As an aquafeed processor, you are always aware that your drying operation has a significant impact on your bottom line. Drying is a very energy-intensive operation; that’s why most aquafeed producers diligently track the cost of fuel used in their drying operation.

1. A
s an aquafeed processor, you are
always aware that your drying
operation has a significant impact
on your bottom line. Drying is a
very energy-intensive operation;
that’s why most aquafeed
producers diligently track the
cost of fuel used in their drying
operation.
However, are you aware that your dryer could be costing you
10 times as much in terms of lost production compared to energy
costs? You may be sending over 3 percent of your production out
through the exhaust stack without even knowing it.
Downtime for dryer cleaning and maintenance may also be
costing you more than you know. This article will describe the
economics of drying in more detail, and outline three operational
areas that could significantly improve your bottom line.
Energy Consumption
The cost of energy used in operating the dryer is the most
obvious cost of drying. One need only look at the larger burners
in the dryer, or at the boiler providing steam to the dryer, to
realize that money is being spent to remove moisture in the
drying operation. This energy is essentially needed to cover four
heat loads in the drying operation, these are:
• Warming up the product.
• Evaporating water from the product.
• Heating the make-up air.
• Steady-state heat losses.
Since extruded aquafeed leaves the extruder at a high
temperature, very little energy is used in warming up the product.
The pellets cool as they are conveyed to the dryer. Typically,
How you can improve your bottom line profits
by Justin Hamm Applications Engineer, Bühler
ECONOMICS
OF DRYING
AQUAFEED
32 | January | February 2016 - International Aquafeed
FEATURE TECHNOLOGY

2. the pellets enter the dryer above 65 °C. Therefore, most of the
energy in the drying operation is used to evaporate water. The
energy required to change liquid water into vapor (latent heat of
vaporization) is about 2,350 kJ per kg of water evaporated (the
value varies very slightly with temperature).
This is a thermodynamic constant and you cannot change this
value in typical aquafeed dryers, or any other convection dryer
for that matter. Only vacuum dryers operating under an extreme
vacuum can reduce this value, but these dryers are not practical
for extruded aquafeed. However, you may be able to reduce the
energy consumed in evaporating water by reducing the amount of
water that the dryer must remove.
Table 1 shows the tremendous impact that inlet and outlet pellet
moisture can have on the drying load in your dryer. Even a small
increase in product inlet moisture from 24 percent to 27 percent
on a wet weight basis increases the load on the dryer by 27
percent. Naturally, your inlet moisture must be driven by product
quality considerations, but do all of your extruder operators
operate the extruder the same way? If one shift runs the extruder
“wetter” than the next shift, you are wasting energy by adding
excessive water to your product only to spend money in the dryer
to remove the water.
The third heat load category, heating the make-up air, is
typically the second highest heat load. However, in a poorly
operated or designed dryer, this heat load can be the largest. This
is the area where you as a dryer operator as well as your dryer
supplier can have by far the most impact. Your dryer needs a
steady supply of dry make-up air.
Without this influx of air, the process air in the dryer would
become saturated with moisture and unable to dry your product.
The trick is in knowing how much air to bring into the dryer.
Excessive make-up air will lead to excessive energy
consumption. This is because the make-up air enters the dryer
at a relatively low temperature, and after picking up water in the
dryer, eventually leaves the dryer at a higher exhaust temperature.
Clearly, energy is consumed in heating the make-up air to the
exhaust temperature.
It is important to understand that the exhaust air in your dryer
is basically the make-up air leaving the dryer along with the
evaporated water. If you want to reduce the amount of make-up
air in most dryers, you will typically reduce your exhaust, either
by closing the exhaust damper or turning down the exhaust fan.
The temperature of the exhaust and make-up air also affects
the energy used to heat the make-up air. If the make-up air is
preheated before being introduced into the dryer, the dryer used
to heat this air will then use less energy. In an extruded aquafeed
line, preheated make-up air can be drawn from your cooling
operation. Spent cooling air used to cool the pellets is still
relatively dry and can be used as make-up air to the dryer.
Improper dryer operation, such as excessively shallow
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International Aquafeed - January | February 2016 | 33
TECHNOLOGY FEATURE

