Turbulent Air Flow as a Factor in Kiln-Drying Operational Control

      The Wood Doctor explains why controlling humidity and temperature, not managing airflow turbulence, is the way to maintain optimum drying conditions in the kiln. June 13, 2014

Question
Does anyone know if there is any drying control system (with sensors) that can control air speed between timber while drying? I think this (the air speed in lumber stack) can be more important than the cfm of vents. The reason is that evaporation ratio depends on the air flow type and speed, more than its overall cfm value. With a low air speed in a lumber stack the flow is laminar, this means that near the wood surface air speed is near zero. Increasing air speed make the flow to become turbulent. The turbulence keeps high air speed on wood surface and generates hi-low pressure zones that mechanically extracts water. So I think that the best air speed is probably the one that guarantees turbulent flow. I read a book that told ideal drying air speed should be about 3-4 feet/second for a stack with about 1' spaced stack. I have no experiments data to confirm this. Since I'm quite new to the wood drying field I'd like your comments very much.

Forum Responses
(Commercial Kiln Drying Forum)
From contributor W:
Air flow is certainly important. The depth of the load, moisture content, accuracy of controls, sticker spacing are all factors. Water is not removed "mechanically" but by evaporation and that requires heat transfer. Turbulent flow allows better heat transfer than laminar flow. Whether you need it or not depends on several factors. Generally kiln designers plan air flow based on product. As controls have improved, air flow has increased, especially for green lumber for example.



From Gene Wengert, forum technical advisor:
You need to consider what offers the greatest resistance to drying of lumber. Except when the wood is really wet, the resistance that controls the drying rate is the resistance within the wood and not the boundary layer resistance. Hence, changes in the boundary layer as a result of air speed, laminar to turbulent, and so on are of no importance. Incidentally, the ideal flow rate is four-ten feet per second for most hardwoods and softwoods are often dried at 20 feet per second.

You will also find that the surface roughness of lumber and the bumps between pieces result in no laminar flow in a dryer. It is always turbulent or in transition. If you are really interested in this subject, you can look at flow in a rectangular channel. Again, the boundary layer is not what is controlling the rate of drying (except when really wet). Your concept of mechanical removal is not correct. In short, drying rate (or evaporation) for lumber depends mainly on temperature and RH. In fact, kiln schedules give only those two values and not air flow. Finally, we do indeed have variable speed fans with a range of zero-ten feet per second. They are used at lower speeds to save energy, as low or fast, we get the same drying rate (at lower MC’s).



From the original questioner:
I explained the mechanical effect of air flow in a bad way. I’ll try to explain better what I mean. Sure water leaves wood by evaporation – the speed of evaporation I think is a function of:

-Temperature of the air
-Temperature of the wood
-Humidity of the air
-Pressure of the air inside and outside of the wood
-Humidity of the wood and its distribution
-Structure of the wood

All these parameters (and probably others) should give the vapor pressure gradient. When the flow is turbulent we have moving low and high pressure zones inside the flow. This causes pressure variations on the wood surface. By theory it could effectively extract water from the wood caused by the different pressure inside and outside of the wood. Anyway since its value is probably very low this effect is very small.

Moving air particles in a circular turbulent way guarantees an air exchange on the wood surface. This exchange causes a better heat transfer and new fresh unsaturated air on the surface.

If the flow is laminar we have no new air on the surface so it can easily saturate and slow down very much with evaporation. I think this effect is very important.

In case of high moisture content, I think that with high air flow speeds (as Gene indicated) we have faster evaporation. The reason can be the mechanic hurting of air-water particles that transfers kinetic energy.

It's a little like after a rainy day. There is a big difference in the time roads dry out if there is wind or not. Anyway, Gene, if the air flow is always turbulent my considerations are not important and the boundary layer do not affect so much the drying rate. If it is laminar I think there is a visible slow down. So, is the ideal air flow speed the smallest that guarantees turbulent flow? I'll do some experiments in a little wind tunnel to check entity of pressure variations on surfaces at different speeds.



