PDA

View Full Version : liquid insulator



darryl
01-05-2008, 07:18 PM
I had a couple questions, but this is the only one I can remember right now. Is there such a thing as a liquid insulator? Maybe it would be a liquid of sorts that's filled with microbubbles of something- this is just academic at this point, but I do have an application if it passes the mental design stage.

Ideally, the carrier liquid would be much lighter than water, and the microbubbles would maintain their affinity to the carrier liquid, in other words, wouldn't separate and join with the water.

ckelloug
01-05-2008, 07:32 PM
3M makes a wide variety or ceramic and glass bubbles. These are often used in making something like man-made tile or concrete more insulating.

Does the insulator need to stay liquid or does it just need the liquidity to get to where it is going?

Off the top of my head, 3M glass bubbles mixed into some type of silicone engineered fluid from Dow Corning sounds like what you might want. Whether it insulates well is another question although without knowing what you're trying to do, it's difficult to say anything rational.

--Cameron

Arcane
01-05-2008, 07:51 PM
Hydraulic oils are often highly dielectric (when new), even PURE water is. Imperial Oil's Univist N22 is one I know is definitely highly dielectric, since it is used in the bucket trucks hydraulic systems that we use when we do rubber glove work on 25KV and barehand work on 240KV energized power lines.

bob ward
01-05-2008, 07:51 PM
It depends a lot on how good an insulation material you need for your application.

Its hard to imagine a liquid system, assuming you can make one with long term stability, that comes anywhere near the insulation values of the standard board foams, ie polyurethane or polystyrene foam.

You can get around that if you can make the insulation a lot thicker to counteract the lower insulation value.

What sort of hot/cold temperatures are you thinking of?

Lew Hartswick
01-05-2008, 08:23 PM
Yea as the previous posts have shown. Do you want an electrical or
thermal insulator? Or for that mater there are several other things
that could be "insulated".
...lew...

2ManyHobbies
01-05-2008, 08:35 PM
I can't imagine a fluid making a good thermal insulator. Maybe for something interesting (plasma inlet on a supersonic centrifuge?), but in general use, you would wind up with convection currents that would transport heat better than insulating it. That is also why they don't make double pane windows 4" thick. Air starts convection in 0.5"-1" spaces with very little effort. As far as a liquid, you would also have containment concerns when looking at longterm use.

As far as fluid making a good electrical insulator, there are tons of examples. Coolant in line transformers comes to mind first...

Fasttrack
01-05-2008, 08:38 PM
An electrical insulator is easy - pcb's, mineral oil, silicon based oils, etc. Of course these all conduct heat well which is why they fill transformers ;)

Heat insulators... thats much more difficult because even if the material doesn't usually conduct heat it will due to convection. You thermally excite one atom and (since its not bound very tightly) it bumps into another atom and so on conducting the thermal energy fairly well.

<edit> oh just saw the previous post... what he said :)

Evan
01-05-2008, 08:50 PM
Most transformers are being filled with vegetable oil now. Far fewer hassles if it leaks.

Fasttrack
01-05-2008, 09:03 PM
vegetable oil? now thats an interesting one ...

all of the transformers owned by com-ed are still mineral oil.

Evan
01-05-2008, 11:37 PM
World's first vegetable oil filled transformer at 132,000 volts

13 Sep 2006
EDF Energy Networks will be trialling the world’s first major transformer filled with vegetable oil in a cutting edge development for the electricity industry.

The insulating liquid, made from edible seeds, will replace the use of mineral oil in a 132,000 volt transformer serving thousands of customers in Luton.

Made from a natural, renewable resource the fluid is an environmentally friendly alternative to depleting mineral oils. The green liquid is also biodegradable, less flammable and is reputed to extend the life of the transformer.

http://www.edfenergy.com/html/showPage.do?name=edfenergy.media.news.item.til&cmsPage=/opencms/export/www.edfenergy.com/media/news/20060913.html

They are switching as fast as they can because it eliminates toxic waste concerns and potential lawsuits when leaks happen.

J Tiers
01-06-2008, 12:35 AM
Biodegradeable, eh.......? Maybe like the insulation on some european cars? That started to biodegrade a little early...........

