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Rising energy costs are forcing operators
to optimize all industrial processes which
rely on high power consumption. One such
application is the continuous flow dryer or
drying oven, which is used in lacquering,
coating and finishing processes to dry
freshly coated surfaces quickly and reliably.
This desire to reduce energy costs, however,
must not conflict with the need for
operational safety.
Explosion hazards in continuous dryers
Inside a continuous flow dryer the solvent
which is essential for the coating process is
removed from the coating material (paint,
adhesive etc.). These solvents, which tend
to be combustible, vaporize and are removed
by a continuous flow of air. The amount
of solvent depends primarily on the process
used, and is also affected by the proportion
of solvent in the coating material, the
applied coating thickness and the speed of
the conveyer belt or feed rate of the foil to
be coated. If too much solvent is placed
in the oven or the air flow is insufficient,
concentrations of solvent vapour can build
up inside the continuous flow dryer which
exceed the LEL (lower explosive limit)
applicable to the solvent in question. Such
hazardous situations arise as a result of incorrect
operation or process errors. Studies
have shown that increase rates of 10% LEL
per second can be reached if, for example,
material starts piling up on the conveyer
belt or the ventilation system fails. This
extremely quick change from safe to unsafe
operation was the reason for standards to
be drawn up which have to be complied
with when planning and constructing continuous
flow dryers.
Protective features
EN 1539 stipulates that the maximum
vapour concentration must not under any
circumstances exceed a value of 50% LEL
and that the alarm time must not be longer
than 1.5 seconds unless other appropriate
measures are in place to maintain safe
operation. The alarm time relates to the
time which elapses from the moment a
hazardous situation arises until such time as
counteraction takes effect. For this reason,
the atmosphere inside the continuous flow
dryer must be permanently monitored by
a suitable gas detection system and any
necessary counteraction must be triggered
automatically. Suitable measures may involve
simply increasing the air flow rate or,
in extreme situations, fully shutting down
the plant and all electrical devices.
Process control
To avoid such critical situations, the operating
parameters of continuous flow dryers
are calculated for maximum safety, i.e. with
a large reserve. The values calculated for
conveyer belt speed and air flow rate generally
result in much lower concentrations
than the applicable standards require. Continuous
flow dryers, whose parameters were
calculated for an operating concentration of
25% LEL, for example, often achieve concentrations
of below 15% LEL in practice.
Such continuous flow dryers are not nearly
as economical in operation as they could
be. They either work with too low conveyer
belt speeds or too high air flows – and it
is this high air flow which is responsible
for the high energy costs, because more
air than necessary has to be heated to the
operating temperature of the dryer.
The only possible solution to this problem is
to move away from open-loop control of the
continuous flow dryer process and establish
a proper system of closed-loop control,
where the vapour concentration inside the
dryer must be the controlled variable and
the setting variables are the material and air
flow rates. A process with this type of control
means that values can be reliably kept
well under the threshold values specified
in the respective regulations, while at the
same time increasing the cost-effectiveness
of the dryer operation. Naturally, a prerequisite
for the safe functioning of this type
of control is the reliable and meaningful
measurement of the concentration values.
Measurement systems
For accuracy and speed, the ideal situation
would involve taking a measurement directly
above or very close to the conveyer belt.
The disadvantage of this type of set-up,
however, are the high costs of fitting a
large number of detectors along the dryer
and the fact that the information obtained
about local concentrations which cannot
be controlled would in some cases be fairly
meaningless due to the fact that not every
measurement point can be assigned its own
independent ventilation system. Using fewer
detectors to take measurements in the exhaust
air ducts, on the other hand, provides
a somewhat delayed picture of the concentration,
but the various local concentrations
have already balanced out so the process
control system can use this value. With the
aid of the process control, it is now possible
to set the concentration in the continuous
flow dryer such that it remains sufficiently
below the shut-down threshold of 50%
LEL on the one hand and, on the other
hand, that the air flow rate is reduced to the
necessary minimum.
According to the provisions of the aforementioned
EN 1539, gas detection systems
which ensure operational safety are not
allowed to be used for closed-loop process
control at the same time. This means that
twice the number of measurement points is
needed, though this is an investment which
quickly pays for itself through lower energy
costs.
The situation becomes more complicated
when a continuous flow dryer is used for
different coating processes. This generally
also means different solvents, which in the
majority of gas detection systems also
requires recalibration to the new substance. |
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Expensive but good
One exception to this are FIDs (flame
ionization detectors) and FTAs (flame
thermal analysis), which do not require
recalibration due to the fact that they react
to all hydrocarbons with more or less the
same degree of sensitivity. This benefit,
however, involves significantly higher
purchasing and running costs, and both
systems require a permanent gas supply
(H2) to keep their "pilot light" running.
