Kiln Drying on Crop Drying Floor

[Will76]

I have access to a farm with biomass powered crop drying floors. They have spare capacity.

I am considering how I might go about using these to kiln dry logs.

The main issues I can see are that the drying floor is not in a sealed room, and has no method of retaining heat and air moisture control etc.

Does anyone have any experience of this and would be happy to discuss with me? Thanks!

 

[Steven P]

How long wold you have access to the floor for and how much could you get in there?

 

Best - cheapest - method of drying is to keep the rain off (a roof) and to let lots of air flow, it would be quicker than an outside stack, slower than a kiln, but cheaper than a kin too.

 

I might throw in a question, how long does a log have to b in a kiln for to be 'kiln dried'?

(dry the logs naturally on the crop drying floor, put them in a kiln for a couple of hours, 'kiln dried', you don't need to heat the whole space)

[Will76]

Thanks for the reply.

 

We'd have as much access as we wanted in essence. We could get approx 200 IBTs at a time, assuming stacked 2 high.

 

I am envisaging putting tarps or similar over the IBCs to create a temporary kiln. But other experiences or ideas would be appreciated.

[openspaceman]

My understanding is these floors have a plenum underneath and then slotted floor for the warm air to pass up through. When there is loose grain on the floor the air is distributed fairly evenly through the grain, also only 2-5% of the mass is exhausted as water vapour.

 

With wood, especially in IBCs I wonder how much of the air will take an easier path between the containers.

 

Also the air only passes through once, with grain that is ameliorated because there is a very high surface area and the moisture doesn't take too long to pass to the surface of the grain. With wood unless you have sufficient moist surface the air will leave the top less than optimally saturated. If you are blowing ambient air the only issue is the fan power but if you are heating the air then it could be leaving the surface still too far above ambient temperature which means heat is being wasted.

 

My old boss built a number of wood chip dryers based on this principle of dry warm air fed in at the bottom and colder saturated air being exhausted  and it's an art deciding when the wood at the top is nearly dry  because whilst the bottom layers become in equilibrium with the incoming warm air  saturated colder air can actually redeposit moisture into the top layers. With woodchip this is visible and obvious but I don't know with logs.

[BARNSLEY BOB]

My understanding is these floors have a plenum underneath and then slotted floor for the warm air to pass up through. When there is loose grain on the floor the air is distributed fairly evenly through the grain, also only 2-5% of the mass is exhausted as water vapour.
 
With wood, especially in IBCs I wonder how much of the air will take an easier path between the containers.
 
Also the air only passes through once, with grain that is ameliorated because there is a very high surface area and the moisture doesn't take too long to pass to the surface of the grain. With wood unless you have sufficient moist surface the air will leave the top less than optimally saturated. If you are blowing ambient air the only issue is the fan power but if you are heating the air then it could be leaving the surface still too far above ambient temperature which means heat is being wasted.
 
My old boss built a number of wood chip dryers based on this principle of dry warm air fed in at the bottom and colder saturated air being exhausted  and it's an art deciding when the wood at the top is nearly dry  because whilst the bottom layers become in equilibrium with the incoming warm air  saturated colder air can actually redeposit moisture into the top layers. With woodchip this is visible and obvious but I don't know with logs.

Same with grain.
That's why they put stirring gantries in grain stores.

[Squaredy]

I used to run a kiln to dry logs.  You need to heat the logs to 80 degrees centigrade for about three days.  And you need to constantly vent the air and therefore heat new incoming cold air.

 

 It is a very inefficient process that requires s huge amount of energy and usually has no heat recovery.

 

I suspect grain dries quickly with low heat.  Logs need high heat for a long time as they are very good at retaining their moisture.

 

I am not being negative but my guess is the grain drying floor would be a total non starter for logs.  I don’t sell logs any more by the way.

[openspaceman]

2 hours ago, Squaredy said:

I used to run a kiln to dry logs.  You need to heat the logs to 80 degrees centigrade for about three days.  And you need to constantly vent the air and therefore heat new incoming cold air.

 

 It is a very inefficient process that requires s huge amount of energy and usually has no heat recovery.

 

I suspect grain dries quickly with low heat.  Logs need high heat for a long time as they are very good at retaining their moisture.

 

I am not being negative but my guess is the grain drying floor would be a total non starter for logs.  I don’t sell logs any more by the way.

That's an interesting post @Squaredy; plainly you don't *need* to heat the logs to a high temperature because passing low relative humidity air past the log surface will remove water. What the higher temperature does is to increase the rate at which the moisture moves out of the log. So yes high temperature leads to quicker drying but also increases heat loss.

 

When running at the higher temperature if you don't  pass the hot low RH air over enough logs it remains warm and unsaturated, hence if you exhaust it you waste energy. This is why many kilns will recirculate air and exhaust a fraction as cooler saturated air.

 

With the grain floor you only have one pass so the air needs to pass a lot of wood surface area to saturate, as it saturate it cools so even if you pass 80C air at the bottom if the stack is deep enough it will become saturated  and cool before it exhausts at the surface. I suspect with logs it would need to be infeasibly tall.

 

My old boss's design took low grade heat at 40C so that vented at ambient after doing its drying with very little energy loss other than in the enthalpy of the water vapour.

 

 

[Will76]

Thanks for the comments.

My idea is to cover the IBCs with tarps to help retain heat (and therefore drying rate) but to do so imperfectly so that it can vent moist air. I might need to include fans inside as well.

Obviously this is sub optimal, but I do have access to a £500k drying facility without having to invest capex, and large quantities of chip, so I am looking for a way to utilise it for log drying!

 

Has anyone got experience of this?

[spuddog0507]

Hi are you having to pay for this service or is it for free ?

we have access to same sort of floor, a price is sorted out but we havent used it yet as its not as good as i thought it would be, for drying logs, 

some one else has used it and i have been and robbed a few logs out of a crate and i was very dissapointed at the result, we tested several logs out of several crates and from different positions in the crate, ok not covered up just the heat n draft going through,

The results after 10 days on the floor where, logs from the outside of the crate where down to 8% on the outside but when split 32% on the inside, logs from the centre of the crate 18% on the outside and still at 42% when split, but this guy who the logs belong to is addvertising and selling them as super dry kiln dried logs, i think he might just get 1 or 2 complaints, so if you cover yours the results might be slightly better but i know i need to come up with a different plan for getting my logs dried, i have an idea and i think it will work and i think i could get logs dried down to 15% from fresh cut in 5 days but time will tell,,,,

[openspaceman]

26 minutes ago, spuddog0507 said:

Hi are you having to pay for this service or is it for free ?

we have access to same sort of floor, a price is sorted out but we havent used it yet as its not as good as i thought it would be, for drying logs, 

some one else has used it and i have been and robbed a few logs out of a crate and i was very dissapointed at the result, we tested several logs out of several crates and from different positions in the crate, ok not covered up just the heat n draft going through,

The results after 10 days on the floor where, logs from the outside of the crate where down to 8% on the outside but when split 32% on the inside, logs from the centre of the crate 18% on the outside and still at 42% when split, but this guy who the logs belong to is addvertising and selling them as super dry kiln dried logs, i think he might just get 1 or 2 complaints, so if you cover yours the results might be slightly better but i know i need to come up with a different plan for getting my logs dried, i have an idea and i think it will work and i think i could get logs dried down to 15% from fresh cut in 5 days but time will tell,,,,

What would be interesting is to put a temperature and RH data logger just under the top layer of logs, one on top of one of the floor vents would be handy too.