What to consider when selecting a convection dryer

What to consider when selecting a convection dryer
www.powderbulk.com
As appeared in April 2013 PBE
What to consider when selecting a
convection dryer
Dan Poirier
Buhler Aeroglide
A convection dryer is by far the most commonly
used drying equipment in bulk solids plants. Because the dryer is available in several types, choosing the right one for your process can be challenging.
You can simplify the task by following this article’s
advice about what factors to consider during dryer
selection and the importance of working closely
with the dryer supplier. A sidebar provides additional guidance on selecting a convection dryer to
meet food safety requirements.
B
ulk solid materials are dried to meet a range of
product goals. Many materials are dried to bring
about physical or chemical changes or reduce shipping costs. Food products are usually dried to enhance
shelf life. No matter why you need to dry your material,
there are many factors to consider when selecting a dryer
and its features. This article focuses on what to consider
when choosing a convection dryer, in which heat is transferred by the direct contact of a hot gas (typically air) with
the material.
In a convection dryer, the hot air typically flows across the
material or through a stacked material bed. Several types
of convection dryers are available, and no one type can
handle all applications. For instance, some dryers are wellsuited to drying small particles, some can’t handle sticky
materials, and others can handle only liquids. Your mate-
rial’s characteristics will dictate the ideal convection dryer
for your application.
The following sections discuss how to match the dryer
type to your material and other application factors when
specifying the dryer. During the selection process, it’s best
to work closely with an experienced convection dryer supplier. The supplier’s understanding of the science of thermal processing will help you choose a dryer with the
features you need to achieve your desired drying results
with the best possible efficiency.
Consider how your material will be
handled in the dryer
Table I briefly summarizes the operation of common convection dryers and their applications. As you can see, the
most obvious difference among the dryers is in how they
handle material.
Some convection dryers, such as conveyor, tray, and tunnel dryers, handle the material very gently. In these dryers,
the particles are stacked on a moving belt or on trays, and
drying air circulates around the particles. Other dryers,
such as fluid-bed and rotary dryers, are much rougher on
the material: The drying air fluidizes the particles — in
some dryers, with high-energy air jets — or the dryer tumbles the particles. While some materials will be damaged
if they’re fluidized or tumbled, other materials may require
a rotary dryer’s tumbling action to break up clumps so that
all surfaces will contact the drying air. These differences
often make a convection dryer’s material handling method
the primary consideration in selecting a dryer type.
Table l
Operation and applications for common convection dryers
Dryer type
Operationa
Applications
Conveyor
Particles are stacked on a perforated conveyor
belt that moves through the dryer as air flows
through the particle bed; dryer may consist of a
single conveyor or multiple conveyors that are
stacked or staged.
Materials consisting of stackable particles from 2
to 25 millimeters, such as extruded foods and feeds,
cereals, fruits, vegetables, nuts, seeds, industrial
fibers, and synthetic rubber and polymers
Flash
Small particles are entrained in a fast-moving hot
airstream and dried as they’re pneumatically
conveyed through the dryer.
Materials consisting of particles below 0.5
millimeters that are easily entrained in air,
such as wood fibers and tobacco
Fluid-bed
Particles are stacked in a bed that’s fluidized
by upward airflow through the bed.
Materials consisting of discrete particles usually
below 5 millimeters with relatively uniform size
distribution, such as polymers and pharmaceuticals
Impingement
Material is conveyed through the dryer on a flat
conveyor or rotating drums while high-velocity
air jets impinge on the material.
Flat materials, such as sheets, webs, and
flat snack products
Rotary
Particles are tumbled inside a rotating drum while
air passes through the drum.
Friable (crumbly) materials, such as biomass
and many waste by-products
Spray
Liquid material is sprayed into a hot airstream and
forms a powder as the atomized droplets dry into
solid particles.
Liquids such as dairy, dyestuff, and detergents
Tower
Material is introduced through the top of a tower
and flows through a heated section, then a cooled
section, while air passes through the tower.
Grains such as rice, corn, and soybeans
Tray or tunnel
Material is laid on trays (often manually) that are
stacked and introduced into the dryer and air
flows across or through the trays; tray dryer
operates only in batches and tunnel dryer can
operate in batches or continuously (with tray stacks
moving through the dryer).
Hard-to-handle materials or materials that must be
processed in batches, such as pharmaceuticals, fruit,
and natural pet treats
Note: All dryers operate continuously unless otherwise noted.
a
Determine the best heat source for the drying air
The convection dryer’s drying air can be heated by any of
several heat sources, including burners, steam coils, thermal oil coils, or waste heat from another process. Fuel
availability at your location will often dictate which heat
source you use.
Natural gas is a clean-burning fuel that can often be directfired in the airstream, even in food dryers. As a result, it’s
the easiest fuel to use in convection dryers. Other fuels,
such as oil or biomass, are often burned in a boiler instead
of directly in the dryer. While steam coils can also be used,
they can only heat the air to less than 347ºF (175ºC) unless
they have very high steam pressure. This can make some
convection dryers, such as rotary dryers, impractical when
only steam is available because they require higher-temperature air to operate efficiently and reduce the dryer size.
