Material Properties

Since steel is a stronger material and has a modulus of elasticity which is considerably more than 2.1 times higher than cast iron, while designing  the pressure vessel safety codes allow for better design parameters on steel. The much higher allowable stresses for steel mean that the shell wall can be thinner, which gives the positive benefit of higher heat transfer rates.

Lower shell root thickness for steel means the resulting heat flow is higher through the cylinder. This gives a higher evaporation rate which can flexibly either be used: to increase the production rate at constant energy input or, when the machine speed is kept constant, to reduce the drying energy.

Furthermore, during casting, pinholes are often developed in body, which need to be filled or repaired; otherwise, these create different quality issues on paper. Such an issue is never seen in steel dryers.

It is interesting to note that CI has a better compressive strength compared to steel, and that makes it more suitable for compressive loads like machine framings, sole plates, base plates etc. Probably, this is the reason that many old papermakers still consider CI a better option than steel. However, the ultimate tensile strength of steel is much higher (even 4 times) than that of CI. To add further to it, one must also keep in mind that specially developed high strength steels now have even more strength than the conventional ones.

Higher heat transfer and lower energy costs

Cast iron and steel have essentially comparable thermal conductivity, so as the shell is thinner, better heat transfer can be assured.

Another benefit of the reduced shell thickness and higher heat transfer is that lower steam pressure can be used for drying. In such a case, the same production rate can be reached using lower steam pressure with higher Latent Heat of Vaporization.

With the grooving inside the shell, overall heating area is considerably increased resulting in better conductivity. The result is lower total energy consumption. Depending on the cost ratio between the different energy sources, significant energy cost savings are possible in tissue machine with fabricated dryers, in fact the energy cost shall be lower in steel Drying Cylinders compared to a C.I. one. The investment cost for steel cylinders are also favorable while compare to the cast iron variety, and the physical weight is also less as well helps less engine power consumption.

Automation, today, is considered the most significant part of any process. Having a dryer of lesser thickness means having lesser heat capacity of the dryer, which leads to faster and efficient control. Hence, you can see better and quicker response to your actions (change in set point of dryer steam pressure, resulting in change in paper moisture). This results in better quality and quality consistency of paper being produced. Not only this, this also results in quicker start-up of the machine after a long stoppage, like a wire/felt change.

Metalizing benefits on carbon steel yankee

It should be mentioned that steel Yankee & dryers can be metalized prior to increase the surface hardness, life and water resistance.  Metallization eliminates the need for re-grinding of the shell wall base metal, which on cast iron can eventually result in de-rating. Metallization is also a key reason that steel Yankees are far less prone to developing chatter problems.

Safety

Safety is of course a very prime factor for consideration for all organizations of the industry. Failure with Explosions on cast iron drying cylinders unfortunately have sometimes happened due to the poor material properties & brittleness of cast iron. Thus the cylinder can explode due to excess pressure, loaded with excessive load, or in case of an accident for other reasons. Steel, on the other hand, will have a better performance in the event that internal stresses exceed than allowable limits.

In the event that internal stresses on the steel shell exceed the yield strength, the steel shell may deform but stay intact.

If the stresses inside a cast iron dryer exceed the yield strength, the dryer may explode.

Top former is a part of the PM wire part. It is installed between the under-pressure and suction boxes, approximately in the second third of the wire table. Its most important part is a three-chamber suction box with different under-pressure values in particular chambers. Through separators this suction box is connected to the vacuum system. On the bearing structure, type cantilever, there are driving roll, guide roll with positioning, wire run regulator, wire stretching device, spraying showers and wire cleaning system installed. All the rolls are provided with doctors. Under the bottom wire, on the bearing structure, there are white water trays with discharge branches installed and used for splashing. The top former itself is completed also with service bridges. It is used particularly in new and reconstructed PM wire sections with operating speeds up to 1.500 resp. 1.600 m/min. Dryness on the bottom wire is 10-12%.

Design

The most important part of the top former is its suction box with diverse vacuum capacity and gradually increasing under- pressure values.

Under the suction box of top former and under the bottom wire there is a forming box installed with height-adjustable and pneu-matically controlled bars in order to improve formation of the paper on the wire. Behind the forming chamber there is a suction box keeping the paper web on the bottom wire in the place of division of the top and bottom wire.

