1/18/2015
Kiln Humidity and
Air Psychrometrics
Bill Smith
SUNY ESF Wood Products
Syracuse, NY
Temperature
Relative Humidity
Dew Point
Vapor Pressure
EMC – equilibrium
moisture content
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Temperature
• Temperature— Degree of hotness or
coldness.
• Dry-bulb—The temperature of the kiln air.
• Wet-bulb—The temperatures indicated by
any temperature measuring device, the
sensitive element of which is covered by a
smooth, clean, soft, water-saturated cloth
(wet-bulb wick or porous sleeve).
Temperature
• Temperature— Degree of hotness or
coldness.
• Drop across the load—The reduction in
the dry-bulb temperature of the air as it
flows through the load and is cooled by
evaporating moisture from the load of
lumber.
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Relative Humidity
• Humidity, relative—Ratio of the amount
of water vapor present in the air to that
which the air would hold at saturation at the
same temperature. It is usually considered
on the basis of the weight of the vapor, but
for accuracy it should be considered on the
basis of vapor pressures.
• Humidity, absolute—The weight of water
vapor per unit volume of space.
Dew Point
• Dew point—The temperature at which
steam or water vapor begins to condense.
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EMC
• Equilibrium moisture content—The
moisture content at which wood neither
gains nor loses moisture when surrounded
by air at a given relative humidity and
temperature.
Interior use applications must be:
• Kiln dried hardwoods to 6-7% MC
– (North American market; export perhaps
somewhat higher)
• Eastern white pine perhaps to ~10-12% MC
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Equilibrium Moisture Content
and Relative Humidity
• RH %
– 90
– 80
– 65
– 50
– 30
–0
• EMC %
– 20
– 16
– 12
–9
–6
–0
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RH and EMC “sorption isotherm”
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Kiln Drying Schedules • The purpose is to dry as rapidly as possible,
to be economical.
• However, drying must be controlled to
avoid value reducing defects.
• Typically, the temperature and the relative
humidity (wood EMC) are controlled.
• Temperature is raised and relative humidity
lowered as wood dries.
Kiln Drying Schedules •
•
•
•
•
So, we know what we want.
And we know what we need.
Do we always get what we want?
Really, you can’t always get what you want.
But, do we get what we need!
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Some typical schedules -
Some typical schedules -
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Some typical schedules -
Some typical schedules -
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Some typical schedules -
Schedule Control -
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Schedule Control -
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Maintainining %MC in
Storage (warehouse)
really the same issue
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Temperature
Relative Humidity
% EMC
– Relationships
Temperature – down?
Air “holds” less H2O
Relative Humidity - higher
% EMC – higher
lumber %MC - higher
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Temperature
Relative Humidity
% EMC
– Relationships
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Add heat
Temperature – higher
air can “hold” more H2O
Relative Humidity - lower
lumber drier
Remember –
outdoor RH
- 100% in early morning
- 30-40% in later afternoon
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Enclosed storage
- Keep doors closed,
especially when %RH and
vapor pressure is high.
- warm air can be dry
- cool air can be humid
Thank you!
• Questions?
• Comments?
– Bill Smith
– wbsmith@esf.edu
– 315/470-6832
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