firewood kiln - How do you build one? does anyone know

[BARNSLEY BOB]

 

Ditto!! I know it is a derail but it's hard to forget a friends very poor experience of them

 

 

 

glen farrow or your friends.?

 

do tell us.

 

how are we supposed to learn or improve if you keep everything top secret.?

[arboriculturist]

Just about to get started with having a punt at this. This is what we have:

 

40ft shipping container, insulated on all sides (bar the door and floor) with an average of 6 inches of kingspan.

233kw rated (though not sure it will actually pump this much out) biomass boiler. Pumps out 4 cubic metres of hot air per second, tested and producing hot air at a steady 120 degrees celcius. Burns sawmill offcuts.

20 potato crates of western red cedar firewood, cut to 25cm.

 

I realise that potato crates are a bit crap for drying, but I'll see how it does. If it works well, I'll maybe get some steel crates or IBCs. If it doesn't, I'll sell the constituent bits and write it off as a failed experiment!

 

You're there Jon and we sensible enough to do at a fraction of the cost of most. Potato boxes ok for now, but you will need to move some slats to improve airflow.

 

Good luck.

[Ambisawrous]

Interesting, how are you getting 120 Celsius that's pretty hot!

[Big J]

You're there Jon and we sensible enough to do at a fraction of the cost of most. Potato boxes ok for now, but you will need to move some slats to improve airflow.

 

Good luck.

 

Should be doing a test run next week so let's see how it goes. I've got a couple of loads ready to go in.

 

Actually had 160 celcius for a while coming out the vents, but that was a bit too hot.

[openspaceman]

Should be doing a test run next week so let's see how it goes. I've got a couple of loads ready to go in.

 

Actually had 160 celcius for a while coming out the vents, but that was a bit too hot.

 

 

Is it recirculating or straight through? That's an air change every 15 seconds but if you’re heating the air through 100K at that rate it looks like 400KW(T).

 

You've got good insulation so heat losses may only be around 4kW, the biggest losses will be in the mass flow being vented.

[Big J]

Is it recirculating or straight through? That's an air change every 15 seconds but if you’re heating the air through 100K at that rate it looks like 400KW(T).

 

You've got good insulation so heat losses may only be around 4kW, the biggest losses will be in the mass flow being vented.

 

Useful information! I'm confident in drying sawn timber, but firewood is new to me.

 

The boiler is rated to 233kw though I'm not sure it will realistically reach that. Observation was that the average air output temperature was around 100 celcius with hourly stoking of the boiler (small firebox of around 0.8 cubic metres and forced airflow). Using fairly fresh offcuts from the sawmill as I have tonnes of the stuff and it burns well.

 

I'd be putting 20 crates into the kiln, which would occupy around 30 cubic metres (albeit only about 20 once airspace is accounted for). So that leaves 54 cubic metres of airspace and an air change every 13.6 seconds. On the basis that the air is effectively coming in at a humidity of 0%, the air would only have to pick up a tiny amount of moisture (relatively speaking) to dry the timber very rapidly. Very rough calculations indicate that a cubic metre of air at 95c can carry 1160ml of water. Assuming it reaches 5% RH, it strips 58ml with every cubic metre of air, which is 835 litres an hour (14400 cubic metres an hour). 30 cube of firewood has at least 4500 litres of moisture to lose, so theoretically it's 5hrs, which it won't be in practice, but does a day sound possible?

[se7enthdevil]

can you not then condense the water coming from the wood and sell it as some sort of wood whiskey?

[log on tommy]

can you not then condense the water coming from the wood and sell it as some sort of wood whiskey?

 

I like it, can I boil my p**s and sell it as man brandy!

[openspaceman]

 

The boiler is rated to 233kw though I'm not sure it will realistically reach that. Observation was that the average air output temperature was around 100 celcius with hourly stoking of the boiler (small firebox of around 0.8 cubic metres and forced airflow). Using fairly fresh offcuts from the sawmill as I have tonnes of the stuff and it burns well.

 

OK then efficiency is not a problem?

 

On the basis that the air is effectively coming in at a humidity of 0%, the air would only have to pick up a tiny amount of moisture (relatively speaking) to dry the timber very rapidly. Very rough calculations indicate that a cubic metre of air at 95c can carry 1160ml of water

 

Even saturated air at 100C only contains 600 grammes of water but this is immaterial as once the vapour pressure of water exceeds the air pressure it is boiling off. Then other considerations apply, like whilst you are putting in nearly 3m3 of ambient air it now occupies 4m3 and every 18grammes of water you drive off increases this by 22.4 litres. In effect the hot air only becomes a heat transfer vehicle and of course the limiting factor is the thermal conductivity of the wood to its interior compared with the heat arriving at its surface.

 

It's very easy to demonstrate that given an average 1kg log dries to below 10% mc in an oven in 24 hours, so if you can keep every log in your kiln at 100C for 24 hours it will work but what will be the temperature of the air+steam mix you are venting be? This is the measure of the efficiency and also the reason I feel a high temperature dryer like this, whilst being fast, should cascade its waste heat to dump it into a lower temperature load, like underfloor heating.

 

I did have a reason for asking about recirculating and it is this; at these temperatures for the air flow the hot secondary surface of your heat exchanger will be way above 120C, a few specks of sawdust landing on this may carbonise, and nascent char will burn at 200C, so a fiery spark of carbon may reach your now hot dry surface of logs, what might happen next? I certainly would know.

[Big J]

OK then efficiency is not a problem?

 

 

 

Even saturated air at 100C only contains 600 grammes of water but this is immaterial as once the vapour pressure of water exceeds the air pressure it is boiling off. Then other considerations apply, like whilst you are putting in nearly 3m3 of ambient air it now occupies 4m3 and every 18grammes of water you drive off increases this by 22.4 litres. In effect the hot air only becomes a heat transfer vehicle and of course the limiting factor is the thermal conductivity of the wood to its interior compared with the heat arriving at its surface.

 

It's very easy to demonstrate that given an average 1kg log dries to below 10% mc in an oven in 24 hours, so if you can keep every log in your kiln at 100C for 24 hours it will work but what will be the temperature of the air+steam mix you are venting be? This is the measure of the efficiency and also the reason I feel a high temperature dryer like this, whilst being fast, should cascade its waste heat to dump it into a lower temperature load, like underfloor heating.

 

I did have a reason for asking about recirculating and it is this; at these temperatures for the air flow the hot secondary surface of your heat exchanger will be way above 120C, a few specks of sawdust landing on this may carbonise, and nascent char will burn at 200C, so a fiery spark of carbon may reach your now hot dry surface of logs, what might happen next? I certainly would know.

 

Bloody useful again. You imput is much appreciated

 

Realistically, due to the lack of a capacious burn chamber or buffer tank, the kiln will be on in 10 hour blocks. It doesn't take long to get to temperature (about 20 minutes) and would be pumping hot air at around 100c for all that time.

 

There is no heat exchanger - it's a simple case of hot air pumped into one end and vented out the other. Efficiency is not an issue for me as the whole set up only cost a couple of grand, and one day's cutting on the sawmill produces enough offcuts to fuel the biomass boiler for several days.

 

Thank you for the figures on what saturation point would be. Late last night I just couldn't find the tables I needed!

 

Do you reckon I'm going to be at risk of spontaneous combustion with an internal temperature of 80-90c?