WPA NewSteambox

 A c o m p arat ar atii v e st s t u d y o f n ew ste st eambox mbo x de d esign sig n and performa perform ance

 A Metso White Paper © 2011 Metso

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Introduction Steamboxes have become an indispensible tool on many paper machines to improve drying efficiency and cross directional moisture profiles. By the 1990s, virtually all paper machines making containerboard, pulp, or newsprint had one or more steam boxes on the fourdrinier or in the press section to increase sheet temperature and improve dewatering. The steam from a wet end steambox heats the water in the sheet, reducing its viscosity and allowing it to drain more readily. In a well designed configuration, the improved dewatering can be translated directly to steam savings or, on a dryer limited machine, increased production. The steambox can provide a more uniform moisture profile and avoid the need to over dry the sheet to eliminate moisture streaks. However, many papermakers hesitate to install a steam box in the presses because of the lack of space or simply maintenance and cleaning concerns. New application areas Metso steambox designs are now more compact and engineering design has eliminated many of the problems experienced with conventional constructions. This has led to a wider application of the technology to save energy on grades not previously considered. Metso reports that tissue makers have discovered the energy saving potential of its new technology steamboxes resulting in a dramatic increase of tissue installations over the last two years. Many new m achines are now being designed with sufficient space for the steambox, this is especially true for board machines, where lack of space previously ruled them out.

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Background The main purpose of a steambox is to increase sheet temperature by applying steam. This helps lower the viscosity of the water, which leads to additional dewatering and increased press solids. The main principle is the same for all steamboxes. What differentiates one steambox design from another is the efficiency with which it increases sheet temperature. If the efficiency of a steam profiler is good, most of the energy available in the steam is transferred to the web. Typical experience shows that a 10-14°C increase in sheet temperature in the press section will improve sheet dryness by 1%. This translates to a 4%-5% reduction in drying load or possibly an increase in production. The second objective of the steambox is to reduce CD moisture variation at the reel. This is done with segmented steam sections across the steambox to selectively control the temperature profile.

Figure 1. Water viscosity decreases with temperature

The potential of dryness improvement depends on web properties and pressing conditions Moisture content at operation location Types of fiber Basis weight Sheet temperature Permeability of sheet Amount of fines Beating level   Freeness Ash content • • • • • • • • •

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Steambox performance should not only be j udged by the sheet temperature increase obtained, but also by how much steam is used to achieve the temperature increase. Otherwise, it is just moving the steam consumption from the dr yers to the steambox. Although drying accounts for 50% of energy use on the paper machine, its share of total water removal is minimal. The potential for savings is huge. Steambox basics The optimum sheet temperature increase is obtained when the steam gives up the most energy (its latent heat) in the vapor to liquid phase, also known as condensation. There is a very specific range for the degree of superheat as it comes out of the diffuser plate so that the steam condenses to a maximum extent and gives up its latent heat in the paper web. The required degree of superheat is based on the dynamics of the installation location. If the temperature is too high, the steam will not condense effectively in the sheet. A steam cloud maybe seen around the steambox as the steam flashes on the colder sheet surface but heat transfer is poor and very inefficient.

Figure 2. A steam cloud around a conventional steambox shows poor heat transfer. (Compare this to figure 6!)

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Many conventional steamboxes cannot be run with the optimum degree of superheat as it leads to runnability problems as the steam starts to condense in the approach piping and the steambox itself, or in the air before it reaches the sheet. The design of the steam delivery system and, in particular, the degree of superheat in the steam is very important for the energy delivery efficiency and profiling effectiveness.