Main parts

• Supporting structure – cantilever design (1)
• Guide rolls (2)
• Wire stretching units (3)
• Wire run regulator (4)
• Suction box – top former (5)
• Forming box – bottom wire (6)
• Oscillating system (7)
• Suction box for wire cleaning (8)
• Doctors
• Suction box – bottom wire (9)
• Sensing of wire stroke
• Collecting tanks (10)
• Covering and walkways

Accessories

• Under-pressure system
• Drive
• Control system with control panel
• Dewatering wire
• System for vapours suction
• Fixtures for rolls and wire replacement

Construction design and machine safety are in compliance with EU and EAC standards.

Rope threading system is used for rewinding and longitudinal slitting of the paper web from the paper machine reel spool into paper rolls of required sizes.

Design

The winding part of the machine consists of two rolls, each of these carrying rolls has its separate drive. A hydraulically controlled ejector of the reeled up rolls and hydraulically controlled cross cutter, which separates the paper web from the wound up roll, are installed in the first carrying roll bedding in the paper web run direction. The carrying roll guard bedding is done in the same way on the second roll, it is controlled pneumatically and serves for a safer roll manipulation in the area between the tilting table and the smooth carrier roll.

Both rolls are dynamically balanced according to the required winding speed. Surfaces of both rolls are plasma-sprayed. Both carrying rolls are driven by AC variable-speed motors. The paper roll is laterally guided by mandrels that are pneumatically lifted or dropped down. They are retractable hydraulically or pneumatically. Every paper roll, while being reeled up, is pressed down from the top by a rider roll, its loading is controlled according to diameters of the paper roll while being reeled up. Pressure roll guiding is equiped by the system
against the roll crash.

The slitting section of the machine is arranged according to required width of strips to be cut, max. operating speed and type and width of the paper web. It is controlled pneumatically or electro-pneumatically on principles of so-called shears slitting with a tangential slit or a slit with embracement on the slitting roll. The machine is equipped in front of and also behind the slitting section with spreader rolls used for correct paper web guiding and cross stretching in the input part just before cutting and when coming to carrying rolls.

These spreader rolls are installed on feeders for an optimal adjustment of the paper web run. The slitting section comprises also a device for direct measuring of the paper web tension located on the guide roll with its constant embracement by means of which the required tension of the reel can be controlled. In the slitting section, there is also an equipment for a dust exhaustion which significantly reduces the amount of impurities on the paper surface.

The paper web threading system serves for a transfer of the paper web from an unwinding stand through transfer, spreader and guide rolls, the slitting section and its spreader roll up to the space between carrying rolls of the rewinder. It comprises V-belt conveyers, chutes, air nozzles, clamping roll device and underpressure system with a sucking of the paper web onto the first carrying roll of the rewinder.

The unwinding stand is arranged as a twin frame with two stands on which reel spool beddings with locking levers and an ejector of empty reels spool, combined with hydraulic dampers on ends of unloading arms, are installed. The brake for unwinding is manufactured either with an AC generator motor, a pneumatic disc brake with air or water cooling, or a drum brake. A clutch installed between the brake and the reel spool is equipped with a grooved shaft on which the coupling part is pneumatically displaced in dependence on the reel spool carrier. The entire unwinding stand is equipped with a cross and longitudinal feed for correct adjustment of the paper web run when entering the machine.

The machine can be also delivered with a complete electric installation, including the control system, automatic paper web threading system, equipment for dust exhaustion from the paper web surface, equipment for suction of trimmings in the slitting section, and reel spools. Besides production of new machines, the company offers also complete overhauls and reconstructions of rewinders and their parts. Machine design and work safety are in compliance with EU and EAC standards.

Main parts

• Bearing structure – individual stands made of profile steel, attached to each other by fixing bolts and locks (1)
• Supporting rolls (2), pressure roll (3)
• Core clamping mandrels
• Paper rolls ejector (5), tilting table (6)
• Spreader rolls (7)
• Slitting section (8), paper web threading (9)
• Unwinding stand (10)
• Machine hydrostatic drive
• Machine pneumatic circuit
• Machine electric part (switchboard boxes, cabling, control board)
• Control system incl. software
• Machine guards, machine drive (11)

Accessories

• Equipment for suction of trimmings
• Automatics for paper web threading
• Equipment for dust exhaustion
• Reel spools with rubber or polyurethane cover
• Automatic roll core threading
• Cross knife
• Vacuum on a grooved carrying roll
• Barrier increasing safety of reel spools handling on unwinding stand
• Central lubrication system

Scope of delivery

• Machine according to main parts description
• Accessories optionally

Material

• Bearing structure – common steel
• Roll coats – common steel with a surface treatment

• Open Head Box for Machine speed up to 350 meters/ minute.
• Slice width up to 3 800 mm are regularly manufactured.
• Profile variation across the deckle in maximum ±3 GSM
• Cylindrical Tapered and Rectangular Tapered with Polypick diffuser, Stainless Steel diffuser, Manifold are provided depending upon the quality and production capacity.
• Two numbers Properly designed Evener rolls for evening effect and better deflocculation.
• Machined and Ground Top and Bottom Lip for smooth flow of stock.

