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Drying
•Introduction
•Background
•Why is it Important?
•Drying Theory
•Equipment Operation and Control
RELATIVE COST OF WATER REMOVAL
FORMER PRESS DRYER
0
5
10
15
20
(Drainage) (Evaporation) (Compression)
WATER REMOVED (kg water/kg fiber)
CONSISTENCY
1.3
95%
825
10-13%
5.3
40%
Rela
tive C
ost
Exhaust Air
Recirculated Air
Supply Fan
Combustion Air
Condensate
Hood
Exhaust Fan
Fresh Air Steam
Burner
Yankee
Fuel
Evaporated Water
Air
Tissue Machine Drying System
YANKEE & HOOD SYSTEMS
Air Impingement
Dryer Shell
Steam
Drying: Simultaneous Heat & Mass Transfer
Sheet
Mass T
r. (
Evap
.)
Heat
Tr.
Heat
Tr.
YANKEE
HOOD
Drying
•Introduction
•Yankee Dryer
•Steam In
•Condensate Out
•Drying Theory
•Equipment Operation and Control
•Hot Air In
•Evaporated Moisture Out
•Yankee Hood
Backside (Drive Side)
Frontside (Tending Side)
Steam In
KC Stayed Head Yankee Dryer
Stay Bar
Blowthrough Steam & Condensate Out
Blowthrough Steam & Condensate Out
Shell
Journal
Head Head
Condensate Removal Pipe
Yankee Dryer Heat Transfer Coefficients
Sheet
R Coat = 1/h Coat
R Shell = 1/h Shell =t/k Shell
R Cond = 1/h Cond
R Sheet =1/h Sheet
1/U D
Condensate
Dryer Shell
Coating
Steam
Heat
Tra
nsfe
r Sheet/Coating
Adhesion
Dryer/Coating
Adhesion
Motive Steam
Warm Up Steam
Atmosphere
Condensate
Dryer
Hood
Flash Tank
Yankee Dryer Steam & Condensate System
Sight Glass
Model of Condensate Behavior
Ponding Cascading Rimming
0 0.1 0.2 0.3 0.4 0.5 0
500
1000
1500
Condensate Rimming Thickness (in)
Hong & Appel
White & Higgins
Condensate Rimming Velocity D
rye
r S
pe
ed
(fp
m)
0
0.020
0.015
0.010
0.005
0.025
Dryer Speed (fpm)
Heat Transfer Resistance of Condensate
Rimming Speed
1000 2000 3000 4000 5000
Co
nd
en
sa
te R
es
ista
nc
e
(Hr
Ft
F
/BT
U
2
Ways to Avoid the Rimming Effect
Minimize the condensate layer
Bypass the condensate layer
Create Turbulence
Condensate Out
Ribbed Dryer With Soda Straw Condensate System
Shell Ribs
Grooves
Soda Straws
Dryer Rotation
Movement in
Relation to Dryer
Shell
Condensate
Movement in
Relation to Dryer
Shell
Minimum Condensate Thickness
Maximum Condensate Thickness
GRAVITY
Top
12
3 9
6
Spilling Over Behind Bars
Wave Advances Ahead of Bars
Spilling Over Ahead of Bars
Wave Advances Behind Bars
Spoiler Bars
Rotation
Basic Hood Design
100,000 cfm
100,000 cfm
Typical HTHV Hood
100,000 cfm
100,000
cfm
25,000 cfm
?
25,000 cfm
?
Infiltration / Exfiltration
Suck / Spill
100,000 cfm
100,000
cfm
25,000 cfm
?
0 cfm
?
Infiltration / Exfiltration
Suck / Spill
100,000 cfm
100,000
cfm
0 cfm
?
25,000 cfm
?
Infiltration / Exfiltration
Suck / Spill
Typical Counter-flow Hood
Identified Problems on B1
• Make-up air dampers closed
• This condition results in the hood not being
balanced (infiltration/exfiltration)
• This condition make the system less energy
efficient.
