Question
Is there any good rule in regards to thawing frozen wood before kiln drying it? My first impression is that I should heat up wood quite slow to avoid evaporation of superficial water leaving a block of ice inside wood.
Forum Responses
(Commercial Kiln Drying Forum)
From Gene Wengert, forum technical advisor:
With white woods you must be certain to use the correct RH no matter what the temperature. There is no information in any drying texts about thawing the wood. It is not done and has not been done.
There are two (similar) reasons. One is that if you start drying frozen wood with ice inside you risk overdrying the surface while keeping ice inside. With fast drying schedules I suppose there is water going from the wood core toward the outside. If it is frozen water can't move. The other reason is that inside wood water is tough to follow regarding the colder zone (because of lower pressure). This makes it harder to extract. Regarding Contributor P's thought I understand the philosophy - heat up wood but avoid water evaporation. I think that's good, but when did you stop the thawing phase?
I have never experienced any degrade without using a thawing schedule (I do track degrade in all of our lumber), but if you use a thawing schedule, the elements are there for degrade by stain or discoloring in white woods. To prevent stain/discoloration the moisture must be removed rapidly. When drying for profit every hour of kiln time is money and wasting time by thawing is sacrilegious. When deciding what schedule to use consider all of the pros and cons along with your goals, then decide the best schedule. The 1/2 billion board feet that I have dried for profit over the years is proof that Gene is 100% correct.
This will make certain that the core dries at the same rate as a normal schedule. Contributor D, the instrumentation will be correct using a wafer or sock. That's why you see a spike in the chart recorder when the fans reverse, the kilns are recording the actual and adjusting to the desired. This is due to the moisture build up on the exiting side of the lumber and this is why the fans should be reversed every two hours for uniform drying. What I mean by correct instrumentation is that what the actual environment in the kiln is should be what is being displayed. I have seen many operations that have not had all of their kilns calibrated to display what the actual is. This is important because a heat valve may be stuck open and the dry bulb could be off by many degrees or the charts are working and the operator doesn't know what to look for.
"Water will follow the colder zone..." What does this mean? This is meaningless unless you have a non-adiabatic source of coldness internally, which we do not. Someone needs to go back to basic heat and mass transfer in a porous capillary bed to learn what really is going on.
As further proof, in North America, we dry about 7 to 8 billion board feet of wood per year and have dried that amount for many years and thawing is not used. It is also important that one appreciates that water in wood that is called bound water and is rough 0% to 30% MC) never freezes, as it is chemically held water molecules and is therefore not a liquid, vapor or solid form like free water. Also, when free water is in the living tree, does it freeze? Rarely. This is because of the various chemicals in the water. So, when the tree is cut, it is rare to have the water in the lumber freeze, except on the surface. If this internal water did freeze, it would expand greatly and then cause all sorts of internal failures before drying even begins. Yet this does not happen. Again, some basic understanding of water in wood is needed by anyone who claims that there is a block of ice inside the wood. Of course, if the water does not freeze, then it does not need to melt and therefore will warm up rapidly. I suggest that anyone interested in this from a technical standpoint take a class in heat and mass transport phenomena, read Transport Phemomena by Byrd, et al. and also read Water In Wood by Skaar.
2. At about 271ºK (-2ºC) (pure water freezes at 0ºC) most free water inside wood is ice (some variations with species). Moreover an experiment in Romania monitoring wood core temperature shows that warming up from -10ºC to 25 ºC wood boards stay much time near 0ºC: looks pretty like a melting process. "Again, some basic understanding of water in wood is needed by anyone who claims that there is a block of ice inside the wood." Sorry, not a block of ice, many little ice blocks.
3. "If this internal water did freeze, it would expand greatly and then cause all sorts of internal failures before drying even begins. “Inside the wood there is much air: volume of ice goes up without wood damages.”
4. "Water will follow the colder zone..." What does this mean? This is meaningless unless you have a non-adiabatic source of coldness internally, which we do not. Someone needs to go back to basic heat and mass transfer in a porous capillary bed to learn what really is going on." Heating wood creates a temperature gradient from core to outside (ice or not). If you take a Mollier diagram you can easily see that at the same relative humidity lowering temperature you get a lower vapor pressure. That's why with HighFrequency you cut drying times: you heat up wood from inside and water is pushed out by pressure.
