30
deviations from normal will indicate a problem in
loading, baffling, airflow etc. That will affect drying.
These figures will always vary but being familiar
with them, the operator will be able to detect a
malfunction early.
Record should be kept for comparison throughout
the year. It is wise to compare summer loads with
summer and winter loads.
Drying Rate Index (DRI)
The dry kiln industry has never had a method of
predicting drying rates. This is remarkable as one
main reason for having a kiln is to bring about
predictable production rates. The EMC value gives
an end point of what the moisture content of the
lumber would eventually become but it does not
give an indication of how long it will take to reach
that moisture content.
Drying schedules for dehumidification drying were
developed in the late 1970’s. It became obvious that
a method of adjusting kiln schedules to meet drying
time objectives within the limitations of the operation
range of the dehumidifiers had to be found. It was
common to look up a drying schedule in the Dry Kiln
Operators Manual or some other reference, and then
find a set of conditions at which dehumidifier could
run using the same EMC. This resulted in unnecessarily
long drying cycles and was quickly shown to be an
ineffective method of doing kiln schedules.
The Drying Rate Index was developed in response
to that but time has shown that the Drying Rate
Index is very useful in both conventional
and dehumidification kilns. With the trend to
control kilns by monitoring drying rate to get
maximum productivity and quality, the Drying
Index becomes a very valuable tool.
The drying rate is a function of the vapor pressure
deficit. Everyone k nows that things dry faster in
hotter, drier air. Every fluid h as a v apor p ressure
associated with it that varies with the temperature
of the fluid. Air has a vapor pressure that is of function o f
temperature and humidity. The difference b etween
the two determines the rate drying. This is how
everything in the world dries whether it is paint, the
ocean, the lumber or perspiration. When the
humidity of the air is 100% no evaporation takes
place regardless of the temperature. As relative
humidity drops the rate the fluid evaporates
increases. The problem is that a way of predicting
the change in drying rate with changes in temperature
and humidity was needed.
The Drying Rate Index is a relative number.
Example: A kiln is operating at 120º F. dry bulb and
110º F. wet bulb, that means the depression is:
10º (120º F.- 110º F. = 10º F.) According to the
Drying Chart, The relative humidity is 72% the Equilibrium
Moisture Content (EMC) is 12.1% and the Drying
Rate Index (DRI) is 1.0. The EMC indicates where the
lumber will end eventually. The EMC is also an
indication of where the surface moisture content of
the lumber will go fairly quickly. The whole board
will eventually be 12.1 % but the surface will reach
12.1% much quicker.
In the above example, the lumber might be drying
at 3% per day but it could be dried faster at 5% per
day. In order to change the drying rate from 3% per
day to 5% per day, it would be necessary to find a
DRI that is 1.67 times the existing drying rate.
Desired Drying Rate/Existing Drying Rate = Multiplier.
(5/3=1.67)
Multiplier x Existing DRI= Desired DRI
The existing DRI is 0.9 so the new DRI should be 1.5
(0.9 x 1.67)
Referring to the Drying Chart (Pg 27), it can be
seen that if the temperature were increased
to 130º F and the depression 13º F (read
between 12 and 14 on the chart), the DRI will be 1.5.
Other combinations of dry bulb and depression
will give the same result.
Example: Dry bulb could be left at 120º F and the
depression increased to 17º F, or the dry bulb
could be raised to 140º F. and the depression left
at 10º F. All of these would result in a 1.5 DRI and
thus would dry the lumber at 5% per day.
Note: The above three choices, 130/13, 120/17
and 140/10, the EMC is 10.5, 9.1 and 11.9
respectively. It is clear that EMC has nothing to do
with how fast lumber dries but it does mean that
low temperatures and bigger depressions may
mean the surface moisture content will be
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