Mental Exercise! Hybrid kiln design (solar drying concept w/o solar heat)

[scsmith42]

We have some very smart folks on the FF, and I would appreciate your insights (especially Gene's, Pineywood's, other folks with significant solar kiln experience along with our normal contingent of really sharp folks that are good at bouncing ideas (Ian, Ron, Danny, YH, TPL Rob, etc.).

I'm in the process of building a new 40 x 80' x 18' tall covered air drying structure, and am planning to incorporate some kiln chambers onto one end of the building.  I presently have two Nyle L200 kiln units (one in service in a shipping container and the other on a pallet ready for install), plus a VT design solar kiln.

My plan is to eliminate the container kiln and build a pair of 26' wide, 12' deep, 12' tall chambers that will allow me to dry up to 4K bd ft in each chamber via two stacks end to end of 2K bd ft each (8' - 12' board lengths), or a single stack of 16 footers.  Having spoken with the folks a Nyle a few months back, this seems to be the optimum chamber size for drying 4/4 and 5/4 oak with an L200, and have the fewest compromises on air flow.  I'm also looking forward to being able to set stacks directly into the kiln via a side door as opposed to loading carts from the end as I'm convinced that I can reduce my labor costs by taking this approach.

I'd like more kiln capacity than just the two L200's though, so I'm engaged in a mental exercise to figure out a way to obtain it w/o buying an L500 sized unit (or paying to power it).  So here is my concept....

A solar kiln works on the concept that the sun will heat the lumber up to around 135F or so during the day, with fans circulating the air in the chamber so it absorbs moisture from the lumber while lowering the RH% in the chamber as the air heats.  During the day a small amount of the chamber's air is exchanged with outside air, thus releasing some of the absorbed moisture.  At night the fans shut off, the air cools back down, RH% goes up and conditions the lumber overnight and a percentage of moisture was released from the chamber via the vents and/or air leakage.  The next day the cycle repeats.

What I'm thinking about is building a chamber inside a building, with a vent system for controlling the amount of air exchanged and an OWB providing a heat source instead of solar.  The OWB can be fired with waste slabs from the sawmill and the heat exchanger inside the kiln can be an old car radiator with low speed fan behind it.

Every evening, the OWB can be stocked with a fixed amount of wood slabs and kindling.  It will have a self lighting propane burner inside on a timer that is set to come on around 5 am.  When the timer kicks off, the burner will light for 15 minutes, causing the slabs to get started burning.  The OWB will run flat out for however long it's fuel lasts, storing the heat in the water jacket.

Around 8 am a timer will turn on the kiln fans, including the fan on the radiator, along with a recirculating pump to circulate the hot water from the OWB through the radiator, thus heating the chamber during the day.  The system should be sized so that there are an adequate amount of btu's available in the water to heat the chamber and the charge of lumber to around 135F for two to three hours.  I figure that the water temp will drop by 15 degrees or so per hour, so by late morning the chamber should be at max temp, and then slowly decrease through the afternoon.  At some point in time a set of powered vent fans can come on for a set amount of time to exchange some of the moisture laden air in the chamber with cooler air from outside (late afternoon?), thus removing some moisture from the chamber.  Then all fans shut down for the kiln to condition overnight, and the next day the cycle repeats.  At the end of the cycle the OWB can be stocked continuously for a day in order to heat the chamber adequately for lumber sterilization.

I'm wondering about the validity of the concept, and also wondering about the best match of a well insulated chamber (so that less wood is required for heating it), but one that will not retain so much heat overnight but rather let the lumber cool enough so that the RH% will rise and condition the boards. 

I'm also wondering about the energy costs.  Hypothetically the fans exchanging air inside the kiln are required for DH or hybrid, but with the hybrid they only run 12 hours a day instead of 24.  The DH kiln requires energy for the compressor, heat strips and fan, but the hybrid requires a smaller fan, no heat strips and a low HP pump to circulate the water, so it should consume significantly less power than a DH unit.

The biggest advantage of hybrid versus solar is that I can use it effectively 12 months a year, instead of the more limited effective range for solar in the late spring, summer and early fall.

Thoughts?

Scott

[Brad_S.]

I have a Keotter Kiln that heats the lumber to a relatively low temp and drys it by constantly exhausting the moisture laden air. The amount exhausted is controlled by the size of the vent opening.

Sounds like the same type of theory you are proposing without the daily temp swings and complicated timers.

[Gary_C]

Scott with a building of that size you can extend the capacity of your proposed kilns by just air drying ahead of the kiln without even going the solar route. Of course just orienting the building properly, providing some translucent panels on the side facing the sun and providing some air flow will be a plus. Seems like the benefit of enclosing another chamber and heating it somewhat would give a small return over just air drying.

