卫生纸纸机的干燥技术、扬克烘缸和气罩(英文版)

142 | Metso Paper Technology Days 2005
For a conventional dry crepe tissue machine to be competitive today, it must have the ability to effi ciently produce high-quality paper at a high utilization rate and at the lowest possible total production cost. The drying section, consisting of a Yan-kee cylinder and an AirCap plays an important role. Its dimensioning and design should be optimized for the specifi c process. It is also important that the components work together in order to guarantee the required drying load at the minimum energy cost. The Yankee dryer and AirCap must also be designed and manu-factured for safe and reliable use with the least possible downtime for maintenance and service.
1. The drying process – a brief introduction
The drying process is a combination of cylinder (Yankee dryer) and im-pingement drying (AirCap) (Fig. 1).Energy for the drying of the tis-sue sheet is supplied both by high-
pressure steam condensing inside the Yankee cylinder and by hot air blown onto the sheet in the AirCap, which covers a large area of the Yan-kee surface. Conductive drying on the Yankee cylinder initiates from the fi rst press roll nip and contin-ues as long as the sheet remains in
contact with the dryer. Convective and radiant drying begin under the area covered by the AirCap. The Yan-kee drying process is very intensive with drying rates of 150 – 240 kg H 20/hm 2, versus 20 – 30 kg H 20/hm 2
Fig. 1 Yankee drying is a combination of conductive cylinder drying and convective and radiant impingement drying. 1. Yankee hood air, 2. Diff usion
vapor, 3.Conducted heat.
Vice President, Sales Tissue Business Line Metso Paper
General Sales Manager Metso Paper Gorizia
Sales Manager, Yankee Dryers Tissue Business Line Metso Paper
Metso Paper Technology Days 2005 | 143
on a conventional cylinder drying section. During drying the sheet is fi xed on the dryer and not subject to shrinkage as with other drying sections.
The drying capacity of a tissue machine is mainly aff ected by the size of the Yankee cylinder which, in turn, gives a larger hood-covered area. Higher speed machines and the need for higher drying capacity have resulted in bigger and bigger Yankee cylinders. Today the largest Yankee dryers for tissue measure 5,500 mm (18 feet) in diameter. In addition to larger Yankees and Air-Caps recent developments take us towards higher hood impingement speeds and temperatures.
The drying of tissue occurs through a process of evaporation, and it is infl uenced by the hygro-scopic nature of fi bers. Fibers con-tain water in the form of:• free water (inter fi bers and intra lumen)
• bound water (inside fi ber wall pores and chemically bound to fi ber molecules).
When the wet sheet is pressed onto the Yankee surface, a short period of high heat transfer occurs until the sheet reaches a steady state temper-ature of about 90°C (heat-up period). The drying process continues and remains constant until all free water is evaporated (constant rate period) and the only remaining water is bound to the fi bers. The fi nal stages
of drying are then slower and more
energy-demanding (Fig. 2).
More energy is supplied from the AirCap than from the cylinder on modern tissue machines. The AirCap can supply between 50% and 70% of the total energy today.
2. Yankee dryer
The main elements of a Yankee cyl-inder include its shell, which is con-nected to a through-going journal by two heads. This through-going journal is a typical hallmark of a Yankee cylinder. Besides its primary function as a drying cylinder, the Yankee cylinder also functions as a press roll and transports as well as supports the sheet during the crep-ing process.
Energy for the drying process is normally supplied into the Yan-kee through the front side journal in the form of superheated steam. This steam condenses on the inside of the shell and its heat is transferred through the shell into the paper sheet.
Yankee dryers are pressure ves-sels and are subject to standards and inspections stipulated by gov-ernmental authorities. They need to be designed, manufactured and operated with very close attention to safety and reliability.
The higher temperature of the condensed steam inside the dryer means greater heat fl ow and thus higher drying capacity, all things be-
ing equal. However, higher steam temperatures require higher steam pressure and thus a thicker shell. This relationship varies between dif-ferent Yankee cylinders (geometry and design conditions) and needs to be analyzed case by case.
Modern Yankee dryers for tissue paper are usually designed for steam pressures of 800 – 1,100 kPa.
