Does redoak take longer to dry than white oak?

[kelLOGg]

I put 700 bdft of shed dried RO in the kiln on 5 Sep. The MC was 17% and today (17th) it is 11.4%. Seems slow to me compared to WO. 700 bdft is more than I have ever dried but it still seems slow. I vent when the temp gets to 100 - 105 F and the RH is down to 22%; drops of H20 are still coming but quite slow now. Am I impatient? My kiln is home made w/ a room de-hum, fans in an insulated chamber.
Bob

[WDH]

I only have experience air drying oak.  My red oak definitely dries faster than white oak.  The tyloses in the white oak impede water movement in the wood thereby slowing the drying rate.

[kelLOGg]

I guess that's the same as saying the open pore structure of RO enables it to dry faster then WO. My experience is just the opposite so I hope it is due to the size of the load of RO I have in the kiln now. Does that make sanse?

[red oaks lumber]

heat realeses water, by venting, only sets back your drying process.turn up the heat and run the d.h.full cycle. air dried oak you can't screw up drying. you have a better change of wrecking wood during the air dry cylcle for reasons you can't control.
another thing you might not have enough air flow in your kiln. heat realeses water ,air flow gets the moisture out from within the rows of wood.

[shelbycharger400]

venting at 100?

i have read elsewhere on here people are keeping their kilns at 140 to 160

[kelLOGg]

Maybe I am misinterpreting the following posting by ianab. When I read this I thought I should vent to dump the heat and try to keep the humidity low. It made sense - A de-hum should be more efficient at condensing water vapor at ~90 F than at 140 - 160 F.
When I dried WO (before I read this post) I was using much higher temps (120 - 140F). So I need to up the temp.
Ianab,  can you clarify your post and set me straight?
Bob


This page has a table for working you the Equilibrium for wood at various humidity and temperature.

http://www.csgnetwork.com/emctablecalc.html

Humidity is the most important, you can pretty much ignore the temperature.

So if your average humidity is 30%, then dry wood will be approx 6% moisture.

It doesn't need to be exact, so if it's 7 or 8, probably close enough. Same thing with stickers, take the wood off them when it stops loosing moisture, or is "near enough" anyway.

Ian

[Ianab]

My post would have been referring to equilibrium moisture and air drying I think.

In a kiln you bring the temperature up to force the moisture out of the wood faster, and more evenly. You need that heat to move the moisture from the centre of the wood, and then you keep the humidity up to control the speed that the surface dries at.

So the drying speed is related to both the temperature and the humidity. Higher temp, the water can leave the wood faster. Higher humidity, that slows the evaporation of the water.

BUT - High heat coupled with too low humidity and you get the surface dried first, and that caused all sorts of grief like surface checking and case hardening.

Different wood can be dried to different schedules. Like White Oak is naturally slow drying so you must use a cautious schedule with that, and keep the humidity up to control the drying. Pine dries easily and quickly, so you can pretty much turn everything to max and cook the water out as fast as the DH and remove it.

In your question it would seem that the DH unit may be able to remove moisture from the air in the kiln better at a lower temp, but unless you have the heat there to evaporate it from the wood in the first place, then drying is actually slower?

Ian

[red oaks lumber]

if the m.c going in the kiln is at 17% any damage has been done already. temp and humidity only need apply when wood is above 25-40%. the best assurance against surface checking on greener wood is run a higher kiln temp with a slower drying cycle, by doing this the heat will still realse the water but more moisture stays in the kiln in the form of humidity keeping the surface damper. you can also  manipulate your rate of drying with bigger or smaller kiln charges.
 drying greener wood to slow without good are flow your chances of having more cupped wood increases greatly, so there is a fine line betwwen drying to slow or to fast.

