Drying Softwoods for Value Added Markets
Drying Softwoods for Value Added Markets
Professor and Wood Products Extension Specialist
North Carolina State University
There is a great opportunity for softwood mills to enter into the value added markets arena, including lumber used for moulding, doors, windows, and furniture. However, to be successful in this market, quality drying is a prerequisite. That is, the lumber must be dried to a customer’s target moisture content with little variability in the final moisture content. Also, in some cases the lumber needs to be stress relieved. In this paper, suggestions on how to select a proper target moisture content, conventional temperature schedules for quality drying, equalizing for a uniform final moisture content, and conditioning for stress relief will be discussed.
Why The Concern
Why the concern over final moisture content? Small changes in moisture content can add up to big problems for re-manufacturers. An example is a manufacturer making a table top that is 30 inches wide. Wood will lose moisture until it comes into equilibrium with its environment, that is the temperature and relative humidity of the surrounding air. If the table manufacturer uses 15 percent moisture content lumber and the table top eventually dries down to 8 percent moisture content, the top will shrink over one-half of an inch. You can imagine the problems created by this shrinkage. There are similar examples in moulding, doors, and windows. What happens when a window manufacturer uses lumber that is too wet to make windows? As the lumber dries in use to the equilibrium moisture content, the wood warps resulting in a window that is difficult to open or close. The problem of wood shrinkage and swelling due to not drying the lumber to the target moisture content is aggravated by juvenile and compression wood. In both juvenile wood and compression wood, longitudinal shrinkage is much greater than in normal wood. The only way of avoiding these problems is to dry wood to the moisture content that it will come into equilibrium with when it is in use.
Relieving drying stresses is critical if wood is to be machined after drying. If the wood is not stress relieved prior to machining, the stresses in the lumber will cause the parts to move during machining. This results in moulding blanks that are warped, window and door parts that do not fit, and panel parts that will not glue properly.
Today’s manufacturers seek to consistently manufacture products of good quality. If their raw material does not let them manufacture a good quality product on a consistent basis, you can expect them to change their raw material. One must also consider that the closer a manufacturer is to the final consumer of a product, the more expensive quality problems become that are not detected early in the manufacturing process. Let’s get back to our window example. What happens when the window warps and can not be easily opened or closed? The cost of this quality failure is not the cost of equalizing the lumber in the kiln longer, but rather the cost of replacing the defective windows in service. The window manufacturer will have a long memory when it comes to a raw material problem.
If you are planning to enter the value added market for softwoods, proper drying can not be over emphasized!
Know Where To Start
A good place to start when drying softwoods for value added markets is to read the literature and visit other operations. I have included three tables in the appendix that were extracted from the Dry Kiln Operators Manual. Table A1 is the specific gravity of various softwoods found in North America. Woods with similar specific gravity often dry in a similar fashion. Table A2 lists the heartwood and sapwood moisture content for various softwood species. What one can see from this table is that sapwood of most softwood species is very wet. When designing a kiln schedule this should be taken into account. Also, the heartwood of many of the softwood species is much dryer than the sapwood. If a uniform final moisture content is expected, we will have to equalize the lumber in order to avoid over drying the dry boards or having wet boards.
Table A3 lists the common drying defects encountered in drying different softwood species. This table is very handy for identifying specific problems and gives you some clues for modifying schedules in order to avoid some of the listed problems.
Two species that I will discuss in detail are Southern pine (loblolly) and Eastern white pine. From Table A2 you can see that the heartwood moisture content for loblolly pine is 33 percent while the sapwood moisture content is 110 percent. From studies that I have conducted, the moisture content ranged from approximately 40 percent to 140 percent. Common drying defects for loblolly pine include brown sapwood stain, checks, and splits attributed in part to excessive drying temperatures (Table A3). Experience has shown that brown stain is usually not a problem in thinner stock. Checks are caused by high initial wet bulb depressions. Splits are often caused by over drying, and are usually amplified when the over dried, often cupped lumber is run through a planer. Considering the split problem and high variability in initial moisture equalizing is critical.
