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Foolproof relative humidity for drying/not wrecking red oak?

Started by IndyIan, May 25, 2005, 03:39:03 PM

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IndyIan

Hi Everyone,
I've just had about 400 bf of red oak sawn up to 4/4 and I don't want to mess it up.  I've stickered it and put it into the house I'm building about a week ago.  I put a fan blowing into the side of the pile and over night I'm pretty sure that all the lumber drove the relative humidity to near 100%.  I've got a dehumidifier going and now have a hygrometer set up.  How much can I drop the relative humidity with the temperature being around 65F with out degrade problems? 
I am trying to learn about how to dry this wood properly (:P all I can at the USFS website), but until I do is their a safe relative humidity to keep the room where the oak is at?  Dry enough to prevent mold but not run into checking problems?
Thanks,
Ian

Don P

I'm no expert but this time of year with fans on I doubt you need the DH for quite awhile yet  ???

Brad_S.

You need to turn that fan off ASAP. Hopefully, it's not too late. Oak must dry slowly, and the air across the surface this early in the drying stage will cause severe surface checking.
"Life is what happens to you while you're busy making other plans." J. Lennon

Den Socling

Since I was thinking about predryer settings in another thread, I'll suggest the same here. If your temperature is 65, you won't be removing water very fast. A humidity setting of 75% would keep the wood safe. And air flow doesn't cause drying. Air is the vehicle that carries the water. Heat is needed to evaporate the water and a humidity less than 100% makes room in the air to carry away the water vapor.

jimF

The standard schedule calls for ~85%RH at 110F and 400ft/min of air flow.  Much less than this mold will grow.  If you have bacterial infected, I have never seen a schedule that is suggested for it.

IndyIan

Thanks for everyones help.  I have moved the fan so it doesn't blow directly into the pile anymore but still keeps the air moving in the room.  Also I have turned off the dehumidifier as the relative humidity is 70% at 65F.  The boards have been in for a week now and no longer feel "wet" to the touch but now just very slightly damp.  No mold far either so hopefully I'm OK. 
Eventually the lumber will be outside when I get a shelter set up and mother nature can take it from there.

Ian

kilndry

I think some clarification is in order. Air flow is not necessarily a bad thing, as Jim alluded to above with the 400fpm reference.  Plenty of people put green oak into a predryer that has both air flow and humidity control with good results.  TOO MUCH air flow will certainly increase checking, but if you can maintain uniform air velocity throughout the pile, with a very high RH, you can accelerate the drying without adversely affecting the wood. Easier to do in a kiln or predryer than in your living room, obviously.

And as for air flow not drying the wood, that's a little unclear, too. Above FSP the air flow over the surface plays a more important role in drying than it does below FSP. You can dry the wood quite a bit if you have a breeze blowing through the pile even at low temps. When you get below 25-30% mc, heat becomes more important because helps move the bound water out to the surface, where the moving air can carry it away. It's common practice when drying many hardwoods to use higher air velocities early in the drying schedule, and the reduced air speed below FSP. Not because it helps the drying, but it can be an energy saver if you can slow the fan speed down.

Probably more than you need to know when drying it in your house, huh?

P.S. A good moisture meter would be helpful, too, if you don't already have one.

Den Socling

Air flow is necessary to dry wood but air flow does not do the drying. Yeah, that sounds confusing. My point is this: if RH is 100%, I don't care if you blow 1000 fpm. It isn't going to dry wood and that means that you won't be causing any checks.

jimF

Air flow is the means to transport both the heat to the wood and water away from the wood.  If the air is saturated with water, RH 100%, than fast air cycling through it will not move the water away.  If you replace the saturated air with drier air then drying will occur.  If the Evaporation of water to the air cools down the temperature, even though the RH is low, very little drying will occur.
If the air is very dry, slow moving air will attain to a high RH quickly and little drying will occur.
I hope this provides some more understanding.

Don P

I did prove to myself the other week that at 5000 fpm, ~65F, and 65%RH that you can check the mess out of some white oak in about 20 miles  :-[
Airflow does accelerate surface drying and at high mc's and low RH's causes more checking. 

QuoteIf you have bacterial infected, I have never seen a schedule that is suggested for it.

Page 90 of my '78 copy of "Drying Eastern Hardwood Lumber", McMillen/Wengert  has a 4/4 bacterially infected oak schedule, it looks to be less severe than a regular schedule. The note says that if severely infected it should be air dried in a sheltered location or forced air dried to 22% or lower before kd.

