Cool Enclosure for High Temperature Application

Why on earth would you want to do this?

Are you thinking along the lines of some sort of self-contained capsule so that a person can observe a hot process from within, or is the furnace being used to simulate some sort of harsh environment for the self-contained life support capsule you are designing?

Can the capsule have an umbilical connection to the outside (cool) world for life support (oxygen), heat transfer cooling fluids, etc or does it have to be fully self contained?

Does the capsule need a window?

Does the environment consist of hot air or gasses, or is it molten steel?

What type of thermocouples were you thinking of that could handle 1000 deg C? How about using non-contact Infrared thermocouple or pyrometers? (which are commonly used for remotely monitoring temperatures of molten steel crucibles, etc)?

Yes, you are right. The idea is to build an enclosure to have all instruments inside the enclosure and send this inside a very large furnace with zig-jag path and some moving arrangement to see how on each point the temperature distribution exists as the material moves. That takes 6 hours. I want my instrument to get isolated from furnace heat and keep recording the data on the way. It is like Columbia capsule that keeps men safe in space flight from friction temperature equivalent to furnace temperature. This is real need.

It is just hot air around. Some external excess to high temperature sensor is required to sense environment.

Yes, This is possible.

If you can tell me the maximum heatup you can have during this 6hr period and the mass the capsule has + typical heat capacity I can calculate the amount of insulation you need.

It will be a serious layered insulation as the 1000°C require some special products and those don't insulate well.

I suggest that you build in a heat absorber, ice is a perfect one. If you have more money to spend you could work with fluid CO2 that can be released to control the temperature.

In your calculations: don't forget the own dissipated heat of the instruments and the power supply.

If you need assistance to calculate and design this, you're welcome.

So lets say we have a walk in container that's 2m x 1m x 1m which is about as small as you can get and be able to walk in to it. This would have a volume of 2m³ and for this calculation lets say it mostly filed with air. The volumetric heat capacity of air at 20ºC is 1.0KJm^-3Kº^-1 and we have a temperature differential of 1000ºK. That means that for every 2.0KJ that leaks in will raise the temperature by 1ºK. If we let the temperature rise to 50ºC which is getting close to the upper limit for a lot of electronic equipment then the maximum amount of energy that we can let leak in would be 60KJ.

You stated that you wished to have the unit soak for 6 hours or 21,600 seconds so if you divide the 60KJ by 21,600 seconds the maximum leakage will be 2.778 Watts. Now the surface area of our container is 10m² so we can now calculate the thermal conductivity of the insulation 2.778 ÷ 10m² ÷ 1000ºK = 0.0002778 or 0.2778mWm^-2ºK^-1 and this is roughly 100 times smaller than the value of the value for that of air.

I havn't been able to find the thermal conductivity of the silica tiles on the space shuttle but from what I have read I would say that with current technology it not possible. Remember that even in the space shuttle it gets hot inside during the few minutes of re-entry and once on the ground they need to get out of it and ventilate the thing otherwise they cook.

I may have made an error in my calculations so please feel free to correct me if I am wrong.

Masu: You are perfectly right,

But I think that the size is over estimated + use the free space to fill it op with ice.

The only thing that will go into this furnace is a data logger of 100gr with 10kg if heat absorber and 50kg of insulation (at least)

The space shuttle is build to wheight as less as possible to reduce the amount of fule needed to crank it up. That is why it heats up to the limit and nothing can go wrong.

It all comes with the safety factor and some risk analysis, Here in Europe we decided not to go for humans in space after we did this risk analysis. For some countries a human has no value any more after turning into a corpse.

The instrument that is to go inside the casing is about 20"x20"x20", so you can say 2ftx2ftx2ft space is OK. Using any cooling material will affect the external environment and hence can not be used. This instrument sits inside the enclosure and is pulled slowly from one end to another. It is not exactly for me to go in but for the system which is a black box to go in. It has lots of sensors that protrude out of the enclosure and these sensors can withstand the temperature. What I need a very good heat isolator that will not let heat conduct in within this period. Using any voporiser to cool inside may end up in explosion due to build up pressure. Perhaps vacuum walls isolator OK. I can get quartz capsule made or SS isolated walls with vacuum in between. Only radiation part is to be handled and conduction can be stopped to say 99.9%. At 1000C IR radiation may be high.

Has any one worked on flask like structure? These structures also used in satellites to keep electronics cool from high solar radiation. I am not sure of their exact design but they do use such things.

