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Grain drying
Process and applications in modern agriculture
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Introduction
Ever-increasing cultivated areas for growing cereals
(also by mergers) require specific harvest plans to
finish the harvest in the shortest possible time with
most effectively utilized machines.
The harvest plans can be drawn up early considering the
sowing time and the sort. These schedules must be kept
as exactly as possible as otherwise the complete
operation for the rest of the year will be delayed.
On the other hand, more and more supply contracts are
concluded with a fixed date. The weather situation is
and remains the great element of uncertainty.
In Europe, a lot of farmers still rely on a "dry" summer
to harvest the grain with a storable final moisture.
With this method, however, you depend completely on the
weather and risk your economic success totally.
Harvesting in time, in connection with a powerful drying
plant, makes you independent of the risk factor weather.
Especially annoying are thunder storms and hail a few
days before the official harvest date, as the year 2002
showed in an extreme way.
Furthermore, the harvest has been recently reduced
locally to less than 14 days by the permanently
increased performance of the harvesting equipment.
Therefore, the trend towards more and more powerful
drying plants continues. Mobile and easily movable
systems make it possible for the operator to react more
flexibly to the demand on the market.
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Drying methods
Grain is dried by means of hot air. This classical drying
process is simple and has gained acceptance in the field of
agriculture, as a powerful, high-quality and careful drying
is achieved. The investment and operating costs are low
compared with other methods like freeze, vacuum or radiation
drying.
An essential division of the drying method results from the
hot air generation.
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Hot air generation
There are the options of direct and indirect hot air
generation for the drier.
The modern burner systems distinguish themselves by a very
clean combustion of the used fuel. The combustion gases
consist almost completely of carbon dioxide and steam; both
are main constituents of the air around us. With direct air
heating, these combustion gases are mixed with approx. 60
times the amount of fresh air and are led into the drier as
hot air.
A lot of investors also think about an indirect air heater
when buying a new drying plant operated with fuel oil. With
indirect air heating, the combustion gases are drawn
off separately from the fresh air to be heated through a
chimney. The air heater works on the heat exchange principle.
Comparing both systems direct / indirect air heater makes
the functioning principle clear.
With direct air heating, the heat of the combustion gases is
added to the heated process air. Thus the firing efficiency
is 100 %. The direct air heater consists of an outer casing,
an intermediate casing against radiation losses and an open
combustion chamber made of highly heat-resistant steel,
which is arranged concentricly.
1a) direct air heater
With indirect air heating, combustion chamber and gas passes
as heat exchanger areas are divided constructionally from
the process air areas. The indirect air heater consists of
an outer casing, an intermediate casing against radiation
losses and a closed combustion chamber with gas passes made
of highly heat-resistant steel. On demand, there are even
indirect air heaters which can be changed over at direct
heating in almost no time.
1b) indirect air heater
The indirect air heater has a disadvantage compared to
direct heating: a firing efficiency of 100 % will never be
achieved because of physical principles.
Therefore, the achieved efficiency has to be minded most
when an indirect air heater is bought. If, e. g., 5000 t of
maize are dried in one season (from 35 % to 15 %) and an oil
consumption of approx. 25 liters is assessed for 1 t of wet
maize, so a worsening of efficiency of e. g. 2 % at the
indirect air heater means an increased consumption of 0. 5
liters per ton wet maize and for the whole season 2500
liters!
Consequently, indirect air heaters with an efficiency of
approx. 80 % have a seasonal increased consumption of fuel
oil of 12 500 liters – money that is really burnt.
Direct or indirect air heater – it is a fundamental decision
which should be preceded by a market analysis. A detailed
consultation by the manufacturer, whose technical design
should be scrutinized most exactly, is recommended anyway.
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Drying
principle
Drying processes are based on the principle of energy
transfer to the product to be dried, which gives off
moisture as a countermove. This moisture is taken up by the
passing air and taken away. Consequently, the product to be
dried gets warmer and drier during the process, the drying
air gets moister and cools down.
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Constructive designs
Generally, the construction of the so-called roof drier has proven
itself. Specially it means that the air is taken in and carried off
through horizontal roof-shaped air ducts, which are open downwards. In
between, the product trickles through the drier by means of gravity.
Horizontally arranged hot air and exhaust ducts run through the vertical
product ducts. The optimized roof shape of these ducts guarantees an
even product flow.
The roof-shaped ducts are open downwards at full length. The hot air
streams into the column at the front side of the hot air roofs and then
flows through the piled-up product. The product heats up, gives off its
moisture to the passing air and dries. As a countermove, the air cools
down and takes up moisture up to the saturation limit, depending on the
product. The moist air escapes through the neighbouring exhaust roofs.
This air is sucked out of the drier through the exhaust fan. The
constructional design of the roof ducts guarantees an optimum and even
drying result with the highest product care.
The throughput quantity is adjusted by means of a computer-controlled
discharge. Furthermore, the construction guarantees easy transversal
mixing and thus an even drying result. Then the desired operating mode
of the drier is the main difference in process.
There is the possibility to operate the drier in continuous-mixed-flow
mode or in the so-called circulating batch mode.
Modern drying plants are built in modular design, which makes it
possible to meet exactly the customer's requirements concerning capacity
and design.
Construction of the roof drier. Red: hot air
supply (= hot, dry air), green: exhaust air (= cool, moist air), orange:
product
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Environment
A certain amount of dust is in the exhaust air when grain is
dried. A drier, however, can be equipped today according to
the legal environmental rules without any problems.