There is also a Forced Air drying schedule on pg 27 of that same book, Red oak is listed as a FA-2
step 1, above 40%mc- db=75-85F, 4F depression
step 2, 40%mc- db= 95F, no control of wet bulb


The first steps of a low temp DH schedule for red oak is
above 50%... 90F...85%RH
50-40%.........90F...81%RH
40-35%..........95F...75%RH

If the oak has pre existing surface checks each step happens 10% lower mc.

Through all those schedules, and the standard schedules, the constant is RH. Those moisture contents correspond to those RH's.

??? ???

Don P

 If I put together what I think everyone is saying and try to apply it in a uncontrolled room...help me if I'm seeing it wrong.
If the rh in the room with green RO is >85%, run fans, vent the room and if that doesn't get it down, maybe dh
If the rh in the room is < 85% you have to decide.
If the air is very dry (like this time of year is for us), no breeze or very little, you want to maintain closer to that saturated boundry layer jimf was talking about. As we go into summer, the rh gets a whole lot closer to 85% and mold is likely, more breeze would be a good thing.

If that's right, is there a cheap way to monitor exit air rh from the stack and use it to switch fans?

kilndry


Dan_Shade

I'm guessing Don P has some stickered up white oak on a trailer and drove up the road with it at 55mph or so...

bummer that it all checked up on you, Don.
Woodmizer LT40HDG25 / Stihl 066 alaskan
lots of dull bands and chains

There's a fine line between turning firewood into beautiful things and beautiful things into firewood.

Don P

 :D You got it Dan, should have tarped it but it was just a short hop, what could happen. Luckily it was for fencing boards.

Its also an example I've heard used when people think kiln drying is a harsh environment. The trip to the kiln can be the harshest part of the whole drying process.  The next day you could go out and the checks would have closed up as the moisture worked its way to the surface again...but the tear is there.

VA-Sawyer

This is something I haven't thought about. I've protected wood for long trips, but never considered the drying problems from short ones. That might explain some checking problems I recently had on  WO trailer planking for a customer.  I need to remind my customers to cover their wood when bringing it to the kiln or when taking green wood home.  Thanks for the great tip.
VA-Sawyer

jimF

Don P.
That is about right.  Like thermostats that controls the temperature there are humidistats that controls the humidity.
Technically, mold will grow on the wood if the surface is saturated.  So you want the saturated boundary layer to be below the surface.  But don't have the surface too much lower than saturated too quickly or you will get surface checks as is being talked about here when you ride down the road with no tarp.

IndyIan

Thought I would give an update, so far things are going ok.  No surface checks, some checks on the ends but nothing major.  I put some foundation paint/tar on the ends as it doesn't seem to leach into the wood at all and should help with the end checks.

I've got the humidifier set to kick in at about 70% relative humidity now as I am getting a bit worried about how the humidity is effecting the house even though its under construction.  I was cutting drywall and it wasn't really "popping" when I snapped a cut...  Also I've got some prefinished hardwood in boxes that is probably sucking up some moisture but I have lots of time to dry it again before I install it.

I am reading everyones comments, and I think I am building an understanding of how drying works.

Thanks,
Ian
   


 

Don P

I guess this is off on a tangent  ::) ;D
We've had homeowners that have prestained green log siding, it seems to have checked considerably less than where we put up the siding unfinished. That's a very harsh environment for green material. I've noticed on some siding a homeowner had sprayed with Seasonite and then dried for 2 years, it had dried really well. I've also heard that wood treatments that contain PEG have the same effect of lessening seasoning checks. I'm wondering if some dip or spray could moderate the moisture movement right at the surface and "protect" wood through a wider than optimum range of drying conditions...Anchor pretty-permeable Seal?

james04

Quote from: Don P on May 27, 2005, 10:10:12 PM
.

The first steps of a low temp DH schedule for red oak is
above 50%... 90F...85%RH
50-40%.........90F...81%RH
40-35%..........95F...75%RH

If the oak has pre existing surface checks each step happens 10% lower mc.

Through all those schedules, and the standard schedules, the constant is RH. Those moisture contents correspond to those RH's.

??? ???

[/quote]

Hello Don,

Could you please advise the source of the above schedule. Is it available on the Internet? Do you have the rest of the schedule for red oak?

James

DR_Buck

So,  this air flow thing........................

        If I load my DH4000 (Nyle L200)  with fresh sawn green oak, set my starting MC at 85% and 90°,   I should not turn on my fans?      This is not something I've found written in the manual.   I have always turned on the fans.  They run 100% of the time the kiln is in operation.   Am I to understand this is a bad thing?
Been there, done that.   Never got caught [/b]
Retired and not doing much anymore and still not getting caught

Don P

DR_Buck,
I don't think that's what was being said, read JimF in post #4.