In addition to conductive resist-ance through the shell, which in-creases with shell thickness, heat fl ow is also restricted by the heat transfer resistance from steam to the shell inside the dryer and heat trans-fer resistance from the shell surface to paper. The transition resistance from cylinder surface to paper is generally very small compared to the other two.
The transition resistance from steam to shell is determined prin-cipally by the condensate fi lm on the inside of the shell. A thicker and more stationary condensate film means higher resistance and less effi cient heat transfer.Heat fl ow could be signifi cantly increased with the introduction of a ribbed shell. As ribs increase the rigidity and strength of the shell, the distance from the bottom of the groove to the outside of the shell can be less than the thickness of a plain shell for the same pressure. Thermal resistance will thus be low-er. The vertical sides of the grooves will also have no condensate fi
lm,
Fig. 2 Drying of tissue.
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which gives them very low thermal resistance. The resultant heat trans-fer from steam to shell increases (Fig. 3).
To attain an eff ective heat fl ow, the inside of the shell must be kept as free from condensate as possible. This is done with internally mounted condensate siphons or headers op-erated by means of blow-through steam. These headers collect and transport condensate into the center shaft through riser pipes. From there it is normally evacuated through a rotary joint in the drive side journal.
3. The Metso Yankee dryer
The Metso Yankee dryer (Fig. 4) has been developed and optimized for safe and reliable high performance operation. It features a highly con-ductive internally ribbed shell, two curved heads, and a through-go-ing journal in two or three sections. The philosophy behind the design is to have as few sections, and ac-cordingly screw joints, as possible and to match them in stiff ness and strength.
The internal condensate removal system consists of headers with long straw pipes mounted in both direc-tions, or headers with short straw pipes mounted in the radial direc-tion. Both are well-proven designs
for effi cient and uniform condensate removal, and are selected case by case depending mainly on cylinder size and machine speed. The sys-tem is mounted inside the Yankee dryer without attaching the shell or other stressed or steam-atmosphere boundary elements (Fig. 5).
All parts of the Yankee dryer are dimensioned to fulfi l Metso Paper’s stringent and extensive design crite-ria and, of course, the requirements set by governmental authorities. Metso Paper is certifi ed as a supplier by ASME and by European and Chi-nese pressure vessel authorities.
3.1 The manufacturing process
The Yankee dryer is made of cast iron. Molding and casting, especially of the shell, make up the heart of the Yankee manufacturing process and represents its most critical mo-ment. It is a very demanding and complicated process, and only very few foundries in the world are ca-pable of it. Even fewer master this process fully.
The molds are hand-formed us-ing highly specialized methods and materials developed over several
decades. The mold for the shell cast-
Fig. 3 Heat fl ow as a function of steam pressure and required shell thickness for
a Yankee dryer with three alternative shell designs.
Fig. 4 Sectional view of a Metso Yankee cylinder.
ing takes a couple of months of care-ful work to fi nalize.
When the mold is ready, the actu-al casting is done. The iron and alloys are melted in several large furnaces to exactly the right composition and temperature, after which the mixture is distributed to a number of ladles. Up to four ladles, maneuvered with overhead cranes, are used for large shell castings. At the critical moment all ladles are simultaneously poured into the mold through runners. A spectacular sight follows during the 60-90 second casting process (Fig. 6).
All castings are heat-treated be-fore machining to release all stresses and strains. This is to secure that the fi nished Yankee dryer will retain per-fect dimensional stability during its entire service life. This is especially important for a large high-speed creping cylinder.
All machining of the shell and heads is carried out on a giant verti-cal turning machine with high toler-ances (Fig. 7).
Since the shell is machined standing up in a vertical position, the fl anges will be perfectly plane and the fi t between the shell and heads gets very tight so that steam leaks are avoided even after many years of service when the sealing compound may have lost some of
its eff ectiveness. While the perfect fi t at the shell head joints may be the most noticeable eff ect of vertical machining, all other shell tolerances, such as thickness variations and out of roundness, benefi t as well.
3.2 Quality control
The fi rst major quality control fol-lows after the initial rough machin-ing of the shell. The shell is 100% ultrasonically tested to verify that the material is sound and fulfi lls all specifi cations. A number of X-ray im-ages are taken as well based on the UT plot of the shell.
