Advice needed. My rocket won't draw!

Hi Peter.
Ok, here is my first list of questions-

Starting from the beginning,

I will have to build the firebox from firebrick so I am trying to work out exact dimensions. If you have those it would save me a lot of guesswork. I have already downloaded the free version of the Sketch up viewer. I now see the P chanel but the cross sectional view does not really give me a good view of where exactly it opens into the riser.

Could I use a metal pipe for the riser? If so how thick should the metal be to withstand the heat?
The diagrams show a double pipe which I assume would be in-filled with insulation, what would be a reasonable diameter for this outer pipe?
At what height would the riser end? In other words how much of a gap should I be leaving between the top of the riser and the brickwork above? (or how long should the heat riser pipe(s) be)

The top part would be tricky but do-able in brick but it could also be cast in fire-proof casting cement (not sure if this is what you guys call refractory cement?)creating a nice curve to roll the heat over and down the sides & yes, I would certainly use mortar between the bricks all the way down the sides as in the original Finnish stove design. I have old, hand made bricks which I salvaged & which I wanted to use (?).

I don't see the door being a problem, I have several old cast iron doors for these ovens in different sizes and most have an adjustable air inlet at the bottom which I can modify to let in the required amount of air, the metal skin of the oven is good grade steel so very weldable if I should need to put in a smaller door than is standard.

There is also a clean-out hatch at the base of the oven (see previous photo). so I will have to add some different brickwork at the base, also to hold up the fire-brick built fire-box. Chimney sweep must have access right through the stove to the base of the chimney for a vacuum hose.

Directly above the stove is the horizontal slit in the wall for the 'Pelti' (is this what you refer to as a damper?) in normal operation of the original stove one would burn one load of wood and when it is down to just embers the 'pelti' is slid in closing off the chimney and retaining the rest of the heat. Are you saying that I should forget this and seal up the slit in the wall?

Looking forward to your reply & a very interesting project which I will fully document.

J

Highlander Anderton wrote:I will have to build the firebox from firebrick so I am trying to work out exact dimensions. If you have those it would save me a lot of guesswork. I have already downloaded the free version of the Sketch up viewer. I now see the P chanel but the cross sectional view does not really give me a good view of where exactly it opens into the riser.

This will be a description with a lot of details, I will upload the link to the drawing in due course...
OK, The dimensions of the firebox and whatever are linked to the system size, the diameter of the riser, in this case 120 mm. Take 72.36% of the diameter, this is 86.8 mm, being the base number for most of the rest.

Firebox dimensions for width, height and depth are two times, three times and four times base number. This would be 174 mm, 260 mm and 347 mm respectively.

Height of the port is 2.2 times base, which is 191 mm measured from the firebox floor, width is 0.5 times base, this will add up to 43.4 mm. All dimensions can be rounded off plus or minus 5 %, whichever is convenient.

Riser height is 8 to 10 times base, being 695 to 868 mm, measured from the firebox floor. Riser cross section should be as close to round as possible, an octagon is a reasonable approximation. Don't forget the sloped sides left and right in the firebox, and the bottom rear side of the riser.

Main air intake is at the firebox lower front, 20% of the riser's cross section area, 2262 mm2. P-channel opening is 5% of system csa, 566 cm2, width as close to the port width as convenient, 43.4 mm. Depth of this channel 566 devided by 43.3 = 13 mm. The channel can be made physically from a steel or stainless steel rectangle duct of 50x20x2 mm, close enough. The channel opens above the port which is the opening between firebox and riser. The channel maintain an overhang over the port ceiling by as much as 13 mm in this case, the backside of the duct facing the riser cut away the same as the overhang. The metal of the channel could be made out of mild steel since it is constantly air cooled.

Highlander Anderton wrote:Could I use a metal pipe for the riser? If so how thick should the metal be to withstand the heat?

This riser pipe should be stainless steel, 1 mm thickness at least. Mild steel will burn away in no time. This riser will be red-hot during every single burn.

Highlander Anderton wrote:The diagrams show a double pipe which I assume would be in-filled with insulation, what would be a reasonable diameter for this outer pipe?

diameter of the insulation canister could between 220 and 250 mm. Insulation material preferably perlite, bonded with a little bit clay/sand mixture.

Highlander Anderton wrote:At what height would the riser end? In other words how much of a gap should I be leaving between the top of the riser and the brickwork above? (or how long should the heat riser pipe(s) be)

The more space above the riser the better, use the dimension given above for the riser.

Highlander Anderton wrote:The top part would be tricky but do-able in brick but it could also be cast in fire-proof casting cement (not sure if this is what you guys call refractory cement?)creating a nice curve to roll the heat over and down the sides & yes, I would certainly use mortar between the bricks all the way down the sides as in the original Finnish stove design. I have old, hand made bricks which I salvaged & which I wanted to use (?).

Refractory castable, sometimes called refractory concrete is fine for the top segment. Last row of bricks at the top preferably firebricks, the temperature coming off the top of the riser easily get to 700 degrees Celsius and more.

Highlander Anderton wrote:I don't see the door being a problem, I have several old cast iron doors for these ovens in different sizes and most have an adjustable air inlet at the bottom which I can modify to let in the required amount of air, the metal skin of the oven is good grade steel so very weldable if I should need to put in a smaller door than is standard.

That would be nice, respect the inlet dimensions and location.

Highlander Anderton wrote:There is also a clean-out hatch at the base of the oven (see previous photo). so I will have to add some different brickwork at the base, also to hold up the fire-brick built fire-box. Chimney sweep must have access right through the stove to the base of the chimney for a vacuum hose.

Maybe you could position a T-pipe behind the stove in order to provide for a clean-out port.

Highlander Anderton wrote:Directly above the stove is the horizontal slit in the wall for the 'Pelti' (is this what you refer to as a damper?) in normal operation of the original stove one would burn one load of wood and when it is down to just embers the 'pelti' is slid in closing off the chimney and retaining the rest of the heat. Are you saying that I should forget this and seal up the slit in the wall?

That would be a chimney damper, that one could be operated following this conditions: the fire has to be completely out, no glow to be seen anymore. The glowing phase isn't very long by the way, about 20 to 25 minutes in my case. The character of this stove is like a ventilator charged fire.

Here's the link to the SketchUp file.
The p-channel is a little bit different at the port end, because in this casted version the channel is flush with the backwall of the firebox. This is not very practical in a firebox made of firebrick, so this channel can be mounted against the back wall instead.

Edit:
To give an idea how to build a firebox and riser like this using firebricks, see this video. The one shown is a 150 mm system, so you have to cut more bricks.

Wonderful
I will take some time to digest the info. Have to go off and do a few days work but will try to source materials at the same time.
Many thanks.

J

Hello Peter and others.
We last spoke some time back, I am sorry for not continuing our conversation and project. I started my own business and it has taken a while to build it to the point that I can get back to my projects.

But now I am back and as the ice and snow are melting outside I am planning to start the big batch fed rocket. I have spent a couple of days reviewing everything from before and will soon begin. A lot of time has passed & I know you guys have been tinkering in the mean time so one question before I start...have there been any significant developments while I was 'away' that I should be incorporating?

I noticed a few youtube videos dealing with pre-heating intake air, for example.

Hope to hear from you soon.

J

Terve, man from the high lands.

For the latest developements on the batch, look there

http://donkey32.proboards.com/

There's been side windows, side heat risers, S portal. Matthewwalker's channel. And may be a few other things i haven't seen!

Highlander,
How is your rocket doing? With a few years under your belt do you have anything you would care to report about how things are working?
--Mud