kiln drying firewood, batch #3

jimbarry · April 09, 2020, 09:53:17 AM

batch 3, day 5, 9:50 am

This reading is after I performed a 30 min air dump by setting the DB to 85º and let the exhaust fan run, with opposite corner manual vent open.

jimbarry · April 09, 2020, 09:54:59 AM

batch 3, day 5, 10am

This reading was taken 10 minutes after I reset the DB to 120º (above) to shut off the exhaust vent and closed the manual vent.

doc henderson · April 09, 2020, 10:01:38 AM

WOW, the ambient temp really makes a difference on the probe MC. moves in the same direction as air RH (wet bulb depression). Is the compressor able to keep up ? be nice when you can maintain the DB/WB span but retain the heat. If I recall, this wood started much wetter. are you back to getting lots of water from the compressor.

jimbarry · April 09, 2020, 10:26:58 AM

20200409 kiln drying firewood - air dump - YouTube

jimbarry · April 09, 2020, 10:34:16 AM

Compressor running all the time, water coming out. It doesn't appear to be coming out as fast as it was prior to the incident yesterday were it stopped dripping for a couple hours. Still don't know what happened there. Maybe now there's less MC in the air so less is being drawn out. Its still a steady stream though.
When I was doing this 30 min air draw this morning, the DB stayed mostly at 105º for the duration. When it started dropping to 100º I figured that was enough time, result was 30 min.
Wood was green as green can be. Logs cut in Oct last year. Only cut and split over the course of a week, seems like I only get time to do about 1/2 cord at a time. Then it was ready last week but we got snow and rain, ground was too soft to move things into the kiln.

doc henderson · April 09, 2020, 10:58:17 AM

lots of work going on to remove water. curious to see how long it takes. great motivation to get ahead if able and air dry some to avoid having to air dump. what is the highest DB temp you can set. looks like another kiln in the works in the video. thanks again for all the data and video.

K-Guy · April 09, 2020, 12:44:53 PM

It looks like the free water and surface moisture has been removed and from the the drying will be slower.

jimbarry · April 09, 2020, 05:31:04 PM

Quote from: doc henderson on April 09, 2020, 10:58:17 AM
lots of work going on to remove water. curious to see how long it takes. great motivation to get ahead if able and air dry some to avoid having to air dump. what is the highest DB temp you can set. looks like another kiln in the works in the video. thanks again for all the data and video.

Yeah, some air drying would be more advantageous. Like a year at least.
The next floor you see is a dedicated bldg for our sharpening equipment (sawmill blades). Its taken over my workshop and I want it back!

Compressor trips out at 130º. I have the DB currently set to 125º. Probe temp seems to be dropping 1/10 of a % every hour since I did it this afternoon.

jimbarry · April 09, 2020, 07:29:41 PM

batch 3, day 5, 8 pm

doc henderson · April 10, 2020, 12:12:25 AM

nice DB/WB separation. great drop in MC, headed in the proper direction. 6% in 12 hours.

jimbarry · April 10, 2020, 07:36:34 AM

batch 3, day 6, 8 am

I noticed the previous day the DB was set to 120º the cost to run was $16.76 for that day. This past night's DB was set to 125º and the cost to run was $16.71 for yesterday. Which supports my theory that running higher temps does not mean it will cost more. The only additional cost is bringing the air up to the higher temp.

doc henderson · April 10, 2020, 07:47:31 AM

looks like the higher temp gets you a larger WB depression, so the hotter air can hold more water. also helps motivate the water at the core to move. 8)

YellowHammer · April 10, 2020, 08:13:53 AM

Thats the key to faster drying, as the ability of air to hold water greatly depends on its temperature. So air at 68F can hold twice the amount of water vapor at 50F.

Thats why it's so important to vent or other wise get fresh cool air in the kiln, get the moisture laden air out the kiln, and use the mass of the already heated wood and stuff in the chamber to heat the air without significantly dropping the temps of the basic kiln and wood mass. So the more mass in the kiln, the faster the dry incoming air will reheat. Hot rocks.

When I'm really trying to maximize drying capability, (pine, cedar, poplar) I start bumping right up the the 130F pressure limit. I've tripped mine more than once. Stan always gives me heck about it, but its not a temperature switch, its a pressure safety, generally in the three kilns I have had, the pressure switch really pops at 133 or so. I routinely run to 125F to dry faster, just creep up on it. Resetting the pressure safety is no big deal, and if you are monitoring your unit, you will now know where the limit is. Then operate closer to the limit, just try to not go over it.

jimbarry · April 10, 2020, 11:24:01 AM

What YellowHammer said about using thermal mass confirms what I was thinking. I really only started to see progress after I did that 30 min dump. Moving forward, I think what I'll try on the next batch is once the thermal mass is sufficient, I'll be doing ><30 min air dumps. Or whatever deemed sufficient.

I took a quick peak at 12 noon. 35.9% probe avg. The diff between DB and WB was 15.4.

jimbarry · April 10, 2020, 11:24:59 AM

The only other questionable thing going on with this load is the back half if drying much slower that the front half.

doc henderson · April 10, 2020, 01:25:58 PM

Is it mixed species? as long as the kiln can maintain that DB/WB spread, I think it means the DH is keeping up with the water.

jimbarry · April 10, 2020, 07:19:24 PM

Mostly maple and birch.

jimbarry · April 10, 2020, 07:22:53 PM

batch 3, day 6, 8pm

jimbarry · April 11, 2020, 07:33:24 AM

batch 3 day 7 8 am

What I am noticing especially with this batch is as the wood gets drier, the kiln uses less energy per hour.

doc henderson · April 11, 2020, 11:16:00 AM

that makes sense due to the energy used to convert liquid water to water vapor. Have you come up with any ideas regarding the wood in the back vs the front. in terms of the different drying rates? Hoping with less overall water in the wood, that the drying rate may pick up. easier to maintain temp, and dryer air. thanks for all the info.

jimbarry · April 11, 2020, 11:33:17 AM

Regarding the uneven drying, my guess is the firewood in section (probe) 2 is not stacked as well to allow for sufficient airflow.

jimbarry · April 11, 2020, 07:58:50 PM

batch 3 day 7 8 pm

DB 124.7 WB 104.5

Probes: 41, 37, 24, 23, avg 31.2

doc henderson · April 11, 2020, 10:49:39 PM

can you put a couple box fans on the area drying slower. ? not sure if it would help.

jimbarry · April 12, 2020, 05:22:44 AM

Box fans wouldn't help, not powerful enough. I think its because the tarp that's draped over the entire stack is flapping in that area. When I raped the tarp first and turned on the 3 overhead fans the entire tarp was flapping up and down. I tossed a couple pieces of wood as far as I could reach but could not access to the back.