As the name suggests the recovery section was installed to recover the cooking chemicals from the Black liquor received from the Brown Stock Washing section. Although, this section is much cost-intensive it needs to be installed to avoid the use of 100% fresh cooking chemicals. As the losses in the Brown Stock Washing is 8-12%, the requirement of the fresh chemicals will be only 8-12%.

The main subheads in the Recovery plant are,

• Multiple Effect Evaporators
• Low-Temperature Incinerator
• Recaustisicing Plant

Multiple Effect Evaporator Section

The Black Liquor received from Brown Stock Washing Section is having a concentration of 0.8- 1.5% depending upon the use of Raw Material. The Concentration of Black Liquor is increased to 25-30% in the evaporator section. There are different types of evaporators being used in the paper industry depending upon the quality of Black Liquor. These are Long Tube Vertical evaporators, Falling Film Evaporators, Natural Circulation Evaporators, etc.

The most common evaporator being used in the paper industry are Falling Film Evaporators which are having advantages over Long Tube Vertical Evaporators, which are as follows.

• In Falling Film Evaporator, the black Liquor is pumped from the Vapour Head to the Distribution Plates located above the Tubes allowing the liquor to fall down within the tubes.
• The Falling Film Evaporator working at a low Temp. Difference and which allows many effects to be installed in series. It also has a better steam economy than other types of evaporators.

Low-Temperature Incinerator

The basic concept of this system is the use of fluidized bed technique for the treatment of black liquor received from Multiple Effect Evaporator to recover the Pulping chemicals as well as to reduce the pollution load on the environment. The Low-Temperature Incinerator is used in the paper mill, where the Black Liquor solids are less than 300 tons per day.

IN LTI, The solids in the liquid waste containing heat energy are amenable. The advantage of LTI is,

• Elimination of Air as well as Water Pollution.
• Removal of BOD & color, etc. from residual organic products.
• Removal of all particulate matter & gases before discharged into the atmosphere.

The system consists of mainly

• Low-Temperature Incinerator Proper With Accessories
• Venturi Scrubber System with Cyclone Separator
• Attrition Mill and other Accessories
• Air Pre-heater with HSD Burner

The Semi Concentrated Black Liquor from the evaporator is fed to Venturi Scrubber, which works on the Direct Contact Evaporator. The Concentration of Black Liquor is increased to 45%. This Black Liquor is fed to LTI- where the Temp. Is around 600-700 Deg. C is maintained. The water in the Black Liquor is evaporated & the solids are converted to Soda Ash.

The Soda Ash produced is withdrawn from the side of the reactor at a controlled rate by a variable speed Rotary Valve. The Soda Ash is conveyed through Water Cooled Screw Conveyor before being stored in a Silo.

Recaustisizing Plant

The Soda Ash (NA2CO3) produced in the LTI- is further treated with milk of lime (Ca(OH)2) to get Caustic Soda (NaOH) which is used in the Digester for the Cooking of the Raw Material & the Calcium Carbonate (CaCO3) produced is either used as Landfills or sold to Cement Industry.

The Major Equipment in Recaustisicing Section is as below.

