Using Air for Heat Insulation

http://en.wikipedia.org/wiki/Vacuum_flask

One might try putting ambient air in the pipe and heat it the far end, doing away with the insulation and the need for a pipe jacket.

One might source the air at the far end of the run, doing away with the pipe.

Hair Dryer comes to mind. But it realy depends on how much air is needed, how hot it is supposed to be and what the current process is to create it.

i dont need hot air. i have about 280oC hot air and need to carry it 300 meters with less losses.

You did not say that in your first post did you? So now that is concluded: How hot you think a heat gun gets?

It comes down to how much air you need. You only specified a pipe. This could mean a lot. But I dont know this. So will 10 heat guns produce the Air flow you need? Is there any pressure in your pipe? Will the air be allowed to expand and loose Temperature in the process?

What is the process that creates the hot air in the first place?

Just bear in mind the higher the temperature the higher the losses.

Good luck!

The purpose of the rockwool is to slow down the movement of air across the pipe. Just as in a wall insulation retard the movement of air. The convection of air across the heated surface cause heat loss. In a closed environment the air will still move across the pipe. To use your insulated glass example. Air cooled on the outside pane drops. Heated air on the inside pane rises. You start a circular motion. The greater the difference between temperature between inside and outside the faster the circular motion. The greater the heat loss. Insulated glass uses gases to slow this down.

If the heat loss is too great wrap more insulation around it. If you still want to enclose it do so but pack the inside with insulation.

my heat losses are still considerable after insulating it with high density rockwool. maybe aerogel will solve my problem but it is so expensive.

i think myself that if the thermal condictivity of rockwool is 30% more than air, why dont i use air.

i need help about designing "inner pipe-air-external pipe" system

http://www.engineeringtoolbox.com/air-properties-d_156.html

A standard IGU consisting of clear uncoated panes of glass (or lites) with air in the cavity between the lites typically has an R-value of 0.35 K·m²/W

i compare air with rockwool. so i have 280oC hot air in pipe and ill insulate it. which is more efficient insulator air or rockwool. thermal conductivity of rockwool is about 0.1W/mK at 300oC when air is still 0,045W/mK. because of this i think insulating with air is more logical.

we all try to make closed micro or nanopores in insulating materials. aerogels are the best and they are nearly pure air.

i wonder why we dont use only air for insulating.

thank you for understanding what i'm talking about. i dont have any idea how i can calculate how much air i have to trap between two pipes. you said its not easy.this is my first problem.

the other one is, if i find the maximum amount of air i have to trap between two pipes, than how can i trap it in a 300 meters long pipe? i have a design problem about it. how can i fix the external pipe?

thank you PWSlack and Anonymous poster.

i know about thermos and it inspired me to think about insulating pipe with air. "inner pipe-air-external pipe" is what i want. but the pipe is horizontal so i have to fix the external pipe to vacuum the air between pipes.i wonder how can i do it.

there will be thermal bridges between two pipes after fixing the external pipe to inner pipe.

so i need smart ideas maybe one will say "you dont need to fix it" :)

im the guy who asked the question by the way:)

Hello Giray.

If you calculate the heat lost by conduction, for a pipe coated in solid rockwool, you'll find it's quite low (there's insufficient info so I can't do it for you). So the thermal conduction of rockwool isn't the main problem.

Usually most heat is lost from a hot pipe by convection, so wrapping the pipe in a thick layer of rockwool traps the air and reduces convective losses.

More details would help get a better answer.

6" DOUBLE WALL STOVE PIPE

:)thank you. i can explain it easily with this product:)

think that im carrying the air (280oC) in this pipe. this has an insulation material between two walls. i try to say wont it be more efficient if i dont use any materials between two walls?

No, it won't be more efficient.

There is sufficient space for circulating air currents to develop that act to transfer heat.

Putting some rockwool in the space stops the air currents and reduces the heat transfer.

Double wall stove pipe does not insulate. It relies on air flowing between the inter and outer pipe to Convecition to keep the outer pipe cool.

