Whether to Spray Wood During Kiln Warm-Up

Is it wise to spray wood with water or steam while the kiln is warming up? June 13, 2014

Question
We have always been told to never use spray during warm up of a kiln. Recently I read in the Dry Kiln Operator's Manual that brief, intermittent spray can be used for green, check prone lumber during warm up. This is on the same page that indicates to not use spray during this time. Has anyone used spray during warm up? What were your results?

Forum Responses
(Commercial Kiln Drying Forum)
From Contributor D:
I use spray during warm-up only on certain species/thicknesses, such as thick timbers for log homes or when warming frozen lumber (use low temperatures, around 80 degrees F). I never use spray during warm-up on air dried lumber, especially oak and other highly refractory species.


From Gene Wengert, forum technical advisor:
Be aware that the DKOM was written in about 1950 when we did not have the tight kilns that we have today. With today's kilns and high steam spray capacity and high pressure on some kilns, always avoid steam spray. The steam spray temperature is over 212 degrees F so when the steam condenses on the wood, the surface will be really hot, which is bad. At the same time, avoid heating up the kiln too fast as this will create rapid drying of the surface. At the surface the wet-bulb will be the wood's temperature (30 to 40 F maybe) while the air is at maybe 90 F, so you will have a large depression. When the DKOM was written, the kilns did not have high heat capacity, so rapid heating was not an issue. Today's kiln controls do usually have a ramp feature to allow slow heating per hour. Incidentally, do not use a thawing schedule, as this is not supported by any research or deemed necessary.

From Contributor K:
I started doing this ten years ago on ponderosa pine (West Coast 4/4 com, 5/4-6/4 and 8/4 Shop and Mldg grades) only in the Summer season. This is for fresh cut (less than three days old) lumber only. I start with ambient outdoor temperatures as Gene recommends (here it's around 80 degrees F and the average in kiln temp will easily drop 20 degrees below that at start-up), live spray comes on about two hours into the run once a 88F d/b setpoint is achieved for about 90 minutes (spray valve is open approximately 30%) and quickly gets the d/b and w/b up to beginning setpoints of 105-100 during a six hour startup ramp. Having a temperature above 90 degrees (d/b) will also help in preventing kiln corrosion. I have 100' double track Wellons CM/Z multizone kilns that run on 30# steam.

My initial theory was if I could connect the link between moisture vapor in the air with the surface moisture of the wood to help the capillary action of the drying process begin much quicker. I basically call it wet the wick as in the principle of osmosis. Hope I don't offend anybody with that comparison of terms. I started experimenting with this in response to surface season checking (mostly razor check) and it does reduce the problem in my opinion. There are many different circumstances that predict when to use it and when not to use it. It is not a one size fits all by any means. Air dried or re-dried lumber is definite no-no! I don't know if this will work on hardwoods or not.

Gene makes a very good point of surface temperatures to take into consideration here, Temps are temps (d/b and w/b) high or low and they equal real time depressions that must be applied to the equation of drying methods. I think the very short duration and the lower temps during the start-up of the schedule help eliminate a lot of problems in this regard. I can only say it has helped me, attention to the details during the drying run along with kiln samples, record keeping and monitoring during the surfacing process will help confirm results and when to make adjustments.


From the original questioner:
During ramping we find that the WB does not rise as fast as the DB. It takes the WB longer than the DB to reach set point. So, during ramping the depression gradually gets wider. We have found that using brief intermittent spray (after two hours of initial warm-up) for one-two minutes every half to one hour keeps the WB depression where it should be. This procedure has worked for us for several kiln charges. We ramped the DB 4-5 degrees per hour. If we do not use the brief intermittent spray, how slow should we ramp? Is there such a thing as ramping too slow?