• Lime Slurry Preparation
  • Table Feeder: A table Feeder is installed to feed the lime to slaker at a controlled rate.
  • Rotary Drum Slaker: In Drum Slaker, the water is added along with the lime to make the slurry. The slaker rotates at a fixed speed & required time is given to complete the reaction. The grits in the lime are removed from the slaker.
  • Classifier: The green liquor from LTI Section is mixed with Lime Slurry in Rake Classifier. Any grits in the system are removed in the Classifier.
  • Causticizer: The Slurry from Rake Classifier is pumped to Causticizer, which is installed in series to provide the required retention time to complete the reaction. The Causticizer has an agitator & the size depends on the size capacity.
  • White Liquor Clarifier: The slurry from Causticizer is pumped to White Liquor Clarifier, which is provided with a rake mechanism having blades to ring the sludge in the center. The Overflow from the clarifier is taken to the Storage Tank, from where it is pumped to the Digester House to be used as Cooking chemicals.
  • Recausticizers: The underflow from White Liquor Clarifier, is taken to Recausticizers- where it is mixed with Washed Liquor from Ist stage Lime Mud Washer.
  • Lime Mud Washer: Lime Mud Washers are similar to White liquor Clarifier, which are provided with Rake Mechanism. The Lime Mud Washers are provided in Series to recover the chemicals from the Lime Sludge & the wash water is used in the Counter-Current system.
  • Lime Mud Filter: The Sludge from the final Lime Mud Washer is pumped to Lime Mud Filter, which works on the vacuum. The inlet consistency of lime sludge is around 1-1.5% and outlet consistency is around 60%. The lime sludge from the lime mud filter can be used either in landfills can be sold. Lime Mud Filter is provided with a filtrate tank.

As the name suggests the recovery section was installed to recover the cooking chemicals from the Black liquor received from the Brown Stock Washing section. Although, this section is much cost-intensive it needs to be installed to avoid the use of 100% fresh cooking chemicals. As the losses in the Brown Stock Washing is 8-12%, the requirement of the fresh chemicals will be only 8-12%.

The main subheads in the Recovery plant are,

• Multiple Effect Evaporators
• Low-Temperature Incinerator
• Recaustisicing Plant

Multiple Effect Evaporator Section

The Black Liquor received from Brown Stock Washing Section is having a concentration of 0.8- 1.5% depending upon the use of Raw Material. The Concentration of Black Liquor is increased to 25-30% in the evaporator section. There are different types of evaporators being used in the paper industry depending upon the quality of Black Liquor. These are Long Tube Vertical evaporators, Falling Film Evaporators, Natural Circulation Evaporators, etc.

The most common evaporator being used in the paper industry are Falling Film Evaporators which are having advantages over Long Tube Vertical Evaporators, which are as follows.

• In Falling Film Evaporator, the black Liquor is pumped from the Vapour Head to the Distribution Plates located above the Tubes allowing the liquor to fall down within the tubes.
• The Falling Film Evaporator working at a low Temp. Difference and which allows many effects to be installed in series. It also has a better steam economy than other types of evaporators.

Low-Temperature Incinerator

The basic concept of this system is the use of fluidized bed technique for the treatment of black liquor received from Multiple Effect Evaporator to recover the Pulping chemicals as well as to reduce the pollution load on the environment. The Low-Temperature Incinerator is used in the paper mill, where the Black Liquor solids are less than 300 tons per day.

IN LTI, The solids in the liquid waste containing heat energy are amenable. The advantage of LTI is,

• Elimination of Air as well as Water Pollution.
• Removal of BOD & color, etc. from residual organic products.
• Removal of all particulate matter & gases before discharged into the atmosphere.

The system consists of mainly

• Low-Temperature Incinerator Proper With Accessories
• Venturi Scrubber System with Cyclone Separator
• Attrition Mill and other Accessories
• Air Pre-heater with HSD Burner

The Semi Concentrated Black Liquor from the evaporator is fed to Venturi Scrubber, which works on the Direct Contact Evaporator. The Concentration of Black Liquor is increased to 45%. This Black Liquor is fed to LTI- where the Temp. Is around 600-700 Deg. C is maintained. The water in the Black Liquor is evaporated & the solids are converted to Soda Ash.

The Soda Ash produced is withdrawn from the side of the reactor at a controlled rate by a variable speed Rotary Valve. The Soda Ash is conveyed through Water Cooled Screw Conveyor before being stored in a Silo.

Recaustisizing Plant

The Soda Ash (NA2CO3) produced in the LTI- is further treated with milk of lime (Ca(OH)2) to get Caustic Soda (NaOH) which is used in the Digester for the Cooking of the Raw Material & the Calcium Carbonate (CaCO3) produced is either used as Landfills or sold to Cement Industry.

The Major Equipment in Recaustisicing Section is as below.

