kiln dryer

[Duffryn]

I've just put a 200kw heizomat woodchip boiler in to power a house, cottage and a log drying kiln. The kiln is based on a 10ft HQ container ( which I insulated myself) with two 54kw heat exchanger's ( heat exchanges cost about £500 each). It will dry 10m3 of logs from 40% to 20% in about 36 hours. I'll post some pics in the next week or so if anyone is interested.

 

Here are some pics of the setup (almost complete) just need to install the heat exchangers on the kiln and cut the vents.

 

 

 

[mikeyne]

This is a very interesting and potentially flexible design Mikeyne. Build the kiln round the stack!

I would be very interested to know how you get on.

I have neither the space nor the throughput of material for a permanent installation but the ability to set up a temporary kiln to deal with loads of firewood or cut boards is very appealing.

If you have any further reflections having used it please keep posting.

Andy

 

This is probably too much information. Scroll down to the end for the summary!

 

It ended up being a steep learning curve as I redesigned it three times before I'd finished but it ended up a lot better than I thought it would.

 

The first thing I changed was to improve the recycle path of the air as it was getting constricted with the weight of the insulation on top. I pushed in a few long 100mm drain pipes and that helped a lot.

 

After a few days, I opened it up and could see that the wood at the start was drying really nicely with lots of cracking and shrinkage, but the final wood at the end of the kiln was still wet (and starting to grow some nice mould!) - not enough air movement - so I bought a really big fan and put it where the dehumidifier was and left the dehumidifier at the start to dry the air from the recycle loop before it was then taken by the fan and blown through the stack.

 

Unfortunately, that didn't work out so well as the dehumidifier was overheating because it was working in such a small space, it also didn't seem to be all that efficient.

 

The final redesign involved moving the dehumidifier to the end of the kiln so it's own fan acted to pull the air through the stack. I then put a long flexible duct on the output of the dehumidifier and pulled the duct back through the recycle path. So the air was being pushed round by two fans one at each end of the stack and dry warm air was being delivered back to the main fan.

 

As the drying progressed, it became harder and harder to drive the water out. I knew that there would come a point where the electrical cost of extracting each liter of water would make it no longer worth continuing. I was maintaining a temperature of about 20 degrees C (it was about 4 degrees C ambient) inside the kiln with the heat coming from the electrical power used to dehumidify and move the air, but I knew the dehumidifier was rated to work up to 38 degrees C so in the final week, I decided to really push it and put a small 1kw fan heater at the start next to the main fan.

 

I knew the fan heater would blow my cost per liter sky high but I wanted to see what it would do to the water extraction rate. Each time I switched it on for about 8 hours, the temperature would skyrocket and I'd see a marginal improvement in water extraction but not much.

 

After 19 days of running the kiln I stopped it and took it apart to see the result. The wood at the front was great. 6.8% moisture on the outside, 21.5% in the middle of a freshly split log. The wood at the back not so good: 11.4% on the outside, 33.7% in the middle. But the wood at the end only really started drying after I'd redesigned the kiln for the umpteenth time so it only had about 10 days of effective drying.

 

Total cost to do this ended up being 375 units of electricity which on our tariff ended up being about £65. I ended up with 1300 kg of (mostly) dry wood, so the cost of drying it ended up being 5 pence per kg and when I bunged it through the boiler it burnt reasonably well and we got just over 2kwh of useful heat per kg so 2.5p per kwh for the energy cost (not including the original cost of sourcing the wood).

 

So not too bad, but I know that next time I could do better. I would use the final design I came up with but instead of letting it build up to temperature naturally (it took about 5 days), I would bung the heater on straight away for the first 24 hours and give it an initial boost, then I would leave it alone. I reckon this would mean less energy used but all the wood moisture would be down to the best I achieved at the start of this kiln. It would probably also take only 14 days instead of 19.

 

I would also switch the power supply to a cheaper economy seven tariff and save some money there too.

 

My main learnings were (sound obvious in hindsight)

 

Get the airflow as fast as you possibly can

Make sure the air moves through as usefully as possible - stop it from going around the wood - pack air gaps.

Get the kiln up to temperature fast and let it marinate.

It could be very cost effective.