For Eastern white pine, the sapwood moisture content is listed as 175 percent while the heartwood moisture content is 50 percent. Common defects for Eastern white pine are brown stain and ring failure, both attributed to wetwood. Brown stain is termed a chemical stain. Studies of Eastern white pine show that one of the largest factors in eliminating brown stain is moving the felled tree through the lumber manufacturing process into the dry kiln as rapidly as possible. Other contributing factors to brown stain are high ambient temperatures and relative humidity and high initial kiln temperatures and relative humidities. Wet pockets are also a common kiln defect in drying Eastern white pine which can be eliminated by a long equalization period. Over drying becomes a problem when equalizing at too low an equilibrium moisture content (EMC). Over drying is manifested at the planer in the form of splits.
Know Where You Want To Go
The target moisture content is the average moisture content one hopes to achieve at the end of a kiln charge. The target is going to depend on the type of end product that will be manufactured from the lumber and the environment in which the product will be used. In general, the closer to the 10 to 12 percent moisture content range, the better the machining with softwoods. The EMC inside a house may range from 4 to 12 percent depending on the location (the desert versus the coast) and season (winter versus summer). Some processes, such as radio frequency gluing, are extremely difficult with lumber over 12 percent moisture. Another important quality characteristic in many secondary operations is that the pitch is fixed, that is, all of the volatile resins should be driven from the wood.
The decision to stress relieve lumber is based on whether the wood will be machined after drying. If the lumber is to be ripped, run through a moulder or glued, it should probably be stress relieved during drying. Suggested target moisture contents for various softwood products are given below.
|PRODUCT||TARGET MOISTURE CONTENT||CONDITIONING REQUIRED|
|Furniture Stock||7 or 8%||Yes|
A conventional kiln schedule can be divided into four stages. In the initial stage, a small wet bulb depression is used to prevent surface checking while a moisture gradient is established. To prevent stain and mold from developing on the lumber in the kiln, it is important to get the dry bulb temperature above 130 degrees Fahrenheit. During the second stage, drying is sped up by raising the dry bulb and increasing the wet bulb depression. The third stage is equalization which prevents the further drying of the dry pieces while letting the wet pieces dry down to the target moisture content. Many drying manuals separate the drying of the lumber from equalization. That is, the dry bulb is raised and the EMC is lowered throughout the drying schedule. Once the driest part of the charge is dry, equalizing is initiated. Because the moisture content in many pine kilns is not accurately tracked, we use time schedules versus schedules controlled by moisture content. It may be better to never go lower than an EMC that would be used during equalizing throughout the entire kiln schedule. This will be illustrated later on. The last stage of drying is conditioning the lumber to obtain stress relief.
Equalizing And Conditioning
The next step in the drying process is equalization. Equalization is a method of preventing over drying and for bringing all of the pieces of lumber in a kiln charge to a nearly uniform moisture content. It is the surest way to reduce the variability in final moisture content between boards in a charge. Equalization is usually done near the end of the drying cycle, and is accomplished by adjusting the kiln operating conditions to achieve an EMC that is 2 or 3 percentage points below the target moisture content. Whether you use an EMC 2 or 3 below the target depends on how closely grouped you want the final moisture content of the lumber to be. For extremely uniform final moisture content use 2 percentage points below your target, however, the time to dry the remaining wet boards will be slightly extended. If you can live with a little more variability and need faster drying times, use an EMC 3 percentage points below the target. The kiln is maintained at these conditions until the moisture content of the wettest board reaches the target moisture content.
The tables below present equalizing data for six different target moisture contents. To use the table, find the line corresponding to the final desired moisture content. The second column gives the moisture content that the driest sample should reach before the equalization process is started. During equalization, kiln operating conditions should be adjusted to achieve the EMC given by the third column. The fourth column shows when the equalization process can be terminated based on the moisture content of the wettest board.