[kelLOGg]

My situation is probably different from others. I never put green wood in my kiln because I don't have the controls for humidity except on/off. I shed dry to ~20% then charge the kiln.  I have the de-hum and vents on a timer and am constantly adjusting the on/off time. When the de-hum is on the vents are closed and when the de-hum is off the vents open and exhaust fans come on. The cycle repeats until I intervene which is frequent. I do have thermostatically controlled heaters for a final heating to 140 F to kill bugs. I dry wood to 6 -9% for my own projects. I have dried WO, hickory, river birch and maple and nothing I have made with the lumber has cracked over time in air conditioned spaces. My current approach to the red oak was based on a mis-interpretation of IANAB's post where I vented at ~100F; in earlier drying I vented at ~120 - 140F. Given the controls I have how would you guys operate it?
Bob

[red oaks lumber]

if its not a steam kiln why vent? all your doing is losing heat and bringing in moisture. imo 

[kelLOGg]

I assume my room de-hum is not designed to condense water at high, (say 140F) efficiently so I vent to a lower temp where it can be more efficient. I realize i am introducing moisture but when the de-hum comes on again the RH rapidly falls presumably because the lumber has not had time enough to absorb the moisture and it is just being removed from the air. Half-baked idea, huh?

[Charles]

I have a home built kiln with a house DH unit. It would take me about 2 weeks to dry 250bf at 17% red to 7%
I don't go any higher than 90deg  then the DH kicks out and I cool down the kiln for 20 min then the DH starts back up and runs for 40 min.
A house DH can't run much past 90 or the evaporator coil can't keep cold enough to dehumidify when the RH starts to get low in the kiln. At 35% MC in the kiln the temp of the coil has to be in the 50 to 55 deg range in order to keep dehumidifying, because the dew piont of the air is now about 56 deg. or so ( I think I don't have my charts in front of me) I have a temp probe fastened to the condensing coil (evaporater) and mounted outside the kiln so I can watch the coil temp when the RH starts to get low
If you take the temp up in the kiln then the condensing coil temp goes up to but so does the dew point but the coil can't get low enough to evaporate
Also the DH's  are famous for getting low on freon. I install a external charging port on the suction side and check it each time a load goes in the kiln
By doing this I can accomplish the 30-35% RH needed for 6-7% lumber and have excellent results
You have had 700bf of 17% lumber in for 12 days I think you have another 20 to go
If the water stops at around 40%MC your condensing coil is not cold enough and probley low on freon
I have been drying for myself for over 10 years and have had excellent results if I watch the temps andMC closley because most house DH's aren't strong enough to run at high temps

Hope this helps
charlie

[kelLOGg]

Charlie, That was very helpful.
Sometimes I will run the DH for 4 hours and vent for 1 hour. If I skip my vent cycle and let the kiln cool on its own it my require as long as 12 hours to cool to the 90s. (The insulation is working against me here. It is insulated so well that running only the fans will increase the temp.) If you have a DH/cool cycle of 40 and 20 minutes your kiln must not be insulated?

Bob

[Charles]

Bob
I'll see if I can get a couple of pictures to-gether and explain as briefly as I can what I do
that may help a little
charlie

[Charles]

Here are some pics with a short description

 




This a picture of the kiln itself without the doors installed. The plastic separator that is hanging down  is used to fill the void if there is any after the lumber is stacked. It ensures that the air is forced through the lumber not over it. In front of the separator  are two  wires attached to two nails driven into the lumber at the same spacing as my pin moister meter. I do this at the bottom, middle and near the top so when the door is shut I can check the moisture content of the lumber
Right side has  the elect. panel to turn on the DH, heater, fans, lights and A/C. The two little boxes by the top door are external temp probes that give me DH evaporator coil temp and incoming air temp from the A/C unit
The A/C unit sits on top. The DH unit sits in the center, the wet/dry bulb sits next to the DH and there are two fans on each side tipped on a 45 angle. They blow down in the kiln at the front and the air is forced through the lumber and back up behind the DH unit









This is picture of the elect stuff. The big box turns on the lights, A/C and DH unit The white box is a variable speed for the fans. The unit above is a temp on off relay. The DH runs until temp reaches 90f then DH shuts off and A/C starts, you can see the A/C unit on top of the kiln. It blows in to the top of the kiln just above the DH unit.  The air is cooled 7 degrees  to 83f which takes about 20 min. When the relay hits 83 the A/C shuts off and DH starts again and raises the temp to 90f which takes about 40 min. . Above that is a hydro meter that I use to track the hydro used so I can figure a cost









This pic shows the DH, a water hose runs off the DH and A/C to an outside pail with graduations on it so I can keep track of the water pulled from the lumber. The little fan on the DH creates a turbulence of air around the compressor and helps the cooling. The little V above the DH is the deflectors for the incoming A/C air. The DH pulls in air coming up behind it from the bottom of the kiln, what it can’t pull in simply goes around it and back into the fans.







