Troubleshooting Wood Discoloration with High Pressure Steam Equalization
Advice on setup details to keep liquid water out of the situation when using high-pressure steam to condition wood in a kiln. June 13, 2014
Question
Is there any danger to lumber when equalizing with high pressure steam in a kiln with no upper fan deck? We dry red oak and white oak lumber and notice that the top layers are very discolored. It appears that the steam causes this to happen. Color is not important to our process, but what about the quality of the lumber?
Forum Responses
(Commercial Kiln Drying Forum)
From Gene Wengert, forum technical advisor:
Are the fans running? If so, then I would expect the conditions to be fairly uniform top to bottom. During fan reversal do all the heat and spray valves shut as well as the fans? It is unusual to use much spray during equalization. What settings are you using?
From the original questioner:
Yes the fans are running. During fan reversal all valves are shut. Our EQ setting is 160/141. Our MC target is 10%. The spray valve is located behind the fans and sprays directly over the top layers when the fans are cycling that direction. When the fans reverse it sprays directly down the plenum on the back wall. The valve opens at 1.5 degrees below set point and shuts off at 2 degrees above set point at which time the vents open. Also could it be boiler chemicals causing the discoloration?
From Gene Wengert, forum technical advisor:
It is not boiler chemicals. It is likely that you do not have an adequate system to remove the liquid water from the spray line - slope of the line downward from the entry point and a drain without a trap are commonly used to remove liquid. Are the holes pointing upward? Is the spray line close to a corner?
From the original questioner:
The holes are pointed downward and the spray line is along the back wall of the kiln under the fans.
From contributor G:
If the holes are pointed down, then you are putting water into the kiln as well as steam. As Gene suggested above, the spray line should be sloped away from the boiler so condensate in the line will drain out. With the holes pointing down this isn't happening. Point the hole vertically or slightly off vertical away from the fans, and you'll get only steam and not liquid water when you open the spray line.
As for the no fan deck I believe this creates a problem with the kiln's airflow. A fan deck helps create the proper plenum for the air coming off the fans, and keeps maximum volume going to the load of lumber. If you can go in the kiln with the fans running, take a handkerchief or something similar and check the air velocity and direction from top to bottom on the exiting air side of the load. The results may surprise you. I have seen kilns like this where the airflow is quite different between the top layers and the bottom layers.
From Contributor R:
Definitely slope the spray pipe down from the entrance and point the hole upward. Put an elbow on the end of the spray pipe, pointed down, run the pipe outside the kiln, and add a small float type steam trap. You can add a manual valve just before the trap so you can see how much water is in the line. You might be surprised. Be sure your spray valve isn't leaking.
