Peter van den Berg

fraser stewart wrote:Can anyone help me. I need to know what sheetrock plastic wrap is. I have googled but not reaching anything. Anyone know what this is and can send a link

Since you are in the Netherlands, try stucgaas. Knauf Gitex is one of those.

thomas rubino wrote:How hot is an 8" batch?

Just checked: measured 1173 ºC equals 2140 ºF in a 6" version. A larger one like 8" would top that up a bit, might be 2300 ºF tops inside, especially against the rear wall. No experience with even larger systems, sorry.

fraser stewart wrote:I made the secondary air intake a while ago. I think its based on peters sketch up model measurments  Tonight i will take it out and measure it and post it..

The p-channel is the same size rectangle duct throughout, the floor channel is not. It's possible that you've mixed up the properties of the p-channel and the floor channel.
It has happen before, have a look at this information, please note the 2019 modification.

No soot to form due to evaporating moist then. I mentioned the required space around the firebox already, maybe you should have a look at that whether or not it's wide enough.

So both air inlets are separated from each other and it's a 150 mm system. In the one picture of the firebox' inside I can see a floor channel. The vertical part (stub) should be about 35x35x2 mm, the horizontal part (feed) twice as wide. Two ducts of that same square of the stub would be sufficient, or a duct of 60x40x2 mm. In your configuration it's 50x25 mm, that would be about half the required size.
You might think this isn't that important but in reality it is. I tried 5 different sizes during the development of this specific part and this provided the best performance. The reasoning behind the feed being twice as large is that the stub itself is acting as a narrowing in the gas path and thereby as a venturi. That will result in higher gas speed inside the stub as well as better cooling of that part.

The primary air inlet is round 50 mm, this comes down to 1963 mm². This inlet should be around 15% of the system's csa which is 2655 mm². So in your heater this is too cramped again, although I would regard the floor channel as the main bottleneck.

Fraser, there are a couple of parameters that are important for the working of the system. Space inside the bell is one, air inlet size is another and let's not forget the bench you have there. So here we go, in no particular order:
The space left and right of the firebox, inside the lower bell seems cramped to me. In order to keep the heater low-friction there should be at least 4 times the system's csa around the firebox area.
Now in order to see whether or not the air inlets are correct one need to know what the size of the system is. So, what is the system size?
Furthermore, of what refractory material is the firebox made of? When this is firebrick, moist need to be driven out and during this process soot will be produced.
Are all the other proportions respected?

Gerry, a slot might or might not be better, I simply don't know. As I said, nobody tried this on a bog standard batchrocket before plus there's always the possibility that it won't work at all. Assuming it could work though, it's a sliding scale antway between too sluggy and overfuelling. The difficult point here is to find the sweet spot where both effects are in a reasonable balance.

fraser stewart wrote:"caulk the seams" is that with heat proof stove silicone kit. The black stuff?

The heat proof silicone kit is not up to the job in my opinion. What you need is the black stuff, I have good experience with that. The best out of three was the one from Bison saying "Openhaarden Kit" and "Mastic Refractaire" on the tube. Resistent up to 1250 ºC, 530 grams in a tube.

Luke Perkins wrote:What is your experience using fire brick splits in a batch box lined with fiber board? Do the bricks tend to last for a long time or do they break and need to be replaced within a year or two?

I did only one experiment using split fire bricks inside a CFB box, pushed to its very limits as usual. The bricks seemed to hold well, don't forget those are able to get red hot again and again without any visable damage. My concern would be about the CFB.
Do I expect the splits need to be replaced within a year or two because of heat damage? The answer will be a firm no, not likely, think of counting in ten or more years.

Fraser, I know of one online shop in the Netherlands which sells the ceramic fibre board. https://kachelmaterialenshop.nl/53-keramisch-board- Before you order online, phone them up and ask if the material you want to buy is available. I'd suggest you line the firebox with firebrick splits, it is too easy to damage the walls just by shoving fuel in. The riser doesn't need to be protected. Over time, the riser's walls are starting to sinter and a rigid layer is formed.

More than one of my experiments were built out of 50 mm (2") material and were just screwed together. Use long stainless steel screws and caulk the seams.

It could either the shorter riser or the smaller diameter throat, or both. My guess would be start with the smaller throat, that's more or less how I did it.

Important: if and when it's too small the heater becomes very sluggish and no flame at all comes out of the riser. A little bit larger throat might make the difference. In my view, there's a trade-off between slugginess and overfuelling, a little bit held down is enough to prevent thermal runaway in 90% of cases.