|Home » Forums » Commercial Kiln Drying » Message||Login|
You are not logged in. Consider these WOODWEB Member advantages:
EMC Basic question...12/11
1.an average value of moisture content at the equilibrium (so that the effective MC will be dependent by wood's adsorbing or desorbing behavior)
2. The final moiture content (at the equilibrium) of drying (desorbing) wood.
Looking in internet and in some documentation I can found "weakly" both.
EMC is not a static value but a dynamic value where the number of water molecules vaporizing equals the number of molecules condensing.
your #2 definition is the standard definition. The standard curve used is from sitka spruce. The actual MC of the object in hand depends on species, any treatment it has recieved(heat or chemical) and MC history. Hysteresis describes how the MC is dependent on the MC history. The percent reduction Of the MC do to hysteresis can be as much as 20%. Den gives an explanation of what is happening on the atomic level at the surface of the object.
EMC refers to a condition of the air and not to wood. That is, air has an EMC and not wood. However, wood will eventually lose or gain moisture so that the wood is in equilibrium with its conditions...and that might be called an equilibrium moisture content. But strickly speaking, EMC va,ues in the various tables refers to the air and not to wood. Therefore, your definition #2 is fairly close ...EMC is numercially equal to the moisture content obtained by wood (spruce shavings) in oscillating (slightly varying temperature and humidity as the chamber used was not well controlled) desorption. This defintion and appropriate values of EMC at a given RH and temperature are a fixed values as defined nearly 100 years ago by the US Forest Products Lab (except for a small correction made at higher temperatures in the 1960s).
I have to disagree with Gene. I would not say anything but just the other day a student came to me with a problem concerning his research and his problem was directly related to the difference Gene and I have concerning EMC.
To explain: Consider thermal expansion of materials, take steel and copper , they have different volumetric thermal expansion per lb of material that responds to changes in temperature. Thermal expansion is not a property of the air but of the object in interest. If it was a property of the air the two metals would have the same volumetric thermal expansion value, but they don't.
Wood gains or losses moisture in response to the partial vapor pressure in the air. Just like the metal responded to the temperature of the air. The chemicals in wood attract the water molecules while the water molecules try to escape the attraction of the wood. This occurs while the amount of water vapor in the air creates a pressure that forces the water on to the wood. If the water has enough thermal energy to overcome the woods attractive forces and the pressure of the vapor in the air, it then become vapor and goes into the air.
Because different pieces of wood attain different MC at the same partial vapor pressure, standard mechanical testing procedures stipulate a specific temperature and RH not a specific EMC.
When Den vacumn dries wood and reduces the pressure of the air around the wood he reduces the partial vapor pressure at the same time, enabling the water to escape from the wood at a lower thermal energy as well as creating a bulk pressure difference between the internal wood and air, allowing mass flow of water to occur.
EMC is a defined term. It is not a property, like thermal expansion. It is important to avoid confusing properties and defitions. The fact that a single EMC value does not represent the behavior of wood in all cases has nothing to do with the definition.
The definition of EMC and the exact number value associated with a given temperature and RH were determined almost 100 years ago. There are not a bunch of EMC values for different conditions or different species or wood types or whether the wood is adsorbing or desorbing. Certainly any piece of wood has a unique moisture content at equilibrium. But the definition of EMC does not change from the value given 100 years ago. Again, EMC is not a basic property of wood; it is a definition of some measured values that refer to air.
Your comment about wood testing in the 2nd paragraph from the bottom: if you have an EMC in the air, you can expect a certain moisture condition in the wood, give or take depending on history, the wood itself and so on. To achieve uniformity amongst various testing labs, one can specify a temperature and an EMC. For example, one could say 80 F and 65% RH, or 80 F and 12% EMC. Certainly temperature is important when preparing samples for testing. Of course, when one tests samples that have been equilibrated at 12% EMC, the actual MCs are varaible. How to accomodate such variable MCs at equilibrium when testing is discussed in the standard. Incidentally, Bruce Heebink of the US Forest Products Lab and the world's authority on plywood and veneer at the time, wrote a paper on this EMC and RH subject. He proposed that it would be better to define an "Equilibrium RH" or ERH. EMC values, based on first drying of spruce shavings etc., did not provide good MC estimates for plywood or veneer, and therefore were misleading.
Your last paragraph: Appreciate however that there is no definition of EMC at any pressure condition other than atmospheric pressure, under 212 F. Someone could indeed develop EMC values at other total pressures.
I am not sure what you mean by having water escape from wood at a lower thermal energy in a vacuum. Since this is not taking about EMC, maybe you (or Den, if he agrees) could start a new thread about this and educate me (or us).
I think that 'lower kinetic energy' is what Jim meant instead of 'lower thermal energy'. But there is a direct correlation between temperature and kinetic energy.
If you use 110'F in a conventional kiln, you can expect the wood to be close to 110'F. If I use heating water that is 110'F in a vac kiln and the chamber pressure is 50 Torr (50mmHG), I'll find the wood temperature to be closer to 100'F. The energy that I am adding is leaving with the vapor.
If you increase that conventional kiln to 120'F, you can expect your wood to increase to 120'F. If I increase water temperature to 120'F and other parameters are held constant, my drying rate will increase but the wood temperature will stay near 100'F.
Den, you are right. I was referring to kenetic energy. Not everyone realizes that heat is kenetic energy. Most people think kenetic energy is only an object in motion like a baseball being thrown. But heat is motion of the molecules. So I described it so everyone can understand.
Gene, I never said both situations would be at the same temperature. As the pressure is reduced and the wood will lose water and cool down - lossing kenetic energy. But since it is exposed to a lower pressure it will continue lossing water for a longer time. If you heat the wood up to maintain the temperature, the water in the wood will have a greater partial pressure than the "air" and evaporate at a faster rate.
EMC is a condition of the air, not the wood.