What to do when wet bulb reading is too low to kick on compressor?

TBrown99 · January 04, 2024, 10:53:04 AM

We're finally starting to use our L-53 after setbacks the past two years. Our chamber is constructed quite well to standards and at the correct overall size.

On our second test load of poplar, we're using 120 dry bulb & 90 wet bulb drying schedule for 500 bf of Group 2 poplar (mostly 2x4s and 1.5x varied widths with some live edge slabs). Half of the load dried down to 6-8% within a week, starting at 30 average moisture. The first two days we threw off 40+ lbs of water each day. But the other half is still high 15-20%. We reloaded the remaining unfinished 250 bf and are trying to complete drying.

Our measured wet bulb doesn't rise above 81-82 so the condenser doesn't kick on as we don't reach 90 (which signals I believe a dryer room vs. 80 level). We use a Govee temperature and relative humidity device and it shows the chamber at 19-20% RH. This seems low.

What should we do? Should we simply set down the wet bulb to force the compressor to start working? Will this take the chamber too low for humidity and produce drying damage?

Thank you,
Toby

K-Guy · January 04, 2024, 12:55:40 PM

No, don't lower the wet bulb, the wood has to release more moisture to bring the temperature up. If it stays there for several days, the wood has locked up and isn't releasing moisture. To treat that you must be below 25% moisture, turn your compressor off, set your dry bulb to 140° and leave for a couple days till you see the moisture moving out of the core.

doc henderson · January 04, 2024, 02:44:53 PM

is the temp (dry bulb) 120°?

TBrown99 · January 04, 2024, 08:18:36 PM

Yes, we were at 120F for a couple of days with wet bulb at around 80-82. We initially had flow with WB set below 80 but I reset to 90 because that's what the schedule calls for and b/c I'm concerned about low humidity damage. Anyway. I took Stan's advice, so we'll see in a couple of days. Already at 131 degrees. The RH devise is showing a further fall in RH from 18 to 17.5. WB is at 85.6, DB 131.3.

doc henderson · January 04, 2024, 09:07:53 PM

the higher the temp goes with the same amount or water in the air, the lower the RH humidity goes. If you dropped the temp to 80, the RH would go up and you could pump more water out of the air, but you need to move water out of the wood, so the increase in temp will "boil" or motivate the contained water to the air so it can then be pumped out.

blackhawk · January 05, 2024, 11:09:59 AM

Are you monitoring the actual EMC of the lumber? At 120F and 20% RH, your kiln is just below a 4% EMC condition. That is fine if the kiln condition is at 4%, but I don't let the actual lumber get below 5% EMC. I monitor my lumber in the kiln real time with Lignomat probes. If your driest lumber is getting close to 5%, you need to remove it or start adding water to the kiln.

TBrown99 · January 06, 2024, 09:23:36 AM

To update, we are continuing to "boil" this lumber charge although the kiln isn't quite at 140 yet, as the image below shows. A second image below shows the L-53 bulb readings. (Dry is at 137.1)

To answer Blackhawk's question: No, I'm not monitoring EMC. I don't have probes yet this seems like a good thing to have and I'm trying to not open the kiln door as it takes a long time to build up temperature.
I did just calculate EMC of the room and it's very low 2.x.

I did remove lumber that was dried to about 6-10% and I wrote MC range for each remaining piece.

I'll probably let the heat remain high until this evening and then begin cooling and turn compressor on. However, I expect the wet bulb will quickly go below 90 and I'll be back to where I was...

Does a fall under 90 wet bulb at a given temperature (say 120 dry bulb) represents an INCREASE in humidity level? If so, why doesn't this represent a good time to extract this moisture? Why would I wait until web bulb increases (space becomes dryer) to run the compressor?

I'd like to emphasize that the first half of this load dried very well and we were very happy with the results (a first for us!).

blackhawk · January 06, 2024, 05:54:04 PM

You need to download the humidity graph from the Drying Hardwood Lumber manual here: https://www.fs.usda.gov/research/treesearch/5710

If your dry bulb is at 120 and your wet bulb is set to 90, as the wet bulb drops the humidity is getting lower. The difference between the DB and WB is called your wet bulb depression. This is on the graph that I linked. Your kiln EMC is extremely low right now. I would turn off the heat now. Monitor your Govee and if your Kiln EMC condition stays below 5% you need to throw some water in there on the floor to be safe.

What I've seen on my L53 is that if you keep the kiln at 5% EMC the lumber will keep drying even though my compressor never comes on. The moisture is coming out of the wood. I see this as I monitor my EMC probes in the lumber. I assume that even though the compressor is not cutting on and removing the water, the moisture is escaping through the small leaks in the vents and door.