Economical alternative
A much cheaper alternative in terms of
purchase price and costs of ownership are
sensors which function according to the
heat of combustion principle – so-called
catalytic Ex sensors. They work as part of
a Wheatstone bridge which is unbalanced
depending on the gas concentration. Because
the sensitivity to the different solvent
vapours varies, recalibration is generally
required when the substance is changed.
Unfortunately, it is not only the sensitivities
to different substances which vary, but also
the individual sensitivities between the
sensors. This makes it virtually impossible
to work with easy to handle substitute
calibration gases to facilitate the calibration
process. The calibrations would be too
imprecise to operate a closed-loop control
system with the measurement results. A
further disadvantage of the catalytic sensors
is their tendency to lose their sensitivity
relatively quickly due to poisoning of the
catalytic material at the measurement
elements. Only sensors with operating
currents specially adapted to this measurement
task are able to achieve acceptable
lifetimes.
Low-cost but good
Measurement systems based on infrared
absorption are much better suited to the
task in hand than catalytic sensors. They
are poison-resistant and can withstand the
conditions of process application. Like the
catalytic sensors, IR sensors also react with
varying levels of sensitivity to different substances,
which means that the sensor will
need to be recalibrated when there is a
change of solvent in the continuous flow
dryer. However, calibration of IR sensors is
much easier to carry out because substitute
gas calibration can be performed. Instead
of complex apparatus for target gas calibration
with solvent vapour, IR sensors can use
calibration gases like propane. This is possible
because the sensitivity ratios between
the different substances do not change as
a result of poisoning or ageing, as is the
case with catalytic sensors.
In conventional IR transmitters, however,
the measured values may under certain
circumstances be relatively imprecise. This
inaccuracy results from a non-optimized
linearization of the IR sensor signal by the
transmitter electronics in use. As a rule,
the circuits only linearize the signal for a
few standard gases. When the sensor is
used for a gas other than one of these
standard gases, the only possibility is to
use a linearization which is as close as possible
to that necessary. This means that the
measurement is only really precise at the
calibration point, i.e. at the concentration
at which the transmitter was calibrated.
An additional inaccuracy occurs as a result
of the individual production tolerances between
the sensors of a particular type.
Slight differences in the absorption wavelength
can produce significant differences
in the sensitivity to a particular substance.
This aspect is non-critical if the aim of
the measurement is to ensure operational
safety, but for improving energy efficiency
this type of measurement is only of limited
value.
Low-cost but even better
Dräger Safety followed a quite different
approach to preparing the measured value
for its Polytron IR transmitter. This transmitter,
unlike other IR transmitters, functions
not only with the linearizations for a
handful of standard gases, but currently
uses 38 different internal absorption
characteristics. The characteristics are
determined individually for each transmitter
and stored in a data memory called the gas
library. The user can switch freely between
any of the characteristics, without compromising
the quality of measurement. As confirmed
in the technical measurement report
compiled by test and certification company
EXAM BBG Prüf- und Zertifizier GmbH,
the measurement error is within the limits
required by EN 50057. This also means
that substitute calibration of the transmitter
takes on a whole new quality. The measurement
is no longer correct in just one point,
but offers sufficient accuracy across the
entire measurement range to allow closedloop
control.
In addition, rapid switching between substances
has been made simpler with automatic
remote configuration by PC. Software
allows different formulation-specific system
configurations to be defined which can be
selected at the click of a mouse and transmitted
to the transmitter when the coating
process changes. Subsequent calibration
is no longer necessary. It is even possible
to calibrate the transmitters on one dryer
to different substances if this should be
necessary in a so-called multizone dryer.
Besides the measurement quality of the
transmitter, however, the sensitivity distribution
vis-à-vis the different solvents is also
a measure of the quality of the targeted
concentration control inside the continuous
flow dryer. Only measurement systems
with evenly distributed sensitivities can
be used to set up a closed-loop control
system, because otherwise deviations from
the measured values will occur when
solvent mixtures are used, making troublefree
operation impossible. To maintain a
sufficiently even distribution of sensitivities,
DrägerSafety offers the Polytron IR with
different absorption wavelengths. The wavelengths
of 3.34 μm and 3.44 μm which are
used optimize the transmitters for specific
substance groups.
To avoid confusion and incorrect calibration
when using different types of the Polytron
IR transmitter at the same time, the configuration
software is in this case a particularly
useful tool because the security
prompts effectively prevent any incorrect
calibration of the transmitters. Studies conducted
by the EXAM BBG Prüf- und Zertifizier
GmbH confirm the safe function of
the software, and the technical measurement
report of the transmitter has been
extended to include this accessory.
Ingo Edler
Dräger Safety AG & Co. KGaA
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Dräger Safety AG & Co. KGaA |
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Revalstrasse 1 |
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23560 Luebeck, Germany |
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Tel +49 451 882 0
Fax +49 451 882 2080
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