Figure 3. Optimum superheat is needed High heat transfer efficiency is achieved by pr oducing the optimum steam velocity to g et full penetration in the sheet and optimum steam temperature so that all of the steam to condenses within the web. The steam velocity vector is mainly a function of the steam pressure, internal and diffuser plate design of the steambox, machine speed and vacuum in the suction roll. If the steam velocity is not optimum, steam can blow through the sheet or not have enough penetration. Steambox design Compared to older units, the new design is compact, more rigid and features various features to prevent fiber buildup, avoid cold areas and eliminate drips from condensate on the surfaces or internal water buildup. The most important design considerations are how the steam is presented to the sheet for maximum heating efficiency, reliable control of the steam flow to the profiling sections and how the steam flows within the steambox without heat loss. In the following paragraphs, conventional steambox designs are compared with Metso’s IQ Steam Profiler, a steambox that is setting new standar ds in performance and ease of application. The diffuser Perhaps the biggest noticeable difference in various steamboxes is in the design of the diffuser plate and the pattern of the steaming holes. Many steamboxes for some reason have a lot of dead space and use only a small part of the available area for steam application. A limit to the condensing rate is the available web area, on medium to fast machines, the dwell time, the time the sheet is exposed to the steam box can be less than 20 milliseconds. Metso’s IQ Steam Profiler, with specially designed diffuser plate, features th e longest machine direction steaming area of any available design. While others use several rows of steam holes or slots to direct steam towards the sheet, IQ Steam Profiler has a matrix of nozzles in a pattern modeled to reduce interaction and provide good steam flow. The steam hole matrix, patented by Metso, 5

provides an even steaming action over the sheet to improve condensation and heat transfer. The increased number of steam holes allows lower steam velocities and the smallest distance from the web of any design, which also reduces turbulence at the sheet surface and spilled steam. This aids in much better heat transfer efficiency and allows the minimum of superheat needed to carry the steam into the sheet. Fewer holes will p resent much higher steam velocities to the sheet and require a higher degree of superheat to ensure even condensation.

Figure 4. IQ Steam Profiler (right) maximizes the steaming area in the machine direction with a matrix of nozzles for an even steaming action and improving the energy transfer compared to another design on the left with only four rows of nozzles. Sealing zones IQ Steam Profiler has integrated sealing zones that eliminate the need for separate preheating zones or shower curtains. Some other designs use a preheating zone or steam shower curtain across the entire web to reduce the effect of laminar air (air travelling with the web at its surface) and possibly, because the heating area of the profiling section is so short in the machine direction. The use of a preheat zone limits the amount of CD controlled steam available for profiling. The longer MD steaming ar ea of IQ Steam Profiler is totally segmented so that all applied steam is controlled for superior cross direction profiling and maximum press d ryness.

Figure 5. Effective sealing zone technology, and profiling chamber design (seen here with part of the diffuser plate removed) reduces air entry and steam spillage.

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Figure 6. Even at full power, this close-up of IQ Steam Profiler shows no steam spillage Better accuracy and reliabilit y wit h electromechanical actuators The majority of available steambox use pneumatically actuated diaphragm valves for control of steam flow to the sectioned diffuser p late. Unfortunately, the positioning accuracy and repeatability of pneumatically actuators have fallen far behind the accuracy of modern moisture profile measurements. Manufacturers offer incredibly long warranties for pneumatic valves but in reality, failure can only be observed visually, requiring downtime and disassembly of the steambox. The value of having a free replacement part is doubtful when taking into consideration the cost of downtime and mechanical inspection. Online bump tests can show total valve failure but a sticky actuator is very difficult to find. Regular preventive maintenance including changing O-rings and seals add considerably to the total cost of ownership. Early Metso steamboxes utilized pneumatic actuators but advances in technology have now provided maintenance free electromechanical actuators with online diagnostics and troubleshooting capabilities that are far more robust and perfectly suited to the paper machine environment.

Figure 5. The hysteresis in a pneumatic actuator (left) makes them unsuitable for pr ofile controls, the maintenance free design of the Metso actuators (right) with position feedback offer more accuracy and better reliability.

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Motor driven stepper actuators provide steam flow control in the IQ Steam Profiler, a vast improvement on the non-repeatable characteristics of pneumatically actuated diaphragm valves. The high temperature valve design offers 250 accurate and repeatable flow control steps that are directly accessed by electronic means. The valve provides continuous position feedback to ensure control integrity and diagnostics give instant notification of a hardware problem. The better actuator response, in terms of response width and amplitude, can be seen in better profiling efficiency. Virtually maintenance free, more than 8000 actuators have been delivered since 2002 and during the last 9 years, only 29 of those have been replaced.