3. bed depths (shorter retention time in the dryer) or internal
air bypasses can lead to high exhaust temperatures, thereby
increasing the energy used to heat the make-up air to the exhaust
temperature. Figure 1 illustrates the significant impact that
heating the make-up air and various operating parameters can
have on your dryer’s overall energy consumption.
Unless your dryer is very poorly insulated, losses from the dryer
should be low. If, however, you have a dryer design with large
areas of un-insulated or poorly insulated surfaces, thermal losses
from your dryer could also be costing you.
So, how much does it cost to run an extruded aquafeed dryer?
Typically, aquafeed dryers will use between 3 000 kJ per kg and
4 500 kJ per kg of water evaporated. A very poorly-operated or
poorly-designed dryer might even use much more. Based on a
typical value of 3 250 kJ per kg of water evaporated, an extruded
aquafeed line producing 75 000 tons per year will typically use
about 5 x 1010
kJ/year.
If natural gas is used as the heat source in the dryer, and if
the cost of natural gas is 5 x 10-6
US$ per kJ, the yearly cost of
natural gas to produce this tonnage is US$250 000. Naturally, this
cost will change depending on the type and cost of fuel used in
the drying operation.
Figure 1 and our experience in evaluating extruded aquafeed
dryers show that many dryers in the industry are using over 50
percent more energy than they should due to improper design or
operation. This represents a tremendous potential savings for the
aquafeed producer. Imagine saving US$100 000 per year, simply
by adjusting a few dampers and making minor changes in the
way you operate your dryer. Believe it or not, many aquafeed
producers have done just that, after learning how to properly run
their dryers.
Moisture Uniformity
Most extruded feeds are sold by weight. For this reason, if the
product is overdried, it is as if you are sending water at the value
of your product – which should have gone to packaging—straight
out your exhaust stack. Suppose that your extruded aquafeed
cannot contain over 10 percent moisture limit and the product
coming out of your dryer has a moisture variation of ± 3 percent
moisture on a wet-weight basis.
This means you must dryer your product to 7 percent moisture
on average in order to ensure that no product is over the specified
10 percent moisture limit. The result is a 3 percent loss in
production compared to drying the product only to 10 percent
moisture. If your dryer could be made to dry more uniformly,
you could raise your discharge moisture and get more out of
the production line with no additional cost or additional dry
ingredients.
The value of this lost production due to non-uniform drying can
be surprising. Consider the same aquafeed line as in the previous
example, producing 75 000 tons per year. A 3 percent loss in
production from this line represents a possible 2 250 tons per
year of lost production. What is the value of this lost production?
At US$400 per ton, it is US$900 000. At US$600 per ton, it is
US$1.35 million. The numbers are staggering.
The bottom line is that you cannot afford to overdry your
product. A well-designed, well-adjusted aquafeed dryer should be
able to dry your product to within ±0.75 percent or better. When
was the last time you checked the moisture uniformity on your
dryer?
Maintenance and Sanitation
Many extruded feed producers struggle to keep old, high-
maintenance equipment running. The cost of purchasing
replacement parts for the dryer may be well documented, but
what about the cost of downtime for cleaning and maintaining
the equipment? A dryer producing 10 tons per hour of extruded
aquafeed can be producing well over US $100 000 worth of
product each day.
If you are spending even three hours a week of unplanned
maintenance or cleaning downtime, you have lost 30 tons of
final product per week, which equates to US $650 000 per year
of lost production using the above parameters. The cost of this
downtime must be considered when looking at the economics of
your drying operation.
Drying is a critical operation in the production of dried
aquafeed. Your drying operation does not only determine
the quality of your finished product, but it also significantly
affects your cost of operation. Most aquafeed producers are
well aware of the energy cost for their drying operation;
however, not all producers track the cost of over-drying due to
non-uniform drying, or the cost of downtime for maintenance
and sanitation.
Many producers will significantly reduce their drying costs and
increase their production rates by taking a closer look at their
drying operation. If you want to find untapped potential savings
in your dryer operation, a field engineer specializing in dryer
optimisation can give your dryer a full mechanical and process
evaluation.
Bühler Aeroglide has a long history of teaching customers
how to identify and implement process improvements. The
company’s field engineering group has provided expertise and
evaluation services to processors around the world for over 15
years, driven by three guiding principles: educate, evaluate and
enhance.
Education is provided by Bühler is through a series of drying
theory and dryer maintenance seminars called“Drying Theory
Put to Practice.” These seminars explore the basics, including
parameters, balancing, evaluating, mechanical inspections and
troubleshooting using a dryer simulation.
By learning how to adjust and regulate the process for the
desired outcome, participants will be able to find previously
undiscovered areas of opportunity.
www.buhlergroup.com
34 | January | February 2016 - International Aquafeed
FEATURE TECHNOLOGY

4. Innovations for a better world.
Aquafeed – complete solutions from a single source. Turn to Bühler for one of the
most comprehensive lines of aquafeed process technology available anywhere:
from raw material handling, cooking and shaping through extrusion to drying
and coating of finished products. With an extensive know-how and a passion
for quality we ensure not only product uniformity and production efficiency, but
also maximum sanitation and safety. Bühler – gentle processing at its best.
More at nutrition@buhlergroup.com