TBrown99 · January 07, 2024, 07:41:21 AM

Okay, I will check this drying manual. Thanks.

To update, I let 140 F. run until this morning (maxing out at 138.5 F) and then reset to 120 F. quickly venting excess heat and settling presently at d121.4 and w85.6.
Relative Humidity by Govee reading is very low at 7.5%.

I have realized that I have a big problem with fan air distorting (decreasing) the wet bulb readings. I have a 4-fan bank with a hinged baffle hanging below (800 cfm each). I also have two 800 cfm and one 500 cfm independent fans trained on about 10' of baseboard radiation (hydro from wood boiler) fin sections -- back side of the load near bulbs -- as supplementary heat source. I noticed that after cool down this morning when I turned the fan bank back on, web bulb decreased rapidly. (This was without even the influence of the other 3 fans.)

Should I try to totally isolate the dry and wet bulb lines? I was thinking of screwing a small box to my wall, mount bulbs inside and hanging light fabric over the front. What type of material or design would work well here? I need to cut the draft while allowing some air flow. Is this an essential modification?

doc henderson · January 07, 2024, 08:53:41 AM

Have you been able to check the MC of the wood? I think you do not want the wet bulb in direct breeze from the fan but should be in a representative environment (air) of the rest of the kiln. The dry bulb will not matter as there is no moisture to evaporate. the wet bulb should have evaporative cooling depending on how dry the surrounding air is. that is how it works and the dryer the air, the wider the depression when compared to the dry bulb. I would think putting it (wet bulb) in the path of fan air would facilitate evaporation and further lower the wet bulb temp distorting the depression and giving a false drier reading. Sounds like the gradient is high based on the schedule. Maybe time to add some water to the kiln so the surface to core gradient is not so large, or at least let the wood catch up. Disclaimer, I do not have a kiln, but love thinking through these issues. I do have a science background. good luck.

doc henderson · January 07, 2024, 09:01:28 AM

you also dumped some moisture suspended in the warm air. so it may be even dryer in the kiln. Look at your graph. If you just turned down the heat without running the DH or vent, the RH would have come back up to where it was at the prev. temp or higher. If you had an empty kiln with a certain amount of water in the air, the relative humidity will go up and down opposite the temp reliably. It seems dry, but if you cooled the air temp, you could reach 100% RH inside the kiln with the current water in the kiln. The air can hold more water the warmer it gets and the is what the relative humidity is. how much water is in the air compared to the max. the air could hold at that temp. I am just thinking outload and sorry if you already know all this.

blackhawk · January 07, 2024, 10:39:20 AM

The wet bulb needs to be somewhat out of the air flow. Stan at Nyle suggested that I shield mine and that is what I did. It definitely helped but I could never get it perfect. That is why I use the Govee to help me know exactly what is happening in the kiln. See my last post in this thread to see what I did to shield my wet bulb: https://forestryforum.com/board/index.php?topic=121782.0

doc henderson · January 07, 2024, 11:29:45 AM

TB remember you could put this wood in a heated airconditioned shop, and it will dry fine. It will take longer. also, a solar kiln has nighttime temps of near ambient outside temp, with a high humidity and that relaxes the stress in the wood, but also takes longer and has/requires less control. you want to get this wood moving, but do not want to produce stress and surface defects. you can be more aggressive as the MC decreases. What is the current MC of the load? to get back to a schedule you can turn down the heat or add water back to the kiln, or let it go and see how it goes. air movement across the surface also becomes less important/effective as the wood dries.

TBrown99 · January 07, 2024, 07:25:55 PM

Just checked the charge... I had thrown in 3 pieces of boxwood elder (very soft maple sample with full red streaks), and I was impressed to see 6 readings at 6.2 with a high of 6.5. (Started at 20 and 25-27.) The poplar wasn't that amazingly low but 16 samples showed 8.1 with 10.6 as the high. So, I'm pretty surprised.

This round helped me understand that our chamber can go to 140 F while I was thinking the limit was 130 from previous tests. Just have to wait a while. Though I can't say I was entirely comfortable going to bed with those temps (which is probably irrational fear on my part) but the chamber is attached to our 30x50 shop (with a 25' loose housing extension in the back... which is filling up with some fine lumber) containing a non-trivial amt of tools & machines.

I will provide details of the outcome soon.

TBrown99 · January 08, 2024, 07:01:09 AM

Indeed, I had some amazing drying with some hard maple boards standing up in the main shop area, which has a Modine hydronic heater circulating a bit of air, but also 3 DYI air cleaner fans help to move air as well. Presently, RH is 30%. Our kiln is housed in our shop side shed within a slightly larger chamber which I also aim to use for pre-drying.