Sometimes, free heat from another process is available to
heat the dryer air. For instance, conveyor dryers for drying
charcoal briquets use hot exhaust air from upstream furnaces as the sole heat source.
Consider energy efficiency
Unfortunately, evaporating water from wet material takes
a significant amount of energy, so whatever dryer you
choose, it will likely be the highest energy consumer in
your plant. There are many ways to minimize your convection dryer’s energy use. Recirculating the drying air to
minimize the amount of warm air exhausted from the
dryer is the first step. Beyond that, you can also preheat
makeup air to the dryer or recover heat at the dryer exhaust
to significantly reduce energy consumption. Another
often-overlooked but key way to reduce energy consumption is to make sure that the dryer is operating properly.
Meeting food
safety
requirements
with a
convection
dryer
Food safety doesn’t end with the
design of the dryer itself: The dryer
is just one step in your process.
Choosing the wrong dryer can result in unforeseen problems else-
where in the production line. If
you’re selecting a convection dryer
for food products, partner with a
supplier that understands food
safety requirements. —D. Poirier
F
ood safety is highly regulated and critical for food
producers, so drying food
products comes with more requirements than drying other products.
Food safety considerations for
convection dryers include minimizing flat surfaces where fines
can accumulate, ensuring good access for cleaning, and designing
the equipment for washdown.
This conveyor dryer for drying breakfast cereal uses multiple conveyor beds and is equipped with large access doors to simplify
cleaning.
[Editor’s note: Find more information about reducing a
convection dryer’s energy use in the author’s previous
Powder and Bulk Engineering article “Big dryer, small
footprint: Minimizing your convection dryer’s environmental impact,” April 2008, pages 27-31; also see “For
further reading” later in this article.]
Consider drying uniformity
Drying your material to a uniform moisture content is critical. Nonuniform drying can lead to product spoilage or inferior quality if the dried product has more or less than the
required moisture content. It can also lead to lost revenue
if you’re forced to overdry products to compensate for
nonuniform drying. The revenue gained by reducing lost
product weight due to overdrying can often pay for a new
dryer in less than your required payback period.
A simple way to determine if a dryer will dry uniformly is
to ask the supplier whether every particle will experience
the same processing parameters as it passes through the
dryer. If some particles spend less time in the dryer than
others, they’ll be underdried. Likewise, if some are exposed to lower-temperature air or less airflow than other
particles, they’ll be underdried.
Consider long-term supplier support
Proper dryer operation is critical not only to achieve good
energy efficiency, but to ensure good product quality and
drying uniformity. You’ll need support from your dryer
supplier to ensure that your dryer is operating properly.
The supplier should offer training and come to your plant
to help workers learn how to operate the new dryer. Because downtime costs can rapidly add up when a dryer
breaks down, the supplier should also be able to supply
spare parts quickly.
When you select a dryer, you’re not only buying the equipment, you’re entering into a long-term relationship with
the dryer supplier. You’ll be living with the equipment and
the supplier for decades to come.
Consider life-cycle cost
Finally, of course, cost will come into your dryer selection
equation. Be sure to consider not just the dryer’s capital
cost, but, more importantly, its life-cycle cost. The cost of
running a dryer is far greater than the capital cost. For instance, you could easily spend over $6,000,000 in today’s
dollars on fuel alone to run a $500,000 dryer over its 20-
A biomass burner (at right) is the drying air’s heat
source for this rotary convection dryer.
year lifespan. The costs of maintenance downtime, revenue losses resulting from nonuniform drying, and inefficient energy use could surpass the dryer’s capital cost in
less than 1 year if you choose the wrong dryer.
PBE
For further reading
Find more information on convection dryers in articles
listed under “Drying” in Powder and Bulk Engineering’s
article index (in the December 2012 issue and at PBE’s
website, www.powderbulk.com) and in books available on
the website at the PBE Bookstore. You can also purchase
copies of past PBE articles at www.powderbulk.com.
Dan Poirier is director of process engineering at Buhler
Aeroglide, 100 Aeroglide Drive, Cary, NC 27511; 919851-2000, fax 919-851-6029 (dan.poirier@buhler
group.com, www.buhlergroup.com/drying). He holds a
masters degree in applied sciences from the University of
Ottawa, Canada.
Drying workshop to be held at PBE’s
2013 Midwest Conference & Powder
ShowTM
A 3-hour technical workshop on drying, “Understanding factors that affect dryer performance,” will
be held at PBE’s upcoming Midwest Conference &
Powder ShowTM in Columbus, Ohio, May 21-23.
Workshop instructor Karl Jacob from Dow Chemical will take a practical approach, focusing on key
aspects of reliable and optimal dryer operation.
For more information and to register, visit
www.pbepowdershow.com.
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