2. The article you cite indicates that there is still liquid at - 15C. "Liquid water is apparently present in the stems of trees at even the most extreme winter temperatures based on our TDR measurements."
3. Indeed there is a lot of gas (probably not air) within wood. However, once ice is formed, it is a solid and then it expands in all directions. The "air" in the cells would not absorb the radial and tangential swelling of this sold; only the longitudinal direction would be affected. Consider a hollow tube with some water in it or even a glass of water that is 1/2 full. Freeze it at -1C, and then when frozen, cool it further to -10C and you will see that the glass breaks even though there is no lid on the glass. The ice can expand vertically without damage, but not transversely.
4. Indeed heating wood creates a temperature gradient, but this gradient in most cases is quite small (just a few degrees) with wet wood. Certainly above 100C or with a high mass flow process such as RF drying, you can get different results. But this discussion has been about drying at much lower temperatures (50C or so) and not 120C.
2. At -15C the article you quote reports that over 25% is still liquid (assuming that their interpretation of their data is accurate). In other words, it is an ice/water mixture that we call slush. But they do not say that as you go under -15C that it turns to solid ice. (Your initial posting used the term "block of ice.") Further, the water in the cell wall does not freeze as it is not a liquid, solid, or gas, but is chemically bound water. This means it does not behave like free water. This is based on a basic understand of adsorbed water and is not something I dreamed up.
3. I do indeed know that wood is not glass, but you have ignored the expansion in the transverse direction which will cause internal damage. I was merely using the glass as an analogy. I recall that Erickson at the Univ of MN froze wood at -40 and this then showed internal damage from expansion. The process is called pre-freezing. Perhaps some of the literature on pre-freezing has more information about internal structural damage.
4. If there is a large temperature gradient, with the high conductivity of wood, there will be a lot of heat flowing to the interior to melt any ice, so any gradient will quickly become very small and of little consequence. In conventional drying, the temperature gradient is not the controlling process of drying. Rather it is the mass flow or diffusion gradients that control the process. This is the point of calculating the Lewis number.
Please note that at -40C, the free water will freeze solid, but this is an extreme condition. On very cold days (-35 C or colder) in the woods, with a little wind, you can hear the trees snap as the ice breaks. The stems will then have a crack which is called frost damage.
A dynamic controller (not a schedule) runs according to what the lumber is doing and not by time. A dynamic controller has may advantages some of which are faster drying times, energy savings, and better lumber grade. It is energy in the form of heat that dries lumber. With high humidity more energy can be supplied to the lumber in respect to time. Keeping in mind when humid air is raised in temperature the relative humidity goes down. To bring the relative humidity up you need more water vapor. The lumber adds water vapor to the air but only at the rate water is flowing from the center to the surface of the lumber. This rate is different for all wood species. Since water can only exit lumber from the surface, removing the surface moister from the lumber too quickly will case harden the lumber making it almost impossible to remove the water in the center. Transferring heat to lumber allows the water in the center of the lumber to flow to the surface and exit into the air. A dynamic controller will automatically adjust set points to keep the rate to a maximum limited to the species of lumber being dried. Back to the thawing out frozen lumber question a dynamic control has a thawout function. This thawout function is based on the humidity in the kiln and the outside temperature.
It is true that humid air has more energy, but it does not have more heat. The specific heat of dry air is roughly 1.009 kJ / (kg C). The specific heat of moist air at 40 C is about 1.013. This is not much difference - less than 1%. Understand that most woods, except softwoods, are dried at about 40 C when very wet, which is when there would potentially be ice in the core - the subject of this posting. At 40C, the air at 100% RH would have about 46 grams of water per cubic meter. Dry air is at a density of 1.2 kg / cubic meter. So, although the specific heat of water is four times that of air, the amount of water in air is very small. So, I have trouble with your statement about humid air having much more energy (unless you go to very high temperatures). But even at high temperatures, having more energy does not mean more heat transfer unless there is also mass transfer.
You state that "More energy in the form of heat is carried to the wet lumber in a humid environment." How can this be unless the moisture in the air is condensing on the lumber, releasing the heat of vaporization (or the lumber's surface temperature is below the dew point of the air)?
