6" System Heat Riser - can I shorten the length without affecting performance?

I'm working on the core of my outdoor Batch Box RMH/heated bench project - it's a 6" (150mm) system.

On the Batchrocket resources site it lists the heat riser length for a 6" system as 43 3/16" (1080mm). I think this may be too high for my project if I allow enough clearance for the exhaust from the top of the riser to the ceiling of the bell. I'm proposing to build a '5 minute riser' using a 25mm  'Superwool' blanket inside an 8" ducting pipe.

I have two questions :-

1/ What is the minimum clearance between the top of the riser and the ceiling of the bell?

2/ Can I shorten the length of the riser and if so, by how much?

Many thanks as always for any help or advice.

Hi John;
Well about 12 " is what is recomended as a safe clearance.
You might be able to reduce that to 8". However the top of the bell over the riser will need to be able to deal with  1800+ temps.
One way to deal with that would be to use ceramic board or blanket as a roof insulator.
Once that heat leaves the riser it is cooling rapidly.
With the roof insulated you can bring the riser within 6" or so if need be.

Yes , you could shorten your riser but draw will be affected.
How much can you shorten it?   Well Matt Walker builds and uses a "riser-less" core and he gets clean burning &  super hot temps from his builds.

Many thanks for your reply Thomas.

I was going to roof the top of the bell with a 3" granite slab with a one inch (25mm) layer of Superwool between the riser and the granite.
The  'chimney' for the whole system is about  11 feet (3.5 metres) high of which over 7 feet of it is insulated. I'm hoping that this will provide a good draw so I could possibly compromise on the riser/bell clearance and the heat riser length without too much affect on performance.

Thanks again for your help.

Minimum figures for the plain vanilla batch box rocket: riser length 8B or 8 times base number, top gap (between riser and top of bell) equal to riser diameter. So for that last point I second Thomas.
In your particular case: riser length 34.7" (86.8 cm), top gap 6" (15 cm) but keep an eye on that granite slab.

Many thanks Peter for your detailed reply and figures for my application.

I'm factoring in easy access to the top of the bell so that I can monitor how the 'Superwool protected' granite slab performs and also to keep a check on the Superwool liner of the 5 minute riser. I'm planning to rely on a friction fit of the Superwool in the riser - would that work or do I need to fix it in place with some sort of adhesive or hardware fixing?

Thanks again.

Friction fit is all you should need on the riser.

Great Thanks again Thomas for your swift reply.

I may be wrong as I am far from experienced but I am sure that if you are following PVB rocket stove dimensions, it is not recommended to shorten the riser?
Also I think  granite  is prone to cracking and I would think you might need more than 25mm of fibre ?

Peter and others have experimented with shorter risers and found that a certain amount of reduction will still function fairly well as long as the rest of the system is optimal. Peter just gave his expert opinion on the minimum riser height.

I would be concerned about the granite slab - the more massive it is, the more risk I would expect of differential expansion cracking. I especially would not allow more cool edge extension than absolutely necessary.

Thanks Glenn.

As regards the concerns with the granite slab, I have an option to use a larger, thinner slab as the bell ceiling. I could protect this with 25mm Superwool and cut out a section that would line up with top of the heat riser. I could then use, say a 10 inch square, thin or insulating firebrick to withstand the intense heat from the riser. Would that work better than the granite slab protected by a layer of Superwool?