In Nuclear instruments we use that liquid N2 Diwar to store liquid N2 for several days. Something like that is required. Perhaps pushing instrument in may be again a problem.

The instrument that is to go inside the casing is about 20"x20"x20", so you can say 2ftx2ftx2ft space is OK.

Your need reminds me of the conditions that a fire safe has to withstand. I worked at a major safe & vault company, and we had to run tests on large, heavily insulated safes made to hold documents through a building fire; something like 6 hours at high heat, a drop onto a bed of broken bricks (simulating what often happens in a fire), and a long cool-down cycle during which heat continued to leak into the container. Find someone manufacturing fire safes, and see what they can offer as a starting point. They will not have the feed-throughs, but may be a big help with structure, insulation, and methods to seal the opening.

You did say "I want a walk in casing" so that's why I used the 2m³ size but the smaller you make it the worse it gets because as you reduce the volume it will heat up faster. If you reduce the size to the 216L capacity you suggested and plug in the figures in you get 216JKº^-1 with a surface area of 2.16m² so

30ºK x 0.216m³ x 1.00KJm^-3Kº^-1 = 6.48KJoules

6.48KJ ÷ 21,600 seconds = 0.3Watts

0.3W ÷ 2.16m2 ÷ 1000ºK = 0.000138 = 0.133mWm^-2ºK^-1

Which is twice as hard to achieve than with the original walk in container. The only thing I can think of that could possibly withstand these sort of temperatures ii the flight data and cockpit voice recorders on commercial aircraft. Even these though are only rated for something like 1 hour and use ablative insulation which would contaminate what you are trying to measure. I agree that the fire proof safes are a good starting point but I just can't see you finding something with that sort of thermal conductivity. I would however be happy to be proved wrong.

You are right. What I meant is that my instruments are as sensitive as me inside the chamber. The flight data recorder idea is closer to what I need. It is for an unmanned data recording situation. In flight data recorders, the data recorder itself may not survive and only data tape need to survive. I can easily do that but for the kind of use, wasting the data recorder may not be an accepted idea to say funrnace analysis each day three times.

I can try some high temperature electronics that can take 250C to 500C temperature but again that will cost me 100 times more. ICs from Honeywell can take to that temperature. I have used their amplifiers, switches, oscillators, memories and 8051 uCs. Other alternative is to use supper high temperature transistors as RF oscillator and transmit infor. There is one problem as furnace casing is metal.

How about Kaowool from Thermal Ceramics? http://www.thermalceramics.com/

If I use vacuum isolator with heat reflector then perhaps I can cut down that radiation more effectively. I really need that 6 hours safe housing. I need to have technique other than NASA uses. they actually melt the tiles to cool the surface if temperature exceeds. I can't do that.

If I use reflector inside to outside then it will heat the wall more than external temperature due to double heating mechanism. Or will it not be that way? Actually I need to stop the heat flux going inside the box cavity.

Returning to earlier parts of the thread, you will need to use multiple layers of insulation, with each transition adding a bit of barrier to heat flux. Alternating layers of thermally conductive materials force the heat to spread from each hot spot (such as struts or standoffs between layers, the cabling, etc.) so that all of the next insulating layer is being used. You need something that can withstand 1000 C indefinitely outside, perhaps a ceramic shell. I know I've seen reference to a ceramic foam, which might serve as the next layer; if clad inside & out with a non-porous ceramic, you could [theroretically, at least!] draw a vacuum on the foam volume yet have structural strength. Nesting similar "thermos bottle" devices would give additional time delays; as the final temperature expected on the inside of a given layer decreases, you can build the next from a better insulator which cannot handle the higher temperature. This would be a slow, iterative design process, but would tell you how big the box would have to be to get six hours worth of delay of heat transfer. The actual size would be larger due to losses at door edges and via the cables out to the sensors - I wouldn't be surprised to find that the volume needed to double to handle these factors. If this is larger than your furnace doors (or larger than the furnace itself!) you know that the initial concept won't work. Good luck on this one!

1000C withstanding is not a problem. I can use Alumina Al2O3 which can easily go far more higher. Yes, Alumina powder is also an heat insulator to some extent. I used to fill this in furnace cavity such that heat does not come out easily. Perhaps I used 6" thick layer of this poweder. Alumina tube are like glass tubes. Mica can also withstand temperature but is highly heat conductive. I need heat insulator. Quartz or fused silica also with stands 1000C but not more than 1100C. Alumina is better.