To fine tune equalizing, a kiln operator will have to get some feedback on the final moisture content distribution. In-line moisture meters are a great tool for this process. Equalizing time will depend on factors such as the length of air travel in a kiln before reheating and air flow.
Desired Equalizing Conditions for Various Target Moisture Contents (Target – 2 percentage points)
Desired Equalizing Conditions for Various Target Moisture Contents (Target – 3 percentage points)
The table below gives the wet bulb temperature needed to achieve an equilibrium moisture content for equalizing from 4 to 11 percent at various dry bulb temperatures. For example, if the desired EMC is 8 percent, and the dry bulb setting is at 180 degrees Fahrenheit, a wet bulb of 161 degrees would be needed. In terms of speed it is best to equalize at the highest temperature possible. Looking at the EMC tables for dry bulbs over 212 degrees one can see that above the boiling point one can not obtain higher EMC’s required for equalizing and conditioning.
Wet Bulb Temperature Needed to Achieve Specific EMC Using a Given Dry Bulb
|Dry BulbTemperature||——— Equilibrium Moisture Content (percent) ———|
As mentioned above, equalizing through the entire kiln schedule is not a bad idea. That is, the EMC in the kiln schedule should never be more than three percentage points below the target moisture content.
The final step in kiln drying is conditioning. The purpose of conditioning is to relieve the stresses induced by drying (to relieve case hardening) so that the wood will not warp when the lumber is resawn or non-uniformly machined. This step is extremely important if consistent results are to be obtained in the re-manufacturing plant. To properly relieve drying stresses or condition the lumber, the lumber first must be well equalized. If it is not well equalized, the results of conditioning will be sporadic. Like equalization, conditioning is accomplished by controlling the EMC of the kiln. The EMC required for conditioning is 3 percentage points higher than the target moisture content. This will raise the moisture content of the outside shell of the pieces of lumber. When lumber is properly conditioned, one can expect that the average board moisture content will be increased by 1 to 1 ½ percentage points compared to the moisture content prior to conditioning. For example, if the target moisture content for the lumber is 9 percent, the desired EMC for conditioning is 12 percent.
The table below gives the wet bulb temperature to achieve EMC’s of 10 to 14 percent that are commonly used in conditioning.
Wet Bulb Temperature Needed to Achieve Specific EMC’s Used in Conditioning
|————- Equilibrium Moisture Content (percent) ————-|
When conditioning lumber a very common occurrence is the dry bulb temperature inside the kiln rises. This rise in the dry bulb will alter the actual EMC versus the desired EMC. In other words, the actual EMC will be lower than the desired EMC and conditioning will not occur. It is important that the kiln operator remember the actual dry and wet bulb dictate the conditions in the kiln. If one has a difficult time obtaining a high EMC, one method of trying to obtain a higher EMC is to let the kiln cool down prior to conditioning for several hours. Then just turn on the steam spray by raising only the wet bulb on the control. Observe the controller and see what EMC the kiln conditions achieve.
Conditioning is completed only when the drying stresses are relieved. Conditioning may take from 4 to 8 hours for 4/4 softwood lumber depending on the amount of stress and how quickly the desired conditions are obtained in the kiln. When boards are being conditioned, the moisture content of the surface of the lumber will be about 3 percentage points above the core of the lumber. To determine if the lumber is conditioned, stress samples should be cut. The thickness of the prongs should be 1/4 the thickness of the lumber. The stress samples should be evaluated several hours after they are cut. If after several hours the prongs are standing straight up, the lumber is properly conditioned. If the prongs are pinched together, the lumber still contains stress. If the prongs are spread apart, reverse case hardening has occurred. Reverse case hardening occurs when the EMC used in conditioning is much higher than the moisture content of the lumber. This is one of the reasons having the lumber well equalized prior to conditioning is critical.
A basic conventional temperature schedule for Southern pine for the first two stages in drying is listed below. One item to note in this basic schedule is the low equilibrium moisture content (EMC’s) at the end of the schedule. The schedule is also time based, not based on the moisture content of the wood or drying rate.
