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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 % •EMC % –90 –20 –80 –16 –65 –12 –50 –9 –30 –6 –0 –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 – [email protected] – 315/470-6832
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