Most veggie oils will oxidize/degrade at lower temps and in less time than mineral oils, and may become somewhat conductive if they do.

Well, presumably they know what they are doing..... better than the Soviet experiment of saving power by running at lower voltages........ motors didn't like it very much.

darryl
01-06-2008, 02:01 AM
Without divulging more than I should about this- the application is for heat insulation, not electrical- I should have realized to mention this. The space it has to occupy is about two inches thick, so we're thinking air might convect too much and transfer heat across this gap. We were hoping that an insulating fluid would give better insulation value in this case. Water will at times displace this fluid to actually conduct the heat when required. That's all I can say about this.

As far as electrical insulation, I like the vegetable oil in the transformer idea. They should put shredded potatoes in with it. Then if it overheats, the hydro guys will have something to snack on while they work on it. :)

ptjw7uk
01-06-2008, 04:43 AM
You will have a bit of a problem with this as air has about the lowest conductivity as seen by most heat insulation materials the active ingrediant is air you can find a list at http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html
You may be better off if you could get a vacuum section around the object that is the best although will suffer from radiation heat losses.

Peter

Evan
01-06-2008, 10:30 AM
I don't think you will find a solution where the liquid is less dense than water. However, Fluorinert FC-72 has a latent heat of vaporization of 88 j/gram whereas water is 2260 j/gram. This implies that as long as it is kept under pressure to prevent the vapor phase forming it has a very low capacity to transfer heat. It is virtually insoluble in water, <5 ppm, and environmentally friendly. It is however much denser than water at 1.68.

bob ward
01-06-2008, 11:09 AM
The space it has to occupy is about two inches thick, so we're thinking air might convect too much and transfer heat across this gap. We were hoping that an insulating fluid would give better insulation value in this case. Water will at times displace this fluid to actually conduct the heat when required.

Two thoughts on improving the insulation value of a 2" air gap and still allow cooling water to flow through the void when required

Multiple layers of aluminium foil with a .5" air gap in between them make a very efficient thermal insulation, getting towards EPS values. This assumes of course that we are dealing with flat panels.

For irregular shapes, a coarse open cell foam of the type used in race car fuel tanks, installed in the 2" void would reduce convection losses and allow water flow.

Paul Alciatore
01-06-2008, 11:11 AM
As I understand it, you want to prevent heat flow across a 2" gap and gas or fluid are both possibilities. So, I would conclude that the volume is closed, but may change shape and possibly size.

I think gas would be the better solution just from the possibility of fluid leaks that may be problematic. And air is probably the cheapest gas you can use so without knowing of any reason not to, I would suggest sticking with air.

But you probably need to decrease the thermal transfer through the air. The primary mechanism for this transfer in a space like this is going to be convection currents. This simply means there will be a bit of wind inside there. Air on the hot side expands and rises and on the cold side it will contract and fall. This creates a circular air current which transferes the heat from the hot side to the cold side. So, what you need to do is add something to prevent this circulation. Depending on the types and amount of changes the volume can undergo and on the temperature range it may experience and on other substances that may come in contact with anything in the volume, this may be something like foam rubber, or a loose fiber type of insulation, or fixed baffels, or many other possibilities. The insulating material they sell in hobby and crafts stores for things like quilting is very resiliant. It can be crushed very easily and yet will easily recover to it's former size when allowed to. Or fixed baffels could be constructed from an insulating material like card stock or plastic. They would be roughly parallel to the hot and cold surfaces of the volume. Air would circulate in each of the chambers created by them, but each such chamber would be a further hinderance to heat flow. The more such chambers, the better. You could get ten or even more in a space of two inches. The baffels could have a small hole in the center to allow for changes in volume.

Air is still the actual insulating material. The added material or baffels is just to prevent thermal currents. Either of my suggestions would also work with a fluid, including water. The primary transfer mechanism in a fluid would also be circulation and they would cut down on the circulation there also. A very viscous fluid would also be a possibility. High viscousity equals less flow. There are some oils that are almost solid. Or perhaps grease. I would bet the manufacturers could provide thermal transfer properties. Small insulating beeds could even be mixed in with the grease. Or in an oil if they had the same density so they wouldn't float or sink.