• Lime Slurry Preparation
  • Table Feeder: A table Feeder is installed to feed the lime to slaker at a controlled rate.
  • Rotary Drum Slaker: In Drum Slaker, the water is added along with the lime to make the slurry. The slaker rotates at a fixed speed & required time is given to complete the reaction. The grits in the lime are removed from the slaker.
  • Classifier: The green liquor from LTI Section is mixed with Lime Slurry in Rake Classifier. Any grits in the system are removed in the Classifier.
  • Causticizer: The Slurry from Rake Classifier is pumped to Causticizer, which is installed in series to provide the required retention time to complete the reaction. The Causticizer has an agitator & the size depends on the size capacity.
  • White Liquor Clarifier: The slurry from Causticizer is pumped to White Liquor Clarifier, which is provided with a rake mechanism having blades to ring the sludge in the center. The Overflow from the clarifier is taken to the Storage Tank, from where it is pumped to the Digester House to be used as Cooking chemicals.
  • Recausticizers: The underflow from White Liquor Clarifier, is taken to Recausticizers- where it is mixed with Washed Liquor from Ist stage Lime Mud Washer.
  • Lime Mud Washer: Lime Mud Washers are similar to White liquor Clarifier, which are provided with Rake Mechanism. The Lime Mud Washers are provided in Series to recover the chemicals from the Lime Sludge & the wash water is used in the Counter-Current system.
  • Lime Mud Filter: The Sludge from the final Lime Mud Washer is pumped to Lime Mud Filter, which works on the vacuum. The inlet consistency of lime sludge is around 1-1.5% and outlet consistency is around 60%. The lime sludge from the lime mud filter can be used either in landfills can be sold. Lime Mud Filter is provided with a filtrate tank.

The pulp after washing contains uncooked shives which are required to be removed before the pulp is used for kraft paper making or to be used in the bleaching section to make white grade papers.

Screening is carried out in 3 stage screening process with a centrifugal screen in the primary stage. The accept from the primary screen is further cleaned in the centricleaning system & the accept of centricleaning system is thickened in the Vacuum Thickener to be stored in the High-Density Tower at the consistency of around 10%.

The rejects from the Primary screen are further cleaned in the secondary screen to recover the pulp from the rejects.

The aim to install Brown Stock Washing System is to wash the pulp after the cooking process to recover the chemicals & other organic compounds. The washing system runs on the Counter-Current washing system, where the hot water is used on the final washer and the black liquor is sent to the Recovery section from the First washer. The washers are designed to work on Barometric Leg Vaccum System.

The inlet consistency of the washer is generally 1.5-2.5% and the discharge consistency is around 10%. Shredder Repulper is installed at the inlet of the next washer to make a homogeneous mixture of 1.5- 2% consistency by using the filtrate of the previous washer.

The Filtrate from each washer is collected into the Filtrate Tank & is used in the previous washer for the Dilution purpose. As the liquor is very dilute in nature, there is a tendency for Foam Generation, which is controlled by installing Foam Breaker on a Foam Tank.

Chips from the storage silo are loaded into the digester, where it is cooked with cooking liquor (Caustic Soda) at a Temperature of 170-175 Deg. C & Pressure of around 9 kg/cm2. The cooking is carried out by an Indirect heating process to remove lignin & other organic compounds. After attaining a desired pressure & temperature, It is kept at the same pressure & temp. for 2 hours. Then, the whole mass is blown to the Blow tank under pressure. The Vapors coming out of the blow tank are utilized to heat the water in the blowing heat recovery system.

Vertical Stationary Diester With Preheater:

Digester has got cylindrical portion in the center & a conical shape at the bottom & top end. It has got screen plates at the top, middle & bottom. Material of Construction of digester is SA 516 Gr. 70 & the thickness is approx. 36 mm. The designed pressure is 10.5 kg/cm2 & test pressure is 16.5 kg/cm2. The screen plates MOC are AISI 304. Digester body has got connection for putting liquor in the center & discharge at the top & bottom. Blow line to the digester is connected at the bottom, from where the complete mass is blown after the cooking cycle completes.

As the cooking process is carried out by Indirect heating method, Digesters are connected with Liquor Preheater- where the steam is injected at the outer body of the preheater & the liquor is circulated in the preheater through liquor circulation pumps in the tubes of the preheaters.

Blow Heat Recovery System:

Blow Heat recovery system is installed to condense the contaminated vapors originating from the blow tank during blowing of pulp from the Digesters. The water from the Accumulator tank is circulated to condense these vapors & raise the water temperature. This hot water is used to heat the fresh water from ambient Temperature to 70 Deg. C in plate type or Tube type heat exchanger. The heated fresh water is used in washing section.