100,000
50,000
10,000
40,000
Exhaust Tubes
Supply Plenum
Exhaust Plenum
Exhaust Duct
Supply Ducts
Burners or Heat Exchangers
Supply Fans
Exhaust Air
Exhaust Fans
Fresh Air
Fresh Air
Tissue Machine Dryer Hood
FEATURES SUITABLE FOR:
- New machines
- Rebuilds / replacements
- No limit for paper width
VALUES
- lower energy consumption
at same (competitors’)
performance
= Less cost per ton
TECHNICAL DATA
- Impingement speed up to 37400 fpm
- Impingement temp. up to 950 °F
- Specific evaporation up to 41 lb/hft2
Metso Paper Basic SC- Yankee Hood
Drying
•Introduction
•Condensate Removal System
•Steam and Blowthrough Control
•Hood Air System
•Drying Theory
•Equipment Operation and Control
29 Gravities
Condensate Removal
7000 kg/hr Steam Input
7000 kg/hr Condensate Output
How do you get Condensate out of a
Yankee Dryer ?
• Differential Pressure
• Blow Through Steam
– Velocity Control
0
Pressure Drop vs. Blowthrough Flow
Blowthrough Flow (kg/hr)
0
600
500 1000 1500 2000 2500 3000
150
300
450
Density
System
Blowthrough Curve
Density + Friction
(1)
(2) (3)
Pre
ssure
Dro
p (
kP
a)
Friction
0
PRESSURE DROP VS. BLOWTHROUGH
EFFECT OF SPEED INCREASE
Blowthrough Flow (kg/hr)
0
600
500 1000 1500 2000 2500 3000
150
300
450 1600 m/min
1500 m/min
Friction
Density
Dro
p P
ressure
(kP
a)
Two Phase Flow Regimes Bubbly Flow
Slug Flow
Churn Flow
Annular Flow
Pressure Drop vs. Blowthrough Flow Effect Of Condensing Rate Increase
Blowthrough Flow (kg/hr)
0 500 1500 2500 3000
0
150
300
450
600
700 kPa
800 kPa
Pre
ssu
re D
rop (
kP
a)
1000 2000
Controlling Blowthrough Flow
•Stable condensate removal -
Annular flow velocity > 21 m/sec.
Reduces the density with more steam.
Makes condensate removal easier
•Pressure drop is not a reliable indicator of
blowthrough flow.
Blowthrough should be controlled by
controlling flow.
(Or in the case of B3 – Velocity)
Effect of Yankee Pressure
On Density and Blow-Through
•Steam density changes significantly with pressure
100 psig steam (7 bar) -- 0.24 m^3/kg
50 psig steam (3.5 bar) – 0.42 m^3/kg
•The higher the density the more blow-through
steam is required for good evacuation.
For consistent condensate removal if we
operate at various Yankee steam pressures
we need to have various blow through
(Velocity) set points.
Ribbed Yankee With Soda Straws
Kimberly-Clark Condensate Scoop
Steam & Condensate In
Condensate & Steam Out
Dryer Shell
Scoop
Condensate Layer
Yankee Dryer Steam & Condensate System
LIC
DPI
PIC
TT
PI
Motive Steam
Warm Up Steam
PIC
Vent Valve Blowthrough Valve
Thermocompressor
Warm-up Valve Steam Make-up Valve
LIC
Steam/Condensate Separation
Blowthrough & Vent Valves
Steam Supply Control
F.T.
Condensate
Dryer
Sight Glass
FIC
Function: Allows blow through steam and condensate to separate.
Condensate Separation System
LIC
Sight Glass
TT
PI
LIC F.T.
Condensate
Dryer
DPI
Blowthrough and Vent Valves
Vent Valve Blowthrough Valve
F.T.
Functions:
> Control Blowthrough Flow
> Vent Non-Condensible gases to Atmosphere Dryer
FIC
Yankee Dryer Steam Supply System
Motive Steam
Warm Up Steam
Warm-up Valve Steam Make-up Valve
Functions:
> Control dryer steam pressure
> Recover blow through steam
Thermo Compressor
Blowthrough Steam
Dryer
PIC
Thermocompressor
Discharge
Nozzle
Diffuser
Body (Mixing Area)
Suction Inlet
Motive Steam Inlet
Actuator
Spindle
Inlet
Diffuser
Nozzle
Thermocompressor Zones
Discharge
Suction
Expansion Mixing Compression
-100
55
60
65
75
70
80
85
90
95
100
50 -60 -80 -40 -20 0 20 40 60 80 100
Effect of Thermocompressor Nozzle Opening
on Suction Flow
% Suction Flow
Discharge
Nozzle
Diffuser
Suction Inlet
Motive Steam Inlet
Actuator
Spindle
% M
otive N
ozzle
Open
Thermocompressor Sizing
The thermocompressor spindle can regulate
dryer steam pressure.