What is good heat reflector coating that can take to 1000C for long?

If alumina powder makes a decent insulator for this temperature, alumina foam should further reduce heat flow, ESPECIALLY if the pores are evacuated (the Dewar / thermos principle). Google Ceramic Foam and you'll get a huge number of responses, many for commercial filtering materials used for molten metal. You will find other useful references, too, such as http://www.alliedfoamtech.com/Appcera.htm, where firebrick-like materials are offered. Consider using them to separate non-porous walls of alumina while providing structural support. Companies like Cotronics (www.cotronics.com) have alumina adhesives (Resbond 901, e.g., that would help to fabricate structures; they also have castable ceramics worth a look).

Another thought: if you built one layer at a time around your instrument, sealing edges and holes as you went, drawing a vacuum and closing off chambers when appropriate, could you make up an enclosure without doors, eliminating edge leakages? This would be a single-use structure, sacrificed after a pass through the furnace to obtain the recordings and access the instruments for re-encapsulation and re-use. You'd need to have a larger number of instruments, and a more-or-less continuous process of building housings on the side . . . Or maybe the main structure could have layers added sequentially to "build" a closing plate / door, in such a way as to permit breaking it out after use, with the main structure re-usable.

I can't answer as to what would work as a reflector at these temperatures - I don't know. Perhaps someone else can help.

Dear Ron

Yes, these were good references and I will look into their material. Perhaps more ideas of vacuum furnace design will also help me.

In this application, we are measuring the actual temperature profile the material experiences and hence this box moves along with material on a trolly inside a 300m furnace room. Locating thermocouples etc in fixed condition is not a solution as sensors are placed on material that moves. This material type differs considerably and service provider can't have idea in advance.

I will be doing some research on actual materials and some structures will be tried. However, it is a very nice idea to discuss something in advance to rule out likely problems.

In one of my early experiments with 1000C range I used a Platinum crucible hanged with SS316 wires and cucible had Lithium Fluoride powder with some moisture. This moisture reacted with Fluorine and made HF acid that eaten away the wire and cucible fell into the Furrnace along with material which otherwise Supprapur. I was shocked to find the crucible missing. Thereafter, I always take care of most of the possibilities ahead of time as creating mesh inside the furnace may lose its quality for ever. This actually mean destroying a factory in one go.

While most of the people do not have actual design idea, it is always valuable to read some comment that may give a clue to the likely problem or solution.

If you are going to profile a furnace, why not just install type K thermocouples at each point you want to read. Type K thermocouples are good to 2300F and 1000C is only 1832 degrees F.

I would use the sheathed Thermocouples (Inconel) and buy them long enough to reach each point.

You can rent or buy as many recorders as you need, the company that I work for has a 240 point data acquisition unit. Just how many points do you need to read?

jmart23

Furnace temperature is monitored. However, as material moves in it, the profile is affected. I need to look into the moving material to see what is its actual temperature.

Before going any further I would contact NASA and ask them what the thermal conductivity coefficient of the heat tiles on the space shuttle was and see if it comes close to the 0.133mWm^-2ºK^-1. The silica tiles have the lowest know thermal conductivity and if they are close to the figure above then you're in with a chance but if they aren't then I think you would have the same chance as a snowball in hell.

NASA uses special foam tiles. They are also looking for new tile material as the current technology is prone to damage.

Some work also was done at NIST in the area of fire retardant building structures after 7/11 world trade center disasters.

Some of the high temperature material were developed for the Fusion reactors. As much of the momentum is lost in nuclear research due to lots of public opinion against it, we also lost scintific work in those lines due to funding problem.

Actually Aluminum softens around 700C but Alumina Al2O3 which is also used in hybrid electronics as white ceramic place, lasts more than 2000C.

We generally call these high temperature material as functional materials as they have special properties for a purpose. We actually nee more of such materials. Diamond can take 3000C. I am not very good at material science so can't tell which material will be functional material and which one not.

Actually NASA have given up on the heat tile problem with the shuttle. The next generation of spacecraft are going back to the ablative heat shield one use throw the entire space craft away principal.. The space shuttle seemed like a good idea at the time but turned out to be a extremely expensive diversion. We just don't have the technology to make it economically viable yet. There are several problems with the concept one being the silica tiles another is that thee are periods during the flight, like while the solid fuel boosters are burning, that there is no way to abort. The next generation consists of two spacecraft one for cargo that burns up on re-entry and uses solid fuel boosters and another for the astronauts that uses only liquid fueled motors. Both of them are really scaled up Apollo type spacecraft and to an untrained eye look much the same.

