World Grain - July 2005 - 43

feedoperations

* Product ingredients - If the product ingredients change, then you have
a different product. Although the new
product could behave similar to the
original, remember every product has
its own drying curve.
* Product size - As with the ingredients, changing the product size
ultimately creates a new product.
You should expect a quicker drying rate in a product with more
surface area relative to the size of
the product.
THE AIR
A fine balance exists between the
amount of air used, the humidity of
the air, and the temperature of the
air to achieve the desired drying
rate. Most dryers re-circulate the drying air to build humidity and increase
efficiency. Too much recirculation
will cause the humidity to climb to
the point that you could make the water vapor condense within the drier;
too little and you are throwing air
out of the exhaust stack that still has
good drying potential. You need to
find the optimum balance that works
for your product.
Five air-related characteristics to
check:
* Process air temperature - This is
the temperature of the air that will
do the drying. Check to see if it corresponds to your baseline.
* Process air humidity - Air is like a
sponge: the wetter it is, the less it will
absorb. This can typically be adjusted
by controlling your exhaust volume
via a damper or VFD on the exhaust
fan. The more exhaust, the lower the
recirculation air humidity will be and
the more drying you can do. Remember, this may come at a price since
you will have to replace that excess
exhausted air with fresh make-up air,
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which will require energy to heat.
* Process air volumetric flow rate
- This is typically hard to measure
in the field. One good estimate is to
take the static pressure across the recirculation fans and use the fan curves
to estimate the airflow. A good rule of
thumb is "more airflow is better as long
as it does not entrain fines into the air
stream." There are exceptions, but this

haust." You should consider having it
checked by an expert.
* Air flow short circuits - You are
losing valuable airflow anytime the
air can leave the fan and return to the
fan without being forced through the
product. A small short-circuit will
have a significant impact on your
capacity. A common short-circuit is
a gap between the inlet cone and the

is normally the limiting factor.
* Exhaust air volumetric flow rate
- This is the air that is taken out of
the drier. The less air you take out
at the lowest temperature, the more
efficient the drier is. However, remember the drier still needs to do the
job, and if you do not have enough
exhaust, performance will diminish.
This is too product specific to make
recommendations, but a very loose
rule of thumb is "if you open the door
to your drier and your safety glasses
fog up, you may not have enough ex-

Aeroglide 3-pass sanitary drier.

fan wheel of the recirculation fan.
These should overlap. Refer to the fan
manual for more details. You could be
losing 40% to 50% of your airflow
without even realizing it.
THE DRIER
With all types of driers, a common
theme is that they are often the most
energy intense piece of equipment in
the process. Day in and day out they
run as a big "black box" where opera45
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World Grain - July 2005

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