EMC What is it and Where Did it Come From? - ESF Home EMC What is it and Where Did it Come From? Peter Garrahan NEKDA – October 30-31, 2007

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EMCWhat is it and Where Didit Come From?

Peter GarrahanNEKDA – October 30-31, 2007

OVERVIEW OF PRESENTATION

• Basic Wood Science

• Investigative Work on Origins and Derivation of Tables

• Current Situation and Significance of EMC Tables

EMC - Equilibrium Moisture Content

EQUILIBRIUM MOISTURE CONTENT


In relation to the environment around

MC usually defined based on oven-dry basis

EMC


• Is it a property of the air?

• Is it a property of the material?

Some Basics


• Wood (and other organic materials) is hygroscopic

• At any RH > 0% wood will have some moisture present

• Water can exist as “free” water or “bound”water

• Limited space (sites) available for bound water




FSP – Fiber Saturation Point

• Achieved when all sites are full and no free water present.

• Sitka Spruce• FSP = 27%• ranges from 23 to 33% depending on temperature


Drying (i.e. Low RH high MC)


Wetting (i.e. High RH low MC)


In Equilibrium (i.e. @ EMC)

Determining EMC• Hold a piece of wood at fixed RH and temperature

• It will eventually reach equilibrium

• It will stop losing or gaining weight

• It’s MC will now be equal to the EMC


Factors Affecting EMC

• Wood Species• Wood Temperature• Drying Temperature (air dry vs. KD)• Drying vs. Wetting• Heartwood vs. Sapwood


Species Effect on EMC


@ 50º F. & 75% RH

• Highest EMC = 15.7%

• Lowest EMC = 11.6%

Wood Temperature Effect on EMC


@ 50º F. & 75% RH - EMC = 14.5%

@ 150º F. & 75% RH – EMC = 12.0%

Drying Temperature Effect on EMC


• Wood dried at high temperature will equalize at a lower EMC

• Thermal modification process results in better dimensional stabilization.


Effect of Drying vs. Wetting on EMC

- hysterisis effect

Effect of Heartwood and Sapwood on EMC


• Effect of chemical constituents (extractives)

• Lower EMC values for heartwood

U.S. FPL Tables on EMC


Work by Stamm and Loughborough

• Exclusively on Sitka Spruce - why?

• Relatively small number of samples equalized over a range of temperature and relative humidity

• Resulted in graphic representation



1949 Keylwerth

• Extrapolated Data to Temperatures Over Boiling

• Extended range of data produced by Stamm



By the 1950’s we had EMC Tables That Were in the Wood Handbook and the Dry Kiln Operator’s Manual

• These tables were generated by visual inspection of the graphs to look up values at various intersection points of DB temperature and RH or WB


Starting in the 1970’s Bill Simpson of the U.S. FPL analyzed data in EMC tables to develop a mathematical model:



By 2007 Peter Comes Along and Needs to Find an EMC Table for his new Kiln Operator Manual

Which one to use:

• U.S. original version?

• U.S. version with Hi Temp data?

• U.S. version determined by Simpson’s model?

• Canadian version of tables from 1940’s?

Purpose of EMC Table in a Kiln Manual

• Compare drying conditions• Relate one schedule step to another• Establish potential for MC gradient


In kiln drying we only use EMC as a measure of the relative ability of the air to dry the wood further


Different tables in different manuals only creates confusion.

More important to standardize with a specific table rather than try to use species specific values


• New tables are based on Stamm’s work and as expressed mathematically by Simpson

• Tables have been compressed to cover range of temperature and depression on a single page

• Version of table in both Fahrenheit and Celsius

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Thank You

Documents

EMC What is it and Where Did it Come From? - ESF Home EMC What is it and Where Did it Come From? Peter Garrahan NEKDA – October 30-31, 2007