Hydrophobic material to stop frost 3

[HDS]

Is there a material or coating that is hydrophobic enough to prevent the formation of frost on cold objects?

One of the challenges with any cryogenic system is that there are always parts that are cool enough for frost to build up. The thing I am looking for is a foam insulation that would shed water to the point that it does not condense on the surface or freeze. Coatings for glass or metal would also be valuable.

 

[Demon3]

There has been a lot of work on this. I.e. millions of dollars spent to prevent icing on airplane wings. So far attempts have been fruitless. Hydrophobic coatings do not work. A fluoro coating as I just described yesterday in a thread would decrease adhesion of the ice (technical term is abhesion, i.e. lack of adhesion). However, for a foam that would not help, as the ice is interlocked in the pores if you have an open cell foam.

My advice is not to try, it's not possible.

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry

 

[HDS]

It was your comments in the other thread that lead me to start this thread. Wing icing is a more difficult problem to solve because of the amount of water. I am thinking about a environment with just condensation from humidity. However, I agree it is very difficult.

My experience is that open cell foams are worse than closed cell as you suggested.

 

[IRstuff]

Presumably a totally non-polar molecular surface would have the least adhesion to the polar water molecules, and while that wouldn't prevent frost from accumulating on such a horizontal surface, and sloped surface might be sufficiently "slippery" to get the frost to slip off under its own weight, but some level of frost would probably still accumulate.

An interesting experiment might be to coat some surfaces with cooking oil and see what happens.

TTFN

FAQ731-376

 

[Demon3]

And before someone asks, no Lotus Effect surfaces do not repel ice. I've tried it and ice sticks extremely well to them. I was surprised too.

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry

 

[HDS]

Is there a word like hydrophobic that describes surfaces that shed ice?

 

[IRstuff]

Not if there isn't a material that can actually do that. If there were, airlines would be jumping over themselves to have it installed on their planes. Deicing is a very expensive operation.

TTFN

FAQ731-376

 

[MikeHalloran]

Since a terrestrial cryogenic plant doesn't have a weight restriction, I suppose you could use a resistive heater blanket over the cryo insulation to keep the surface temperature above the dewpoint.



Mike Halloran
Pembroke Pines, FL, USA

 

[IRstuff]

Mike,

The reason for the frost is because something cryo is flowing through them. Therefore, a heater blanket would be contraindicated.

TTFN

FAQ731-376

 

[unclesyd]

The only way to mediate frost and moisture from Cryo lines is to use specifically designed vacuum lines and valves.
Here is one (Vacuum Barriers) of several manufacturers of this type equipment.

http://www.vbseurope.com/htmle/news-stainless-semiflex.htm

 

[MikeHalloran]

The reason for the frost is that the laminar sublayer of still air adjacent the surface allows the surface temperature to drift downward past the dewpoint, despite the fact that the cryo insulation keeps the actual heat flux low.

This may be counter-intuitive, but a very low- powered heater blanket should be sufficient to prevent frost, without seriously affecting the load on the refrigeration plant. To the extent that the frost compromises the insulation, there may be a net energy saving associated with heating the insulation's surface.





Mike Halloran
Pembroke Pines, FL, USA

 

[HDS]

The vacuum insulation and heaters are both common solutions to the problem. However, at some point there is always a fitting or other piece where those options can't be used. For example the outlet of an LN2 tank or the lid on a dewar.

 

[Demon3]

You could try combining a low energy smooth, non-stick surface with some action to shake off the ice. For example a piezoelectric material like PVDF could work.

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry

 

[HDS]

Chris

That is a novel solution. It is easy to vacuum form a film of kynar around the parts (I wonder if pvdf paint would also work). I just have to figure out how to make them shake

You should see if someone has tried that for deicing.

 

[btrueblood]

HDS,

Chris is pointing out how to make it shake - PVDF is a piezoelectric material. I.e., put an electrode on each face of the PVDF, and apply a sufficiently high voltage - and spung! goes the ice (well, that's the idea anyway).

 

[HDS]

I understood the piezoelectric part. What I have to figure out how to get the effect initiated and the electronics required.

 

[patprimmer]

From memory, PVDF is quite expensive andfragile.

Will the piezoelectric effect work if it is part of a multi layer film, with maybe PTFE or similar surface. PTFE is probably impossible to produce as co-extruded or laminated. PE would also release easy and be possible to make.

Regards
Pat
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[HDS]

PVDF is expensive but there are films used for skin card packaging that are made in high volume at reasonable cost. (I even know of a source for recycled PVDF film.) Those films are reasonably tough. They hold the parts in place but you can rip into them to get the product out. I do not know if those grades are piezoelectric yet.

If it works I can justify the cost because of a performance increase. It will probably be cheaper than a heater or vacuum insulation.

 

[Demon3]

Any grade of PVDF should be piezoelectric as that as I understand it is an intrinsic property of the material.

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry

 

[IRstuff]

Is there sufficient displacement of the film to actually shake the ice loose?

TTFN

FAQ731-376