You can insulate with anything you want. The better the insulator, the less space it will take to insulate. To know if your insulation choice will work will require an understanding of thermodynamics and differential equations. Remember, insulation just slows down energy transfer, it does not stop it. Silver coated walls of a vacuum chamber still radiate heat. If they didn't radiate heat then this would be a very, very cold planet. So if you're moving hot air 300 meters it will always be cooler after this movement unless you add a heater at the far end of the movement.

You might also consider moving your air faster so that there will be less time to transfer energy.

Is there something magical or mystical about this particular air?

Is it not more practical to, say, use a boiler and use the hot water to temper the air where it is needed? 300 meters is a long way, and losses will be large no matter what you do.

Hey,

Don't tell 'em that. I want to sell him some LynDoor™Industries SuperDumper® Double walled pipe.

At $50.00USD/foot the pipe would cost nigh on to $45,000.00.

Oh, the sweet smell of money.

:) when i loose my hope about air i'll find you.

Running heat 300m is a poor idea with a low heat capacity medium as air. High heat capacity media as water, oil, mercury needs small diameter pipes (easier to insulate). Add some reasonable high speed pump, and heat exchanger on both end.

Or simply dispense with all the complication, and heat the far end straight. As a concept, think on-demand water heaters.

i have waste hot air. i'll use it as combustion air for the burners.i mean i dont try to carry heat only, i want to use my waste hot air.

Very well my friend. Sally forth and conquer the ferocious giants.

:) my idea is simple but hard to design. i'm afraid if i success, all heat insulation materials producers will chase me:)

Why is the facility wasting hot air at the moment? What can be done to eliminate the waste? Is there a facility whereby the outgoing hot air could be used to preheat the combustion air for a burner without transferring it 300m? Is there a Process Engineer at the facility that can provide some practicalities based upon local observations?

How much heat are you loosing? If the layer of rockwool allowed the air temp to drop 40 degrees, would adding another layer of insulation produce an additional decrease in temp loss? Or add a booster heater. If you only need to recoup the thermal loss, the wattage for the heater should not have to be great.

i insulated as well as possible with rockwool.insulating with any material is not problem. im asking an idea "why dont we use air for insulation?"

If... well, you... we have... did you... What?

Go to this site: Thermal Conductivity of some common Materials and Gases and compare the thermal conductivity of various materials to air.

This may be of interest:How is heat transferred? Conduction -- Convection -- Radiation

Heat Transfer, Conduction, Convection and Radiation

"why dont we use air for insulation?"

I do, in a different application.

Here are some questions:

1. What pipe size?

2. Where is it placed?

3. How hot is it after insulating with rock wool?

4. Is it one straight line?

5. How is it usually done in the industries were it counts?

6. Could you put the pipe in a long tank (ground level) of water and store the heat for longer and make it part of your heat recovery?

7. Were are you?

8. Can you post a photo?

I have a method/system but will not mention it in detail here. After receiving your answers I might be able to help you and maybe many others.

Good luck, Ky.

1- dia- 30cm, length- 300m

2- it will be placed on ground inside the factory.

3- external temp- 50oC, starting temp- 295oC, at the end- 225oC

4- yes

5- its a heat recovery project so it would be all industries which have waste hot air

6- no its impossible

7- i'm from Turkey

8- i will send you a drawing

thank you:)

Insulation only slows the transmission of heat from the inside of the pipe to the outside of the pipe. It doesn't stop it. If your pipe was in contact with pretty much anything, but lets say, it is enclosed inside a wall, then eventualy the outside of the pipe would rise to the same temperature as the inside of the pipe. This is a problem with wood stoves which have the chimneys "boxed in". The best you can hope for is to provide ventilation to the outside pipe so that the outside pipe will cool off as quickly as it is being warmed from inside...in which case it will rise to the stable point...a rather warm point, but perhaps not as hot as the inside.

Air by itself will develop convection currents, and will not do a good job of insulation. The rock wool prevents those little micro convection currents. A double pane window filled with rock wool is thus a better insulator than that same window filled only with air. But you would not be able to see through it.

You should really think about not using air. A vacuum is the way to go. When you have a vacuum heat loss will be only by radiation. I heat treated steam generators that had tubesheets that were 22-inches thick. We were able pull a vacuum and achieved a deltd T of less than 50-degrees! Y

what was your temperatures? you advice vacuum but i cant imagine how to vacuum the air between two pipes while its connected to a fan?