[scsmith42]

Scott with a building of that size you can extend the capacity of your proposed kilns by just air drying ahead of the kiln without even going the solar route. Of course just orienting the building properly, providing some translucent panels on the side facing the sun and providing some air flow will be a plus. Seems like the benefit of enclosing another chamber and heating it somewhat would give a small return over just air drying.

Gary, I'm definitely planning on air drying as much as possible, but my problem is that I work extensively with oak, and it takes a long time to AD QS oak.  Plus we are seeing more demand for local custom milling and drying and my current kiln capacity is the road block.

Too often the architects and engineers will see that I get the logs at the beginning of the project, and then delay for months in finalizing their demands until I'm crunched to get everything dry.  The additional drying capacity - over and above the air drying - should help me to secure more business.

Solar is a great augmentation to the Nyle system, but it is not dependable from October through March; hence my inquisitiveness.

[boardmaker]

Scott, you've nearly got a conventional kiln.  I think I'd rather go that route instead of mimicking a solar kiln(however I definitely appreciate thinking out of the box.).

[YellowHammer]

From my prospective, running both solar and DH kilns, the solar kiln is excellent for slow dying species because it has what amounts to a foolproof 50% duty cycle.  Of course in the winter, the duty cycle drops to much less as there is a distinct lack of solar. 

Also, I run my fans 24/7 so if you did that, and opened the vents at night, cool moist air would enter the kiln and negate the OWB supplied heat.   So during the day, it heats, at night it's cools and equalizes. 

However, if you didn't want to do that, with the heat source being fed by wood, you only will have semi control over half of the duty cycle, i.e. heat input and the duration will be subjective to, as you say, how much wood you load in the OWB and how the chamber is insulated, or vents opened, in order to stop heating.  So the cooling cycle would be difficult to control and the more you insulate to retain and store heat, the longer it will take to cool down.  So in order to be effective and predictable, you need to have control of the cooling cycle, also.  I would take the radiator concept to the next step and incorporate a 2 way valve in the system to shut off the hot water flow and allow cooled or chilled water to flow through the internal kiln chamber radiator on command.  This would allow you to bleed heat out of a well insulated kiln while keeping the vents closed, if you wish.  So where would you get the cooler water?  Maybe another heat exchanger in a nearby pond or stream or well.  I suppose it could even be a heat exchanger in the ambient air, and as the day cools at night, water would flow through the system and the heat exchanger would cool the kiln chamber air to that of the outside air, in effect reducing the insulation value of the kiln by using the heat exchanger to provide temperature loss.   

[xlogger]

Scott, I control the heat on my kiln with the OWB, you are welcome to come up and take a look. But my kiln is a long ways from what your are planning. I've not had to cut on heat supply from the Nyle unit yet.

[WDH]

It should work if you have some way to control the conditions in the chamber like YH described.  However, that sounds complicated, and you would have to monitor it carefully, which might eat up your labor savings. 

[scsmith42]

Many thanks for the great feedback thus far. 

Brad, it sounds like my concept is not too dissimilar to what your Koetter does, albeit a little different.

BM - my experiences have been similar to YH in that the solar kiln method is very forgiving for difficult to dry species such as oak (which is what I predominantly work with).  Thus I'm thinking that my  hybrid solar concept may allow me more safety margin when drying oak as opposed to a conventional kiln, albeit at a somewhat slower rate.

Y'all's comments have helped me to solidify the concept a little more.  Where I'm at is close vents and heat during the day, and open vents and exchange air during the evening (with the fans circulating 7/24).  Basically the same thing that a solar kiln dies but with using a wood fired boiler for heat instead of the sun.

Gene kindly posted some great info in a post last month about btu's per solar collector size, etc.  Using his numbers, here is what I've come up with.

Target - evaporate 1/10 lb water per day per bd ft of 5/4 oak.  This works out to be a 100 lb evaporation per 1K bd ft of oak per day.  It also works out to be 100,000 btu's per day to evaporate 100 lbs of water from 1K bd ft of oak (3% MC reduction).

Gene also stated that it required about 1/5 cord of wood to provide 30 days of 100K btu/day of heat.  That equals 700 lbs of seasoned oak per month, or 23 lbs dry oak firewood per day per 1K bd ft of 4/4 oak lumber in the kiln.  That's probably one log's worth of dry offcuts per day - not a significant amount.

Assuming firewood prices of $200 / cord, my cost of lost firewood sales would be $40.00 per month per 1K bd ft of 4/4 oak (which is less than the utility cost for an equivalently sized DH kiln.)

From what I've seen with my kiln, a solar kiln will raise the temps on the lumber by around 40 degrees or so each day during the spring, summer and fall.

Based upon YH's experiences, in a highly insulated hybrid kiln during the day the vents would be closed, the kiln fans operational and the heat from the boiler added to the kiln chamber gradually during the day to bring it up to around 135F.  At night leave the fans running but open up a pair of vents to exhaust the moisture laden air and allow the lumber to condition over night.  Repeat the next day.

