In recent years, high intensity electrical-infrared coating drying has gained acceptance
in the paper industry to replace or supplement more conventional drying methods.
Pure high-intensity infrared (HI-IR) drying has the potential to evaporate eight to ten
times more water per square meter than typical air drying systems, thus achieving fast
immobilization and IMPROVING THE QUALITY and also saving space & simplifying
the sheet run, while allowing remarkable increases in production.
What is Infrared?
Infrared energy is a form of electromagnetic energy that falls between ultraviolet and
microwave energy in the spectrum. Emitter temperature, emitter emissivity, paper
emissivity, and the emitter surface area govern the heating effectiveness.
The IR energy being highly penetrating, the drying rates provided by the IR dryers are
much higher than that possible by convection (e.g. Hot air dryers) or conduction (e.g.
Steam Cylinders).
Advantages of Infrared drying
- Enhanced quality of the coated surface due to quick coating immobilization.
- Elimination of reduction of scale on initial dryers
- Simplified sheet travel because of compact IR dryer.
- Improved porosity.
- Suits any requirement.
- Provides better coverage or reduce fiber swelling and show- through.
- Allows lower coat weight and increased amounts of synthetics in the surface
coating.
- Reduces the restoration time following sheet breaks/stops.
- Improves control over process because of fine-tuning ability.
- Instant cool down limits the possibility of web damage during emergency stop.
- Fast response to product temperature, speed and moisture variation (if moisture
sensor is provided) ensures invariant quality and enhanced production.
- Instant ‘switch on’ minimizes waiting time and under processed products.
Dynamics of Drying
High-Intensity IR Systems:
Performance characteristics of HI-IR drying on the coated
sheet differ significantly from conventional energy sources, such as Hot Air dryers or
Steam Cylinders.
The high intensity and short wavelength provide penetrating capabilities that eliminate
negative temperature gradients in the sheet as a high percentage of the IR passes
through the sheet. With radiant heat transfer, infrared energy penetrates the sheet.
As The energy hits the sheet it is either absorbed or reflected when it interacts with
each particle of the coating/ sheet. It is this uniform internal heating that is more
advantageous from QUALITY & EFFICIENCY point of view than the
convective/conductive surface heating of air.
IR drying enables the coating colors to be dried rapidly to the immobilization point. THE
RESULT IS A PREDICTABLY UNIFORM COATING SURFACE WITH TYPICALLY
EXCELLENT PRINTING CHARACTERISTICS.
IR dryer
Salient features of IR dryers
- Proven with several prestigious installations throughout India.
- Indigenously developed system to meet world standards.
- Used reputed make Quality infrared emitters, which are world class.
- Steel construction.
- High intensities of IR radiation (up to 600 KW/M²).
- Gold / White coated reflectors.
- Quartz sheet cover-, which is transparent to infrared radiation. Protects the emitters
from splashes, can be cleaned easily.
- Enhanced life of components due to air-cooling.
- Air impingement and exhaust of vapours for increased efficiency.
- Compact- typically 0.5 m in the direction of sheet travel (inclusive of exhaust).
- Instantaneous ‘On’ & ‘Off’ switching of emitters OR Thyristorized (SCR) power
regulator for stepless 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 IR dryer is lightweight and is designed for optimum
energy utilization. Its unique pivoted type design makes it suitable for installation in any
position and direction. Even after installation, it can be further swiveled through any
angle to suit the sheet run.
The frame design is such that it can be totally enclosed to eliminate any external
airflow, which is ideal for coating application.
Rugged steel construction minimizes cross-sectional deformation and corrosion.
The compact size and concentrated energy of the Short-wave IR Dryer allows it to be
applied in existing machine location with minimum or no mechanical modifications.
IR emitters:
The reputed make IR emitters are used and are of special type so as to
facilitate universal mounting. These lamps are placed close to give high heat intensity.
Being Short wave, the IR energy is highly penetrable and give very quick response to
the ‘ON’ & ‘OFF’ switching.
