building.

Ken