You state "Heating a kiln up too fast at first can do a lot of damage to the outer cells on the surface of the lumber causing twist and warpage." Although excessive heat can cause damage, the rate of heating does not. That is, heating cold lumber at 0C to 40C quickly or slowly at the same humidity will not make any difference in wood quality. In fact, some kilns actually heat the lumber with saturated steam (over 100C) at first to relieve stresses and obtain flatter lumber. Further, some lumber is steamed before drying (the Elder process or steaming for color enhancement). You state "removing the surface moisture from the lumber too quickly will caseharden the lumber making it almost impossible to remove the water in the center." This slow drying of the core due to casehardening has not been shown to occur. First, casehardening is a condition of stress; actually the technical term for casehardening is tension set. It has nothing to do with moisture movement restrictions. In fact, tension set means that the wood cells are actually stretched out large than if they had been free to shrink. This stretching would seem to encourage moisture flow. Further, when drying Southern pine, for example, the temperature in the kiln will be in excess of 100C within an hour or less. In veneer drying, it is over 100 C within minutes in many cases. There may be casehardening, but the core has no problem drying. Appreciate that in conventional drying, the factor that limits the rate of drying is not heat transfer but moisture loss. In other words, the process is controlled by mass transfer and not heat transfer.
Your idea of using certain temperature and humidity settings to control the drying rate is indeed a good idea and is used quite often in conventional drying. This concept was initially patented by Dallas Dedrick in the 1970’s and was called CRT (Constant Rate Drying). He used sophisticated controllers to achieve CRT. Experience with the CRT drying system resulted in some changes and improvement in drying, so it is not used as pure CRT today in many facilities.
Specific Enthalpy of Moist Air = Specific Enthalpy of Dry Air + Specific Enthalpy of Water Vapor
H=Ha+Hw
Specific Enthalpy of Water Vapor „³ Hw = Cpw T + Hwe
Cpw = specific heat capacity of water vapor (kJ/kg.oC)
T = water vapor temperature (oC)
Hwe = evaporation heat of water at 0oC (kJ/kg)
Specific Enthalpy of Dry Air Ha=CpaT
Ha = CpaT
Cpa = specific heat capacity of air (kJ/kg.oC)
T = air temperature (oC)
You can see that the enthalpy of moist air would be much higher with the above formulas because the specific heat capacities are multiplied by the temperature. Yes my theory does work a lot better at high temperature drying.
You said appreciate that in conventional drying, the factor that limits the rate of drying is not heat transfer but moisture loss - does heat not increase the rate of moisture loss, or even yet the rate of melting solid water to a liquid?
The rate of heat up would cause case hardening when drying at high temperatures like 200(F) (softwood) probably not much at low temperatures 100(F) (hardwood). I do believe how ever that stripping the moist off the surface of lumber to fast will cause the cell fibers to bond very much limiting the water in the center to exit the surface. I am trying to keep on track of the how to dry frozen wood question but I believe that it is important to understand the energy that is used to both dry and thaw the wood.
While this supports Gene's statements I disagree with him on if there can be ice in a tree. The chemicals in sap can act as an antifreeze to just so low of temperature. Also, some of the bound water has more energy than ice therefore it will transfer to ice as the total thermal energy in the tree is decreased. There are hydroxyl sites on bound water to promote hydrogen bonding as in free water and ice. To adjust the volume of water in the tree, trees can allow transporvaporation to occur faster than uptake of water from the roots, thereby reducing the amount of water in the tree. It has been some time since I read Erickson's paper on pre-freezing wood by I don't remember reading about internal damage.
As for heating wood from the center out with high frequency is a misnomer. Most of the high frequency heating energy is absorbed by the outer layer of wet wood. This may be the surface or an inner layer depending on how far one is into the drying process. This type of heating does not "push the water out by pressure. This energy can bypass to some degree the drier outer wood and have the energy absorbed by the outer most layer of wet wood only.
As to casehardening making it almost impossible to remove the water in the center - casehardening has absolutely no effect on the rate water can be transported through wood. If the humidity is too high, water cannot evaporate no matter how easily it has the potential to transfer heat to the wood. If the wet-bulb temperature is the same as the wood surface temperature no heat will transfer. As Gene says no mass transfer - no heat transfer.