Gold Coated Reflector:
The gold-coated reflectors at the back of the IR emitters are
highly efficient because gold has a reflectivity of 99%. The sheet easily reflects shortwave
IR and not all the energy enters the sheet at first time. The gold reflector rereflects
that energy without any absorption until it does finally enter the sheet.
Air circulation System:
At the sheet surface there is a boundary layer of air, which
rapidly becomes saturated and obstructs evaporation when so much moisture is
released from the sheet in such a short distance. If this boundary layer is replaced with
fresh air, further rapid evaporation will take place, until the evaporation process
absorbs all the heat radiated to sheet.
And hence an effective air management system is essential, to maintain thermal
efficiency and to prevent curl and uncontrolled sheet temperature variation.
The blower (at the bottom of the dryer) sucks air from the atmosphere and blows it on
the reflectors as well as the gap between the reflector and quartz plate. Due to this the
lamp seals and the gold-coated reflector get cooled and this heated air further travels
to the top of dryer and on the web through the ducting provided at top.
Air circulation system provides cooling to the IR emitters and internal heat builds up.
Depending upon the application, the air can be directed to flow parallel to the sheet
and retrieved on the downstream side of the frame.
The hot air is impinged on the sheet in the form of a jet to break the surface tension of
water molecules and increase the rate of evaporation.
The impingement air from the IR emitters is guided on the web with such a velocity and
volume that scrubs the boundary layer from the sheet surface and replaces it by fresh
hot air which moves rapidly along the sheet surface. This vapor-laden air is further
exhausted through an exhaust system in order to maintain low level of moisture in
vicinity of the dryer for higher efficiency of evaporation.
Front Plate:
The quartz plate placed in front of the emitters prevents any splashes and
withstands the thermal shocks. This reduces the maintenance & replacement costs
and gives outstanding run ability. The coating splatter can be easily cleaned off the
quartz plate, later on.
The quartz plate fixed on the IR emitters prevents any dust particle lodging on the goldcoated
reflectors and control the impingement air.
Process Control:
The heater modules in the IR dryer can be powered to emit variable
amounts of infrared energy across the sheet to selectively remove peaks of moisture
profile or powered equally to provide incremental drying on paper machines or coating
applications.
The IR emitters are placed close together to achieve high heat intensity. The energy
density has been optimized to provide maximum drying capacity with long service life.
All radiation from the tungsten filament is converted to radiant energy, with the energy
peak in the 1.2 microns range.
The IR dryer is provided with Thyristorized Power Regulator for stepless variation in
power and power saving at the same time. The power is
Proportionally varied with the change in speed, gsm, solid & moisture contents in the
web.
Safety features of IR dryer:
- A break detector detects break in paper web and the heating system is
automatically switched off.
- The heat intensity is automatically varied with the help of speed sensor fitted on
the machine itself.
- Reflector temperature is continuously monitored a number of points with the help
of Reflector temperature sensor and if it exceeds the limit, an alarm is sounded
and the system trips.
- Air pressure in the duct is monitored and if it drops below a predecided level
alarm is sounded and the system trips.
- Emergency switch-off.
Other application of infrared dryers in paper industry
- IR Dryer can be used before the final press section to modify the viscosity of
water so that more water is squeezed out in the press.
- As BOOSTERS at the end of paper machines. At this stage the efficiency of
steam cylinders is low because of fallen rate of evaporation at low moisture levels.
However, the efficiency of IR heating is still very high and addition of a compact
IR dryer at this stage can improve the speed remarkably.
- For EDGE CORRECTION i.e. whenever the edges of paper are not dried properly
and have to be trimmed, IR cassettes can be used to direct extra heat on the
edge and thus to improve output.
- For CORRECTING MOISTURE PROFILE: The IR emitters in a frame can be
electrically connected in small zones in such a manner that each zone will heat
about 75mm to 150mm width of the paper. By connecting each zone to separate
regulator can be set in such a manner as to make that moisture profile uniform
within a specified band.
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