From Gene Wengert, forum technical advisor:
If the air is laminar and the drying slows down, just lower the RH to increase the drying rate. Note that the vapor pressure gradient in conventional drying is very small and is represented by the difference between 100% RH or less (in the wood) and the outside RH. It is this resistance to vapor flow within the wood that slows drying and not the boundary layer. Roads dry out because the water is on the surface.


From the original questioner:
If air flow is laminar and the drying slows down it could be better to increase the air flow (to get turbulence) than lowering the RH. Usually, as you know, in conventional kiln drying lowering the RH causes heat lost. Anyway, if heating energy is less expensive than vents' electricity probably lowering the RH would be more economic.

There is another thing, most important for quality to consider. When developing automatic drying schedules based on DG (drying gradient) DG equals wood moisture/EMC. It is important to know very well the RH (so EMC). If the flow is laminar your sensor measures something different (dryer) from real RH around wood. This is more evident if you are using electronic sensor or special cellulose strips (as our, very fast and precise) to measure RH. Sure, the main water resistance to evaporate is due to the wood internal structure. Anyway the outside RH value is fundamental. In fact we control drying speed by temperature and RH (or EMC) value.

As I mentioned in my last post roads dry out because water is superficial, and (as Gene too indicated) wind can speed up this process because the mechanical hurting of water-air particles.

A problem in my experiment is that I was thinking to measure turbulence in a little wind tunnel using a hot wire anemometer and a fast pressure sensor as a turbulence detector. The most sensitive pressure sensor I found has a sensibility of about 1" H2O (about 250Pa), and I think that is not enough.



From Gene Wengert, forum technical advisor:
I stated before that laminar or turbulent air flow does not matter except for very wet wood. I also stated that the air flow is turbulent due to roughness. All this is predicted by flow numbers such as the Reynolds number. Water does not move due to mechanical removal. You seem to think that the drying process is controlled by mass flow. It is not. It is controlled by vapor pressure gradients. I do not believe that you can use DG equals MC/EMC for lumber drying. Due to mixing of the air, the standard RH measurement is just fine. We have been using this technique without problems for over 100 years.


From the original questioner:
I don't think laminar or turbulent flow matters. The main reason is as I stated above with a laminar flow you have high humidity value around the wood. This can slow down. Drying (sure you can lower the kiln RH to get the same drying speed but this would mean a heat loss). Moreover with turbulence you have still a faster heat transfer from air to wood. I'm going to be quite sure that the best air flow speed is the smallest that guarantees turbulent flow. This is probably notable until the center of the wood reaches the fiber saturation point. Only experiments can really confirm this. It will be hard but I'll try to do them.

Gene has made me think about roughness. After doing a little search I think that the small absolute roughness (about 2000 feet) of sawed wood will not affect low speed flows. Gene, can you give me some other info for this topic? I'd like to know more on this field.

We stated above that the evaporation rate of non-superficial water is function of vapor pressure gradient. I can't understand what you mean with my thinking of mass flow.

About the drying gradient I'm really surprised. In the plant where my friends work there are two types of electronic control systems, one German (very old, about 20 years) and one Italian (modern). In both types schedules for drying are based on DG equals wood moisture/EMC. With this approach the control system can keep constant the drying speed without any step of temperature/RH the wood doesn't like. In this way is also very easy to experiment different drying speeds.



From Gene Wengert, forum technical advisor:
DG is not used in the USA and Canada and has not been for over 100 years (except for a few experimental units of imported controls). Who says that the wood does not like small step changes? They have been used in the USA for 50 plus years. We dry about 6 to 8 billion board feet every year.

For example, in the very successful technique of drying oak in the USA, when the MC is between 80% MC and 50% MC for 4/4 red oak, the RH is 87%. At 50% MC, the RH is lowered to 84%. So, DG starts at 80/17.5 and goes to 50/17.5 and then 50/16.2 to 40/16.2. So, this gives DG values between 4.6 to 2.9 and then 3.1 to 2.5. When the lumber is around 15% MC down to 7% MC, we would use 3.7 EMC, so the DG is 4.1 to 1.9. Incidentally, MC/EMC will not result in constant drying. The USA technique actually will achieve nearly constant drying in spite of the large DG variations. Wood does not reach the fiber saturation point because the surface reaches that level very early and the core very late. The piece of wood is never at the fiber saturation point. Only a small zone is at that level, or none is at the level when the wood is fairly dry. My favorite book is “Transport Phenomena” by Byrd, Stewart and Lightfoot.

