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Any new small charcoal retorts out there?

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Temp probe is fitted through a 7.5mm hole at the top of the door. I've fitted a stainless steel K-probe this morning which was threaded, so it's permanently attached to the door - just need to plug it into the reader as and when required.

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Must admit I had not grasped that fact. So do all steels and stainless steels become dramatically weaker when heated?

 

Steel loses about 40% of its strength at pyrolysis temperatures (say 470C on the hot side of the retort), flame temperatures with dry wood will be 1100C plus.

 

Stainless is weaker than mild steel plus it is a poorer conductor ( so cannot get rid of its heat as well).

 

Having said that thin mild steel manages quite well at 700C on the hot side if it's room temperature on the other, trouble is with a retort you have it hot on the flame side but you need over 450 on the char side. The thicker the steel the hotter the hot side stays.

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Steel loses about 40% of its strength at pyrolysis temperatures (say 470C on the hot side of the retort), flame temperatures with dry wood will be 1100C plus.

 

Stainless is weaker than mild steel plus it is a poorer conductor ( so cannot get rid of its heat as well).

 

Having said that thin mild steel manages quite well at 700C on the hot side if it's room temperature on the other, trouble is with a retort you have it hot on the flame side but you need over 450 on the char side. The thicker the steel the hotter the hot side stays.

 

Sounds like I should not have used stainless angle for the charge chamber supports and some thicker steel would have been better. Not had too many problems with charge chamber but the inner lining of the retort took a beasting but it was some wafer this steel sheet I had laying around. Trying to keep the whole thing as light as possible as I don't want any of the parts to be more than a two person lift. This one definitely gets hotter than my old one and may need some means to crank it back. Was trying to keep it simple as every little bit adds cost :001_huh:

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Sounds like I should not have used stainless angle for the charge chamber supports and some thicker steel would have been better. Not had too many problems with charge chamber but the inner lining of the retort took a beasting but it was some wafer this steel sheet I had laying around. Trying to keep the whole thing as light as possible as I don't want any of the parts to be more than a two person lift. This one definitely gets hotter than my old one and may need some means to crank it back. Was trying to keep it simple as every little bit adds cost :001_huh:

 

Stainless starts off with lower strength than mild steel, depending on which value of strength you measure, but it retains its strength better at high temperature, particularly its creep strength, which is slow, gradual movement under relatively low loads, so it will be stronger for a given thickness than mild steel.

 

For supports, where you aren't trying to get heat through, the low thermal conductivity isn't a problem.

 

Alec

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Thanks Alec

 

So some pictures of the damage. I know the lining and support were lightweight but surprised by the bending of the L section in pic one. Thats 6mm X 50mm L with around 25kg resting on it at the start of the burn. Easy for the base to be much heavier steel with reinforcement on the ends but the lining in pic 3 wants to be as light as possible. From a look at heat chart based on the colour of steel pic 2 got up to around 800C!

DSC06130.jpg.64de5fe9356d27b673aa6b4660f17595.jpg

DSC06133.jpg.0090717434847eeef0e0aa3ca2a33850.jpg

DSC06136.jpg.87e44a6ec0a6dc145acbfa0369d8fab2.jpg

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look at heat chart based on the colour of steel pic 2 got up to around 800C!

 

Which is too hot. In fact the picture shows the steel has burnt and that is iron oxide flaking off.

 

I can't see the need for the firebox temperature to be much above 550C. Can you not line it with tiles or fire brick.

 

I was told a slurry of clay and milk could be applied to protect steel, I was not very successful with it but I probably did something wrong.

 

In boilers the steel is often protected by the water bath and the grate by exhaust gas recirculation but these aren't practical here.

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Which is too hot. In fact the picture shows the steel has burnt and that is iron oxide flaking off.

 

I can't see the need for the firebox temperature to be much above 550C. Can you not line it with tiles or fire brick.

 

I was told a slurry of clay and milk could be applied to protect steel, I was not very successful with it but I probably did something wrong.

 

In boilers the steel is often protected by the water bath and the grate by exhaust gas recirculation but these aren't practical here.

 

Thanks

 

Just ordered a thermometer and thermocouple so can get a better idea of whats going on next burn. At present I have very rudimentary control over combustion air. Presume if I reduce the air supply some of the gas will go up the flue and combust there? At present I think it all gets burned off in the fire box.

 

Other than reducing air supply are there other ways to control temperature?

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Just ordered a thermometer and thermocouple so can get a better idea of whats going on next burn.

 

At these higher temperatures radiation becomes important, thermocouples read low because they are constantly radiating heat away unless in equilibrium with the walls of the surroundings. This doesn't matter if you are just comparing burns.

 

 

 

At present I have very rudimentary control over combustion air. Presume if I reduce the air supply some of the gas will go up the flue and combust there? At present I think it all gets burned off in the fire box.

 

Other than reducing air supply are there other ways to control temperature?

 

The obvious one is to have a smaller fire underneath and a more conventional design with a grate. The power is then controlled by the primary air and the secondary air is entrained by the primary combustion.

 

As I keep saying you need more time to dry wood than to pyrolyse it, once pyrolysis starts its self sustaining as long as the temperature doesn't drop. At this stage lots of offgas is evolved and most of these simple retorts pipe the gas into the firebox where it adds to the heating just when you don't really need it or you flare it off wastefully. When I built one of these after seeing them at the merriworth estate and being developed by Robbie Webster I had two pipes out of the retort with one cap. The idea being to take the offgas under the retort when needed and flare it when not. I intended to use a bimetallic strip to open and close the flues but we moved back toward kilns (having moved away because the Viper produced poor charcoal for barbecues).

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At these higher temperatures radiation becomes important, thermocouples read low because they are constantly radiating heat away unless in equilibrium with the walls of the surroundings. This doesn't matter if you are just comparing burns.

 

Best way round this is to fix the thermocouple against the relevant wall. If you want to measure burn temperature (most useful for controlling product) it goes inside the vessel but if you want to understand what is going on you need it on the outside wall at the hottest part. The easiest way to fix it is to weld a tag on to slip the thermocouple behind but you could bolt it on instead, so long as you either drill and tap blind or you seal the holes over. For your set-up it may be easier to fix it to the SS support bar instead.

 

As I keep saying you need more time to dry wood than to pyrolyse it, once pyrolysis starts its self sustaining as long as the temperature doesn't drop. At this stage lots of offgas is evolved and most of these simple retorts pipe the gas into the firebox where it adds to the heating just when you don't really need it or you flare it off wastefully.

 

My very limited knowledge of practical retort operation relates to the Exeter, from TVI (inc. observing a burn) and from the manufacturers who also operate one. The heat-up phase to the point where the off-gas is combustible appears to be a couple of hours and the retorting phase appears to be around 6hrs, during which there is still drying going on (water is still coming off) but the reaction is self-sustaining. You therefore do want the heating, but you do have an enormous amount of surplus gas and it would be handy if you could have had it earlier! That said, it appears to take a very modest amount of wood to get things going. The more interesting thing for us is looking at using the surplus gas productively in a cost-effective manner.

 

Alec

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