Other quality inspec-
tions carried out during
the manufacturing process
include magnetic particle
testing, dimension and tol-
erance checks, checklists for
all screw joints and fasteners,
and visual inspections, such
as a fi nal shell surface inspec-
tion. A number of controls are
also carried out under the su-
pervision of representatives
from pressure vessel authori-
ties, such as material strength
Fig. 5
Condensate header with long straw pipes mounted in both directions.
Fig. 6
Casting of a large Yankee dryer shell.
Fig. 7 The giant vertical turning machine used for
Yankee shells and heads.
Metso Paper Technology Days 2005 | 145
tests with test bars and hydrostatic testing.
The journals are machined in a horizontal turning machine and a drilling-milling machine. The balanc-ing, grinding and superfi nishing of assembled Yankee dryers occurs in another tailor-made machine. All machinery used is of the highest standard.
4. Advantages with the Metso Yankee dryer
Our fi rst Yankee dryer was delivered in 1895, and since then we have manufactured and shipped 500 Yankee dryers in diff erent sizes, in-cluding the world’s largest Yankee dryers and MG cylinders. Add to this the experience from an additional 11,500 drying cylinders.
During this long and active peri-od the design has been optimized in every detail – not least due to valua-ble feedback from skilled customers. The molding and casting methods have been signifi cantly developed and improved, and some steps have even been patented. Other areas that have been the subjected of lots of successful eff ort and devel-opment work include cast iron alloys (higher strength, thermal conductiv-ity and wear resistance) and quality control methods (e.g. specialized ultrasonic testing).
More important than our ability to lead in the development towards larger, higher performance Yankee dryers are our recognized high qual-ity and outstanding safety record.
Today’s prospective modern Yan-kee dryer buyers should pay special attention to the following advan-tages of using Metso Paper as their supplier:
4.1 Safety
Besides fulfi lling applicable govern-mental requirements, all parts of Yankee dryers are dimensioned to
fulfi l M etso Paper’s own stringent
and extensive design criteria. These
are followed up by careful quality
control. Our cooperative eff orts with
safety-minded customers have re-
sulted in an outstanding safety
record.
This very meticulous and de-
manding inspection protocol is one
of several methods and standards
developed by M etso that other
manufacturers eventually have had
to follow.
4.2 Reliability
The Yankee dryer is, to say the least,
a vital component in a tissue ma-
chine. A very important aspect of
all development and optimization
work has been, and still is, maintain-
ing and improving the robustness of
the dryer design. A simple but well-
performing design, good quality of
castings and other materials, and
high-precision machining are the
main reasons why a Metso Yankee
dryer requires minimal downtime
for maintenance and service.
4.3 Performance
Our special and highly acclaimed
molding technique and cast iron rec-
ipe result in shells of high strength,
rigidity and thermal conductivity
with minimal porosity in the mate-
rial. The rib profi les have been de-
veloped using both analytical and
empirical methods to achieve a
strong shell with high and uniform
heat fl ow.
The heat-treatment of castings
and first-class turning machinery
ensure tight and lasting tolerances,
such as run-outs, thickness varia-
tions and uniformity. This is particu-
larly important for a large, fast-run-
ning tissue Yankee dryer.
Condensate is effi ciently removed
by well-proven internal systems that
are optimized case by case depend-
ing mainly on cylinder size and ma-
chine speed.
4.4 Longevity
The expected service life of a Yankee
dryer has become almost infi nite in
recent years with improved chemi-
cal coating systems and the success-
ful development of methods for full
face metal spraying. However, this
presupposes that the whole Yankee
dryer is built to last. One very im-
portant factor, besides high-quality
castings and proper tending, are
the diff erent screw joints, especially
those between the shell and heads.
Steam leakage through a fl ange joint
is diffi cult to cure and may erode and
corrode the fl anges leading to safety
and runnability issues that shorten
the service life. The key to durable
and steam-tight screw joints, along
with an optimized design and the
foregoing considerations, are high
tolerances of the machined fl anges.