The RH gradient between the lumber and the free stream air is so small it can be ignored, except when the wood is very wet. It becomes important with veneer. What is the RE when drying lumber? I think that you will find it to be about 8000. Incidentally, if laminar flow was a problem, please explain what the problem would be, as the drying time for most species is limited because of the internal stresses and not the transport of heat or moisture to or from the wood. It takes four weeks to dry oak because we cannot have the stresses buildup. The stress buildup is related to the rate of moisture movement (or the gradient) within the lumber. Please refer to a text on drying lumber to understand this more clearly. Again, the resistance to drying is the moisture movement within the wood and not the events at the surface of the wood. So, your idea that laminar and turbulent will affect drying until the core reaches FSP is not at all possible.



From contributor W:
Gene is correct that DG is rarely used in North America. The mills that are concerned with quality generally control the kilns by adjusting conditions to maintain drying rate. If the goal is to dry at 5% per day, for example, then the temperature and humidity are constantly adjusted to maintain that rate. This is often done manually but newer systems are coming on the market that do that automatically. It might be possible to come up with a relationship that does this by adjusting three variables, (temperature, humidity and air velocity) but that seems to be more complicated than necessary for something that will not increase the drying rate or quality.


From contributor M:
In countries with hi cost electric power this kind of control can be good for economic issues. Turbulence is not all: with a big kiln if a 1m/s air speed gives turbulence it may not be enough. The last wood stack would be in a high RH climate. You talk about a special cellulose strip (wafer?) What is it?


From Gene Wengert, forum technical advisor:
Contributor M - why would such a control be good for high cost electricity countries? On the contrary, it will actually cost more. Have you ever heard of the fact that as fan speed is increased the power consumption increases even more - by the cube? Low air flow is an economically desirable event.

Many hardwood lumber dry kilns operate at well under 1 m/s air flow and dry wood nearly as fast as modern kilns with higher air flow when the wood MC is under 30-35% MC. As I have stated above, you can increase air flow when the wood is 25% MC and lower (average MC) and the drying rate will not change at all. Air flow, turbulence, laminar, etc. is not what is controlling the rate of drying. The drying rate of lumber is controlled by RH and temperature, and is limited by the quality that we require. Please read this previous sentence several times to make sure you clearly understand what is really going on.

With big kilns (100,000 BF or about 200 cubic meters) drying hardwoods, there is not a gradient of MC from side to side at the end of the cycle. That is why we reverse the fan direction every two hours. There may be final MC variations due to incoming MC variations or other wood factors. That is why we use equalization.

In softwood kilns, the limiting control variable is often the rate at which heat can be transferred to the lumber. This would be at temperatures over 110 C. Higher air flow assures more uniform drying. Sometimes air flow is 8 m/s. However, the electrical power cost for such high flow is huge and pays only when electricity is at a low price. DG would never be used for such drying anyway. The concepts for drying at these temperatures are totally different than what has been discussed so far.

Regarding the cellulose wafer, it is nothing more than a heavy piece of paper. It will, since the resistance to MC relationship of wood is the same for cellulose as for solid wood, have two electrical contact points and the resistance of this wafer or paper will be measured. (One kiln company actually uses small thin pieces of wood.) It is old technology. I first saw it with Hildebrand kilns 30 years ago or so. The first USA manufacturer I saw was Lignomat. Today, it is used by many companies. One advantage is that you do not need a water supply for a wet-bulb.



From contributor M:
About the RH environment you are right Gene. I forget that with reversing the fan you cancel this problem. I realized it a second after posting. What I meant with the economics maybe is true. The Original Questioner says "the best air flow speed is the smallest that guarantees turbulent flow". With turbulent flow you heat up faster (at the same air temperature).

I was meaning to use the speed measure to keep it as low as possible, saving fans. More: an in-line system that measures air speed in stacks could help me to understand when I place lumber badly in the kiln (so it’s wasting air flow and money). If I place lumber badly I should see high fan speed and low air speed.