Boy, 1400+ titles of free wood stuff, it might have been in here James, that neuron musta died;
http://treesearch.fs.fed.us/pubs/fpl/

This is "Drying Eastern Hardwoods" I don't think that schedule is out of it though. I'll keep trying to remember where it came from.
http://www.fpl.fs.fed.us/documnts/usda/ah528.pdf

james04

Hi Don,

Thank you for replying. No luck with the two links you posted. I am so surprised that I am having so much trouble locating a schedule. I wonder if all of the DH kiln operators are just using the conventional schedules. I have posted this request on three different forums and haven't gotten any replies. Only you and one other reply stating to search on wood web. No luck there either.

James

BBTom

I don't know if there are any internet published schedules for DH kilns, but you can use conventional schedules humidity settings.  I have a hot water kiln that I use modified schedules for.  I have included the humidity readings on these schedules. 

I am attaching an excel file with my schedules and other related ramblings.

2001 LT40HDD42RA with lubemizer, debarker, laser, accuset. Retired, but building a new shop and home in Missouri.

james04

BBTom,

Thank you so much. I really appreciate you sharing that info. I was having trouble getting the water out because I was trying to keep the temperature below 90 and the RH was dropping below 85%. So I decided to just let the DH run regardless if it went over 90deg. That is when the RH started to rise to 89 and the water is now coming out. Today I lost 3.66% MC.

James 

BBTom

3.5% moisture loss in one day is absolute maximum for red oak.  Keep that humidity up there so it doesn't dry too fast and check on you. 

The oaks are the slowest to dry native lumbers.  White oak has 3% max.  These numbers are off the top of my head.  I believe them to be true, someone out there certainly can correct me if I am wrong. 
2001 LT40HDD42RA with lubemizer, debarker, laser, accuset. Retired, but building a new shop and home in Missouri.

Don P

I did some rooting through the pile here. The schedule you asked about is from a class handout from Dr Wengert/Nyle's "Introduction to Kiln Drying" class. The other guys are talking from experience, I'm just acting as a librarian here.

more stuff while digging around;
("Drying Hdwd Lbr")
Air velocity is as important as temperature and RH for controlling
the drying process. The velocity of air when the
wood is above 40% MC (approximately) affects the rate of
drying. Higher velocities result in faster drying (Fig. 2.3),
more uniform RH within the pile, and more uniform drying
within the pile. Of course, faster drying can increase the risk
of checking, but conversely it can decrease the amount of
warp. (The curves in Figure 2.3 vary with lumber thickness,
species, specific gravity, and other variables. The intent of
presenting this information is to illustrate the general affect
of velocity on drying rates.)
Below 20% MC, velocity has very little effect on the rate or
quality of drying. That is, the relationship between velocity
and drying rate is nearly a horizontal line at 20% MC
(Fig. 2.3). Velocity has an insignificant effect because at low
MCs, the drying speed is controlled by the rate at which
water can move within the wood rather than the rate at which
the air can carry the moisture away from the surface. Between
40% and 20% MC, the effect of velocity decreases
proportionally (Fig. 2.3).

There is a direct relationship at high MCs between air velocity
and RH. That is, if the velocity is decreased, which will
slow drying, then the RH can be decreased, which will increase
the rate of drying. In other words, many combinations
of velocity and RH can provide the desired drying rate.
Pages 58 and 95 from "Drying Hardwood Lumber" has some more info and is a good book for the reading list also;
http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr118.pdf

This came out of one of them;
Dehumidification Kilns
For DH kilns, use the same RH, but lower the temperature to the typical operating temperature for the compressor; raise temperature to steam values when convenient and possible.


james04

Hi Tom,

I have seen 3.8% mc loss per day in several references. However I would like to keep my loss at around 3.5% to be on the safe side. After 3 days of slow drying I was happy to get the 3.66 as I have some white mold growing. Will the higher temps slow or stop this?

BBTom,

Thank you for digging that up. Funny I could have sworn that I have read that PDF. I must have let that pass under my nose without noticing the Red Oak DH schedule.

I currently have two 1600 cfm fans in the kiln. The kiln is 8'x11.5'. I have purchased one more fan to add. Do you think 3 1600cfm fans will be enough. Some of the plans I have seen on the net had 4 12" inch fans and advised two would be ok. Well I figured since mine are 14" that two would be ok. Any thoughts on this?

James

James

Handy Andy

  Just noticed this is an old thread, but wondered how that tar worked for IndyIan when you saw into that log.  Or maybe he put it on after the logs were sawed into boards.  Basement waterproofing would be a cheap log sealer.
My name's Jim, I like wood.

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