In the winter the amount of vent opening would need to be controlled so that the chamber did not lose too much temp - probably set the vents to close around 80 degrees or so in order to not have to increase the heat required to bring it back up.  With the heat retention in the lumber, I would imagine that if the vents closed at 80F the chamber would probably recover 5 - 10 degrees before starting to drop due to passive heat losses.

If I wanted to size a hybrid kiln for 8,000 bd ft of oak, I'd be looking at 800,000 btu's / day, or a little over 1.5 cords of wood per month / 200 lbs dry firewood per day.  Most likely I would see a 90 day kiln cycle for 5/4 oak (maybe a bit less), so I'd have to invest 4.5 cords of firewood in order to dry 8K be ft of 5/4 oak.

Drying QS, I find that my drying times are longer than standard oak, and I will use around 700KW drying and sterilizing 1K bd ft of 5/4 green QSWO. 

At .11 per kwh, that equals $77.00 of energy costs to dry 1K bd ft of 5/4 oak.

I have to admit that this seems complicated when compared with running a traditional solar or DH kiln. The primary "hypothetical" advantage over solar is that this could operate 12 months a year in a well insulated kiln chamber.  If I simply augment a solar kiln with an additional heat source there will be a lot of heat loss through the collector - ie much less efficient and more costly from an energy standpoint.

The primary advantage over DH is energy costs.  An 8K bd ft load of QSWO will most likely run me around $650.00 of electrical costs over a two month period.  If it requires 15 minutes of time per day to split and handle the firewood, and another 15 minutes to load the boiler daily, then my monthly labor costs for keeping the boiler fired would amount to $150 or so - about half of the cost for electricity. 

[GeneWengert-WoodDoc]

There is a little problem with you theory. 

1.  A solar kiln works on the concept that the sun will heat the lumber up to around 135F or so during the day, with fans circulating the air in the chamber so it absorbs moisture from the lumber while lowering the RH% in the chamber as the air heats. 

This is not quite true, as with green lumber and good air flow, the moisture evaporation will use the heat energy nearly as fast as it comes into the kiln.  If you achieve 135 F and a low humidity, something is not right as the lumber should be using the heat energy, evaporating moisture, cooling the air, and increasing the humidity if the vents are closed.  In fact, it will take only about 1/8% moisture loss from the lumber to fully saturate the air.  So, we need to vent, and this also will cool the dryReally.



2.  During the day a small amount of the chamber's air is exchanged with outside air, thus releasing some of the absorbed moisture.

The truth is that it is more than a small amount.  So, you cannot close the vents during the day.

3.  The heat during the day will not dry much lumber.  You need low RH as well.  The heat in air at 135 F is rather small, so the amount of drying is small.  That is, it takes very little heat, compared to the energy for evaporating, to heat 70 F air to 135 F.

So, overall, go one way or the other, but not mixed.

[scsmith42]

There is a little problem with you theory. 

1.  A solar kiln works on the concept that the sun will heat the lumber up to around 135F or so during the day, with fans circulating the air in the chamber so it absorbs moisture from the lumber while lowering the RH% in the chamber as the air heats. 

This is not quite true, as with green lumber and good air flow, the moisture evaporation will use the heat energy nearly as fast as it comes into the kiln.  If you achieve 135 F and a low humidity, something is not right as the lumber should be using the heat energy, evaporating moisture, cooling the air, and increasing the humidity if the vents are closed.  In fact, it will take only about 1/8% moisture loss from the lumber to fully saturate the air.  So, we need to vent, and this also will cool the dryReally.



2.  During the day a small amount of the chamber's air is exchanged with outside air, thus releasing some of the absorbed moisture.

The truth is that it is more than a small amount.  So, you cannot close the vents during the day.

3.  The heat during the day will not dry much lumber.  You need low RH as well.  The heat in air at 135 F is rather small, so the amount of drying is small.  That is, it takes very little heat, compared to the energy for evaporating, to heat 70 F air to 135 F.

So, overall, go one way or the other, but not mixed.

Thanks Gene. 

If I'm understanding you correctly, most of the btu's created by the solar activity are consumed by the evaporation process?

I now understand the point about the air saturation and the need to exchange it during the day - thanks.

[GeneWengert-WoodDoc]

Over 50%...maybe 65% or so.  We need a well insulated kiln.  And we need some venting, but not too much.  On the other hand, if you just used a shed with no solar collector, but blew outside air across the load during the daytime (low Rh), then the wood would dry pretty fast.  So, there are times when the solar energy in a kiln is not a factor in drying...vents wide open, fans on, low RH outside.  We call this a shed fan dryer or a single pass dryer.  The Vt kiln can operate in this mode and dry lumber faster than if it was closed vents.

Bottom line:  drying is controlled by temperature, RH and velocity.