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Cooling
of Extruded, Dry Pet Foods
Paul Tedman
, PE
Extru-Tech,
Inc.
Sabetha
,
KS
One
of the critical processing steps in the production of
extruded, dry pet food is cooling. The
cooling step is required after the drying and liquid-coating
processes to remove excess heat.
If enough heat is not removed, condensation can occur
in the product storage bins or in the product package.
This article will discuss the major change in dry pet
food that has prompted a change in the method that pet food
needs to be cooled. The
article will also review horizontal and vertical cooler
designs, typical processing conditions, and how much cooling
is required.
Changes
in Pet Food
In
the last 10 to 15 years, product changes have caused the
cooling process to be redesigned in most pet food production
facilities. In the
early stages of pet food production, specifically dog food,
fat addition levels applied at the coating process were
usually less than 10%. The cooling step was normally done
before the coating process, as the typical plant flow was
extrusion, drying, cooling, coating, storage and packaging.
The pre-cooled product would satisfactorily absorb the fat
added in the coating system, and the additional heat added
from the fat was usually not sufficient to cause storage
problems.
As producers began to make dog food with higher levels of
added fat, problems were encountered in the coating process.
The cool pet food would not absorb the high levels of
fat, resulting in pet food with large amount of free fat on
the external surface. It
was quickly discovered that if the pet food was coated while
it was warm, it would absorb the fat much more effectively.
The major problem this created was that most plants
were not designed to cool the product after the coating
system. Since that
time, some plants have been modified to incorporate the
cooling process after the coating process.
The majority of the new plants being built today are
designed to cool the product after the coating process.
Types of Coolers
Most
pet food production systems built prior to 1990 commonly
incorporated a horizontal cooler (Figure 1) between the drying
and the coating process. These
coolers were normally an extension of the dryer, which is
often referred to as a combination, horizontal dryer-cooler.
Although horizontal coolers can be arranged as
stand-alone machines, it is more common to see them included
as an integral part of the dryer.
Incorporating the cooler into the main body of the
dryer is a very cost-effective method because the capital
investment is reduced compared to a stand-alone unit. In
addition, the heated air recovered from the cooling process
can be used as make-up air in the dryer, making the overall
operation quite efficient.
The general design of a horizontal cooler consists of a
perforated conveyor on which the pet food is uniformly
distributed up to depths of 4 to 12 inches.
The conveyor is sized with sufficient length and width
to provide ample retention time to cool the product.
Ambient or chilled air is pulled through the product
bed with a centrifugal fan to remove the heat.
Figure1:
Illustration of a combination horizontal dryer/cooler.
Horizontal coolers do not perform well when trying to cool
post-fat coated pet foods due to the amount of free fat on the
external surfaces of the food.
The free fat, along with fines present in the product,
will readily plug the perforations in the conveyor.
Once the perforations are plugged, the airflow through
the product bed is reduced, which will result in inadequate
cooling. As a
result, most new facilities, as well as many existing
facilities, have switched to vertical coolers rather than
horizontal coolers.
Vertical coolers, like horizontal coolers, have been
used in the feed industry for many years.
There are many different types of vertical coolers, but
all function in a similar manner.
Figure 2 illustrates a typical vertical cooler.
The product continuously enters through the top of the
cooler through a rotary valve or airlock.
The product is distributed into a square or round
storage vessel where it can be held for a specified retention
period. When the
level of the product reaches a predetermined level in the
storage vessel, product is discharged from the cooler until a
specified low level is reached.
At that point, the discharging is stopped until a high
level is again reached. There
are several different designs of discharge mechanisms,
depending on the manufacturer, used to discharge the product
from the cooler. Ambient
or chilled air is drawn through the product bed via a
centrifugal fan to cool the product.
Product depths are usually much deeper in a vertical
cooler compared to a horizontal cooler, with depths often
exceeding 3 feet. The
increased product depth causes the air pressure drop to
increase, which means the power requirement for the fan is
more when compared to a fan used on a horizontal cooler.