A vertical turning machine, like the
one used in Yankee manufacturing
at Metso, is a big advantage since
when the shell, and heads, are ma-
chined standing up in a vertical po-
sition the fl anges will be perfectly
plane. This ensures an excellent fi t
so that steam leaks are avoided even
after many years of service when the
sealing compound may have lost
some of its eff ectiveness.
5. Yankee hood
The general trend is toward more
effi cient and higher capacity drying
sections. Faster machines for in-
creased production naturally mean
the need for higher drying capac-
ity. But producing a higher grade
premium quality product often
also demands higher drying capac-
ity. For example, the use of single
press confi gurations - suction press
or SymBelt TIS – to increase bulk re-
sults in lower post-press consistency,
and thus places an increased load on
146 | Metso Paper Technology Days 2005
the drying section. Other examples are the increased use of quality-en-hancing Yankee coating chemicals that cool the dryer when applied, and that may result in thicker coat-ing buildup that reduces the heat fl ow, and increased reel dryness for increased tissue softness.
The need for higher drying ca-pacity has resulted in bigger Yan-kee cylinders whose materials and design are optimized for high ther-mal effi ciency. There are, however, some practical and technical limita-tions restricting size and capacity.
A higher evaporation load at the drying section therefore has to be addressed on the air side, and recent developments are moving towards higher AirCap impingement speeds and temperatures (Fig. 8).
Air blowing against the sheet is important in the drying process as it breaks the stagnant boundary layer and produces a high heat transfer coeffi cient on the paper surface. This way heat fl ow takes place mainly by forced convection. The main pa-rameters that can help to increase evaporation are:
• jet temperature
• jet speed
• jet humidity
• system geometry
6. The Metso Advantage Aircap
M etso Paper has devoted a great deal of eff ort to developing a new generation AirCap to respond to tissue customers’ new and higher identifi ed needs and values.
Four key features defi ne the na-ture of the Advantage AirCap:
• Advantage expansion
• Advantage profi ling
• Advantage drying
• Advantage safety and mainte-
nance 6.1 Advantage expansion
Uncontrolled thermal expansion
is an important issue in a conven-
tional air-cap; this eff ect increases
the distance between the AirCap
and the Yankee dryer resulting in
• heat losses
• higher energy consumption
• risk of fi res
The eff ect is heightened especially
when the drying process requires a
high temperature (e.g. 700°C). Many
FEM studies have been conducted
on the Advantage AirCap family for
a better design and completely con-
trolled heat expansion: the blowing
nozzles follow the cylinder at a con-
stant distance, which consequently
helps to reduce heat losses and low-
ers energy consumption.
6.2 Advantage profi ling
Profi ling is another critical feature to
obtain higher tissue paper quality
standards.
A new system with double the
normal profi ling zones has been de-
signed in the wet end toe of the Air-
Cap to enhance profi ling capabilities.
The profi ling area is concentrated in
the toe only because profi ling capa-
bility is more eff ective there, while
the rest of the AirCap wet end sec-
tion remains completely dedicated
to the drying process (Fig. 9). This
way there is no longer any need to
Fig. 8 Higher evaporation load has to be addressed on the airside.
Fig. 9 Profi ling capability is more eff ective in the wet end toe.
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over-dry the paper and dust produc-tion at the dry end is also decreased. The results are up to 3% profi ling ca-pability, faster response, and lower consumption of fi ber for the same quality end product (Fig. 9).
6.3 Advantage drying
As we have seen, the drying area of the Advantage AirCap is larger because the profi ling eff ect is con-centrated in the lower toe, which means that total drying capacity and effi ciency are clearly increased compared to standard solutions.A new design of the cross noz-zle boxes, nozzle patterns and ge-ometry increases heat transfer and energy effi ciency.Uniform air fl ow is a key factor to achieve high-quality tissue: for this reason internal air distribution has been optimized using CDF analysis.
6.4 Advantage safety and maintenance
Producing softer tissue means more dust with the consequent higher risk of fi re and safety implications.
AirCap sections feature Advan-tage Roof with automatic cleaning,
which reduces both maintenance
Fig. 10 Dust is washed away automatically.
Fig. 11 Advantage AirCap is available in compact, basic and high temperature versions.
