PDA

View Full Version : Dehumidifying drying box



bpbuilder
5th Nov 2015, 06:20 PM
Hi All.

Does anyone have any nifty ideas on how to get a purpose made ‘drying box’ down to a certain RH. I’m already in a hot climate so lowering the RH by heating with an incandescent bulb and the like, I’m not so keen on.

I'm essentially after a small electronic dehumidifying device that can keep the RH at a certain level, something along those lines.

RoyG
5th Nov 2015, 07:01 PM
Do a Google Search on "Peltier Effect Dehumidifer" ......

If you want more technical details on the theory, search on "Peltier Effect".

There are a few DIY articles on Youtube that show examples of how it can be done using Peltier Chips. There are also off the shelf ready-made solutions that you could use. Cost wise, a DIY solution will be significantly cheaper. Whether it will be feasible to use a peltier effect device will depend on how big your drying box is.

Hope that info helps,

RoyG

bpbuilder
6th Nov 2015, 10:18 AM
Do a Google Search on "Peltier Effect Dehumidifer" ......

If you want more technical details on the theory, search on "Peltier Effect".

There are a few DIY articles on Youtube that show examples of how it can be done using Peltier Chips. There are also off the shelf ready-made solutions that you could use. Cost wise, a DIY solution will be significantly cheaper. Whether it will be feasible to use a peltier effect device will depend on how big your drying box is.

Hope that info helps,

RoyG

Thanks very much, exactly what I want/need :U

BobL
6th Nov 2015, 11:07 AM
An old small fridge is a simple alternative - think about how quickly veggies go limp inside a fridge.
If you load the spare space with sealed bottles filled with water it will use significantly less power and work just as well.

bpbuilder
8th Nov 2015, 08:56 AM
An old small fridge is a simple alternative - think about how quickly veggies go limp inside a fridge.
If you load the spare space with sealed bottles filled with water it will use significantly less power and work just as well.

Thanks, yeah might be an option. Tested the one we have a should be able to get 30%RH.

Does anyone know if case hardening is ever an issue when you have air dried say down to around 14% or so and then take it down further say to 8% or a bit less? I have seen some documentation suggesting that case hardening occurs in the early stages of drying (first third) but not sure how accurate that is. And typically caused by drying too fast (in a kiln).

BobL
8th Nov 2015, 10:51 AM
Thanks, yeah might be an option. Tested the one we have a should be able to get 30%RH.

Does anyone know if case hardening is ever an issue when you have air dried say down to around 14% or so and then take it down further say to 8% or a bit less? I have seen some documentation suggesting that case hardening occurs in the early stages of drying (first third) but not sure how accurate that is. And typically caused by drying too fast (in a kiln).

The extent to which wood can be dried by air drying (no temp control) in the shade is limited to the RH of the air.
e.g. in Perth the average annual RH is 63% so the most a substantial piece of wood, e.g. 25 mm or greater, can air dry in the shade is ~11%.
Thin pieces can reach 9% in summer and go as high as 15% in winter
Micro climates have a significant effect. In our shady damp back yard, thick exposed wood MCs will rarely drop below ~15%.
At the tree loppers yard where slabs are stored which is only ~1km from the ocean wood is typically in the 12 to 14%
If the wood is going to be worked and used in the same area then drying it below ambient MC is not necessary and can even lead to problems
To get it thick wood to air dry in the shade to 8% you would need to live in a place like Alice Springs.

Air drying in direct sunlight or alongside a large thermal mass like a concrete or bock wall, or exposed to high currents of dry air this is likely to cause case hardening.
If wood is left in the shade and with a limited air supply the loss of water from exterior surfaces during the day will partially equilibrate with water from the interior, or it may even pick up water from the atmosphere.
Apart from the initial rapid water loss after a tree is cut then the rate of air drying is far from constant, and as the wood approaches ambient MC, the MC will oscillate with seasons and other factors.

bpbuilder
8th Nov 2015, 11:50 AM
The extent to which wood can be dried by air drying (no temp control) in the shade is limited to the RH of the air.
e.g. in Perth the average annual RH is 63% so the most a substantial piece of wood, e.g. 25 mm or greater, can air dry in the shade is ~11%.
Thin pieces can reach 9% in summer and go as high as 15% in winter.

Thanks. Yes I agree. Some of my air dried timber has already reached EMC in it's current place (around 12%), I want to dry it down further for various reasons.


Air drying in direct sunlight or alongside a large thermal mass like a concrete or bock wall, or exposed to high currents of dry air this is likely to cause case hardening.

But my question is still can/will case hardening still happen during that last few percent if you are forcing it down with a drying box etc.? (Also considering it was already slowly dried down to a MC of around 12%.) Is it past the point where case hardening can actually happen.



To get it thick wood to air dry in the shade to 8% you would need to live in a place like Alice Springs.


This is the reason I'm looking at this constructing a drying box.

BobL
8th Nov 2015, 01:04 PM
Thanks. Yes I agree. Some of my air dried timber has already reached EMC in it's current place (around 12%), I want to dry it down further for various reasons.
But my question is still can/will case hardening still happen during that last few percent if you are forcing it down with a drying box etc.? (Also considering it was already slowly dried down to a MC of around 12%.) Is it past the point where case hardening can actually happen. .

There will be some case hardening but at those MCs it should not be that great.

For a particular species, hardness is approximately proportional to density, which is in turn proportional to shrinkage.
Shrinkage and MC will are also related by an approximately linear relationship so hardness and MC should be related in the same way.
This means the difference in hardness between timber @ 12% and 8% is about the same as the difference in hardness between 20 and 24% MC
A 4% difference between the inside and the outside of wood is not that unusual for wood while it is air drying.
My understanding of case hardening is when there is a much bigger difference than 4% MC.

Where I regularly come across case hardening is on the cut ends of logs that have not been sealed after being cut and are typically left outside in the sun so the ends shrink and split , and go hard very quickly - even in just a couple of months. The rougher the cut the worse the problem.
This interests me because these hardened ends will knock the stuffing out of a sharp chainsaw chain or bandsaw mill blade just cutting through the first ~100 mm or so of dry end.
I haven't done a lot of testing of this but the few tests I have done with spotted gum shows that ends can rapidly reach 15% MC while the inside is still nearly green as when it was cut.
The way to minimise this is of course to seal the log ends but for those that I have not had access to I dock the first 100 to 150 mm off the log before cutting it.

One way that I have heard of minimising case hardening is to use pulsed drying whereby the drying action is applied in bursts. Basic solar kilns that do not use a heat reservoir do this naturally because they can only dry effectively while the sun is shining. This seems to work well on species that is prone to splitting.