In today's
time, in the paper industry has undergone tremendous change
and the conventional drying method has been replaced by high
intensity electrical infrared drying and coating. The high intensity
infrared drying is able to evaporate eight to ten times more
water the conventional drying method per square meter than the
conventional method. This has given way to fast immobilization,
improved quality, space saving and simplified sheet run. The
final outcome is remarkable increase in production.
What is Infrared?
The electromagnetic energy that falls between ultraviolet and microwave energy
in the spectrum. The heating effectiveness is governed by emitter
temperature, emitter emissivity, paper emissivity and the emitter
surface area.
The highly penetrating infrared energy has much higher drying
rate in comparison to convection (Hot air dryer) or conduction
(Steam cylinders).
Advantages of Infrared drying
- High quality of surface coating from quick coating immobilization.
- Negative reduction of scale on initial dryers.
- Simplified sheet movement due to compact infrared dryer.
- Improved porosity.
- Suitable for various requirements.
- Better coverage or less fiber swelling and show through.
- Allows for lower coat weight and more synthetic on the surface coating.
- Decrease in restoration time from sheet breaks or stops.
- More control over process due to fine tuning ability.
- The possibility of web damage during emergency stop gets limited due to instant cooling down ability.
- Quick response to product temperature, speed and moisture variation (if moisture sensor is available) results in enhanced production and invariant quality.
- The waiting time and under processed products gets minimized due to instant 'Switch On'.
Dynamics of Drying
High-Intensity IR Systems:The
working mechanism of the HI -IR Drying system is on the coated
sheet is different from conventional energy source such as Hot
Air Dryers or Steam Cylinders.
The intense penetrating capability resulting from high intensity
and short wave length removes the negative temperature gradient
in the sheet as high percentage of the infrared passes through
the sheet. The infrared energy penetrates the sheet along with
radiant heat transfer. The energy on hitting the sheet is either
absorbed or reflected back on its interaction with each particle
of the coating or the sheet. Such uniform heating is highly
important from the quality and efficiency point of view in comparison
to convective or conductive surface area.
The IR drying leads to rapid drying of the coating colour to
the immobilization point resulting in A PREDICTABLY UNIFORM
COATING SURFACE WITH TYPICALLY EXCELLENT PRINTING CHARACTERISTICS.
IR dryer
Salient features of IR dryers
- Approved by various prestigious installations throughout India.
- Indigenously designed to meet world standards.
- World class, reputed quality infrared emitters are used.
- Steel construction.
- High intensity of IR radiation up to 600 KW / m2.
- Gold or white coated reflectors.
- Quartz built sheet cover which is transparent to infrared radiation protects the emitters from splashes and easily cleanable.
- Air cooling enhances the life of the component.
- Air impingement and exhaust of vapours for increased efficiency.
- Compact, typically 0.5 m in the direction of sheet travel (inclusive of exhaust)
- Immediate 'on' and 'off' switching of emitters or Thyristorized (SCR) power regulator for step less power variation.
- Automatic safety interlocks against breakage in web, machine stoppage, blower failure etc.
- Trained service engineers for service backup.
- Readily available spares.
Constructional
Features: The light weight IR Dryer is designed for optimum
energy utilization and its pivoted design makes it suitable
for installation in any position or direction. It can be swivelled
at any angle to suit the sheet run even after installation.
For ideal coating application, the frame has been designed in
such a way that it can be totally enclosed to prevent any external
airflow. The rugged steel construction prevents any cross sectional
deformation and corrosion. The Short Wave IR Dryer can be applied
to any existing machine without or no modification due to its
compact size.
IR
emitters: The IR Emitters are uniquely designed to facilitate
universal mounting. The lamps are placed closer to give high
heat intensity. The response to the 'On' and 'Off' switching
is very quick as the shortwave IR energy is highly penetrable.
Gold
Coated Reflector: Gold having a reflectivity of 99% makes
the gold-coated reflectors at the back of the IR Emitters highly
efficient. The sheet easily reflects the shortwave IR as not
all the energy enters the sheet at once. The gold reflector
reflects all the energy without being absorbed till it finally
enters the sheet.
Air
circulation System: There is a layer of air formation
near the sheet which rapidly becomes saturated due to the moisture
released from the sheet at this close distance. This prevents
rapid evaporation. If this layer of air is replaced by fresh
air, then only the evaporation becomes more rapid till this
process absorbs all the heat radiated form the sheet. The maintenance
of thermal efficiency, curl prevention and uncontrolled sheet
temperature variation an efficient air management is essential.
The blower situated at the bottom of the dryer pumps in fresh
air from outside which is blown on the reflector and the gap
between the reflector and the quartz plate. The air cools off
the lamp seals and gold-plated reflector and the heated air
moves up the top of the dryer and on the web by means of duct
provided at the top. This air ventilation system helps in maintaining
the heat build up by cooling the IR Emitters. The air can be
made to travel parallel to the sheet and retrieved for the downstream
side of the frame depending upon the application. The hot air
is impinged on the sheet in the form of jet to break the surface
tension of the water molecule to increase the rate of evaporation.
The impinged air is blown on the web is such a velocity that
it hits the boundary layer of air with great force and makes
room for fresh hot air to move rapidly along the surface of
the sheet. The vapour laden air is removed by means of exhaust
to maintain low level of moisture near the dryer for higher
efficiency of evaporation.
Front
Plate: The quartz plates in front of the emitter prevents
any splashes, withstand the thermal shock, prevents dust to
lodge on the gold-plated reflector and controls the impinged
air flow. The coating splatter can very easily be removed from
the plate. It helps to reduce the maintenance and replacement
cost to a large extent giving it an outstanding durability.
Process
Control: The heater module can be powered to selectively
remove specs of moisture profile by emitting variable amount
of infrared energy across the sheet. It can also be powered
to for incremental drying on paper machines or coating applications.
The IR Emitters are placed closely to achieve higher heat density
which has been optimized for maximum drying capacity with long
service life. All the energy from the tungsten filament is converted
into radiant energy peaking at the level of 1.2 micron range.
The IR Dryer is provided with Thyristorized Power Regulator
for step less variation in power accompanied by power savings.
The power varies proportionally with the change in speed, gsm,
solid and moisture content in the web.
Safety features of IR dryer:
- The break in paper web is detected by a break detector and the heating system gets switched off automatically.
- The speed sensors are fitted on the machine to automatically change the heat intensity.
- The Reflector temperature sensors continuously monitors the temperature of the Reflector and sounds an alarm when the threshold limit is jumped leading to tripping of the system.
- The air pressure in the duct is monitored for a fall below the pre-decided level at which the alarm is sounded and the system trips off.
- Emergency switch off.
Other Application of Infrared Dryers in the paper industry
- It can be used for modifying the viscosity of the water so that more water is squeezed out in the press before the final press section.
- At the last stage of the paper machine, the efficiency of the steam cylinder is low due to reduced rate of evaporation at low moisture levels. It can be used as Booster at this stage as the efficiency of the IR heating is still very high and another compact IR Dryer can improve the speed remarkably.
- Edge Correction The IR cassettes can be used to direct extra heat on the
edge to improve output when the edges of the paper are not
dried properly and need to be trimmed.
- Correcting Moisture Profile To make the zone connected separately to a regulator,
each zone heating about 75 mm to 150 mm width of paper.
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