Other Benefits to Continuous Kilns

[M Gallahar]

I know "continuous kilns" save time. Does anyone have any data showing other benefits such as saving energy?

[MichaelM]

A continuous kiln should save energy.  The total time for wood to be in the kiln may actually be longer, not less, but that is probably not an important consideration.  The boiler will fire at a steady rate.  This may help with its control.  There should be no huge steam draw when a charge heats because this happens continually eliminating poor coil drainage due to high load.  Conversely, there is no low-load period at the end to cause could flooding because of low delta P across the traps.

Does the mill do enough of one or two products to keep the kiln feed?  You can feed two products at different rates, one from each end.  If the mill has 10 different products on 10 different schedules, it might not work well because of the product changes.  Although, you might make it work, a few high volume products would probably be easier to manage.

Do you have good stacking and loading practices?  You don’t want piles falling in the kiln or weights falling.  Adding weights will increase degrade if stacking is not done well.

Part of the energy efficiency is from condensing water.  You must consider how to dispose of the water.  Maybe to municipal treatment?  I would assume it cannot be simply discharged because of BOD.

There are lots of these in the south and I'm sure your vender could arrange for you to see one.

Mike M.

[HencoV]

I would like to see a list of all benefits and negatives on these kilns? Apparently what sells it is the steady energy supply as Mike pointed out. From a control and optimizing point of view I wonder about a few issues:

As seen on the pic, the timber coming out such a kiln looks servery over dried judging by the colour? Is this normal? Overdrying is normally associated with down grade? ...hoping it will be sorted out in the conditioning chamber?

What happens if you need to re-dry boards that comes out wet?...need another kiln?

Are these kilns really so energy efficient?  "lb steam per board ft dried" and "kW hours per board ft dried" ratios versus the same ratios of similar capacity standard batch kiln (volume per month) would be interesting to see?

 Not sure about the control of these kilns. If you get excessive down grade, can you change any conditions, except taking it out sooner or changing energy input/ DB set point..which increase drying time?

[M Gallahar]

Thanks for the response, Michael. Focusing on high volume products makes a lot of sense.

[GeorgeCulp]

Almost all the continuous kilns in the south are direct fired. A direct fired kiln can have considerable varying dry bulb temps within the kiln due to the difficulty of balancing.

In a CDK all the lumber will see the same drying conditions down the length of the kiln even if they are varied which will make a large difference in resulting quality. Any top to bottom variances down the length of the kiln will still be there.

Direct fired burners normally have to operate from a high rate of heat output to a low rate of heat output. As Mike pointed out the heat demand will be steady. The direct fired heat plant can then be diesigned for this "sweet spot" of continuous operation which will helps its operation greatly.

In a CDK moisture migrates from the main drying chamber to the conditioning chamber on each end where the moisture laden air can be circulated through the green track, then heat will be removed by the green lumber and you get liquid runoff. The air then goes through the dry lumber on the other track and is reheated slightly and back around. Here is where the energy savings are as well as a heat plant running steady state.

Since a CDK rarely ever shuts down no time is lost switching out charges which over a week could possibly cost close to a charge depending on your charge time.

You don't necessarily have to have HIGH volume items to run in a CDK, you can just have a shorter kiln which will lengthen the pushrate time therefore causing the lumber to be in the kiln longer. I do agree that it is not good to be changing sizes. In fact, it is optimal to run the same size on each track, as to run different thicknesses on each track will cause some different air flow measurements.

If increased volume is not the primary desire, but increased quality and energy saving are, then a shorter CDK could be the answer.