QWe have trouble controlling the wood’s moisture content, which means relative humid-
ity, in our storage building
and in our shop. Can you talk about how
heating air lowers the humidity?
AIndeed, heating air lowers its relative humidity. To answer your question, I will use an example. If your
temperatures or MCs are different, you
or I can recalculate.
Storage building. Let’s assume that you
want your lumber to stay at 7.0 percent
MC; this means the air must be 7.0 percent
EMC (equilibrium moisture content). To
calculate the amount of heat required, we
need to make sure that the lumber averages 7.0 percent MC when you put it in the
building...if it is wetter, then your storage
building is really a drier.
We also need to assume that there
is only a small amount of fresh air coming into the building. Finally, we need
to make sure that there is no source of
moisture in the building.
One important fact that lets us do
these calculations is that the outside
air in most of the U.S. just before
sunrise is the lowest temperature and
95 to 100 percent RH.
Based on these criteria, we can figure out the amount of heating above
the morning’s low temperature needed
to achieve 7.0 percent EMC throughout
the entire day and night. (See Chart A)
Shop. Let’s assume that your shop
is 70F in the winter and 75F in the
summer (average). Your dust system
recycles some air, but there is always
fresh air coming into the plant. As
you know, the relative humidity (RH)
drops if the outside air is heated. Here
are some values when heating air to
70F. (See Chart B)
I hope you can appreciate why we
encourage adding moisture to the air
in the plant when it is cold outside.
Morning’s Low (F) Heating (F) Daily Building
0 21 21
Chart A: We can figure out the amount of heating above the morning’s low
temperature needed to achieve 7.0 percent EMC throughout the entire day and night.
Morning’s Low (F) Humidity at 70 F (%) EMC at 70 F (%)
Chart B: There is always fresh air coming into the plant. Relative humidity drops if the
outside air is heated. Here are some values when heating air to 70F.