From Gene Wengert, forum technical advisor:
Remember that the wood's surface temperature is not as hot as the air temperature. So, even though you see a large depression during heating, it is not quite that large from the lumber's standpoint. If the wood is quite wet, the wood's surface will be close to the wet-bulb. Also, remember that the WB has much higher air flow across it than the lumber does, lowering the WB temperature a bit. One issue with steam spray is that it is very hot, so when it condenses on the lumber's surface the wood heats up quickly. Also, there is the issue of rapid swelling of the surface fibers, creating tension deeper within the wood and possibly pulling checks into the lumber. Sometimes what happens is that the kiln walls are cold, so even though the moisture is being released from the lumber that would elevate the WB (the wood is drying), the moisture is condensing on the cold walls, roof vent covers, etc.

From Contributor H:
We always use spray during startup. In fact, throughout our drying schedule we have a spray setting that sits about 1 Deg C below vent setpoint.

From Gene Wengert, forum technical advisor:
Assume that you have 50 psi steam at the boiler. Even though you might reduce the pressure, the temperature is still 300 F. This steam hits the cold lumber and the surface is then heated to close to 300 F if steaming continues for very long. Do you really want the temperature of the lumber to be 300 F? If you have a desuperheater, then the steam is at 212 F, and that is still quite hot for hardwoods.

Also, in a 50 MBF kiln, the lumber will be releasing about 150-200 gallons of water for every 1% MC loss. Shouldn't this amount of water be enough to keep the humidity in the kiln at the desired level during the kiln run? It does not make sense that one would have to add more moisture to the air. Of course, if you do add steam, meaning that the kiln RH is too low, then the vents should never open (meaning that the kiln is too humid). If a kiln steams and then vents and then steams and vents, this means that the control system is not working properly. Because of the heat in the steam, a malfunctioning control system, in addition to wasting energy, is also making the lumber extremely hot, which normally we do not want due to strength losses.

If steam is used for the initial heating up, do you really want cold lumber to get a burst of 300 F steam? For hardwoods like oak, which is what the original question was about, steaming will cause the lumber to heat, weakening it and the surface to swell quickly, driving any normal checks deeper inside the wood. Plus, now that there has been water added to the surface, it will take a bit longer to dry the wood. Special note: Although the DB has reached a new, higher temperature, the wood is lagging, so the effective DB as far as the wood's surface is concerned is cooler than the air DB. Hence, your concern about the large depression is not quite so serious. Rather than steam, how about ramping a bit slower so that the depression stays at the level you want?


From Contributor H:
I have no experience with oak species. 99% of timbers we dry are freshly cut. In theory we believe that the outer fiber layers of timber should always be protected. Exposure to too dry air in the beginning of drying, even for a few seconds, or even during air drying could cause outer fibers to micro collapse. Common sense dictates that moisture will move slower through a higher density barrier (micro collapse), even if it is only a few cell layers thick. For this reason we prefer do dry freshly cut timber before any type of micro collapse or drying could take place (or at least very little). We aim to always maintain a minimum RH level to prevent or minimize this micro collapse from taking place. You control the minimum with low pressure spray or wet steam from a steam bath, and the maximum with your vent setpoint.

This theory supports what Contributor K says “My initial theory was if I could connect the link between moisture vapor in the air with the surface moisture of the wood to help the capillary action of the drying process begin much quicker. I basically call it "wet the wick" as in the principle of osmosis.” In my opinion yes, as long as there is no prior surface damage. I believe that intermittent burst of steam spray does exactly this - it softens the surface or expands the collapsed fibers to re-establish diffusion paths blocked by micro collapse.


From Gene Wengert, forum technical advisor:
I have never heard of micro-collapse. Would you happen to know of any explanations or discussions of this phenomena? Wouldn't the cells affected be planed off after drying? Regarding maintaining a high humidity, why wouldn't the moisture coming from the lumber keep the humidity quite high? Normally, the humidity is so high that we need to vent frequently to keep the humidity lower? If we do not vent, where does all the water that is being evaporated go?

The capillary idea is interesting, but are there continuous capillary paths in green wood? I think not, as there are so many air bubbles in the capillaries and the cell lumens that we do not have continuous capillaries full of water only. So, I do wonder how much drying is done by capillary action (mass flow) and how much is done by diffusion - talking about drying well under 212 F. Certainly at 212 or hotter, it is different indeed. If the issue is slow drying, why not just lower the humidity a bit or raise the temperature in order to achieve faster drying?

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