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Solar kiln drying queries7/14
We are designing and building a solar kiln for use in the tropics at 6o north. The average annual relative humidity is 72.5% and average monthly relative humidity ranges from 69% in September to 77% in June. Over the course of a year, the temperature typically varies from 22°C to 32°C and is rarely below 21°C or above 34°C.
We are planning units to comfortably hold 6000 board feet. These will be powered with three DC solar-powered fans for circulation and regulated with vents for internal/external air flow options.
A few questions:
The walls are planned to be concrete block as this is a better insulator than wood – any problem with this?
The roof slope will be around 5 degrees as the sun passes directly overhead during the year, so just for water run-off. This gives us a kiln of around 15 feet in height all round. We were planning to put the fans low down on the input side of the inner wall to suck the air over the heat absorber plate and then down to the fans. But we need to get the air to the upper part of the stack as well, so maybe siting the fans part-way up the inner wall is a better idea – maybe 4 of 5 feet from the ground. Any ideas on this?
Are baffles on the absorber a good idea – will they increase the air temperature by much?
What would be the best recommendation for MC, RH and temperature probes for remote reading in the office close by? These would also read the outside temperature and RH.
Appreciate any answers and experiences – thanks.
I have made several solar kilns for the equatorial tropics. I used the Virginia Tech design.
Regarding the absorber, you do not get any benefit from using an absorber plate versus just painting the interior flat black. You do not have any benefit from baffles on the absorber. The reason is that you are limited by the solar coming in through the clear roof and not the absorber.
For 6000 BF, you need 600 sq ft of roof.
The roof needs to be two clear layers; separation distance is not critical.
We want an insulated wall that also does not store much heat. If it is not well insulated, we lose the solar heat. If it absorbs heat, then the maximum temperature in the kiln and its low humidity is lost. For both reasons, we do not use masonry walls, unless you also put adequate insulation on the inside.
I have not seen moisture probes used with a solar kiln. They would not be useful as their accuracy is probably not good enough. Temperature and RH sensors would be fine if they can withstand frequent exposure to 100% RH...many cannot. But, why do you need such instrumentation? Will you change your operating procedures?
You are correct about the lower fans being poor. However, to comment about the location, we also need to know species, incoming MC, sticker thickness, pile width, vent locations, and fan volume. For simplicity, the standard location on the top near the rear wall seems easiest with vents on the rear wall.
I appreciate your comments and have – since I posted – done some research on design, and especially the Virginia model. I’m now looking at a 2000 BF unit, which will be 200 ft2 roof area. Because we are 6N, the roof must be almost flat, which means that the ground area is pretty large – 30 x 20 feet for 6000 BF, but the wood itself of course occupies a lot less than that inside the kiln.
I am working with a stack width of 6 feet and a stick height of ¾ inch. I take your point on the collector and thanks for that – it saves putting a collector there at all. I can get black plastic sheeting which will form a separation.
I now plan to fit the fans high, near and inside the intake, so they push the air through the system rather than suck it.
I’m not sure what you mean by the roof needs to be two clear layers. I was intending to use corrugated clear plastic sheets for the roof. I figured this would allow the energy to penetrate and heat the air inside. Where does the second roof layer come in?
I am looking at the air space volume above the stack in the Virginia model. It seems that this space needs to be quite large to avoid overheating? In other words, can we build a kiln with say a 2-foot air gap above the stack (and fans pushing air through that space) or must we increase that vertical gap to 3 or 4 feet? I am thinking the larger space is better.
I am using 3 fans at 1000 cfm on a 2000 bf stack based on 12-18 foot boards. This works out at a tad above 3000 the cfm required air flow, so seems enough. I’m now using a totally wooden kiln.
Will RH and temperature readings change operating procedures. A very good question. I had imagined that, assuming we got a high RH outside, then we would shut the suction and recirculate. The converse would also be true. What’s your take on this? I know that we need to measure the MC ideally using accurate scales on test pieces to get a real idea of how well we are drying the timber.
To give an idea for calculation purposes, we would say Crabwood (Carapa Guianensis) at 30%MC from the bush. Sticker thickness ¾ inch, stack width 6 feet, exhaust vent locations spread to give maximum spread, fan volume 3000 cfm.
Really appreciate any input you can give and thanks once again for your help
Glad to hear that some of the comments were very useful.
Two layers will improve kiln efficiency by 50%. The second layer can be under the corrugated and can be flat. If you frame the roof with 2x4, then put the flat on the back side and the corrugated on the top.
Regarding high humidity, if the humidity outside is 100% RH and you bring that air into the kiln, then heating it to about 25 degrees warmer will give you about 30% RH. So, what we want to do is to vent a little bit, when the sun is shinning to exhaust damp air and bring air in cooler moisture air that when it gets heated will be dry, pick up moisture (dry the wood), and then be exhausted as more humid air, etc. So, try and get the kiln 25 degrees F hotter than outside...if it gets hotter, then vent more. Usually, the vets are set once and then stay that way for 24 hours. The fans are not run at night when there is no solar heating.
If you like the smaller kiln, then build a second or third one rather than build a large unit.
With a large slope to the roof, the empty space is automatic. With you kiln, I would make the space on top about 24" in the rear to hold 24" fans and then decrease the slope tot he front by 6 degrees. Blow the fans toward the front.
Again, many thanks for your time. I’ll take all this into account in our design and operation.
I am assuming you mean the lower layer of plastic would be normal Perspex clear plastic sheeting. This is quite expensive where we are – could we use polythene sheeting as a lower layer? Just an idea.
Agree on the smaller kiln – several 2000 bf kilns will work good.
I’ll keep in touch on here and again many thanks,
You can use inexpensive poly film, but it will get brittle and tear from the fans in maybe six months. So, maybe keep your eyes open for a replacement film.
Thanks, yes, I'll keep an eye open.