Kilning theory discussion

[Big J]

I'm always very interested to hear how others on here are kilning their timbers. I would like to propose a discussion of ideas and theories for mutual benefit and to try to get more people drying their timber to the point of maximum value.

 

I've so far run 6 kilns. The most recent kiln is opening on Wednesday and is entirely consisting of Elm. It's also completely presold (circa 300 cubic foot). I'm at the point where I am pretty happy with the way things turn out, but acknowledge that there are improvements that can be made (more air drying - the start of which I'm at now) and that my system has it's limitations.

 

I run a 300 cubic foot capacity 18ft lorry back. I run a combined heat vent and dehumidification system, but am moving away from dehumidifiers.

 

I have a stack 18ft long, 6ft wide and 6ft high in the kiln. A large circulation fan sits in the middle on one side blasting air over the top of the stack. Adjacent to it are several heaters amounting to 2.5kw. At one end of the box is a vent and the other an extractor. The extractor draws cold dry air across the heaters and then the stack, being extracted when warm and wet.

 

The system works well and is quite reliable. The heaters are on a 35 centigrade thermostat and the extractor is on a timer (could be improved). Right at the end of the cycle I put the dehumidifer on as the vents can be a touch gentle for getting the last dregs out of the timber.

 

It will dry up to 2 inch green elm/ash/beech etc (though not Oak) in 8 weeks. 3 inch takes 12-15 weeks but I avoid such thick timber if I can.

 

Talking to an electrician/heating engineer today, he suggested a heat exchanger to reduce the cost of running the kiln. I had always thought that they wouldn't be suitable due to the high temperature/humidity (especially at the start - 35 celcius and 83% RH) but they can come with drains for condensation. A suitable unit would be like this:

 

Heat Recovery Unit/Whole House Ventilation-Airflow-D71 | eBay

 

I honestly don't know why I didn't get one before - I worked out that on a full schedule of kilning (6 kilns a year) it will save me just over £2200 a year. It quarters my electricity bill.

 

So, who else kilns, and what do you do and do differently!?!

[Big J]

Useful moisture carried in air figures (for working out moisture extraction rates via an extractor fan):

 

Moisture carried per cubic metre:

 

0 celsius: 4.7ml

5 celsius: 6.24ml

10 celsius: 9.05ml

15 celcius: 12.5ml

20 celcius: 16.9ml

25 celcius: 22.2ml

30 celsius: 31.3ml

35 celcius: 40.7ml

 

So if outdoor ambient temperature is 5 celcius and 80% RH, and inside the kiln is 35 celcius and 80%, you will be extracting 27ml of water for every cubic metre extracted. Assuming you have a 100 cubic metre/hour extractor, you extract 2.7l hour/64.8l a day.

Edited January 20, 2012 by Big J
More detail

[John Hughes]

Interesting thread.

I still don't have enough shed space to set up my little Ebac kiln. So i have to rely on air drying, a very slow business. Alot of the timber I get is not joinery quality so does not warrant kiln drying, but I hope to get a shed built this year to house the kiln.

John

[Rob D]

I've gone away from kilning in batches.... Now I tend to mill a couple of logs and have them in a stack in the kiln on one side.

 

The other side is mainly free but I put in bits and pieces I'm working on in order to get them dry enough to sand and treat i.e. get them down to around 25%.

 

I still think the cheapest method is the polytunnel but it's limited in that to avoid drying defects you have to get everything cut and stacked winter time in order for the wood to be able to take hot summer temperatures....

 

But if you put 2" timber in there Jan by late August it will be 14% (this is with fans blowing air around the stacks). But you can only do one cycle a year.

[Big J]

I've not had too much success with drying smaller loads in the kiln. Mine needs to be full otherwise there isn't enough moisture in the air to maintain a high humidity to stop premature drying.

 

I think that the heat exchanger is a real winner though. There are bathroom specific models on Ebay for a couple of hundred quid that would suit a smaller kiln (up to approx 150 cubic foot I think). They have tiny running costs and I could see such kilns being run for about £50-100 a cycle. That's not much compared to a dehumidifier.

 

I've no experience with Polytunnels, but I think the length of time required to dry (and the space they require) would be prohibitive for me.

 

Jonathan

[slackbladder]

Hi Big J

Found this on ebay

Is this the sort of thing you are looking at

MANHR100W Heat Recovery Fan Heat Exchanger Ventilation Controls Condensation | eBay

[Big J]

Hi Big J

Found this on ebay

Is this the sort of thing you are looking at

MANHR100W Heat Recovery Fan Heat Exchanger Ventilation Controls Condensation | eBay

 

It is yes, but I need something with a slightly larger capacity (buying two could be an option). Additionally, I didn't see many of smaller units with high efficiency rates. The difference between 70% and 94% is £145 and £29 for the heating of the kiln over one cycle (£696 a year).

 

Speaking to a chap yesterday about heat exchangers is that you just have to be careful to get a wet room heat exchanger (bathroom/kitchen etc) that has a condensation pipe and plastic membranes rather than paper.

 

Jonathan

[bella wood]

ive been running an arrowsmith dehumidifer unit inside a highly insulated and dmp lined garden shed for about 6 years,, shed size 9 foot x6 foot x 5 foot timber stack,, biggest cost in my thinking is the heating ,, 2 inch thick takes 7-8 weeks but the dehumidifier is on timer and is running about 12 hours per day, so in reality i could dry dry twice as much timber ,but dont have space at my workshop for bigger kiln,,

 

ive looked into the heat exchanger route before , and my ideal would be a concrete slab with under floor heating pipes heated by a wood burner with boiler burning workshop plannings and sawdust , high density insulated panel walls and roof , using heat vent with heat exchangers ,, and most important part of any new kiln would be a rolling track to load the kiln,,

as allways ,comes down to costs and time but more importantly the kiln ive got WORKS ,,,,

 

forgot to say ive got 3 10 inch fans to keep the air moving , and inch to 1 1/2 inch timber can be dried in 5-6 weeks

Edited January 21, 2012 by bella wood
MORE TO ADD

[Rob D]

I can see how these could be useful J - let me know which model you research seems to be the best..... Wouldn't mind giving one a try.

[Justme]

What about using an Air Source Heat Pump to provide both the heat & air flow?

 

You should get about 3 units of heat for every unit of elec.