Jaybird Manufacturing - Conditioning Artical

Techniques for Equalizing and Conditioning Lumber
by Eugene M. Wengert
Forestry Facts, November 1992

When drying lumber that will be used in furniture, cabinets, millwork, and the like, the final two steps of the drying process are equalizing and conditioning. Equalizing is a procedure to equalize the moisture content (MC) from piece to piece, as well as equalize the MC within individual pieces. For most hardwood lumber uses, the maximum MC variation within a piece and the maximum from piece to piece cannot exceed +/-1%. Conditioning is a procedure to relieve the drying stresses (sometimes called tension set or casehardening) in the lumber, so that when the lumber is ripped, resawn, or heavily machined, it will not warp. It is important to note that unless lumber has been equalized properly, it cannot be conditioned uniformly.

Oftentimes, the processes of equalizing and conditioning are not done satisfactorily. As a result, the user of the lumber experiences manufacturing problems related to improper equalizing and conditioning. These problems include open glue joints, warping during machining, and movement after manufacturing.

This article describes the requirements and procedures for equalizing and conditioning in practical terms that the kiln manager and operator can adopt into their operating procedures. A special section is included on conditioning to relieve longitudinal stress.

EQUALIZING

It is normal, as lumber is dried, for some pieces to dry more quickly than others. This difference can result because of differences in the lumber (for example, sapwood pieces dry faster than heartwood, and thin pieces dry faster than thick pieces) and/or because of differences in the drying conditions (that is, differences in temperature, humidity and velocity).

Further, it is normal to see differences in MC within an individual piece of lumber. These differences can result because of the reasons cited above and because end grain dries faster than flat grain; because the end of a piece of lumber is drier than the middle; and because knots and the surrounding swirly grain are drier than flat grain regions.

Additional man-mad factors influencing or causing MC variation before equalizing include mixing lumber with different initial MCs in the kiln (for example, mixing freshly sawn lumber with lumber that has been partially air-dried), mixing different species and mixing different thicknesses.

As a result of all these factors, it is common when drying a load of lumber to see some pieces of lumber reaching the target final MC before other pieces do. {Oftentimes, kiln samples do not show much variation in final MC. This is because the samples are mistakenly chosen to represent only the wettest part of the load, rather than representing the true cross section of MCs in the load. Further, kiln samples are almost always dried on the outside edge of the load and, therefore, dry more quickly than pieces in the center of the load.}

General Equalization Procedure

The equalization process involves increasing the humidity in the kiln so that the driest pieces dry no further. At the same time, however, the humidity cannot be raised too high or the wetter pieces will not continue to dry. Specifically, the equalization procedure is as follows:

1. Begin equalization when the driest sample in board (which must represent the driest lumber in the kiln) reaches a MC two percentage points below the target MC. The target MC is the desired final MC for the lumber.

2. Raise kiln humidity to achieve an equilibrium moisture content (EMC) equal to the target MC minus two percentage points. In other words, the kiln EMC equals the MC of the driest sample. This ensures that the driest sample will stop drying.

3. Because moisture in wood moves faster at higher temperatures, the wetter pieces of lumber will dry faster if the temperature in the kiln is as hot as reasonable. Set the dry-bulb temperature during equalization of hardwood lumber to 170°F.

4. Continue equalizing until the wettest sample (which must represent the wettest lumber) reaches the target MC.

If sampling and operating procedures are correct, all the lumber in the kiln will be between target %MC and [target - 2] %MC at the end of equalization.

With lower-valued products and with lumber that will be used for products that do not require extreme moisture content uniformity, it is possible to change the limits given above to "3% below the target." That is, begin equalizing when the driest piece is 3% MC below the target and use an EMC that is 3% below the target. Equalizing stops at the same point - when the wettest lumber is at the target. This "3% below the target" approach will result in faster equalization, but may produce slightly lower quality lumber, especially in terms of warp and machinability.

Equalizing Example

Assume that the final target is 7% MC. Equalizing would begin when the driest sample is at 5% MC (7% MC - 2% MC = 5% EMC). Set dry-bulb temperature at 170°F (unless the kiln schedule calls for a lower temperature). Set the humidity controls in the kiln to achieve 5% EMC. Referring to the data in Table 1, this EMC is achieved with a 35°F depression, or a wet-bulb temperature of 135°F. {Additional wet-bulb temperature settings for different dry-bulb temperatures and different target MCs are given in Table 1.}

Final Equalizing Dry-Bulb Temperature

MC EMC 140 150 160 170 180 190 200

% % degrees F
5 3 92 101 110 120 130 140 150

6 4 99 108 118 127 137 147 157

7 5 105 115 125 135 145 156 167

8 6 111 121 131 141 152 163 174

9 7 115 125 136 146 157 168 179

10 8 120 130 140 151 162 172 183

Table 1. Traditional wet-bulb temperatures for equalizing.