Fig. 12 Higher drying by high temperature toe.Fig. 13 Higher drying by total high temperature.
needs and the risk of fi re, and allows higher machine up-time (Fig. 10).
7. Diff erent versions for optimal fl exibility and performance
M etso’s Advantage AirCap repre-sents new but proven technology applicable to both new and existing tissue machines, and it is available in three versions depending on per-formance requirements:
• Basic (B) 510°C
• Basic with high temperature toe (HTT)700°C
• Total high temperature (HT)
(Fig. 11)
When processes need to focus on higher drying to increase produc-tion or make super soft products, the high temperature toe or the to-tal high temperature version can be selected from the Metso Advantage AirCap family to obtain 10% to 20% more drying capacity (Fig. 12, 13).
Finally, the basic version of the Metso Advantage AirCap can also be provided in a compact design with fans and burners built inside sec-tions to fi t in space-constrained new or rebuilt installations (Fig. 14).
A Compact Advantage AirCap is
an excellent product for customers who need to maximize (increase) their drying capacity, production, and the effi ciency of their machine at reduced capital and installation costs.
They can secure a specifi c evapo-ration rate of up to 132 kg/H2O/m2 of covered surface (to be added to the Yankee dryer evaporation) thanks to Metso Advantage technology and its controlled thermal expansion, which allows operating temperatures of up to 510°C.
The advantages of a compact Ad-vantage AirCap, compared to tradi-tional mono-system or duo-system Yankee hoods with external air sys-tems, can be quantifi ed as roughly
40% savings in the customer’s nor-
mal total Yankee hood investment
costs.
Savings can be seen in
• infrastructure construction and
mezzanine
• Corten air ducts
• insulation to protect high-tem-
perature ducts
• transportation of ducts and
insulation
• installation (cost and time) of
the air system at the mezzanine
level
• fi xed asset depreciation on the
above investment in the subse-
quent years
(Fig. 15)
Finally, since the installation time
of a compact Advantage AirCap is
less than one-half of the normal in-
stallation time, customers can shut
their machines down for approxi-
mately fi ve fewer days in the case
of rebuilds and reduce production
losses correspondingly.
8. Energy savings on
existing tissue machines
Today, when energy costs are con-
tinuously rising worldwide, control-
ling and reducing tissuemaking en-
ergy consumption is no longer an
option but a must to gain, maintain
Fig. 14 Advantage AirCap compact version.
Fig.15 Installing a compact Advantage AirCap saves roughly 40% total Yankee
investment cost.
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and improve a competitive edge in the tissue market. Yankee dryer and hood drying is the most energy-in-tensive process in tissue production, and one of the biggest components of a tissue machine’s variable operat-ing costs.
M easuring drying performance and energy consumption, and ana-lyzing and comparing them against a database of similar installations helps to defi ne process adjustments that provide an immediate effi ciency and service payback.
By adjusting the process, savings of up to 10% of drying costs are pos-sible without any capital investment (Fig. 16).
Small modifi cations with modest capital investment can take drying cost savings up to 20% (Fig. 17).By using other energy sources the drying process can show savings in excess of 30%.
This last point is one of the most interesting today, namely the use of such alternative energy sources as cogeneration (Fig. 18 a and b).
Exhaust gases from the turbine are used both to dry paper and to produce steam for the Yankee dryer and for other purposes, including electricity.
9. Summary
The tissue machine drying process, a combination of cylinder and im-pingement drying, is a very intensive process with high evaporation rates and substantial energy consump-tion and, accordingly, the demands placed on the Yankee cylinder and AirCap are high. A modern and com-petitive Yankee drying section must off er
• High safety
• High performance
• High reliability and durability Metso off ers both Yankee dryers and AirCaps that meet these demands separately or, to fully benefi t from
the M etso Advantage, together. M etso also off ers process studies to increase capacity or to reduce energy costs, as well as full-scope
services.
Fig. 16 Savings by adjusting the process.
Fig.17 Savings by small modifi cations.
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Fig. 18 b Cogeneration: exhaust gases to produce steam.
Fig. 18 a Cogeneration: exhaust gases for drying the paper.-。