fan drying

[xlogger]

How much help does it do when you run fans on lumber like poplar and maple when the temps are below 40° or freezing at night? I put those under fans off the mill like YH suggested.

[WDH]

I run fans on maple for the first 4 - 6 weeks no matter the conditions.  Not sure if it makes sense in the winter, but I did have a very bad bout of gray strain in a stack of maple that was sawn in January with no fans on the stack.  Made me skittish. 

[GeneWengert-WoodDoc]

As a rule of thumb, each 20 degrees cooler means 1/2 slower drying.  So, with the drying rate slowing down, fans still help, but the cost per % MC removes doubles for every 20 degrees F cooler.  So, that leads to a common belief that under 50 F, fans are too expensive to use...but it is just a guideline, of course.

Now the parallel issue is that sometimes in January we might get a really warm day, and if we do not have good air flow, and the resultant fast drying, we can get staining in the white woods.

Remember that above 40% MC, fans help greatly with nearly doubling of the rate of drying when going from 250 rpm to 600 from.  However, 20% MC and drier, the drying rate is controlled by the rate of moisture movement within the wood rather than the removal of moisture from the surface by air flow...fans are not very important under 20% MC, unless the temperature is really high, like over 160 F, and the humidity very low, like under 30% RH.

[WDH]

I had about three warm, humid days in January that did the # on a batch of soft maple for sure.

[Deese]

I run fans on maple for the first 4 - 6 weeks no matter the conditions.  Not sure if it makes sense in the winter, but I did have a very bad bout of gray strain in a stack of maple that was sawn in January with no fans on the stack.  Made me skittish.

WDH,
Do you run the fans 24/7 for that long? Or just during the day?

[WDH]

24 hours, 7 days per week. 

[YellowHammer]

Like WDH, I have been bitten by a few warm wet days spoiling some wood, so now I have a simple RonCo fan strategy.  Set em and forget em.  24/7.

You'll know when to turn them off, because when you stand downwind and feel that cool, wet air comin off though the stacks, the fans are doing their job.

[gfadvm]

As a rule of thumb, each 20 degrees cooler means 1/2 slower drying.  So, with the drying rate slowing down, fans still help, but the cost per % MC removes doubles for every 20 degrees F cooler.  So, that leads to a common belief that under 50 F, fans are too expensive to use...but it is just a guideline, of course.

Now the parallel issue is that sometimes in January we might get a really warm day, and if we do not have good air flow, and the resultant fast drying, we can get staining in the white woods.

Remember that above 40% MC, fans help greatly with nearly doubling of the rate of drying when going from 250 rpm to 600 from.  However, 20% MC and drier, the drying rate is controlled by the rate of moisture movement within the wood rather than the removal of moisture from the surface by air flow...fans are not very important under 20% MC, unless the temperature is really high, like over 160 F, and the humidity very low, like under 30% RH.

I thought that the colder the air, the lower the relative humidity. This led me to believe that wood would dry faster in the winter???

Someone please smarten me up.

[WDH]

I believe that it takes heat to evaporate the water.  The higher the heat, the higher the evaporation, the lower the heat, vice versa. 

[Deese]

I believe that it takes heat to evaporate the water.  The higher the heat, the higher the evaporation, the lower the heat, vice versa.

X2

[beenthere]

I thought that the colder the air, the lower the relative humidity.

The colder air holds less moisture, but it is "relative" humidity (not humidity) and that RH % is higher than warmer air given the same amount of moisture in a cubic foot.

[gfadvm]

I thought that the colder the air, the lower the relative humidity.

The colder air holds less moisture, but it is "relative" humidity (not humidity) and that RH % is higher than warmer air given the same amount of moisture in a cubic foot.

Well, that clears that up. I need to do a little studying on that.

[WDH]

That is why it rains  ;D. 

[YellowHammer]

As the guys have said, relative humidity is not an accurate indication of the amount of water vapor in the air as it is very influenced by air temperature.

The carrying capacity, or the ability of air to hold moisture, is logarithmically proportional to its temperature.  So the cooler the air, the less water it can carry and the hotter the air, the more water it can carry.  Relative humidity, on the other hand, is the ratio of how much moisture is in the air relative to its carrying capacity at any given temperature.  Air at 95F can carry about 9 times more water than air at 32F.  So if the mass of the water in the air remains constant, as the air temperature goes up, its relative humidity goes down, simply because the air has a much higher carrying capacity at higher temperatures.  The actual amount of moisture in the air does not change.

So when fan drying in the winter, the air has a much lower capacity to carry water away from the boards, and it results in slower drying, which leaves the boards in the high moisture content sticker stain danger zone for a longer time.  Whenever the boards have a high enough moisture, and if air temps go up, then sticker stain can result.  So even in winter, I prefer to keep my fans running to try to drive as much moisture out of my air-drying stacks as possible, even though the cold air has less capacity to carry it.

[gfadvm]

Yellowhammer, That really helped me to grasp the whole concept. Thanks, this has been driving me nuts. I'm thinking the same explanation explains dew point? And Danny's rain

I run my fans 24/7/365 regardless of the science.

[4x4American]

x2 Thanks YH u r da man  8)

[GeneWengert-WoodDoc]

The outside air has the same relative humidity, summer and winter in most locations in North America...65 to 70% RH.  So air drying is mainly about temperature.