Temperature at outer side of core.

[Filo]

Hello,

How much of maximum temperature I can expect on outer surface of core like Peter Batchrocket or Yasi Batchblock ?
I need it to choice right insulation like glasswool or rockwool sheets.

Best, Filo.

[peterberg]

Hi Filo, forget about rockwool or glasswool. The firebricks outer side of the core, especially the lower rear end of the riser can get cherry red. Morgan Thermal Ceramics Superwool is the only sensible option here. Or maybe pumice?

[Filo]

So, much more then 250-300 C ? Rockwool start melting (make softer) above 1000 C.

[peterberg]

Jan 25, 2022 14:06:25 GMT -8 Filo said:

So, much more then 250-300 C ? Rockwool start melting (make softer) above 1000 C.

Not only that, there's a binder in there that starts to desintegrate long before that.

[Dan (Upstate NY, USA)]

Not only that, there's a binder in there that starts to disintegrate long before that.

I just had to take my J-tube apart last winter after 8 years of use, all with hard fire brick, the rockwool that I used had scorch marks from the binder burning but the fibers where still all in place. I could not reuse the rockwool, it all fell apart when I pulled it out for the rebuild since the binder was degraded.

Used 2" to 6" around the J-tube core. It worked well enough that I re-built it with more rockwool since I had some lying around.

[fiedia]

Inside my small batchrocket firebox, I measured above 900°C. The temperature on the other side of the bricks will depend on how much you insulate the firebox. With a very good insulation you woukd get the same temp on both sides of the bricks.

I just wonder why one wants to insulate the firebox. Its heat is more usefull at the bottom of the bell than at the top of the HR, no ?

[Filo]

Jan 26, 2022 9:04:10 GMT -8 fiedia said:

I just wonder why one wants to insulate the firebox. Its heat is more usefull at the bottom of the bell than at the top of the HR, no ?

Yes, You guess right. I need a lot of heat at top of heater above firebox, because there will be heat exchanger (air to air). To gain it, I need to insulate sides of heater.
Still no answer, about temperature on outside side of core bricks, nobody measured it ?   

[fiedia]

I did measure it. It was above 300°C. But it is not relevant as I did not really insulate the core. The more you insulate, the higher is the temperature on the hot side of the insulation.

I guess that your solution is to use a high temp insulation. Why do you want to use rockwool instead of superwool, soft refractory bricks or vermiculite ?

[fiedia]

By the way, it would be interesting to have a global picture of your project (air-air exchanger...)

[Filo]

In mine country I can buy easy and for good price expanded clay. But, I afraid that material can thicken in time and will block heater core to free thermal expand. I know that material is very light and between granulate is a lot of free space, but it will be enough to keep core free expanding ? I have read in web that expanded clay was used to insulate riser in j-tube constructions - do You know any experiences with such application of expanded clay ? 
Superwool is many times expensive that rockwool here, and are easy available sheets or blankets (like 5 cm thin) with manufacturer promise of resistance (dimensions stability) about 550 to 660 C degrees. 

[fiedia]

Well, Peter and Dan have been clear about rockwool. I tested it too. It really stinks. The gases coming out of this material are surely toxic.

If you have vermiculite available, it may be a good solution as it is more compressible than expanded clay.

[Filo]

Hmm, I afraid to show it here, because it will be experiment and I not expect high efficiently of this idea. Simply I will tray to take this way so maty heat as it is possible. To exchanger will be delivered air about 20 C with volume about 400 m3 per hour. It will be steel box with square channels divided with thin steel bars as radiators, putted on top open bell of heater. 

[Filo]

Probably I will use expanded clay but for futer use i put here nice calculator of stone wool form company PAROC. I made some calculations with products for HVAC and Industry allowed to use with inside temp. upper to 500 C. Only 5 cm barrier decrease temp. from 500 C to below 100 C. Software alarms when material was wrong choose for such application.


calculus.paroc.com/paroc-calculus/#/