This analysis has been made working with the information you have supplied above. I hope it helps. But, as elements may be missing, please take it with a grain of salt. Designing with incomplete information is difficult.

darryl
01-06-2008, 09:49 PM
Thanks everyone for your inputs. We're actually thinking now that a much smaller gap would be far better all around. I wonder what kind of surface finish would help to impede air convection currents in a gap this small? Hopefully a very smooth surface, unlike a golf ball. At any rate, it's more of a constructional issue to have the gap that small, nothing that can't be overcome. It seems that controlling the water level will be easier as well because less will have to be displaced, even though the pressure required to displace it will be the same. Thanks again for the ideas.

Paul Alciatore
01-07-2008, 08:15 AM
I don't know if the surface finish would have any effect on the magnitude of the air current. It might help slow down a fluid current. But a smooth surface would have less area than a rough one and therefore the heat transfer would be less.

Evan
01-07-2008, 09:57 AM
The golf ball effect isn't applicable to what you are contemplating. A dimpled surface has more drag than a smooth surface. What it does is cause some of the energy of motion to be imparted to the air. That energy is expressed as turbulence which causes the air to slow since the motion is more randomized. That in turn allows it to remain attached to the boundary layer longer which in turn reduces the overall induced drag of the ball to a greater extent than the energy cost of making the air turbulent.

Drag has two forms, induced drag which is the drag caused by the creation of lift forces and parasitic drag which is the flat plate drag of an object. In your application all you have is the parasitic drag of the boundary layers so maximizing that will slow flow just as it does water in a hose. The rougher the surface the slower the movement of the fluid.

Keep in mind that air is a fluid and is subject to exactly the same principles as liquids.

Paul Alciatore
01-07-2008, 12:37 PM
.....

Keep in mind that air is a fluid and is subject to exactly the same principles as liquids.

But air has a lot lower viscosity. Same principles, yes. Different result, definitely. A rough surface would act like a lot of cooling fins, increasing the contact area and thereby add to or increase the heat transfer. Within reason, it would have little effect on the flow with air as the fluid and not a lot with a viscous, liquid fluid. I think with a rougher surface the change in the flow rate will be very small and the net effect would be an increase in heat transfer but it would take actual experiments to confirm this.

Using a viscous fluid would definitely slow the heat flow down. I believe I suggested that already.

Evan
01-07-2008, 04:14 PM
I think with a rougher surface the change in the flow rate will be very small and the net effect would be an increase in heat transfer but it would take actual experiments to confirm this.


Ah, but the rougher surface will tend to trap the boundary layer next to it because of the very pronounced drag near a rough surface. This will prevent convection currents from reaching the surface of the container leaving an insulating layer of fluid that has already adjusted to the ambient temperature of the walls. This effect will also operate very well with air and is how even a small amount of hair on your arms helps to keep you warm.

John Stevenson
01-07-2008, 04:24 PM
Well, presumably they know what they are doing..... better than the Soviet experiment of saving power by running at lower voltages........ motors didn't like it very much.

I thought that was the US with their 110 volt motors ?

.

Paul Alciatore
01-07-2008, 11:46 PM
..... This effect will also operate very well with air and is how even a small amount of hair on your arms helps to keep you warm.

You must have better hair than I do.

aostling
01-08-2008, 12:30 AM
To summarize what I think has been implied: You have an air gap (perhaps 0.5") between two panels. The air gap will be periodically flooded with water. During flooding you want heat transfer between the two panels, otherwise not.

If you are concerned about convection during the air phase, this may not be an issue if the mechanism is natural convection, as distinct from forced convection. Natural convection in an enclosed space occurs due to the buoyancy of heated air rising. If the panels are horizontal, there will be essentially no natural convection, and heat transfer will be governed by the thermal conduction across the air gap. If the panels are vertical, natural convection may still not be very significant if the distance from bottom to top is only a few feet.

Evan
01-08-2008, 01:15 AM
You must have better hair than I do.

I wouldn't know. What do you think?

http://vts.bc.ca/pics3/hair.jpg