Wood-mizer KD150 Series, User Manual

Page: 35 / 47

29
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.
T he Drying Rate Index respon ds 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 knows that things dry faster in
hotter, drier air. Every fluid has a vapor pressure
associated with it that varies with the temperature of
the fluid. Air has a vapor pressure that is of function
of temperature and humidity. The difference
between 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. For
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 32), 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. For
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 that in 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 lower. In some
cases, were this pressed to an extreme, it may make
the surface shrink too much so that factor should be
considered.
The best way to use the drying chart is to check the
kiln each day. Calculate the moisture loss and then
adjust the kiln temperature and humidity each day
to achieve the drying rate desired. Each charge of
lumber will be different and will result in a different
drying schedule. For this reason, it will be clear that
drying schedules are of little use except as a starting
point. None of the drying schedule published state
at what airflow they were developed. So running
a schedule from a manual or another operation is
ineffective as the airflow in the kiln may be different
and may change through the cycle if variable speed
fans are used.
Wood-Mizer
KD150 12/22/17