Conventional Kiln Schedule for up to 5/4 Heavy Southern Pine Lumber
|1 to 24||165||150||9.7|
|24 to 48||170||150||7.8|
|48 to 72||180||150||5.3|
|72 to 96||185||150||4.6|
Before the above schedule can be used it should be modified or adapted to a specific product and/or kiln. Some kiln operators have found the schedule a little too harsh – that is, it produces too many checks in the beginning. To correct this problem, they operate their kiln with a 160 degree dry bulb for the first 12 hours and 150 degree wet bulb temperature. This slight modification lowers the EMC and thus slows down the rate of drying in the first step of the drying process.
Modified Conventional Kiln Schedule for up to 5/4 Heavy Southern Pine Lumber
With a Target Moisture Content of Ten Percent
|HOURS INKILN||DRY BULBTEMPERATURE(F)||WET BULBTEMPERATURE(F)||EMC(%)|
|1 to 12||160||150||12.3|
|12 to 24||165||150||9.7|
|24 to 48||170||150||7.8|
|48 to 72||180||161||7.9|
|72 to 96||185||166||7.9|
The second modification may be to raise the EMC at the end of the schedule, between 48 and 96 hours, in order to insure that you do not over dry any of the pieces of the lumber in the charge. This is especially important if you are targeting a higher final moisture content, for example ten percent. In this case the wet bulb in the final two steps may be raised so that the EMC for these steps is no lower than 2 or 3 percentage points below the target, or 7.8 percent in our example. This will insure the lumber will not be over dried. In effect we are equalizing the lumber during the second stage of drying. One reason this is recommended is that, unlike hardwood dry kiln schedules in which we adjust the schedule based on the moisture content of lumber, pine schedules are based on time because of their short duration.
Based on specific drying conditions, such as actual thickness and air flow in the kiln, the length of the schedule’s steps can be modified. The exact schedule is based on experimentation, namely, how long it takes to dry a specific product in a specific kiln. Setting the pitch is another concern. Pitch is set by vaporizing the volatile oils and resins in the lumber by exposing it to high temperature. The setting of pitch by exposing the lumber to high temperature is also time dependent. This can be illustrated by high temperature dried lumber still exuding pitch after drying. Thus, the lumber must reach sufficient temperature to set the pitch. Finishing with a dry bulb temperature of 160 degrees Fahrenheit is usually sufficient when drying thinner stock in a conventional schedule to set the pitch.
In recent years Eastern white pine has become a player in the millwork market. A marketing representative asked me to assist one of their suppliers in drying Eastern white pine, since much of the lumber being produced by their supplier was over dried and split when surfaced. I visited the operation and saw that the kiln operator knew what he was doing and his kiln was in good working order. The kiln schedule being used when the splits occurred is listed below. The target used by the operator was 9.5 percent.
Original Schedule1 Followed by the Kiln Operator Resulting in a High Percentage of Planer Splits
|2||100 to 85||120||105||9.6|
|3||85 to 60||120||100||7.9|
|4||60 to 40||130||105||6.7|
|5||45 to 30||130||100||5.7|
|6||30 to 25||140||105||4.9|
|7||25 to 20||150||115||5.0|
|8||20 to 15||160||125||5.1|
|9||15 to Final||180||152||6.0|
|Equalize and condition as necessary.|
1 Extracted from Dry Kiln Schedules For Commercial Wood
2 Spray off, vents working
The basic white pine schedule starts at a very low temperature and a relatively large wet bulb depression in order to rapidly remove the free water and not brown stain the lumber. As you can see, the last half of the schedule is very severe in terms of a low EMC. Considering the moisture content of the heartwood was 50 percent and the sapwood was 175 percent by Table A2, much of the heartwood was dry halfway through the schedule. The schedule was modified by raising the EMC to 7.5 during the last half of drying.