Shiam,

I don't see the problem of adding heat absorbers in the enclosure.

It will enable to create a reasonable size.

If you would have 100W that comes through you need 6.5 kG of ice to cope with it.

Also the remark of evaporation is somehow special: build in a leak that enable the expanding gasses to get out, you will have to do this or you will end up with a big hot balloon and cracked insulation. These cracks don't insulate anymore.

To avoid the heat to leak in through the measurement leads you can use longer leads. You will have to use MI insulated leads.

Work on the size of the measurement equipment, try to get is as small as possible as this will determine the final size.

This final size, how big can it be without disturbing the process?

It will be OK to have Ice for a while as long as it does not reach 100C. After that it will expand. Perhaps, if we provide some exit point then it will cool the furnace and prevent explosion within the box. Either Ice is to be loaded or the box to be cooled to make ice in water cavity.

So there need to be a double enclosure arrangement.

I have looked into some more details on Silica fiber technology that is also used as heat shield. They must be stong if they are in bundle. Something like a bundle of optical fibers of Silica which can go to 1100C. It was used as heat shield on the Huygens Probe. Behind the Silica fibers, were the carbon fibers, which is also used as heat shield on tip of the supper fast missiles and fighter planes.

Formula One - Toyota, Ferrari, use Carbon fiber heat shield wall material and it is some kind of soft clay like initially and after baking becomes springlike directional flexible high hardness wall materials. I have seen Toyota documentry on Formula one design and manufacturing. They use baking ovens.

Your customer must really have a serious reason do have this kind of measurement tool. If I read your way of creating the unit it will cost a fortune.

I would go for a unit that is made of different boxes that fit in each other. those boxes can be made of the machinable stuff resistant to the required temp.

This box construction enables each layer to expand as it heats up and you can build in radiation blocking layers.

When the ice would have been completely heated up to vapour, the energy that it has absorbed is 3 MJ/kg. If your measurement device could cope with 100°C and the oven can handle water vapour this is the easiest to do it. I would use this safety for when the process got disrupted and your device is trapped in the oven for a day or so.

How to transfer the water back in ice: refueling the unit. You need to get the data out so you will need to open it to plug in a cable (or you want to use wireless?), you will also need to recharge the batteries and each copper connection is a heat leak path that you don't want.

Putting the unit in a fridge would not really help: it is designed to handle a 1000K difference for 6hr, reducing this difference to 50k would demand a time of at least 200h as the insulation efficacy goes up when the temperature is low.

I would never use the foams that is used on the Space shuttle: they have proven to be a weak point.

Dear Gwen,

Cost is not a real matter for me. What I need is a real functional design. Solid tough design is the best as filling water etc will make things tough. Instruments that will go in the box are about 200,000$ to 500,000$ value. Their safety is very essential. Some are Gas analyzers to atomic level and other are simple thermocouple and optical sensors. Most of the sensors require very small hole and either quartz or SS316 tube which is not a problem even if that conducts a bit of heat.

I am going to work on this line of products and may place some in say forst fire to collect data for a while and then move the box into some safe pit. I may also have to look into other similar area where accident may last for 6 hours and we get back the cricial information. This may be the last valuable source of information to get back to history. I have many sensitive instruments that can go in.

I think this discussion is very valuable. Let me think in the line of only solid shield. Does not matter what the cost will be. I have only one life on earth so can invest everything. Research for real need is what I want to do. While I have few known applications, I may find more in time as I work on. Some of my clients can finance my research. After all no one else will try these ideas in private.

I want to give up the water idea and let us see more on functional materials that can be good for say 6 hours in keeping the inner hardware below 100C. I will avoid every material that may not last that temperature. I may have to worry about the battery and other material that may become explosive at that temperature. Instrument right now uses Lithium battery power source. Data is non-volatile and can be transferred to Flash, or EEPROM if battery is a serious problem. I can use Nicd-H battery used in digital camera.

Can you use a high alumina outer casting in double wall design and use a cooling medium such as water or even a liquified gas pumped through to keep a cool barrier between the outer wall and the inner wall and instruments? We use a system somthing like this to cool our camera lenses as well as other instruments on our large recovery boilers in a paper mill. I would think as long as you have flow through the cavity then you could use alot less insulation in the inner wall. If you have to use cables to get to the instrument then a couple of "hoses" to transfer the cooling medium should not be a problem and could even be used to keep the cables cooler too. There are also some high heat ceramic coatings that can be utilized on the surfaces to reflect the heat away. I use these in my glass furnace at present and they really work well.

