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Structural variations of the
air ducts of roof duct driers in practice
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Grain and maize is dried with hot air. This
classical drying method is simple and achieves a powerful, high-quality
and careful drying with relatively low investment and operating cost.
Generally, the construction of the so-called roof-duct drier has proven
itself. This means that the air is fed in and led off through horizontal
roof-shaped air ducts which are open downwards.
fig. 1
fig. 2
fig. 1: sectional view of the drier. The product is fed into the drier
column from above (arrow at the top) and led through the plant to the
discharge by gravity (arrow below). On the right side, the hot air (red)is
led into the drier column (yellow). On the left side, the water-enriched
air is drawn off from the drier (green). You can see a cooling zone in
the lower area (blue).
fig. 2: functional principle of the roof-duct drier. The red ducts
symbolize the hot air, the green ones the exhaust air. Between them,
there is the product to be dried (yellow).
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The form of the air ducts respectively ducts is an important
constructional feature. In practice, two constructional designs have
prevailed – the conic as well as the straight form of the roofs.
fig. 3: On the left side, the construction with conic roofs is shown, on
the right side with straight roofs. The red arrows symbolize the hot air
inlet, the green ones the exhaust air outlet.
With the conic design, the air ducts have a variable cross section in
the drier column with regard to the depth of the column. Driers with a
straight roof design have a constant cross section in the air ducts
across the total column depth.
A great advantage of the conical design is the easy piling-up of the
roofs during production and especially for transport. This is possible
only restrictedly with straight ducts.
The conic roofs are the cheaper design also with regard to assembly. The
roofs can be pushed easily through the openings in the lateral elements
of the drier columns.
Straight roofs require a more costly assembly. The completely
pre-assembled ducts are fixed separately from inside with numerous
fastening parts when the column elements are mounted. This naturally
increases the production costs.
The disadvantage of the conic design is, however, the missing stability
as there is no tight connection between duct and front plate.
Furthermore, there is no even product bulk height in the column, the
product runs down faster in the middle of the column than on the edge.
This results in a lower total retention time and consequently a higher
final moisture of the product. Practical measurements showed differences
in final moisture of +/- 3% (drying of maize from
35 % to 15 %).
There is no such problem with drier columns with straight roofs because
of an even bulk height, provided that a discharge system is used that
conveys the product constantly across the column surface.
Normally, driers with conic roofs have much lower connected loads than
comparable driers with straight design. This also results in lower
electric consumptions.
With regard to the heating energy, there are hardly any differences
between the two systems. The conic variation, however, has a higher
energy consumption if the irregularities of the final moisture have to
be compensated by overdrying the final product. Then the design with
straight roofs can be more favourable as a whole.
Basically, there are two materials for driers nowadays: galvanized sheet
steel and aluminium. Practice has shown that galvanized or
galvanized/chrome steel plants have an operating life of 5 years only
until they corrode due to moist maize (35%). Driers in aluminium design
do not have problems concerning corrosion. These driers usually have
straight roofs as conic-roof designs require steel materials.
Still both variations of drying plants are produced. The conic design
has commercial advantages, the straight design, however, clearly has
technological advantages. The drying result becomes more constant, the
driers are more stable and you can decide on the higher-grade aluminium.
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