This the kiln buttoned up. The doors slide over little studs and are held in place by a few wing nut. I don’t use all the ones you see in the picture, may 3 on the top and 4 on the bottom. I have a mirror behind the DH so I can look in and see if the evaporator coil is working. Also lets me look inside to make sure nothing is leaking, had that happen once.

I built the kiln this way because I wanted the space, also if I built one again I would not insulate, because the kiln is inside it can generate a lot of heat and that is my biggest problem heat

This gives you an idea of how I do it. There are better ways but I dry usually for me so I don’t have to get too excited about time and it works well.
Hope this has helped

charlie

[kelLOGg]

Charlie,
Thanks for the detailed explanation. If I understand correctly, the AC is used to remove the heat generated by the DH and also provides a little more de-humidification. This way you can keep the temp in the range where the DH unit works best. Unless you use high temp to sterilize the wood to kill bugs, you really don't need high temp or insulation. Correct? I suppose the AC unit is vented to the outside of the kiln to dump heat. So I could get rid of my heat buildup by adding an AC unit instead of venting which adds more moisture.

Before I built my kiln I made a mockup consisting of a huge tarp wrapped around the lumber and a DH unit. It dried hickory to 6% faster than my insulated kiln, presumably because of no insulation allowing heat to escape. Of course I couldn't sterilize the wood.
Bob

[Charles]

Bob

The A/C takes air from outside the kiln and blows the cooled air into the kiln.Remember this kiln is the basement of my shop so the air being cooled is fairly dry already.
If you look at the picture of the kiln that is closed up you will see little flaps so to speak on the ends and the front door. The top flaps are opened to draw air in and the bottom flaps are opened to let hot air out, same as the front flap tp let hot air out. You can't let out what you aren't drawing in so they are usually set close to the same opening. If you could make that would run say 85-90f all the time and have a DH unit that could maintain evaporator or cooling coil temp of 50-55deg. you could easily achieve your 30-35% RH for a wood moisture content of 6-7%, and it would do it at a reasonable rate.
I like to dry anything under 20% prefer lumber at 12%, let nature do most of the drying for you. Once lumber is under 20% it much easier to dry without screwing it up.
Even if you had a kiln the brought in outside air for cooling the little amount you would bring would not likely make a big difference in the drying time. I know of some big kilns that have flaps that open to allow air in for cooling.
The only time you would want to have the kiln really hot is to kill bugs or set the pitch in pine. I can't do that so I don't dry pine. From my experience I did have some post powder beetle that like oak, so make sure all the bark is off the lumber and I found that the few that where in the lumber didn't make the 21 days @ 85 deg. You probely noticed the kiln is not built like a conventional one, it has a second chamber on top. This was more work but I needed the room
I don't know how big of an kiln you are planning to build but 700bdft of lumber is really hard on a home DH if you are try to dry it quick. I get about 1% a day drop in MC and when it gets to about 8-9% MC it falls off  to about 1% every 2 days but at 17% dried to 7% that's only about 2 weeks of drying time.If you could find a DH that could take the heat then I say go for the higher heat but everything slese in the kiln suffers also. All the equipment in my kiln will stand up to 85-90deg. The fans are original and cost 35 bucks and are still going strong after 10 years
The biggest thing to remember is that at 85-90 deg you need a evaporator that can maintain a coil temp of 50-55f and if it is a small kiln your worst enemy is heat

charlie

[kelLOGg]

The A/C takes air from outside the kiln and blows the cooled air into the kiln.