Yes, Alumina casting is possible and we can also get it machined like we do on Stainless Steel. Ceramics are not difficult to work on and we have good industries in that area. Perhaps I will discuss with them also. Alumina is almost like Glass and only some of it non-transparent. I think Alumina glass like conducts heat but grain do not do so. Perhaps a brick of it may not be so bad. Alumina can hold vacuum.

I got this nice reference http://www.3m.com/ceramics/

3M™ Nextel™ Ceramic Textiles and Composites provide innovative solutions in industries from aerospace to petroleum refining to metal processing. The outstanding thermal protection provided by Nextel™ Fabrics, Tapes and Sleevings allows engineers and manufacturers to handle high temperature applications up to 2500°F (1371°C).

Here is an image of 3M ceramic fiber material for Hot Shield. You might have seen glass wool cloth on top of some thermocouples. It is much better material for greater temperature range. It looks similar. Perhaps to be used with some more material to give shape and strength. It is hard to believe that it is ceramic fiber. After we have seen Silica glass fibers. This alpha-Alumina cloth idea can be easily digested.

Picture source http://www.3m.com/ceramics/ (acknowledgement with thanks)

Do they give you a value for the thermal conductivity of these materials?

I am yet to go into complete details of their catalogs. As these materials are used on top of the jet engine to avoid propogation of heat to other sensitive parts like oil pipes, these must be keeping the outer zone cool. They can easily withstand the heat for sure but conduction of heat is another matter. There is also some difference in heat source inside the cavity and cavity inside the heat source. We have no way of cooling and only way to stop the heat is by slow conduction and reflection.

There are some variations in the functional Alumina to reach different temperatures. Some can reach 1300C. These must also be having phase transition so do not know, how long they will take to that temperature. Perhaps space suttle were also using these materials in the from of thick brick to give shape to its body shape.

I think the heat rate may slow down drastically and if we have some thick layer of this material then we may make it safe for many hours. Problem will be late temperature buildup like slow heat wave moving outside to inside intitially and when removed from furnace then from some mid layer to both inside and outside.

I can assume that my entire electronics is MIL grade then 100C is safe temperature inside the box. I think only an hour in the furnace may be possible to hold on the box. What I will do is to measure the entire process using sensors placed in the box with some dummy load.

Good morning,

Quite funny how this discussion goes on through the night,

Shyam, Do you have a list of equipment that needs to go in the enclosure?

Did you list up the power that is used by the equipment, all this power is transformed into heat and as I read through one of the former posts you will have a gas analyser inside. This type of machine usually has a motor to drive a pump that drags in the gasses. This means for me lots of energy.

The first question should be: can I run this equipment in a box for 6hr taking into account that all of the released heat will stay inside of the box (adiabatic heatup)

A simple solid state datalogger will be no problem if you add some mass.

As you described that you will analyse the gasses inside the furnace, evaporation cooling can't be used either.

Are you sure that these analysers can work up to 100°C and are calibratable in this range?

In our lab we have build in special walls to separate the ovens and other hot equipment from the dataloggers and we are only interested in thermal and electrical info. And I don't want the know that the air conditioning costs.

The glass fibre cloth has better K factor than mineral wool in the high temp zone.

This is a link to a supplier of this kind of insulating mats usable till 1000°C:

http://www.klevers.de/html-en/products/needled.php

The outer wall must be made of this alumina stuff, it will take the temperature down for a while, then a layer of this needled glass fiber cloth and again a rigid insulating box in which your equipment can fit.

The thickness will be at least 400mm in total. If you can have more it is always better. If you can add a layer of a material that can change phase (a steel double walled box, with the wall filled up with another metal that will melt, aluminium is a good one) you can add time and take down the insulation thickness. A radiant reflecting coating on the outer box outer and inner wall wil also help you.

Hi

Yes, I am happy about 24-hour link and responses.

Power management to the IMS and other Optical sensor hardware is highly optimized. No problem. These can work well up to 125C and are for tough environmental applications.

All instruments are in-house design and very compact, all MIL Grade parts and data corrected for environmental parameters. Some data is corrected after recording is done and some hardware corrects in real time where changes can affect the measurement conditions.