Practical Ways to Reduce Drying Costs
Five tactics to consider for cutting the cost of drying a load of wood. June 13, 2014
From Gene Wengert, forum technical advisor:
Five practical ideas for reducing drying costs: One of the key numbers I use when meeting with people that want to improve their drying operations is that it costs about $25 per day per MBF to dry hardwood lumber in a kiln, including profit and operating costs, but not quality loss (or degrade). For example, in a 30 MBF kiln, this is $750 per day.
Further, the drying time is almost always determined by the wettest piece of lumber in the kiln. That is, until the wettest piece is dry enough, the kiln is not shut off. This means, for example, that if the incoming moisture content of the wettest pieces of lumber can be reduced enough to shorten drying time by one day, the benefit in the kiln is $750.
Suggestion 1: Spend time in the pre-kiln-drying operation determining what can be done to reduce the MC of the lumber before it enters the kiln. In some operations, this includes using sheds instead of open air drying. In pre-dryers, it includes relocating the lumber stacks, so that the top stacks (slow drying) are moved to the bottom after several weeks. Money spent in lowering the MC has a large payback.
Suggestion 2: The kiln schedule used, if the operation uses the procedures considered standard for hardwood lumber drying it cannot really be accelerated very much. Therefore, do not look for reductions in drying costs by modifying kiln schedules beyond the standard published schedules.
Suggestion 3: If a load of lumber is ready to be pulled from the kiln and another load is ready to go in, it would make economic sense to load and reload on weekends. The loss of not unloading on a Saturday, but waiting until Monday is about $2500 in a 50 MBF kiln. This loss greatly exceeds the cost of paying overtime to get the job done on a Saturday.
Suggestion 4: How does the kiln operator know when the load is completed? Oftentimes, the guidelines he or she uses are not very well defined. Oftentimes, I find that the lumber could be pulled much earlier. Therefore, management and the operator develop a strict set of guidelines for determining the end point in drying.
Suggestion 5: Management and the kiln operator should develop guidelines about the overall drying time for a particular load. If the load is not ready within the time guidelines, then the operator must explain, if possible, why the load took longer than expected. Management must then respond to the reason for delay to avoid such delays in the future.
Note: Although shorter drying times are a good goal, it is also critical to avoid any quality loss. For example, if the operator shortens equalization or conditioning (stress relief) to reduce drying time, but in so doing the quality drops, the end result is not acceptable. Copyright by Gene Wengert 2005.
Forum Responses
(Commercial Kiln Drying Forum)
From contributor D:
That was a very interesting post. What is an acceptable length of time to wait between the time that the conditioning stage has completed until the time that the doors on the kiln are opened and the lumber is removed? It was suggested to me by a kiln operator of a very large operation, that we should allow 12 hours between the end of conditioning and dumping the kilns as a needed cool down time (we are drying mainly cherry and walnut). I have also heard of operators who leave loads of white oak in the kiln for 24 hours after conditioning to cool down before dumping.
However, I read in "Drying Hardwood Lumber" that it is an effective cost saving idea not to let the lumber sit in a kiln overnight or over the weekend (as you just posted). I have not seen much of a quality improvement the longer we let the lumber cool before dumping the kiln. What are your thoughts on this? Is there any reason due to quality concerns to let the lumber cool after conditioning before opening the kiln? We always have material waiting for the kilns, so having something to put in the kilns is not an issue.
From contributor K:
In our operation, species such as cherry, poplar, maple, ash, walnut and hickory are pulled after the conditioning step. We do not cool these species down before pulling the charge. This may not be the suggested procedure but I have not seen any changes in quality due to not cooling down in the kiln.
From contributor F:
Having studied drying stresses and conditioning at Madison I found no benefit to leaving the lumber in the kiln after conditioning has been performed. If you search Madison's publication list you may find the publication. It was also published in Drying Technology.
From contributor D:
Thank you for your replies.
I will search that publication list and look for your report, it sounds like it would be interesting. I also have never found much benefit in letting the lumber cool down in the kiln before unloading. As far as guidelines for the time the lumber is spent in the kilns. In order for a charge of 4/4 cherry to be dried from roughly 40-50%, (dried down to 10%) is it realistic to expect the lumber to be in the kiln for 10-11 days and no more? Or is this asking too much?
From Gene Wengert, forum technical advisor:
Cooling of lumber is an old idea. Actually, it is not so much the cooling as it is the humidity change. When a load of lumber is at 180 F at the end of drying, when it is brought out of the kiln, the wet-bulb outside might be 50 F. So, immediately after pulling the lumber out, the surface is exposed to a 130 F depression, causing very rapid drop in surface MC. The idea of cooling is to moderate this change. Although many people just unload a kiln when it is done, I believe that with thicker expensive lumber it might be prudent to spend 12 hours in the kiln cooling the lumber. But certainly, the details on how to cool lumber are not published, as far as I know.
From Gene Wengert, forum technical advisor:
For 4/4 cherry, that is 40% to 50% initial MC in the kiln, the time is based on the wettest MC. So, the loss is from 50% to 10%, or a 40% loss. I would target about 6% MC loss average, so that is about 6.5 days plus 24 hours for conditioning and cooling. If the lumber is cold, add another .5 days for warming. So, 8 days would be a good target. (Incidentally, 10% MC is rather high for 4/4 cherry for most uses.)
From contributor D:
Eight days? That is a couple of days faster than what I do cherry in. I wonder where I am losing the time. Heat up at 10 degrees per hour to 128 degrees, and then running on basically a standard cherry schedule until conditioning for usually 12 hours which pretty much eliminates any stresses which may have occurred. I would have to guess that our time is lost due to the fact that we do not run our kilns above 160 (even though our boiler can do 180 with no problems) for quality reasons. Are there any studies done on the quality of lumber with higher temps at the end of a cycle (low MC’s)? Also, we dry to 10% for some customers overseas.
From Gene Wengert, forum technical advisor:
I do suggest that you keep the 160 F temperature. Do you start at 130 F and 133 F (DB and WB)?
From contributor D:
Our initial stage is set for 128 dry bulb and an EMC of 14.0%.
From contributor K:
We are in the northern region and have found that exposing hot conditioned lumber to minus degree f. causes the conditioning to rapidly dry off the surfaces of the lumber. Relative humidity goes way down with cold temperature. We use the 12 hours in the warmer temps. We still use the 12 hour rule. For 4/4 cherry our drying times are in the eight days and yes we use the 130 temp, however we ramp the lumber to this for three separate six hour times as follows: db.70 wb. 60 to db.100 wb. 90 then db.100 wb.90 to db.120 wb.112 and the last six hours db.120 wb.112 to db.130 wb.123.
From contributor A:
As time is money in the kiln drying operation and we need time to cool down the lot. I think it will be more effective to remove the wood from the kiln and let it get cool in the shed rather than the kiln and we can immediately use the kiln to load the next lot without any loss of time in the kiln for cooling. I think it should solve our problem. As far as I believe the wood that comes out from the kiln can be given the 12 hour time to cool off before sending it for machining or further process.
