Hello All,
There was a strange situation on the new control system when I checked
it this morning. The first thing I noticed was that there was an alarm
message box (the small 1/2" square at the top right of the screen)
flashing and the system had not taken a wet bulb depression since Friday
night. The normal screen with the circulating fans was showing.
I noted the following items: A wet bulb reading could not be taken
even in manual mode. The wick wetter valve would not cycle, nor was
there any output from the PLC to operate the valve. The screen indicated
that the system was put on hold at about 21:57 on Friday and the system
was resumed at 12:39 on Saturday. The last wet bulb reading was taken at
21:35 Friday. The total drying time clock was frozen at 12 hours and 25
minutes, which corresponds to when the system was put on hold, as the
kiln was started about 9:00 Friday morning. The "Raising kiln to initial
temperature" or "Kiln Drying" was not indicated at the top of the
screen. The screen would log the correct time and date when the system
was changed back and forth between manual and automatic. In manual mode
I could change the stages up and down but could not change the exhaust
speed. I could turn the exhaust drives off and on though.
At this point I pressed the alarm box and a short message popped up
that said there was a wet bulb problem. I acknowledged the message and
the message and alarm box went away. Nothing else changed, the operation
was still as described above. It seemed to me that the system was still
on hold but the screen indicated the the kiln had been resumed Saturday.
I checked the wick, the water supply , and the whole wet bulb system and
all was okay.
Thinking that the system was still on hold, I decided to restart the
system. Before I did that I pressed the hold button just to see what
would happen. The hold screen came on indicating the kiln was on hold
with a time and date of 12/17/04 21:57. It also gave a warning message
that went something like this: "Dry kiln held due to a wet bulb problem
.... Ack alarm then press resume to continue drying." When I pressed the
resume button the kiln went back to its normal operation and the screen
showed an updated and correct Kiln Resumed time and date. The Total
Drying Time clock started counting the minutes again. The "Raising Kiln
to initial temperature" message returned also. I never did have to
restart the system.
The problem that need to be corrected is that the kiln was on hold
with no indication that it was on hold. The screen showed that the kiln
was held on Friday evening, but also showed that it was resumed Saturday
afternoon.
I do not think that anyone pressed the hold button Friday. There is a
section in the program that detects a wet bulb problem such as a loss of
water, and keeps the kiln from reverting to previous stages if the
depression drops very low for an extended period. If the depression
stays below a 2 degree depression for over 5 hours an alarm is activated
and the system is put on hold. I believe this is what happened Friday.
After talking with Jack, we will probably modify the alarm message to
indicate that the alarm may also be due to frozen lumber and or cool
conditions in the kiln during startup during cold weather. As I
mentioned above the real problem is that the kiln was on hold with
nothing indicated that it was on hold. The hold screen should appear and
not go away until the resume button is pressed, without regards to what
the alarm message and box does. None of this explains why the kiln
appears to have been resumed Saturday when apparently it wasn't.

Any ideas? Let me know.
Mike Kley