Pitch Set

[Kierman Dimitri]

What should the Wet Bulb Depression be set at during the Pitch set process?

[MichaelM]

A Tw for setting pitch is not specified in the Dry Kiln Operators Manual.

The DKOM indicates Td > 160F for 4/4 and Td > 170F for thicker material. 

A few points, however,

Setting pitch happens throughout the schedule.  A higher temperature early will help if it doesn't stain the wood.

170F is just a recommendation, not a silver bullet.  It doesn't always work and a higher Td may be needed or a higher Td earlier in the schedule.

The pitch in pockets may never set during drying.

[Kierman Dimitri]

Do fluctuating temperatures affect capillary flow in the lumber, such as during a Pitch set early in the schedule where the temp may suddenly rise up to 60 degrees above the set temp?

[MichaelM]

Not sure I understand.  The kiln should be at the set point temperature and should not be 60 degrees above it.

If you are thinking to raise the starting set point temperature by 60F, I wouldn't do that.  What I was suggesting is that pitch is set throughout the schedule.  If you are running 120F for the first day, 130F for that first day might help set the pitch better than adding 10F at the end.  We usually talk about pitch set at the end because if we raise the temperature in the beginning we might damage or stain the wood.  Sorry for any earlier confusion.

[Kierman Dimitri]

Normally we run a set temp of around 120deg and in order to get a good pitch set we will increase our temp to 170deg. Often times if we do a pitch set late in the schedule we will not have the capacity using live steam injection inside the chamber to maintain a norrow depression. As a result the pitch set is done early in the schedule to utilize the free water coming from the lumber  to facilitate a narrow wet bulb depression during our pitch set process. I do suspect however that this process of increasing the temperature early on (Mainly when above fiber saturation) could break the capillary flow of moisture when the temp is decreased back to the set temp of 120deg then in turn decrease the overall drying rate.

[MichaelM]

The reason a kiln is started at 120F is because the wood cannot handle a higher temperature - because of checking, collapse, or stain.  You will also increase the risk of honeycomb and have more stress to relieve at the end by running a higher temperature at the beginning.  I would not suggest jumping to 170F to set the pitch.

On the other hand, if you can run 170F as an initial temperature without problems, there is no reason to go back down to 120F.

Maybe there's something else going on I don't understand.  Feel free to give me a call. - 541-7 three 7- 421 zero