 

I actually got really serious about turning this prototype into a functioning kiln that would be more permanently built but eventually decided not to pursue it for the following reasons:

 

It is costly to build. The temporary plastic sausage and insulation worked well but ended up being destroyed when taking the kiln apart. You would either have to invest in better insulation than rockwool that can be easily reused, or build a permanent structure with very tight air gaps - I had grand plans for an insulated box which the stillages would be pushed into at one end and pulled out at the other (think a commercial pizza oven with the continuous belt).

 

Controlling it is hard. The dehumidifier has an optimum operating temperature of 30-38 degrees centigrade, and ideally you want it to be working at this temperature from the start. This will give you the highest rate of water extraction per unit of electricity. But, because the kiln relies on insulation to maintain the temperature it will be heavily influenced by the ambient conditions. Basically, in summer, you'd have to be removing insulation or it will overheat - not very practical on a permanent structure.

 

It's not the safest thing in the world. Every time I left it alone I wondered what would happen if it overheated and I started a nice electrical fire next to a big pile of dry wood. I had a fire extinguisher near it at all times, and had a continuous check of temperatures and humidity, but still breathed a sigh of relief when I switched it off.

 

So fascinating experiment, but we've decided to go for the Poly Tunnel route!

[Marko]

So fascinating experiment, but we've decided to go for the Poly Tunnel route!

 

Would it be possible to describe in detail (or with pics) how the Poly Tunnel method works. How is the chip moved through it and at what depth etc etc I have tried it in trays but it is far to labour intensive to be practical. I thought about 1000ltr IBC tanks but thought the lack of airflow would just make fresh chip go mouldy.

[mikeyne]

Would it be possible to describe in detail (or with pics) how the Poly Tunnel method works. How is the chip moved through it and at what depth etc etc I have tried it in trays but it is far to labour intensive to be practical. I thought about 1000ltr IBC tanks but thought the lack of airflow would just make fresh chip go mouldy.

 

Hi Marko,

 

I'm burning cut and split logs not chip, so not what you are after I think.

[Marko]

OK thanks. I can wait for nature to do its thing with logs.... it is chip that I just cannot master drying in a poly tunnel.

[tronman61]

Thanks mikeyne, really helpful to have all that detail. It has generated the germ of an idea using something a bit more solid than rockwool. Will see if I can get anywhere with this. Thanks again.

[arboriculturist]

Here are some pics of the setup (almost complete) just need to install the heat exchangers on the kiln and cut the vents.[ATTACH]155824[/ATTACH]

 

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That is an impressive setup ! Do the heat exchangers have integral fans and is the kiln constantly vented out at the far end or do you have operational vents that are controlled by a 'wet bulb' stat. which open periodically to dump the moisture laden air every few minutes?

[Duffryn]

That is an impressive setup ! Do the heat exchangers have integral fans and is the kiln constantly vented out at the far end or do you have operational vents that are controlled by a 'wet bulb' stat. which open periodically to dump the moisture laden air every few minutes?

 

Yes the heat exchangers have integral fans. The kiln is constantly vented, if it were not then using the current heat exchangers the air would just flow back through the rear of the heat exchangers rather than the vents at the front of the kiln. The constant airflow through the kiln improves drying times. The fans are turned down low at the start of the drying process and turned up as the logs absorb the heat

[arboriculturist]

Apologies, of course there has to be constant venting as you are creating kiln internal positive pressure - I had internal kiln fans in my mind like most commercial lumber kilns.

 

I appreciate that to keep costs down all the Firewood kilns use plate heat exchangers with integral fans - as you know, not as energy efficient as heat passes straight out but to go the route of internal titanium heat exchangers and internal fans with humidity stat. controlled vents with exhaust fans adds massivly to initial costs. With the RHI knowone seems to care about efficiency for clear reasons.

 

You must tire of the endless posts about kiln drying now that you are less involved, however your input is much appreciated on the Forum.

 

Any change of some more images of the finished heiz boiler firewood drying installation?

[Fentiger]

I have access to an empty workshop with a waste oil heater, if I put, say 20 cube bags in and heat it for 8 hours a day for 5 days will I significantly improve the drying speed? I don't intend getting down to kiln-dried levels but somewhere between that and ambient would be good.