What is the material of the pipe carrying 280 Deg C hot air? If it is metal (over 300 meters long) that itself will extract lot of heat. Why should it be a 300 meter long pipe of single material? Say it consists of 1 to 5 meter long pieces each joined by insulation (minimum joint width). Then each 1 to 5 meter section can be double walled pipe with vacuum - if you want a very high level of insulation.

i made it with 6 meters long pieces. the problem is how can i double wall and vacuum the sections? this is what my problem is:)

"i made it with 6 meters long pieces. the problem is how can i double wall and vacuum the sections? this is what my problem is:)"

No. A vacuum is NOT the solution to this.

I completely agree with Doorman here. Theoretically a vacuum will be the ideal method to stop convection and conduction. However, a thirty meters long vacuum chamber is a bomb waiting to explode. If you've never worked at trying to produce and maintain a vacuum system a thirty meter long design is not where you should start.

After seeing various data given by you - it appears that making a double walled vacuum tube of the dimensions mentioned by you seems not practical. Simplest thing would be to build a solid hollow brick cement tube around the pipe- which should also be placed above ground by similar cement bricks. In fact large cement pipes must also be available readily.The pipe carrying the hot air should also be of ceramic (must be available) or similar insulating material. The air trapped between the ceramic inner tube and cement wall will act as your insulator.

It a Post Weld Heat Treatment 1150 F +.- 25F. If you use air, it too would have to in a sealed outer pipe. If air is used you have heat losses through the turbalance convection of air transfering to the outer pipe. Insuation works because it breaks up the convective air currents by containing the air in smaller pockets.

Seriously?!? Are you really proposing as a realistic solution; creating and maintaining a 300m long vacuum chamber?

Bold suggestion sir! You certainly are a free thinker...seemingly unrestrained by factors that limit so many of the rest of us....logistics, economics, logic and perhaps large portions of reality in general. I sit in awe.

Hey...on a similar note, perhaps you care to enlighten us on how your tale of heat treating steam generator tube sheets is supposed to provide a good example of the benefits of insulating with a vacuum?

Isn't a primary purpose of a steam generator tube sheet to efficiently transfer heat...exactly the opposite of insulating?

By bragging about a delta T of less than 50 degrees, aren't you bragging about how well the steam generator is conducting? Doesn't effective insulation tend to increase delta-T?

.

BTW I'm not arguing against the idea that a vacuum is an effective way to limit unwanted convective heat transfer....just that it isn't an appropriate solution here.

As I stated we were doing a PWHT on the tubes to a 22 inch thick tubesheet welds and the head to to tubesheet. the head was 8-inches thick. It was impossible to insulate the back side of the tubesheet because you had a few thousand tubes in the way.

So we put a vacuum on the backside of the tubesheet and was able to a temperature hot face to cold face of less than 50-degrees F with a hot-face temperature between 1100F and 1150F (delta T).

And why would it be harder to make a 300 meter chamber that controls air flow (Convection) than a vacuum chamber.

If you put 300 degree air in the end of a pipe and get 150 degree air out - you must first heat the pipe to some temperature from end to end using that same air flow.

To do this will take time materials have resistance to change temperature and therefore will adsorb a great deal of calories of heat (BTUs), and then if you don't have an almost perfect insulation you will take a long time to have sufficient heat at the other end.

It does depend on the volume of air (CFM), the size of the pipe conducting the air, the material pf the pipe, and heat loss (insulation).

Insulation in its self; fiber glass, glass sponge, RCF (Kaowool, Super Wool, spun rock, (Rock Wool made the same way as fiber glass) all limit the convection losses due to air flow.

Using air as an insulator is the same as having outer walls two by fours and sheet rock and expecting your house to be insulated. Add insulation, fiber glass, foam, cellulose, whatever and you improve the R"R" factor many times.

Windows that are insulating use noble gases not air.

Air is not ever going to replace insulation or a vacuum as an insulating medium!

have you thought about running flexible insulated ductwork inside larger diameter of insulated ductwork to give you a higher r-factor?

A lot of engineers have weighed in on this topic, yet I think they do not have a clear idea of what question they are answering. (I know this because I don't know either.) The answer would depend upon the purpose of this installation.