Figure
2. Typical
vertical cooler arrangement.
Vertical coolers are becoming widely recognized as the
standard cooling device in pet food manufacturing because of
the many advantages they offer over horizontal coolers.
Probably the single most decisive reason that the
vertical cooler is selected is because it will perform
satisfactory with pet food that has been coated with high
levels of fat.
Figure
3. Advantages (+)
and disadvantages (-) of horizontal and vertical coolers.
Horizontal cooler is considered to be a stand-alone
unit.
Figure
3 lists some of the advantages and disadvantages of horizontal
and vertical coolers. One
of the advantages the horizontal cooler has is that it can
handle very large products, such as extruded bones, without
causing damage to the product.
The horizontal cooler also works well for very small
products, such as particles less than 2.5 mm in diameter.
Because it is difficult to get airflow through a deep
bed of very small particles, the product can be spread out
very thin on the conveyor of a horizontal cooler, allowing air
to pass through the product much easier.
The advantages of the vertical cooler, in addition to handling
high fat pet food, is that it usually takes up less floor
space than that of a stand-alone, horizontal cooler.
The vertical cooler is designed to hold the product to
greater depths than in a horizontal cooler, which ultimately
results in less required floor space for the machinery.
However, vertical coolers can be much taller that
horizontal coolers, requiring much more vertical space.
Extru-Tech, Inc. offers a vertical cooler (US Patent Number
5,906,482) which is specifically designed for cooling extruded
pet foods (Figure 4.) Product
enters the top of the cooler via a rotary airlock.
There is no internal discharge grid, which minimizes
product breakage. The
cooler is round in design, which makes it easier to evenly
distribute the product compared to a square design.
A significant portion of the internal cone of the
cooler is perforated, which allows air to be drawn through the
product. Due to
the greater working depths in the cooler, the fat has time to
absorb into the pet food before it reaches the perforations.
As a result, plugging of the perforations is not a
problem.
Around
the outside of the perforated cone is a secondary cone, which
serves as an air plenum and is where the cooling air enters
the cooler. The
advantage of the air plenum is that it can be directly
connected to a ducting system, allowing for outside, chilled
or filtered air to be used in the cooling process.
After the air is drawn through the product, it exits through
the top of the cooler. The
exhaust system consists of a centrifugal fan and a dust
collector.
Figure
4. Illustration of
a vertical cooler manufactured by Extru-Tech, Inc.
Product
is discharged from the cooler with a vibratory conveyor.
The vibratory conveyor is the preferred method as it is
easy to clean and maintain, and product breakage is minimized.
The coolers can be constructed of carbon steel or
stainless steel, with stainless steel being the most common
construction material.
Due to the design of the cooler, it is impossible for air to
bypass the product as long as the product level is maintained
in the normal operating range.
Because air cannot bypass the product, the ratio of the
volume of air to the product throughput rate is quite low (CFM/Ton).
In most cases, the exhaust air volume can be reduced to
the point where the exhaust air temperature is equal to the
incoming product temperature.
This reduces the amount of exhaust air and minimizes
the power requirement of the exhaust fan.
Typical
Processing Conditions
The
pet food temperatures directly after the exit of the dryer
will generally range from 140 to 170°
F. In some cases,
where the food is dried to a very low moisture content, the
temperature will be higher.
After exiting the dryer, the food is usually conveyed
to a screener where the fines and over-sized particles are
removed from the main stream.
After screening, the pet food is usually coated with
fat. The pet food
will flash a significant amount of heat as it is being
conveyed from the dryer to the screener and to the fat-coating
system. It is not
uncommon to see product temperatures dropping 40°
F before reaching the coating system.
The temperature of the pet food after the coating
system will depend on the amount of fat that is added.
High fat additions will cause the temperature to rise
due the temperature of the heated fat.
Typical product temperatures at the exit of the
fat-coating system will range from 100 to 130°
F. From the
coating system, the product is conveyed to the cooling system.