Charlie, what brand/type of AC do you use? Most units re-circulate and cool the air in the room but yours adds new air. If I had a unit like yours I could install it at the intake vent and chill the air in the kiln faster then exit the exhaust vent.
Bob

[Charles]

Bob
Sorry I screwed up in typing it should be 
the A/C takes the air from inside the kiln and blows the cooled air back into the kiln
and I should have added that the heated air from the condensor is vented into the basement

All I use is a 5,000 btu A/C unit., but what I did do is chamber the intake to be separate from the cool air discharge which seems to make the cooling a little more predicable.
When you say you want to cool faster I don't know that is a good thing, my reasoning in having a a 30% cooling time is that the lumber is stressed with the heat and losing water the surface is always dryer than the center. My thinking is that the cooling time is also destressing time where the outside of the lumber has a chance now to partaily equalize which is good for the lumber because the big guys equalize after the drying is done here it is done while you dry cause time isn't important. So while the lumber is cooling the surface is still hotter than the center and water is still being wicked to the the surface as it cools so when the DH starts the water is easier to remove cause it is closer to the surface, untill you get to about 8-8% then things slow down cause now I believe the lumber is heated equally through and now is when you see a slowed drying rate about half of what was happening before.
Now I don't exactly know what you meant by cooling faster but I think 30% cooling cycle or some what close to that should be good.
The only thing wrong with my way is that at first you have be there to watch how long the A/C stays on at a particular setting and adjust untill you get the right time. But once you have the right setting by that that I mean the temp and fan speed setting it works good. Now you fire it up and walk away and check it once a day
Sorry for the mis info

charlie

[kelLOGg]

Now I don't exactly know what you meant by cooling faster but I think 30% cooling cycle or some what close to that should be good.
charlie

My kiln is so heavily insulated (6" fiberglass batts) that it takes ~20 hours to cool from 106F to ~94F. My DH unit cuts out at 112F. In the past I would vent to cool and that has worked well for the species I have dried: WO, hickory, birch, maple. However, RO is taking longer BUT now I am pretty sure it is because the charge is larger. (700 bdft now vs at most 500 bdft earlier.) But, whatever the reason I want to speed it up - present RO has been in since 3 Sep and is now 10.7% MC. The DH unit is still working at high temp: ~1 drop H2O/2 seconds at 107F and ~10 - 11% MC. So, I am considering adding AC air at the input vent to speed up cooling. Or, I may drop the whole idea and go back to venting w/ ambient air. Thoughts?
Bob

[Charles]

Bob

I don't think the drying is taking to long. my thoughts would be that 700 bft of RO would take me 30-35 days to dry to 7%
The fact is your charge is 30% larger than normal and RO dosen't like to give up it's bound water very easily. When you start to dry you are removing all the free water first, that usually dosen't go to bad but getting the bound water gets tougher.So to me you have about 6-7 days left of drying and as I said earlier the last few % of MC slows done to about 1/2% per day for me anyway. You are at 10% and 1/2%  per day is another 6 days witch will be about 32 days which seems right on the money for me anyway.
You are still pulling water which is good, but I think your cooling coil temp may be reaching it's peak. At 110f with 10-11MC your RH should be around 60-65% which gives you a dew point of about 85f. or so and I think your cooling coil is probley around that. I don't know if you can check the coil temp but that would be important to me so you know where you stand as DHing goes. If your cooling coil temp is very close to the dew point as the RH drops you will see things really slow down.

If you have vented in the past and had success  I would stick with it, if you look at the pics of my kiln you can see some flaps on the ends and front of the kiln. Before I put on the A/C unit that's all I did was vent.
That's my take on this
charlie

PS are we the only ones on this post I am sure there has to be someone  out there that may have a better take on this than just me
I don't have a good enough knowledge on the science of drying just what I have experienced in doing it. Maybe somebody out there has a handle on the science end of it and can add something

[Charles]

Bob
Any more info on how the drying is going??
Was wondering if you ever got a chance to check the cooling coil temp and compare it to the dew point of the kiln air

charlie

[kelLOGg]

The MC is in the low 9s now. My timer malfunctioned so I can't rely on it, so I manually turn on the DH and fans for ~4hrs and off for ~20hrs. I'll limp this way til the load gets to ~7% and call it over and replace the timer. I still get water dripping at 105F so I'm still drying.

I have no way to measure thre coil temp.
Bob

[WDH]

I would think that the low 9's would be perfect for NC given your humidity.  I stickered some red oak behind the couch for a project on March 4th and took measurements all along.  The moisture content went from the mid 13's down to the mid to upper 9's and stayed right there until I removed the wood a few weeks ago.  That was with the air conditioner running all summer. 

[kelLOGg]

WDH, I think you're right but I will leave it in the kiln anyway because I'm too busy with garden, building a saw shed and sawing some WO a friend dropped off.
Bob