How to build a DIY timber drying kiln

[Big J]

Big J,interesting results,not enough heat or air flow or a combination of the two? I use something similar to your setup(insulated container c/w dehumidifier+ heat),but I cannot imagine how crates of split logs could be dried uniformly without really high temps to effectively bake the logs, but then what about case hardening ?not sure what "log pro" do, to achieve the results they claim in the time scale quoted..... but it sure would not be suitable for boards, so I can empathize with your last comment

 

I just couldn't get the temperature up to where it needed to be to shift the water. Consequently, it took far too long and was too expensive. I think air flow is also an issue and I'm not sure how you'd resolve that. The fan I have is pretty powerful (over 3000 cubic metres an hour) and it's ample for sawn timber.

 

Jonathan

[Big J]

Rob - I think that the Log Pro would be too aggressive for any kind of sawn timber. Too much heat and a very short drying period.

[openspaceman]

I just couldn't get the temperature up to where it needed to be to shift the water. Consequently, it took far too long and was too expensive. I think air flow is also an issue and I'm not sure how you'd resolve that. The fan I have is pretty powerful (over 3000 cubic metres an hour) and it's ample for sawn timber.

 

Jonathan

 

Drying firewood is a different ball game, one doesn't have to worry about cracking, checking or surface ahrdening.

 

So it's about removing the most water for the minimum energy input, the energy input is in the form of moving air and heat and the aim is to optimise these. Some months ago Brooney posted his finding which concur with mine and on which we based our high speed dryer 17 years ago.

 

Did you measure the temperature and RH of your 3000m3 air flow before and after the wood?

 

My guess is the temperature limited the migration of the water to the wood surface and that the air flow was leaving the kiln both warmer than it should and unsaturated.

 

If you take ambient air on a winter's day at 60% RH it contains 6 grams of water, saturate it and exhaust it at 25C and it contains 23 grams so at 3000m3/hr the maximum you can remove is 51kg water per hour, we removed more like 300kg/hr on a 24 hour load-reload cycle but actual drying time was much less.

 

Energy considerations are something to consider the above assumes a cop of 1. We managed a cop of around .5 but had we stayed in business expected to get 50% better.

 

 

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[Big J]

Drying firewood is a different ball game, one doesn't have to worry about cracking, checking or surface ahrdening.

 

So it's about removing the most water for the minimum energy input, the energy input is in the form of moving air and heat and the aim is to optimise these. Some months ago Brooney posted his finding which concur with mine and on which we based our high speed dryer 17 years ago.

 

Did you measure the temperature and RH of your 3000m3 air flow before and after the wood?

 

My guess is the temperature limited the migration of the water to the wood surface and that the air flow was leaving the kiln both warmer than it should and unsaturated.

 

If you take ambient air on a winter's day at 60% RH it contains 6 grams of water, saturate it and exhaust it at 25C and it contains 23 grams so at 3000m3/hr the maximum you can remove is 51kg water per hour, we removed more like 300kg/hr on a 24 hour load-reload cycle but actual drying time was much less.

 

Energy considerations are something to consider the above assumes a cop of 1. We managed a cop of around .5 but had we stayed in business expected to get 50% better.

 

 

.

 

The temperature and RH is measured on the far side of the kiln to the main circulation fan, but before the air has permeated through the stack. It should be noted that the 3000 CM an hour is the circulation fan, not the extraction through the heat exchanger, which is around 200 CM an hour. The reason I couldn't get up to temperature for the firewood is that the heat loss, intended and otherwise, was too high. This is most likely due to lower efficiency in the heat recovery unit than published (as is often the case, however it's still hugely better than having no heat recovery).

 

Winter average temperature is about 5c here and average humidity about 85%. That means incoming moisture of 5.86g per cubic metre (1.172kg an hour). Outgoing varies - the kiln will usually sit at 83% RH for 2 to 3 weeks innitially and then drop 1 to 2% per day thereafter. At 83% and 30 celcius, it's extracting 25.27g per cubic metre (93l per day total extraction, once incoming moisture accounted for). A full kiln has 250 cubic foot (7.1 cubic metres) and at least 3000l of water that needs to be removed if green.

 

Jonathan

[broonie]

another 3000cfm fan to create turbulent airflow might help Jonathon.

[Big J]

another 3000cfm fan to create turbulent airflow might help Jonathon.

 

Thankyou for the suggestion, but I'm not going to bother with firewood again in it. I just don't think that electricity is the way to go with drying firewood. Fine for sawn timber though.

[openspaceman]

Thankyou for the suggestion, but I'm not going to bother with firewood again in it. I just don't think that electricity is the way to go with drying firewood. Fine for sawn timber though.

 

Yes, with sawn wood you are controlling the RH in order to NOT remove water too quickly. You are also adding a lot more value and the time in the kiln is limited by the rate at which you can safely dry the timber. Drying firewood artificially doesn't have such constraints and to make the most of the capital investment you need fast cycle times, in practice there wasn't any advantage in a load cycle taking less than a day.

 

Even with your recirculating system a knowledge of the RH and temperature either side of the stack is needed to see what is not optimised, as I said my view is that the air was not reaching anywhere near saturated and there was a cost in this in the power necessary to run the fan.

[Big J]

Yes, with sawn wood you are controlling the RH in order to NOT remove water too quickly. You are also adding a lot more value and the time in the kiln is limited by the rate at which you can safely dry the timber. Drying firewood artificially doesn't have such constraints and to make the most of the capital investment you need fast cycle times, in practice there wasn't any advantage in a load cycle taking less than a day.

 

Even with your recirculating system a knowledge of the RH and temperature either side of the stack is needed to see what is not optimised, as I said my view is that the air was not reaching anywhere near saturated and there was a cost in this in the power necessary to run the fan.

 

I do believe that the air was practically at saturation point. In my experience of drying sawn timber in that kiln, the RH never reaches higher than 83%. With the firewood, it sat at that for a couple of weeks, like the sawn timber would, and then started to drop. I took it out at about 65% as it just wasn't worth continuing at 3.5kw/hr. The firewood was mostly very dry, though a bit patchy in places.

 

I've never found there to be a need to have RH meters each side of the stack. Give that the volume of the air inside the kiln is 18CM (accounting for the space the timber takes up), the fan completely circulating the air every 21.6 seconds I think that there would be very little difference between the two. I find having knowledge of internal and external RH far more useful.

[Pinkfoot]

I use a polytunnel for drying firewood. You can create great airflow,use a heater in the winter and heat is free in the summer!

[MaverickGR]

Looking into doing this a bit more, I am thinking about doing it in a container rather than a lorry back.

 

-Is there anything that you would do differently in this case?

-Given the cost of insulated containers would you insulate it yourself? (100mm Celotex works out around £10 per sqm)

-Anything else you would suggest? I am thinking of rails given how painfully large the stuff I have is.

- Would anything have to change drastically if I went for 40ft?