Saturday, 15 March 2014

Making a Rocket Stove from a Beer Keg

Ludi cooking sausages
If anyone isn’t familiar with rocket stoves take it from me that for cooking outside they are excellent!  I use one in summer in the garden and in winter in the courtyard outside the front door.  We also use it for preserving soft fruit by boiling a sterilizer full of Kilner jars on top.

You won’t get barbecue flavours but, for any sort of pan or griddle cooking, a rocket stove gives a high heat, reaches its working temperature very quickly and can burn any type of dry wood cleanly. We always have offcuts left over from construction projects but we've also used branches picked up in the woods nearby.

The original rocket stove
In this family there are three rocket stoves which were bought from a charity. The original design was made from recycled small metal barrels and contains a right angle welded bend, in mild steel, to form the firebox.  They use wood ash as insulation, so they are heavy, especially if they are left out in the rain.  After three or four years of fairly regular use, and kindly neglect, the barrels and the bends of all of them have rusted away.

Click on any picture to enlarge it
For a long time I looked around to find something more durable to make replacements from.  I thought about a stainless steel rocket stove, which would be the Rolls Royce of outdoor pan cooking but I decided that cutting, shaping and welding stainless steel would be beyond my capabilities. Eventually I hit on the idea of using a 30 litre aluminium beer keg (37cm high and 39.5cm diam , 14.5 in and 15.5in) with a commercially bought 150mm (6 inch) stainless steel 90 degree bend to form the firebox.  After checking the dimensions it just so happens that they are perfect for the whole assembly to work well together. Via a family contact I was lucky to get two aluminium kegs for nothing from a small local brewery which has decided to switch to using only stainless steel.  

Beer kegs are pressure vessels so first carefully open the screw top to let out any residual pressure.  A beer keg has no lid, so to fit the bend inside it you have to cut the keg in half.  This is not very easy.  For any job like this you should always wear ear and eye protection.  After contemplating using a 9 inch angle grinder, I decided that cutting into a curved metal surface would be too risky with one of these dangerous machines. They have a nasty tendency to buck and jump and I’ve known several people injure themselves with them.  I settled on using a reciprocating saw and some aluminium cutting blades (10tpi).  You really need to make a jig to hold the keg steady but, however you do it, carefully consider the safest method of stopping the keg from moving around because the saw causes a lot of vibration.  For the cut I marked a line with a marker pen but it’s very difficult to cut it accurately, so it’s just for a guide.  As long as the two halves fit back together again it doesn’t really matter.

The holes for the bend are marked by holding the two halves of the keg together securely and drawing round the end of the bend.  Fabricated bends are rarely exactly circular and are often oval in section, so orientate it with the horizontal end pointing to the side where you want the firebox access. Then use a jigsaw to cut the 150mm holes out as accurately as possible.  Again this causes a lot of vibration and the saw often jumps, so don’t be surprised if you break a blade or two.  Use a file to shape the holes so that they fit the bend snugly.

The finished beer keg rocket stove
To fix the two halves of the keg together I bought a stainless steel hose clamping set (Ref Draper 55591).  For each keg there will be only a small length of the 3m of stainless steel banding left over. You could use any steel banding device but I used stainless because I hope that it will react less with the aluminium of the keg.

The bend needs insulation around it so make a two inch hole in the top of the keg, cut a flap like a keyhole cover and fix it to the keg with a bolt. Fill the keg through this hole with vermiculite.  This was chosen because it is resistant to high temperatures and chemically inert.  DON’T USE WOOD ASH because the highly alkaline residue will dissolve the aluminium if it gets wet.

The metal firebox grid was transferred from the old stove and it was lit.  We have a small oven which fits over the outlet pipe. 
This was bought from the same charity and, although it’s made from mild steel, it’s quite robust and hasn’t rusted.  Once the fire was burning I put the oven on the top and it reached 250 deg C in 10 minutes. 
You do, however, need to keep feeding it because it burns wood quickly!

For pan cooking, I use steel pans which I wash in the stream next to the garden.  They are sterilized before I start cooking by the high heat from the stove.  They sit on a grid which in turn sits on a steel ring supplied with the original stove.  This creates a gap around the pan which allows the fire to breathe.

I’m very pleased with the end result, which you can see in the picture above, it's light in weight, more stable than the original design, has built in carrying handles and should resist corrosion much better than mild steel.

Post Script
20th June 2016
The stainless steel bend gave up a few weeks ago and I have now replaced it with another of the same type.  It had holes in several places. It's interesting that 304 stainless is not very much better at resisting high temperatures than mild steel.  That's not what I expected.


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