The cooler must remove sufficient heat to avoid
condensation problems in the storage bins and the final
package. Safe
storage temperatures are discussed in more detail in the last
section of this article.
The required retention time to cool a product such as pet food
is dependent on three variables: 1) The ability of the product
to conduct heat, which is referred to as the coefficient of
thermal conductivity, 2) The geometry of the particle (this
would include the surface area and thickness), and 3) How
close the temperature of the product needs to be to the
temperature of the air which it is being cooled with.
Typical cat and dog food can be properly cooled with 10
to 15 minutes retention time.
The amount of cooling air required is a function of how much
heat needs to be removed from the food to lower the
temperature to a safe level.
The air will only remove heat from the product as long
as there is a temperature differential between the air and the
product. The
maximum temperature of the air will equal the temperature of
the incoming product. However,
chilled air can be used. This
will allow more heat to be carried away with the air, which
can lower the amount of air volume required.
Chilled air also allows the product to be cooled to
lower temperatures.
Most cooling systems will require 600 to 1200 cubic
feet per minute (CFM) of air per ton of finished product in
order to accomplish adequate cooling.
How
Much Cooling is Required
Condensation
occurs when an air-water vapor mixture is cooled to or below
the corresponding dew point temperature.
Figure 5 illustrates the relationship between ambient
air temperature, relative humidity and dew point temperatures.
As can be seen in the table, as the relative humidity
approaches 100%, the dew point equals the ambient temperature.
So at very high relative humidity, only a slight amount
of cooling of the ambient air will cause condensation to form.
At
30% relative humidity, there is a much larger differential
between the ambient air temperature and the dew point
temperature. At
120°
F ambient air temperature, the corresponding dew point is 80°
F. At 70°
F ambient air temperature, the corresponding dew point is 37°
F. So at lower
relative humidities, larger temperature changes can occur
before the risk of condensation occurs.
Figure
5. Dew point
temperatures for range of ambient air temperatures at various
relative humidity levels.
The
above information can easily be applied to the cooling of pet
food to determine if adequate cooling is being done before
storage or packaging. When
pet food is stored in bins prior to packaging, the free air in
the bin will be heated by the food until equilibrium
temperature is reached. If
the pet food is still warm, it is also possible for moisture
to continue to evaporate from the food.
This can create a condition where the free air in the
top of the bin is warm and moist.
By measuring the temperature and relative humidity of
the air in the bin, the corresponding dew point temperature
can easily be determined from the data in the chart.
If the outside of the bin (assuming it is not insulated) is
subjected to temperatures equal or below the dew point
temperature of the internal air, condensation will occur on
the inside of the bin. Condensation
will form on the internal walls and will drip down and
contaminate the food.
In
the case where pet food is packaged on-line, it is usually
stored in a warehouse directly after packaging.
In many cases, warehouses are maintained at much lower
temperatures than the processing areas.
If ambient air is used to cool the pet food, this means
there will be a significant temperature difference between the
product and the warehouse.
The temperature of the air in the package will equilibrate to
the temperature of the product.
Again, by measuring the air temperature and the
relative humidity in the package, the corresponding dew point
can be determined. If
the warehouse temperature is at or below the dew point
temperature, a risk of condensation exists.
In a lot of cases, the product will spend very little time in
the warehouse and will be loaded directly on a truck.
In this case, consideration should be given to the
temperature that the pet food will be exposed to during
transportation.
A general rule of thumb in cooling is that the pet food
temperature should be cooled within 10°
F of the temperature that the package will be exposed to
during storage or transportation.
This may mean in some cases, such as during the winter
months, that outside or chilled air will have to be used in
order to lower the temperature of the product to safe levels.
During the warmer periods, ambient air is usually
satisfactory.
Summary
A
lot of changes have occurred in the area of cooling in the
last decade, and future changes are inevitable as new products
are developed on a daily basis.
For now, vertical coolers seem to be the preferred
method of cooling pet food, mainly due to the high levels of
fat being applied in the coating process.
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