Changes in spindle opening also change suction
flow (blowthrough).
This can result in blowthrough flow instability
and problems with condensate removal.
Thermocompressors should be sized so that
they remain 100% open 100% of the time.
Hood
Yankee
Remove Evaporated Water
Supply Hot Air
Control Creping Moisture
Functions of a Dryer Hood
Three Main Control Parameters
• Air Temperature – Temperature of the impingement air reaching the sheet
• Impingement Velocity – Nozzle Velocity of the air reaching the sheet
• Return Humidity – Humidity of the air at the entrance to the burner
Effect of these Parameters on Heat
Transfer Rate
Hood Drying Rate Impact
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 2 4 6 8 10 12 14 16 18 20
Hood Rw (# / hr-sq ft)
No
rma
lize
d v
alu
e o
f
Ide
pe
nd
en
t V
ari
ab
les
Effect of Humidity Effect of Temperature Effect of Nozzle Velocity
Effect of These Parameters on
Fuel Cost
Natural Gas Consumption Rate
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Normalized Gas Consumption
No
rma
lize
d v
alu
e o
f
Ind
ep
en
de
nt
Va
ria
ble
s
Effect of Humidity Effect of Temperature Effect of Nozzle Velocity
Exhaust Tubes
Supply Plenum
Exhaust Plenum
Exhaust Duct
Supply Ducts
Burners or Heat Exchangers
Supply Fans
Exhaust Air
Exhaust Fans
Fresh Air
Fresh Air
Tissue Machine Dryer Hood
Main Gas
Combustion Air
Main Gas
Combustion Air
TT TT
TC TC TV TV
Tissue Machine Hood System
Temperature Control
PT PT
TC TC TV TV
Tissue Machine Hood System
SC SC
Air Velocity (h ) Control
Dampers
a
Hood - Yankee Clearance
The hood must supply air in a way to:
•Penetrate the air boundary layer.
•Promote effective mass transfer.
Initial jet diameter depends on nozzle diameters
and the shapes of their edges.
As the jets travel farther from the nozzle exits,
they begin to break up.
Optimum heat transfer occurs when the nozzle
to sheet spacing is 4-7 nozzle diameters.
Hood
Yankee
Exhaust Air Out
Energy In
Cold Air In
Effect of Exhaust Air on Hood Energy
Air Humidity 0.2 0.4
Drying Rate 4%
Drying Cost 25%
Drying
•Introduction
•Yankee Dryer
•Yankee Hood
•Drying Theory
•Equipment Operation and Control
•Operating Centerlines
Drying
•Introduction
•Drying Theory
•Equipment and Control
•Best Practice Development
• The Best Practices
Current Best Practices
Hood System Performance
KPI-01 Hood
Performance Fuel consumption is less than 3.0 MMBTU/MT
KPI-02 Hood
Performance
Pressure distribution difference between crescent headers in the same hood half,
with profiling dampers open, is maintained at less than plus or minus 5%.
Safety
HD-01 Safety Lockout and confined space entry procedures are in place and practiced.
HD-02 Safety No water streams or sprays are used to contact high temperature or insulated surfaces of
the hood.
HD-03 Safety Mechanical stops are checked annually to make sure the hood cannot contact the
Yankee.
HD-04 Safety Personal protective equipment is provided and used when working around hoods.
HD-05 Safety Air Nozzle Best Practices are in use when cleaning around hoods
HD-06 Safety Burner management interlocks are checked annually. (Fuel High/Low Pressure Switch)
(Proof of Combustion Air Flow) (High Burner Temperature Shutdown)
Rate of Rise 1.) Put LIC in Manual 2.) Lower level in FV 3.) Close isolation
valves (manually)
4.) Measure time for
condensate to
rise a given height
Measurement Locations
LIC
Sight Glass
TT
PI
LIC F.T.
Condensate
Dryer
DPI
5.) Restore system to
normal
Measurement Locations
Wet End
Exhaust
Humidity
Operating
Temperature Operating
Temperature
Dry End
Exhaust
Humidity
Measurement Locations
Hood Pressure
Measurements
Combustion Fuel
And Air Pressure
Measurements
Supply Fan Speed