Modified Eastern White Pine Schedule for a 9.5 Percent Target Moisture Content
|2||100 to 85||120||105||9.6|
|3||85 to 60||120||100||7.9|
|4||60 to 40||130||105||6.7|
|5||45 to 30||130||108||7.5|
|6||30 to 25||140||118||7.5|
|7||25 to 20||150||128||7.5|
|8||20 to 15||160||138||7.5|
|9||15 to Final||180||158||7.5|
|Condition at a dry bulb of 180oF and a wet bulb of 172 to 173oF.|
1 Spray off, vents working
By using the above modified schedule, planer splits were reduced from 12 percent down to 0.4 percent. The moisture content was tightened up from a standard deviation of 2.7 percent to a standard deviation of 1.1. Part of the success in the above schedule can be attributed to an outstanding operator. He realized by raising the EMC towards the end of the schedule it would slow down his drying and he would have to adjust the length of time the lumber dried.
Using the basic softwood kiln schedules and an understanding of equalizing and conditioning, stress free lumber with a very uniform final moisture content can be produced. These basic principles can be extended to other softwood species to produce lumber for the value added market.
Table A1. Specific gravity of softwoods
Table A2. Average moisture1 content for softwoods
|Moisture content (percent)|
|Rocky Mountain type||30||112||43|
‘Based on ovendry weight.
Table A3. Common drying defects in U.S. softwood lumber species (extracted from Dry Kiln Operators Manual)
|Species||Drying defect||Contributing factor|
|Old growth||End checks, water pockets||Refractory wood, extractive|
|Young growth||Chemical brown stain||Wood extractive, poor air circulation|
|Alaskan yellow||Resin exudate||Extractive|
|Eastem redcedar||Knot checks, excessive loss of aromatic oils||Excessive drying temperatures|
|– Heavy stock||Water pockets, collapse||Wetwood, excessive drying temperatures|
|– Heavy stock||Uneven moisture content, collapse, honeycomb, chemical stains, iron stains, resin exudate||Wetwood (sinker stock), extractive|
|Coastal||Red-brown chemical stains||Wood extractive|
|Gray sapwood stains||Sapwood extractive, slow drying|
|Ring failure, honeycomb||Wetwood (infrequent occurrence)|
|Balsam||Uneven moisture content||Wetwood|
|California red||Uneven moisture content, shake, splits, warp||Wetwood, compression wood|
|Grand||Uneven moisture content, shake, splits||Wetwood|
|Pacific silver||Uneven moisture content, shake, splits, chemical brown stains||Wetwood|
|White||Uneven moisture content, shake, splits, chemical brown stains||Wetwood|
|Subalpine||Uneven moisture content, shake, splits||Wetwood, compression wood|
|Noble||Warp, splits||Wetwood, compression wood|
|Eastern||Uneven moisture content, warp, ring shake||Wetwood, compression wood|
|Western||Uneven moisture content, warp, chemical stains, shake, iron stains||Wetwood|
|Western||Shake (ring failure, checks, resin exudate)||Wetwood|
|Eastern white||Brown stain, ring failure||Wetwood|
|Western white||Brown stain||Wetwood|
|Ponderosa||Brown stain||Wetwood (less common in ponderosa pine than in the soft pines)|
|– Young growth||Warp||Juvenile wood, compression wood|
|Loblolly||Brown sapwood stain, checks, splits||Excessive drying temperatures|
|Longleaf||Brown sapwood stain, checks, splits||Excessive drying temperatures|
|Shortleaf||Brown sapwood stain, checks, splits||Excessive drying temperatures|
|Slash||Brown sapwood stain, checks, splits||Excessive drying temperatures|
|Virginia||Brown sapwood stain, checks, splits||Excessive drying temperatures|
|Pond||Water pockets, dark chemical stains, honeycomb||Wetwood (infrequent occurrence)|
|Heavy stock||Uneven moisture content, collapse, honeycomb, chemical stains, iron stains||Wetwood (usually in old growth)|
|White||Water pockets, collapse, ring failure||Wetwood (rare occurrence in northern and southern limits of botanical range)|
|Young growth||Checks, splits, raised grain||Fast growth juvenile wood|