From contributor W:
If air flow is to be controlled, it is better to control it on temperature drops across the load as that is a direct measure of both heat transfer and water evaporation. If the temperature drop is high speed up the fans and if it is low slow them down to save money. Trying to make this decision on other factors such as when turbulent flow starts, is indirect and an unnecessary complication. Even basing it on temperature drop has to be determined by experience with a certain kiln arrangement. There are no hard and fast rules that can be applied in every case and no instrumentation that would give a consistent correct answer.


From the original questioner:
Schedules: I'm really not an expert in wood schedules. I can say only that in Europe, many years ago, Keylwerth introduced the DG then people started using it. Maybe there are advantages and disadvantages. I'm going to learn more on schedules as Gene suggests. Is there any paper comparing DG and RH steps?

Contributor W - with the example of 5% you mean MC: 100%-95%, 15%-10%, orr 100%-95%, 15 %-14.25% (the percent of value)? Is there a paper on wood secure drying rates?

Air Flow: The laminar flow can be a problem because (as Contributor M indicated as well) you heat up wood slower (with the same air temp) and RH can be higher on wood surface (I know Gene doesn't trust much on it). It’s interesting about Contributor W’s note about measuring temperature (and RH) in and out of stacks to control speed and RH. It’s also interesting about Contributor M’s note about putting stacks in the kiln the right way

About Reynolds number amount in the kiln I don't know, but I'm quite sure at 0.1 feet/second it is below 2000. Anyway, till now, I didn't find a way to measure flow inline, inside stacks and in a precise way.

Cellulose strips: About cellulose strips what Gene said is correct. There is a constructor (more than one using the same device I think) that uses a threaded piece of paper combined with a particular measurement system so they reach EMC and RH measures near to an electronic sensor. On these strips there is a code but now I don't remember it. My friends have still a graph on paper about a nice comparison between special strips, classical cellulose, and electronic sensor.


Click here for higher quality, full size image


Click here for higher quality, full size image



From Gene Wengert, forum technical advisor:
First, you must note that the pine species mentioned dries extremely rapidly and therefore behaves more like a porous capillary bed than hardwoods. Note that at 50 C, where most hardwoods are dried, the difference is very small. Note that the data for high air flow shows that the temperature of the wood is hotter than the temperature of the air! This is indeed strange. What is causing the wood to get so hot internally? Perhaps this is not good data? If the wood is hotter, then heat will be flowing from the wood into the air!

Note that the temperature difference at 10% MC cannot be supported by theory. At that low MC there is no drying occurring anymore, so the temperature will be at the DB nearly. Also, note that at 110 C the data shows a reversal of the temperature. It does indeed look like bogus data. Please note that data for hardwoods is given in “Drying Hardwood Lumber”. I included two different graphs in my book. Why not study these and see what is actually going on. If you want to study pine drying, then look at my book “Drying Southern Pine Lumber” which has some mathematical equations related to air velocity. Do you know the RE number has a distance term, so even if there is laminar at first, it quickly enters the transition zone? This is true especially at higher air speeds that pine is dried at.



From the original questioner:
I still note some apparent major and minor bugs in the last picture data (the fourth graph I think has traces inverted). Maybe there is some explanation in the paper but it is in Brazilian. I invited the author to give some explanation of the strange data you highlighted. Moreover I can't deeply understand the temperature moisture relation. It was only to highlight some pure differences velocities. I trust more on the first pic. About RH numbers, I hope to be able soon to do experiments in a kiln to see how nature is.


From contributor N:
We have a direct fired 126 ' kiln that has center heat risers which we dry soft wood lumber in. We have a variation in MC from top to bottom in our loads with the bottom loads wetter than the top loads down the entire length on both tracks. Roof baffles and end baffles are in place and the heat supply duct is at one end of the kiln.


From Gene Wengert, forum technical advisor:
Do you also have baffles on the side walls that start at near the top and go partly down the wall? Their purpose is to direct more air flow to the bottom stacks than what would happen naturally. Also, we frequently have a flat baffle between the packs so that the air is encouraged not to go upward when leaving one track and going toward the next one. Bottom line: air flow issues are causing your MC variation.



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