Almost unnoticed by the public, new rules of the German
federal immision law are valid since 01.01.2002. New limit
values for the admissible dust content of exhaust chimneys
and ducts were specified in the technical instructions for
the air; the limit value was reduced by 60% from originally
50 ppm to 20 ppm.
For new plants, this requires a powerful dust extraction
system according to the latest state of the art;
conventional simple dust cyclones will not cope any more
with these requirements.
For this application, there is a proven dust extraction
system on the market – the centro separator.
Zentroabscheider Typ ZA
This is a centrifugal separator which achieves a remarkably
high separation degree by means of its clever double-cyclone
design with internal diffuser (inlet guide vanes) with, at
the same time, a compact design. Compared only to the air
capacity of a cyclone (the increased separation capacity not
yet taken into account), a centro separator is about 70 %
smaller in size.
With the centro separator, the dust-laden air is led into a
vortex chamber and put into rotation with only slight radial
component. The dust particles then move to the side of the
vortex chamber. Together with a small partial air-flow and
by means of a tongue-flap in the last part of the spiral,
the dust is separated from the main air-flow, led into the
secondary separator and discharged.
A cylindrical baffle plate system reverses the direction of
the air-flow in the vortex chamber before the cleaned air
leaves. Thus the remaining dust particles are thrown outside
and also led into the secondary separator.
This secondary separator has the shape of a cyclone the cone
of which is bent. The cleaned secondary air is led back into
the main air-flow through the central pipe.
The centro separator has a further important characteristic,
especially for difficult installation sites: it produces the
same result in every possible position. No matter whether it
is installed horizontally, vertically or even diagonally,
the result is always the same.
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Technical developments
The continuous-mixed-flow drier with roof-shaped air ducts
works according to a proven system, nevertheless the
technical development does not stand still. Today light and
especially noncorrosive aluminium structures can be used
because of an optimized design. The air duct system in the
drier was optimized.
There is an excellent heat and air distribution by means of
the modern burner technologies, gas line burners or fuel oil
burners with air heaters or in small upright air heater
units with several burner units spread out over the drier
width. This has a direct effect on the evenness of the
drying and guarantees a high product quality with low energy
consumption.
Not least on request of the market, the existing product
range is enlarged by new-developed drying plants. Especially
as far as mobile driers are concerned, more and more
powerful models are mass-produced. The control engineering
makes it possible today to automate a drier in such a way
that the operating personal can reduce the time needed for
the drier to a minimum. The computer-controlled product feed
and discharge and the automatic discharge control according
to the final moisture can be mentioned as an example. The
absolute on-line moisture measurement and control makes the
permanent check of the final moisture possible without the
usual measuring effort with conventional moisture measuring
instruments. In the same way, the drying temperature of
circulating batch driers can be varied depending on the
drying time by means of the so-called Multi-Therm automatic
system. Here great importance is attached to a still more
careful temperature course
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Quality and profitability
Both aspects have always be regarded together. Profitability
means maximum throughput rate with the lowest possible
amount of energy. Either the operator himself determines the
marks of quality or they are stipulated by his prospective
customer. Each requirement going beyong the normal quality
demands is cost-intensive. Working economically here means
that only fully ripe, cleaned grain with a harvesting
moisture as low as possible is dried to shelf life just
below 15 %. Special requirements go for very long stock
holding periods e. g. in intervention storehouses. Here
usually final moistures of 12.5 up to 13 % are required.
This means that residual moisture is a quality criterion,
the required longer drying time and the higher amount of
energy reduce the profitability. The future customers partly
prescribe special quality criteria like e. g. the maximum
admissible drying temperature or indirect air heating. Such
parameters, however, justify a higher market price as they
go beyond the normal quality requirements. Then you can
produce quite profitably by means of the price. .
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Buying advice
Generally, the prospective buyer has to decide on the size
and design of the drying plant. Here there are a lot of
questions the prospective user of a drying plant has to
answer himself in advance. Of course, the competent plant
manufacturer helps as well.
A plant should neither be projected too big nor too small as
there arise problems in operation with both extremes.
Nevertheless, the drier should be as powerful as possible as
the energy consumption decreases with an increasing
throughput capacity. Here an active heat recovery by means
of a circulating system has an especially positive effect.
Basically, there is the possibility to install and operate a
drier as mobile or stationary plant.
Mobile driers can be used without much effort for assembly,
installation and permission. They especially distinguish
themselves by keeping their value. A mobile drier can always
be exchanged against a more powerful drying plant.
The customer should insist anyway upon high-quality,
corrosion-resistant and thus long-lived materials like e. g.
aluminium. Purely galvanized driers are not to be
recommended any more for reasons of corrosion. Mixed
constructions made of chrome steel and galvanized sheet
metal should be avoided because of the development of
chemical reactions („Le clanché“ element, „primary cell“).
The life span makes a more favourable calculation possible
when a drier is bought.
By means of the energy cost, the cost budget for the drying
of e. g. grain and a profitability study can be drawn up
quickly. This is also based, of course, on the price that
can be achieved on the market.
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Outlook
The last years have been marked by a concentration and
centralization in the field of drying and processing of
cereals like grain or e. g. also maize. Drier manufacturers
have to be able to meet with this trend by means of a wide
range of drying plants. Besides the increase in capacity,
the operator demands long-lived and high-grade quality.
Mobile or movable systems make it possible to react to the
market conditions fast and flexibly. |
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