Figure 7. The design of the IQ Steam Profiler has been made to eliminate the possibility of condensation within the steambox, all incoming steam is used in a controlled way to maintain the temperature within the individual pr ofiling sections. Profiling efficiency The amount of moisture correction that can be achieved in a particular cross direction position is related to the drying efficiency discussed earlier. It is quite common with conventional steamboxes that the CD response is 3 to 5 times the width of the actuator spacing and depends to some extent on the amount of steam applied to the sheet. In contrast, the Metso IQ Steam Profiler with its higher efficiency, diffuser plate and profiling chamber design together with distance to the sheet has shown response widths of between 1.5 to 2.25 times actuator spacing. This has resulted in customers reporting bet ween 25 to 30% improvement in profiles compared to conventional steamboxes..

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Steambox performance Heat transfer efficiency is understood and explained in many different ways, but mostly in terms of sheet temperature increase only. Two steamboxes may both give a 25ºC increase in sheet temperature but if one uses twice as much steam or a significantly higher degree of superheat then it is clearly not as efficient as the other. Heating efficiency is defined as the amount of sheet temperature increase for a unit amount of steam consumption. Using this definition, Metso has conducted numerous benchmark studies on light weight papers, board and pulp machines to compare performance. On various paper and board machines, the new technolog y IQ Steam Profiler outperformed all other steamboxes currently available, with one additional ton of water removed at the last press for between 0.75 to 0.96 tons of steam depending on application.

Figure 8. In a recent audit of 57 steamboxes, the Metso IQ Steam Profiler was sho wn to outperform the competition by a factor of two to three times. For the benchmarking, the heat transfer efficiency index is defined as sheet temperature increase for 0.1 ton of steam per tons of production. Results have been normalized f or different grades and weights. Conclusion The true value of the steambox is in reducing overall steam consumption or improving the CD moisture profile, preferably achieving both at the same time. The new design of the Metso IQ Steam Profiler has been shown to have considerably better heating and profiling efficiency compared to other available designs in use around the world. The design has gained considerable confidence with tissue producers as the number of tissue machine installations gains a substantial share compared with traditional applications in pulp, board and newsprint. 9

Metso IQ profilers The IQ Steam Profiler is part of the Metso profiler family for paper, tissue and board machines. Innovative solutions and cutting edge technologies are used to provide effective profile management from improved production and runnability to better paper quality and printing properties. Metso IQ profilers feature advanced self-diagnostics and compact designs for easy maintenance and installation. IQ Diluti on Profil er is a basis weight profiler for dilution headboxes that provides rapid and precise positioning, contributing to improved sheet quality and minimized variability. IQ Slice Profiler for conventional headboxes gets the basis weight profile right from the start for higher quality, increased production and efficient use of raw materials. IQ Moisturizer is a moisturizing system with applications for profiling, decurling and calendering. It features an air atomized micro droplet spray nozzle, linear water control valve and electromechanical actuator with position feedb ack. IQ Coat Weigh t Profi ler is the industry’s highest resolution, most compact coat weight profiler producing the most uniform CD coat weight profiles even after blade changes and breaks. IQ Inductio n Profiler provides precise high-resolution CD caliper or g loss control for improved runnability, higher efficiency and better end product quality. IQ Calender Steam Prof iler is a steam profiler for machine, soft a nd multinip calenders. It delivers even gloss and smoothness levels and CD profiles for improved paper quality and better overall printing properties.

Disclaimer The information contained in this document represents the current view of Metso on the issues discussed as of the date of publication. It should not be interpreted to be a commitment on the part of Metso and no guarantee is given concerning the accuracy of any information presented after the date of publication. This White Paper is for informational purposes only. METSO MAKES NO WARRANTIES, EXPRESS, IMPLIED OR STATUTORY, AS TO THE INFORMATION IN THIS DOCUMENT. Metso may have patents, patent applications, trademarks, copyrights, or other intellectual property rights covering subject matter in this document. The furnishing of this document does not give you any license to these patents, trademarks, copyrights, or other intellectual property. © 2011 Metso. All rights reserved.  All trademarks are property of their respective owners.

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