Transcript Document

Specialist in process efficiency & energy conservation
TAPPSA , 19-20 Oct 2010
Maximising Drying Rate through Automation
- Datta Kuvalekar
TAPPSA , 19-20 Oct 2010
Overview of session
-Importance of Drying rate
-Factors affecting Drying rate
-How to enhance Drying rate
-Focus on Automation
TAPPSA , 19-20 Oct 2010
The Drying rate
• The ability of the driers to evaporate moisture in the web
• Measured as the quantity of moisture evaporated per unit drier area per unit time
• Typically measured as kg/h-m2
• Different grades operate with different drying rates.
• TAPPI has published Drying rate curves as a function of Steam temperature
• Dictated by the Moisture profiles, Drier surface area
TAPPSA , 19-20 Oct 2010
Whats a Good Drying rate?
• Obviously Higher Drying rates are better !
• On Indian Machines specially Kraft 10- 15 kg/h-m2 have been measured
• On MG Cylinders 40-100 kg/h-m2 have been measured
• on Writing printing machines , 16-20 kg/h-m2 are normal.
• But can we target 25 kg/h-m2 on driers and +200 kg/h-m2 on Yankees?
TAPPSA , 19-20 Oct 2010
Importance of Drying rate
• Its an indicator of efficient drier operation and Heat Transfer
•Helps assess Drying capacity
evaporating
water
cylinder shell
felt
HEAT
paper
air and dirt
TAPPSA , 19-20 Oct 2010
supply steam
condensate hard sediment
Various Aspects of the drier
Water evaporates from paper
Paperboard
Steam in
Blow-through steam
& condensate out
Condensate out
TAPPSA , 19-20 Oct 2010
Syphon
Dryer shell
The Heat Transfer Equation
The heat transfer from the steam to the paper
can be expressed by means of the formula:
Q=
Q:
U:
S:
Ts:
Tp:
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U· S· (Ts - Tp)
Flow of heat
Coefficient of heat transfer
Drying surface
Temperature of the steam
Temperature of the paper
The most important factor affecting heat transfer :
•
•
•
•
•
•
•
the thickness of the layer of condensate
Cylinder wall thickness
The layer of air between the dryer and the paper
Properties of the paper
Dirt
Incondensable gases
Superheated steam
TAPPSA , 19-20 Oct 2010
Condensate – The Main Bottleneck in Drying rate
Condensate is a barrier to heat transfer and hence should be minimised
The thickness of the condensate inside the dryer depends on:
·
The quantity of condensate
·
The rotation speed
·
The condensate drainage device.
·
Amount of blowthrough steam
·
Dissolved Air circulation
TAPPSA , 19-20 Oct 2010
Condensate behaviour depends on the velocity of the
cylinders and the paper thickness.
1. Stationary
Fig. 1
Steam in dryer,
but no
rotation.
Condensate
shown in blue
colour at the
bottom of dryer.
TAPPSA , 19-20 Oct 2010
2. Puddling
Fig. 2
1st, Stage:
Puddle. As
dryer begins
to move up
dryer shell.
Some
rimming-thin
film-occurs
3. cascading
Fig. 3
2nd Stage: Violent
cascading action
occurs as speed
increases and more
condensate moves
farther up the dryer
shell. Film of
rimming
condensate
increases in
thickness.
4. Rimming
Fig. 4
3rd. Stage:
Complete
Rimming. When
sufficient speed is
reached
cascading action
stops and
complete rimming
occurs.
Condensate –The most critical element of the smooth drier operation
• Removing Condensate effectively and continuosly is key to machine runnability
• Condensate removal depends on :
• Machine speed
• Type of Siphon and clearance
• Differential Pressure between steam and condensate side.
• Good Condensate removal generally leads to high drying rates.
TAPPSA , 19-20 Oct 2010
Other Factors that affect Drying rate
• Drier surface temperature
• contact of sheet with drier surface
• Felt tensions
• Thickness of Paper and internal microstucture
• Pocket Humidity
• Inside hood temperature ( pocket)
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The pocket Humidity Effect
FELT
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POCKET
VENTILATION
DUCT
So where does Automation contribute to maximise Drying rate ?
TAPPSA , 19-20 Oct 2010
Role of Automation
• Automation on the drier section helps maintain steam pressures , Differential pressures and Level
• Condensate removal becomes more constant as differential pressures stabilise across driers leading to
production rates.
• Control of the drying environment through exhaust humidity control. PV supply control and zero level
control.
• Control of moisture setpoints through QCS leads to control on drying rate demand in case of drier limited
machines.
TAPPSA , 19-20 Oct 2010
Steam and Condensate system
Pressure control
Dryers: wet end
Condenser
Pressure
control for
first
separating
vessel
Dryers
Dryers
Motive
steam
MFP14
Level
control
Flow control
valve
Separating vessel
ZOOM IN FOR DETAIL
Meter
Differential pressure control– Key to condensate removal
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Drying Environment control – The Automated Hood and PV system
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Conclusion
•
Drying rates can be maximised through various Interventions
• Condensate removal is the key element.
• Automation supports maintenance and maxmisation of Drying rates on
all machines
• Start with measuring the drying rate on your machine and cross check
dependent parameters.
TAPPSA , 19-20 Oct 2010
Thank you
Specialist in process efficiency & energy conservation