Peter van den Berg

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since May 27, 2012
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forest garden trees wofati woodworking
He's been a furniture maker, mold maker, composites specialist, quality inspector, master of boats. Roughly during the last 30 years he's been meddling with castable refractories and mass heaters. Built a dozen in different guises but never got it as far as to do it professionaly. He loves to try out new ideas, tested those by using a gas analizer.

Lived in The Hague, Netherlands all his life.
+52° 1' 47.40", +4° 22' 57.80"
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Recent posts by Peter van den Berg

I took the time to draw a p-channel that would fit a sidewinder batchrocket. The horizontal part should be outside the firebox in its entirety. The vertical part should be in the firebox, directly above the port and hanging over that U-shaped piece.  Proportions should be fitting for an 8" version.

1 week ago
I know the flap is bent down from the opening in the horizontal channel. But this shouldn't be just a flap, but a U-shape with open sides down and to the riser instead. Again, please have a close look at the picture where it's clearly visable at the lower end of the vertical part.

And pay attention to the text that's directly under the picture which says:

"The downward hanging end stops a little lower than the top of the port, providing a small overhang. To keep things simple, this overhang is the same as the depth of the channel. At the back which is facing the riser a small piece is cut out to promote the suction of air. The same principle that causes the p-channel to work, Bernoulli's principle, means the pressure within the system is lower than that outside. Hence no smoke will escape the p-channel, indeed any other small cracks or the main air inlet itself. Conversely, if smoke does come out of the p-channel or main air inlet, then it means there is something wrong in the system."

That "something wrong in the system" need to be resolved as well before the heater will run properly.
1 week ago

Jane Ashworth wrote:Would it be better to position the channel within the portal with the air ejecting from the top? And do you recommend having a flap here?

Your questions show you didn't get the principles (yet). So I'll try explain it again, although it would be better to stop thinking and just do what is recommended. Air ejecting from the top inside the port itself wouldn't work any better, rather the other way around. Just because the incoming air wouldn't be heated up as much.

The basics are on the website but here we go again: the port act as a temporarily restriction in the smoke path. As such, it is a venturi wherein the gas speed increases and the pressure decreases. It's a law of physics called the Bernoulli theorem, part of the law of conservation of energy. It doesn't matter whether or not you understand how it works, even believing otherwise won't change laws of physics.

Now how to utilize this phenomenon in order to get a sufficient quantity of fresh, hot air into the port. In order to understand this, it is imperative to realize that the lowest pressure in the port happens to be just a fraction of an inch beyond the narrowest point, seen from the firebox' side.

By hanging down a small piece of the channel over the opening, that said narrowest part of the port is the p-channel itself. So the lowest pressure and highest velocity is generated where the air from the channel comes in. That way, air is literally sucked in.

What has been done wrong in your implementation is just a flap hanging over the port, with generous openings left and right. This is so much cross section area together that the narrowest spot in the port isn't where it should be but further downstream instead. That flap should be a U-shape, and the channel should be at least as wide as the port. Entering from the side doesn't help either, none of my drawings shows that and for a reason. The part of the channel  inside the firebox shouldn't be horizontally but vertically instead for best performance.

To conclude: please see that the recommended dimensions, shapes, sizes, proportions, end temperatures and whatnot are adhered to. Just copy the whole of the core design and it'll work, right out of the box. For reference and pictures: see and Both of the embedded pictures show how the p-channel should look like. If at all possible, build just the core outside using bricks and clay/sand to seal the crevices and fire the thing up.

P.S.: the bench together with the water barrel is way too much. As long as it's there the thing won't work as desired so measure the temperature inside the exhaust pipe to the roof, not the outside.
1 week ago
It's not easy to see how it's been built. But Max is right, that bench on it's own is probably already too much. When it's also a piped bench with bends in it you are running into much friction in the smoke path which is something batchrockets are very picky about.
But I do see another blooper that's often made by a lot of people. The specification of the p-channel is: as wide or wider than the port. So the narrowest spot in the port is where the p-channel is. Which means the greatest velocity and lowest pressure is exactly where it need to be.

What you have done is just a flap hanging over the port but it's smaller than the port and is open on both sides. So the highest velocity is deeper in the port and under pressure in the channel isn't as strong.

And yes, exhaust temperature in the heart of the exhaust need to be equal or higher than 60º C in order for the heater core to work properly.
1 week ago
That's true Benen, but the hottest parts of a bell tend to expand during heating up. And contract again while cooling, abeit slow. So there will be cracks developing in a single-skin bell that's fed with a batchrocket, mainly in the upper part above the level of the riser. I've been building a single skin heater a couple of times and I can recommend the method of building with common bricks while there's a lining of firebricks on the inside with an expansion space between liner and wall.

There are two ways to achieve such a partly lined bell.
One is changing to bricks on edge  at the riser's top level so there will be space on the inside to place split firebricks on edge.
The other is do one layer of bricks that are turned around 90 degrees in the horizontal plane so the bricks are sticking out inside and outside. I've used this construction a number of times and it works.
2 weeks ago
Congrats Benen, your little batchrocket performs quite well by the look of it. Do you have a way to measure stack temperature? That would be a good way to determine actual efficiency.

Now I had a closer look at it, it looks like the threshold of the floor channel is missing. The function of this piece of steel is to damp down the most violent top of the burn while placing an emphasis on the floor channel. In other words, the higher the air velocity and temperature inside the firebox the more air is sucked into the channel. Which is a very good thing since at the top of the burn the demand for secondary air is quite large.

About your calculations: I tried your method on the ISA numbers which I have found the hard way. And it does add up, you are right. At the moment I don't have a way to check the load capacity but I think it could be right as well since all measurements are tied to the riser, diameter or csa.

May I add this to the website complete with crediting you?
2 weeks ago
Benen, try the thing first before you change anything. When it isn't working up to expectations alter the riser first, Thomas' suggestion is a very good one. Here's a link to it, it's dead simple.

In general the firebox' insulation is nice to have but not really important, in sharp contrast with the insulated riser. That riser is the spot where it really happens, on the rare occasion that the flame isn't through the port in mid-burn you'll see thick black smoke. All configuration in front of the port is there to ensure the afterburner is fed properly, enough but without over fuelling it.

Again, give it a proper test run and change one thing at the time when necessary.
4 weeks ago

Benen Huntley wrote:All made with 230x115x75mm fire bricks (except for the top of the fire box which is a 305x305x50mm tile and cladded in mild steel.

Benen, I am sorry to say, but you missed a very important point. By scaling the design down, the mass of the core should be scaled down as well. In your case, I would have used 30 mm firebrick or slabs although in a number of cases it proved to be too much. Building such a small device out of insulating firebrick is a better way.

It would probably work like you built it but it'll take a long time to burn without smoke due to the fairly large mass around the firebox and riser.
1 month ago
In that case, you probably can get away with 2/3 of the normal ISA value to start with which comes down to 3.5 m² or 37.7 ft². Remember, the pipe feeding into the bell should be slightly higher than the exhaust. The mass of such a bell is largely irrelevant regarding heat extraction, the more mass the longer heat retention. The other way around, less mass means quicker and higher temperatures going into the room but it won't last until morning.
1 month ago

John Harrison wrote:Would it be worth reducing the ISA of the bell in my proposed application? (the stove output flue is 150mm - 6")

The original stove is extracting heat during the burn, so the bell should be accordingly smaller. How much smaller is anybody's guess, depending greatly whether there're vermiculite boards inside the firebox or not.

In my opinion you could start with half of the recommended ISA for a batchrocket and build it in such a way that you could build it higher later, topping it up as it were.
1 month ago