CONDITIONING

It is normal for lumber, as it dries, to develop drying stresses or "casehardening." These stresses result when the outer cells begin to dry and try to shrink, while the interior cells, which are still wet and not drying or shrinking, will not permit the outer cells to shrink. As a result, the outer cells dry in an enlarged state, called "tension set." Tension set results in casehardening in dry lumber. To relieve tension set, moisture needs to be quickly added to the outer cells, causing them to try to swell. The interior cells resist this swelling, creating a compression set, which offsets the tension set.

Because the process of regaining moisture at the surface is influenced by its MC, variations in lumber MC will influence the conditioning process. This is why equalization is the first step for proper conditioning. In addition, because moisture absorption is faster in low-density species, stress relief will be faster for these species. The conditioning process will take longer for thicker species.

When adding moisture to the surface, there is a risk that the amount of swelling will create more compression set than there is tension set. If this happens, the lumber is said to be "reverse casehardened." Reverse casehardening cannot be relieved or eliminated. It is, however, difficult to create too much compression set with lumber thinner than 1-1/2 inches using high kiln temperatures. The moisture supplied to the surface rapidly moves into the core, causing the core to swell and relieve some of the surface stress.

Appreciate that the amount of tension set varies with a variety of factors. The faster the lumber is dried when at high MCs, the greater the amount of tension set. The thicker the lumber, the higher the potential for high tension set. High-density species with slow moisture movement and high shrinkage tend to have high levels of tension set.

General Conditioning Procedure

The conditioning process involves the rapid increase of humidity in the kiln at as high a temperature as possible. The process is most effective if the moisture is vapor rather than small, liquid water droplets. The conditioning procedure is as follows:

1. Begin conditioning only after the lumber has been properly equalized.
2. Raise the kiln humidity to achieve an EMC equal to the target MC plus four percentage points for hardwoods or, for softwoods, to the target MC plus three percentage points. When longitudinal stress is a problem, raise the EMC an additional 1/2%.
3. Because conditioning is more effective at higher dry-bulb temperatures, maintain the conditioning temperature at 180°F.
4. Continue conditioning until the stresses are sufficiently removed, as determined by stress samples and experience. Note: Allow stress samples cut during the process to dry and cool before evaluating them. Moisture gradients in the lumber must be allowed to dissipate. Cutting samples "hot" and then putting them in a microwave oven for 30 seconds at medium-high power can provide accurate evaluation of stress within minutes.

During the conditioning process, the lumber will regain about 1% MC. The lumber should, therefore, be at a final MC of target MC +/-1%. However, moisture gradients in the lumber should be allowed to dissipate before the lumber is processed further. This waiting period is often called 'cooling' or 'tempering.' Failure to wait long enough will lead to warping after machining.

Most of the stress relief occurs during the first several hours of steaming. After approximately 18 hours with 4/4 lumber, additional conditioning will result in further addition of moisture to the lumber, but with little added stress relief. Thicker lumber often requires longer conditioning time and longer subsequent cooling time to eliminate moisture gradients.

Conditioning Example

Assume that the final target is 7% MC. Conditioning would begin after equalizing brought all of the lumber pieces to a MC between 5% and 7%. Set the dry-bulb temperature to 180°F. For hardwoods, the humidity in the kiln must be 11% EMC ([7% MC + 4% MC] = 11% EMC). Using the data from Table 2, this EMC would be achieved with a 10°F depression, or a wet-bulb temperature of 170°F. Additional wet-bulb temperature settings for different dry-bulb temperatures and different target MCs are given in Table 2.

Final Conditioning Dry-Bulb Temperature

MC EMC 140 150 160 170 180 190 200

% % % degrees F

5 6 9 123 133 144 154 165 175 186

6 7 10 126 136 147 157 168 178 188

7 8 11 128 138 149 159 170 180 190

8 9 12 130 140 151 161 172 182 192

9 10 13 132 142 152 163 173 183 193

10 11 14 133 143 154 164 174 185 195

Table 2. Traditional wet-bulb temperatures for conditioning.

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