From what I understand, 120 F conditions is in the very gentle range and I don't have to worry about case hardening. Though other defects must be a concern...

Adding water has come up twice now so I'll research this.

Quote from: doc henderson on January 07, 2024, 11:29:45 AM
TB remember you could put this wood in a heated airconditioned shop, and it will dry fine. It will take longer. also, a solar kiln has nighttime temps of near ambient outside temp, with a high humidity and that relaxes the stress in the wood, but also takes longer and has/requires less control. you want to get this wood moving, but do not want to produce stress and surface defects. you can be more aggressive as the MC decreases. What is the current MC of the load? to get back to a schedule you can turn sown the heat or add water back to the kiln, or let it go and see how it goes. air movement across the surface also becomes less important/effective as the wood dries.

TBrown99 · January 08, 2024, 07:03:55 AM

Did you find most of this guide to be useful? I'm probably going to order a print copy from my local printer from the PDF. Thank you!

Quote from: blackhawk on January 06, 2024, 05:54:04 PM
You need to download the humidity graph from the Drying Hardwood Lumber manual here: https://www.fs.usda.gov/research/treesearch/5710

doc henderson · January 08, 2024, 07:30:25 AM

sounds like you are moving forward. some will end with some conditioning. More info will be great and we all can learn. thanks. Doc.

blackhawk · January 08, 2024, 09:29:38 AM

Quote from: TBrown99 on January 08, 2024, 07:03:55 AM
Did you find most of this guide to be useful? I'm probably going to order a print copy from my local printer from the PDF. Thank you!

Yes, I read it front to back. It is really the go to document for drying. The schedules in the Nyle manual are fine but they are conservative.

TBrown99 · January 09, 2024, 07:30:47 AM

We checked 50% of the charge with multiple probe readings and found some sub-6% with highest reading at 9.0 with the average around 7.0 to 8.0. There was some "knobby warping" by knots, sorry I don't know the technical term. Our goal is to avoid having to apply high heat (140) and to resolve the problem of WB being to low. More troubleshooting will be required. I think it's time we make a short video of our install and solicit critical feedback.

jimF · January 09, 2024, 09:32:33 PM

The wetbulb needs to be in the full airflow to record the actual wetbulb temperature.
By venting to get rid of the "excess" heat you are ridding the kiln of any moisture that came out of the wood further stalling the wetbulb reading. 120 and 140 is considered low temperatures and would not cause any damage to the wood. Although casehardening can and does occur even at these temperatures.
By increasing the temperature to 140, you are providing more energy that is needed to cause evaporation. With out heat you don't provide the potential for the water to evaporate.

doc henderson · January 10, 2024, 08:40:58 AM

to get the humidity up, might have been better to let the temp come down without venting, so the water remains in the kiln. Kiln drying is controlling the temp and RH (wet bulb depression), so to prevent the potential degrade you follow a schedule. Maybe other could let us know what they do to condition wood at the end. I think some will add water. This would reverse the drying direction adding moisture back to the wood from the outside in. Trying to balance the core and shell. Is the range of MC between differing species or thicknesses of wood?

jimF · January 11, 2024, 07:33:39 AM

The last step in the drying process, conditioning, is not to balance the MC between the core and shell but to increase the compression stresses during moisture gain so that the stresses are reduced ridding the lumber of casehardening. You see, when wood is under high enough stresses and the moisture content is changed the stresses are reduced.
As a added benefit, the moisture gradient is also reduced.
There is usually a step before conditioning, called equaliztion, that is to reduce the majority of the moisture gradient within and between the boards.

K-Guy · January 11, 2024, 10:30:41 AM

You need enough airflow to get an accurate reading but if you have too much, it prematurely dries the wick out.

Larry · January 11, 2024, 11:15:43 AM

My wick was drying out because of too much airflow. I used a piece of foam insulation and moved it around a bit to find the perfect place. It's off to the side a few inches from the wet bulb so it doesn't block the airflow, just slows it down a little.

jimF · January 11, 2024, 06:34:15 PM

The reason the wick dries out quickly is not because the airflow is too high. It is because the water supply is not sufficient. By controlling the airflow you may be recording the readings you want but not the accurate reading. The accurate wetbulb readings is a measure of the potential evaporation rate which needs full airflow to measure. In commercial settings the wetbulb is placed in the incoming air plenum; meaning fulll airfflow and full heat before it enters the stack = most evaporation at the given settings.