WHY do you want to insulate this pipe? And of course, how big a pipe are we talking about? 2 meters in diameter or 2 centimeters in diameter? How much pressure is it carrying? And how hot is "hot air"? Some rock wools melt, making them useless in, say, a blacksmith's forge chimney.

Is it because you want to keep the heat inside, prevent heat loss? Say, to carry energy from one place to another? (steam pipes being sent to radiators.)

Is it because you want to protect people or things from getting burnt by touching it? (wood burning stove chimney application.)

Is it because you want to keep the heat up so you can get the smoke over to the chimney and provide a draw?

Is it because you need to keep the temperature of the smoke high to improve catalytic combustion?

Without knowing your purpose, no answer here is useful.

You are right Yusuf1.

OP want air as an insulating agent in annular space with a double walled pipe. It will not work because he thinks that problem can be solved by restricting conductive heat transfer only. Conductive heat transfer is a fraction of total heat transfer taking place in this case by all three modes- conduction, convection & radiation. OP's proposal is useless here because it allows heat transfer in all tree modes.

If annular space of double walled pipe is kept at high vacuum, first two modes are tackled, still radiation heat transfer will take place. To address heat transfer through all three modes super insulated pipes are used where in vacuumed annular space inner pipe is wrapped with several layers (about 50) of high reflective aluminum foil and thin insulation material sheet alternatively. It is normally used for transferring cryogenic liquids, but it can be used in this case too. It's a costly affair.

From post #20:

i have waste hot air. i'll use it as combustion air for the burners.i mean i dont try to carry heat only, i want to use my waste hot air.

So, for this purpose, at what point is the return diminished?

"what point is the return diminished?"
Doorman, I couldn't get this question.

Hello pradeep44

Our friend is wanting to use this waste heat air to improve performance of this undefined combustion device that is 300 meters away. I assume it will do so, and the improvement will have some numerical value. Less fuel consumed for same output, more output for same amount of fuel... This will be the fruit of this exercise.

Now, what must that improvement be to offset the expense of this 300 meter pipe, and realize that offset in a reasonalbe amount of time?

The Point of Diminished Return is the point (amount of money) where the expense matches what could possibly be saved in a reasonable time, ... that is to say, it will never be cost effective. If you spend $100 to save $80, you have passed the point of dimished return.

Super insulated pipes are of very special construction with vacuum level as low as 10 micro bar (a), normally used for cryogenic liquids like LIN, LOX, LAR etc as you have mentioned. SI pipe sections along with pipe fittings like bends & Tee's are factory manufactured, usually made of SS316, TIG welded, 100% radio-graphed and helium leak tested. They are joined at site with provision of junction boxes filled with perlite. It is to minimize the heat ingress because more heat ingress means more loss of liquid due to phase change, which may not be recovered. But here it is not the case.

You are right in saying- "it's a costly affair." Moreover, SI pipes I experienced is low size range (15 to 150 NB inner pipe). OP require 300 NB, it will be really a costly affair and as Doorman suggested pay back period is to be worked out.

Mr. giray, I suggest you to consult insulation experts in your area for best possible solution. Let them ensure the result you want with cost economics.

Con Edison, the electric company that serves NYC, has been sending steam under the streets of Manhattan for decades. There must be some way to insulate the transfer pipes.

There is an idea that comes to my mind that may help here. I do not know if this is a practical idea or not from the limited information we have but let me offer this anyway.

Can this 300 meter long concentric piping approach instead of being considered insulation, consider this pipe as a heat exchanger design instead. The 280°C air must be replaced with new air. Pipe this air through the outer ring of the 300 meter long heat exchanger.

OP want to use waste hot air as combustion air to save fuel (may be at furnace) by retaining as much heat as possible by reducing heat losses while transferring 300 m away. Your suggestion may not be useful to him because it may increase heat loss many fold.

however, if OP require more combustion air than available waste hot air then your suggestion can be used but by reversing it. It means waste hot air in inner pipe and extra air to be pumped through outer annular space. So, extra air will gain the heat lost from waste hot air which can also be used as combustion air. But the double pipe arrangement is to be designed properly (with provision of expansion bellows) considering differential expansion at different temperatures when in operation and when not.