"Intermediate Conditioning" in the Drying Kiln
Is it a good idea to add moisture to a kiln for a period during the middle of the drying cycle, in order to relieve casehardening stress? June 13, 2014
Question
For some time I have been thinking about doing some experiments (first in lab kiln) during drying regarding eventual reducing of stresses by so called intermediate conditioning (increasing the relative humidity of air during certain period of time when lumber MC is below fiber saturation point). I've heard that similar experiments were done in the past. However, I couldn't find either one paper (or anything else) to find out something more about it. I would be grateful if someone can tell me anything about such experiments or any literature about this problem.
Forum Responses
(Commercial Kiln Drying Forum)
From Gene Wengert, forum technical advisor:
It was tried and its use discontinued due to the large amount of internal checking in oak and maple. The old Moore Dry Kiln Company in Jacksonville had a note about this. I will have to see if I can find this.
From Gene Wengert, forum technical advisor:
The booklet is Production Kiln Drying by John Devine. Incidentally, we usually do not say that lumber is below fiber saturation, as some of the cells are below FSP in a few hours. When the lumber is at average 28% MC, some of the cells in the middle are likely still above FSP, while cells nearer the surface are way below FSP. Only a few cells are actually at their FSP at 28% MC. The problem with re-wetting of many species is that the added moisture quickly causes the outer cells to swell and put the core cells in tension. As the cells in the interior are wet and hot, they are weak and can easily fail, creating an interior check. Further, such a treatment increases drying time.
From the original questioner:
I agree of course with you about FSP. In Europe it's common to use "below FSP" when you think "average MC below 30%". Are there any scientific tests? Do you know some more details? When they have been started with rewetting (average MC around 20% or 15%)? It makes sense that for internal checking, but do you think that idea deserves reconsideration? I am thinking to do some experiments with European beech 38 and 50 mm thick.
From Gene Wengert, forum technical advisor:
I do not know any more, but I am curious why you would want to do this at the higher MC rather than wait.
From the original questioner:
Because you can't eliminate all stresses even with a very long conditioning phase at the end. It's very hard to get a very good drying quality (gap (European new measure for casehardening) below 1 mm) for beech 50 mm, even with 60 hours of conditioning. The solution is to have a very, very slow initial phase of drying or, maybe, to try with re-wetting.
From contributor O:
Rather than thinking of conditioning during the drying cycle, you may want to consider a continuous equalization schedule where you hold your dry bulb values at the desired schedule set-point, but raise the wet bulb values up to the desired final moisture content EMC. This will not only even out moisture contents within the loading but the overall stress at the time of conditioning is significantly less, resulting in reduced condition times. You could begin this when your highest sample is at or near FSP, but you must keep in mind that drying rates will diminish as a result. Do not think that re-wetting the lumber will help.
From Gene Wengert, forum technical advisor:
The standard conditioning treatment used in the US should do better than what you are getting, as we can virtually eliminate all stresses when it is done properly. The first key to is to get the lumber uniform in final MC. Also, we find that you eliminate almost all stress within the first 12 hours, so 60 hours does nothing except raise the MC. You also need to get the correct EMC (4% EMC above the desired final MC) and you must get this value within an hour or less. This means that if the temperature rises use a desuperheater with the steam.
Please describe the actual conditions in the kiln when you condition. How do you measure stress? Some people that have conditioning problems (not enough steam) will cool the load first for an hour or so and then steam it. When you measure stress, typically 1/4" thick, 6" long and 1" wide leg you need to use the same size prongs. As some people measure stress they will plane the lumber first as that is how the customer is likely to use it. Further, the results are read when the prongs are cool.
From the original questioner:
Not sure that I understood the continuous equalization schedule. Is it to rise EMC to final MC value? At that time when the wettest board is around FSP, EMC is around my final MC anyway. Or even higher (average MC 20-25%, EMC 7.5-9.5%, final MC 8%). About conditioning, EMC is MC plus 4%, the same as in the last phase of drying (62C), and most of stresses is eliminated in first couple hours, but not enough. There is no problem with amount of steam, or overheating because cold water is used (it's common in Europe). New European standard for drying quality does not use prong test, but so called gap – the distance between two parts of a re-sawn board measured after 48 hours.
From Gene Wengert, forum technical advisor:
Cold water does not relieve stresses as well as steam. I ran a recent test and it was clear that for excellent stress relief, steam was superior. On the other hand, water was ok in most cases, as most users do not require zero stress. Again, it is critical to rapidly achieve the conditioning EMC. What is the width of the re-sawn lumber when doing the test?
From the original questioner:
The width of the re-sawn lumber when doing the test is 100 mm.
From contributor O:
What happens if after 48 hours your lumber fails your gap test? The prong method, while not instant, gives a quick feedback that you can work with. Why not consider doing both methods for a while to establish a pattern of correlation.
From Gene Wengert, forum technical advisor:
When you cut prongs hot, Drying Hardwood Lumber suggests that you put them in a microwave oven for 15 to 30 seconds on high power and then the reading you get in a few minutes will be the final reading that the customer gets.
From the original questioner:
What do you think about oscillating climate during drying to increase and decrease temperature and relative humidity in cycles? There are some experiments showing good results regarding drying time. Also mechano-sorptive creep is more significant.
From contributor F:
Late in the standard the US schedules you increase the temperature and maintain the same depression. This actually increases the EMC. In the studies I performed on oak the lumber gained moisture and the swelling was able to be measured. The runs that included this increase in EMC before conditioning resulted in much less residual stresses than runs that exclude the EMC rise. There was no increase in open surface checks nor was there any honeycomb with the EMC rise.
From Gene Wengert, forum technical advisor:
What Contributor F says above is true, but the increase in EMC is small (130 and a 45 dep is 3.0% EMC; 160 and 45 is 3.7% EMC.). You might find a limited increase in EMC if you first bring the WB up and then the dry bulb follows (not suggested to do this, but sometimes the equipment does it). Note that with many controls, the DB and EMC are the control variables, so the increase in EMC does not occur. In any case, it is very rare to see oak dried green from the saw in a kiln. It takes too long and therefore costs a great deal of money. Often $35 per day per MBF is the drying profit (gross) for oak and many hardwoods.
