Alois's wood briquette pressAlois Schmid writes:
Until recently, this was my dust extractor. I switched it to barrels from bags because the shavings can be pressed down in the barrels, and the barrels can be stacked.
But the shavings are hard to burn in a wood stove, and I didn't want to throw them out either. So I looked for a briquette press. Industrial machines are available starting at about 10,000 Euros. But there are various homemade briquette presses on the net, for example here: http://www.brikomat.com
For my homemade briquette press, I bought this new dust extractor for 600 Euros, including piping. I also bought this electric log splitter, it cost 200 Euros at the time. I started work on the briquette press two years ago.
By now, the machine runs automatically, with few problems. It could still use improvements, and the rate of production is very dependent on the type of shavings used. The machine works fastest with short hardwood shavings. It slows down quite a bit when running with long thin shavings from softwood. The higher the lignin content (see Wikipedia) the firmer the briquettes become.
The controller has switches at either end of the range, plus a hydraulic pressure sensor. Unlike industrial machines, there is no constriction of the briquettes, so pressure is strictly a function of friction of the briquettes in the channel. This makes controlling the machine more complicated.
The deeper the plunger pushes into the pipe, the shorter the remaining pipe that briquettes must travel through, and the lower the friction. If one plunges too far, the briquettes will not be compressed enough and will fall apart. But if the strokes are kept too short, the friction of the briquettes in the pipe can get too high, and the machine shuts down from excess pressure. I then have to manually clear the stuck briquettes by drilling them out of the pipe.
The stroke length needs to adapt to the type of wood used.
The graph at left shows how the controller operates. After 3.5 seconds of stroke (stage 1), the hydraulic pressure needs to be a minimum of 5 bar. At stage 2 and 3, the pressure must be at least 6 and 10 bar, respectively. If the required pressure is not reached, the cylinder returns for another stroke to press more material. If the pressure peaks out and falls by more than 3.5 bar before reaching stage 4, it also starts a new stroke. Once stage 4 is reached, the cylinder only returns once the pressure drops below 50 bar or when the limit switch is contacted.
1 bar is 100 kilopascal, or about 14.5 PSI.
The above time and pressure parameters can be adjusted, and I don't always operate with these values.
Initially, I used a pipe with an inner diameter of 35 mm, but the hydraulic cylinder had insufficient force for this. I added a smaller pipe inside the original pipe, with an inside diameter of 28 mm. With 7 tonnes of force, this should give me a pressure of 1136 kg/cm2. With the larger pipe, it was 727 kg/cm2. More pressure would be even better.
I cut the pipe open and bent the resulting 'flaps' outward. The container is bolted to the flaps.
A magnetic sensor senses auger rotation. If no pulses are detected for ten seconds of auger operation, the augers are assumed to be jammed and the machine shuts down.
A magnetic sensor detects the screws in the side of the wheel. Two turns of the wheel are almost exactly one meter of briquette. The quantity of briquette and hours of operation are recorded so I can calculate how many meters of briquette per hour are produced.
Another reason for monitoring briquette production: If no rotation is detected for more than three minutes, the machine shuts down. Either the shavings container is empty, or the shavings have jammed or bridged over the augers.
I already had most of the components lying around the house. I only had to buy the pressure sensor and transformers new. The enclosure, motor controllers and magnetic sensors were bought used. So far, this press has cost me between 600 and 800 Euros.
The machine could be optimized further, of course. One could pre-compress the shavings, which would speed up production. But that would be too complicated for the time being. I'm thinking of improving the feed mechanism. My most recent change was to run the augers in the opposite direction.
Changing the auger direction did in fact increase the output, as the shavings are now pushed more towards the opening in the pipe. I'm also thinking of an alternative to augers, but that will have to wait until other projects are finished.
I will write a bit more about my workshop later, but first, I need to do some cleaning up.
I added a kilowatt meter to the press, so I could work out its efficiency.
The barrel contains 51.4 Kg or briquettes, and the machine used 13.5 Kwh of electricity to produce them. With electricity costing about 0.25 Euro/Kwh, my power cost is 3.35 Euro
Wood briquettes cost 2.25 Euro per 10 Kg, but my electricity cost is just 0.65 Euro per 10 Kg. But I'm not counting the cost of the machine or the shavings.
The machine draws, on average, less than 1 Kw, so this took more than 13 hours to produce. I did not watch the time because the machine runs automatically and unattended.
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