Commercial Kiln Drying

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High moisture developing in back of kiln dryer

1/6/17       
Ahmad kurdi

Website: http://www.blackseawood.com

We have installed 3 new kilns with boilers running on wood waste and shavings . All three dryers are having the same problem. When the fans are blowing away from the radiators ( readings are taken from the box infront of the fan direction) slowly the box behind the fans starts building up moisture reading ( this is normal as the air is going through the wet wood and extracting moisture ) , the problem is that the moisture reading builds up to a 5% difference between the two boxes . Now when the fans reverse direction (programmed to do so every 2 hours) the system starts reading this high moisture from the box and immediately put up the alarm of high moisture , this causes the vents to fully open and consequently the temperature drops as the vents are trying to lower the moisture .
In the opposite direction ( fans are blowing towards the heaters ) the moisture in the box behind the fans build up but not more than the 1-2% difference , hence when the fans reverse and take that reading , system stays normal and vents only open 10-20% at which the radiators are able to maintain temperature normally
We checked all the system , loading of wood, however we are suspecting that bcs we don't have a siphon installed at the drainage ( back side ,below radiators ) moisture is building up , OR we are also suspecting the inclination of the flooring of the kiln ? Anyone has any idea why this is happening ? Any suggestions ?

1/21/17       #2: High moisture developing in back of ...
Carlos Levine  Member

How much space you have between the wood and the wall, maybe you haven’t left the right space so the air flow goes smooth, maybe is creating turbulence or build up air can’t flow correctly creating humidity built up, you need more details, you have to create a log of every detail, their are so many reason, maybe you do not have enough space between boards and creating air flow problems and creating moisture build up. You should give a call asap to the guys who sell you not 1 but 3 new kilns to correct this problem, they are the ones should work non stop to correct this problem

1/21/17       #3: High moisture developing in back of ...
Ahmad kurdi

Thanks for your reply
We are using 22mm sticks , we are using 100x100 dunnage under the bundles . We are starting the loading1.25m from the back , not stacking under the radiators .
The manufacturer sent an engineer , he checked everything , literally every single connection , pumps,pipes , boiler , insulation , floor inclination, drainage .....couldn't find a single thing .
We had different ideas , some people in the company are convinced the radiators are not enough , the manufacturer is arguing that if that was the case temperature wouldn't raise and reach setting in one direction but rather would in both directions (which makes sense )
Then we made a trial ( amongst 10s of trials ) and we moved the box at the back slightly ( it is positioned in the middle of the back wall) , we saw a good improvement though it didn't cancel the alarms completely .
Perhaps someone would explain the position of the back box and also the under sized supply of radiator effect
We are getting high moisture in the back under the radiators . When the fans reverse cycle , the system sees this high humidity and opens dampers 100% causing the drop of temperature
Appreciate your help

1/21/17       #4: High moisture developing in back of ...
Carlos Levine  Member

Where are located the humidity sensors?, have you try to place them in a different location?? We always start from the easier solutions, sounds silly but maybe will fix the alarms to open the gates, have you check personally with a different humidity sensor to check they have the same readout?

1/31/17       #5: High moisture developing in back of ...
Gene Wengert

I wonder if the reverse direction gives a higher reading because poor air flow across humidity sensor.

The next time the kiln is empty, lose the doors turn on heat, close the vents and run the fans. Readings should be identical. Reverse fans and again identical readings.

Where are the vents related to the boxes?

It would be helpful to see a sketch of kiln with vents, heat, fans, lumber, etc

1/31/17       #7: High moisture developing in back of ...
Ahmad kurdi

Thanks Gene
This is a clever idea indeed . Well so far , the conclusion from the manufacturer is that the radiators provided are not enough , and secondly the pipes are two closely positioned . Hence , they increased the number of pipes and increased the distance between them . So the new radiators ( they are supplying next week under warranty ) have 2 rows of 12 pipes each instead of 1 row of 18 pipes . Still the same width and same pipe diameter
We will test these in about 10-14 days and I will let you know how it goes should anyone run into the same problem they would know the problem is a poor design of radiators preventing the proper air flow
Thanks for the tip

2/1/17       #8: High moisture developing in back of ...
Gene Wengert-WoodDoc

What type of instrumentation do they have. I hope it is resistance bulbs rather than a filled capillary system. What type of humidity sensor do you have? Also, when they reverse the fans, are you certain that the sensors are actually reversing as well? If one box is against the wall, it might be seeing the cooler temperatures from outside affecting the readings. How much time delay is there when the fans reverse? If more than 1/2 minute, do the heat and vents and steam spray all shut off? How close are the boxes to the vents? Are the fan blades intended to reverse direction?

2/1/17       #9: High moisture developing in back of ...
Gene Wengert-WoodDoc

A simple proportional control system will have three settings. The first is proportional control that opens the vents in proportion to the difference between what you want and what you have. If this is set too sensitive, a small difference causes the vents to open too much...too sensitive...and you will have the problem you mention. More heat will not fix it. So, have them reduce this setting. Next, a control system will note that in order to keep the setting exactly where you need it from one minute to the next, it takes a little bit of opening of the vent or valve. (Similar to an accelerator on a car...at 100 km per hour, on a flat road, you need a little depression of the peddle to maintain your speed.). Finally, the system will look at how fast you are approaching the set value and begin to ease back before you get there so you do not overshoot. (In an car, we begin to reduce the accelerator peddle a little as we get close to the speed we want...the closer we are, the more we cut back. From your description of the problem, it sounds like they did not set the proportional control, rate and reset correctly. With more radiators, you will not totally correct any incorrect control system settings.

Is this the first kiln they ever built of this size or with this control system? It is indeed common with wet wood to have a difference in temperature AND HUMIDITY on each side of the radiators.


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