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High mileage steam

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High mileage steam

July 18, 2010 - 16:00
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BRUSSELS, July 19, 2010 (RISI) -Energy, or more specifically wasted energy, is sometimes visible and you can notice immediately if an improvement in energy efficiency is made. Ask the operators of Ponderay Newsprint's (Usk, WA) PM 1 about wet end steam spillage and they will tell you the air is much clearer around the press section since the installation of a new Metso IQSteamPro steam profiler, replacing an older unit installed about 12 years ago. If the steam is not spilling out of the machine, as it was before, the energy it contains is going into the paper sheet where it helps to drain water in the presses and improve the sheet moisture profile. And that is definitely more energy efficient, as proven by mill steam consumption statistics. Just like a modern car that gets better gas mileage, the Ponderay machine is getting better steam mileage.

The 9.17-m (361-in.) wire width Metso machine, started up in 1989, produces newsprint with a basis weight range of 26.5 lb/ 3,000 ft2to 30 lb/3,000 ft2(43 to 48.7 g/m2). Recently, PM 1 has set a 24-hr average speed record of 4,825 ft/min (1,470 m/min).

Figure 1 - In a post-installation trial, steam consumption was reduced by 3.59 tons/hr. Average mill statistics indicate an overall saving of 3.2 tons/hr which translates into a saving of $610,000/yr

Better than one year payback

The new Metso steam profiler was installed during a machine shutdown on April 8, 2009. It is positioned on the press suction roll of the triple-nip press between the first and second nips. Because of the steambox's more efficient steam jet delivery design and improvements in steam desuperheat control, less steam is being used to put more heat in the sheet. And less steam is more money saved. To date, steam savings of 3.2 tons/hr have been documented. That equates to an annual savings of about $610,000/yr and an investment payback period of less than one year. Steam costs at the mill can be high, as propane is sometimes used to supplement steam generation in the TMP mill and from the biomass boiler.

The new steam profiler applies steam to the full width of the sheet and the deckles are adjustable to the sheet width. The previous profiler could not treat the sheet edges as sheet deckle widths increased over the years.

Additional benefits have been seen in the sheet's cross-direction moisture variation. Keeping the moisture variation to a minimum is important for proper slitting and winding and for Ponderay's customers in the newspaper pressrooms. Since the startup of the new profiler, the 2-sigma moisture variation has been reduced by about 25% compared with the previous profiler.

Figure 2 - Since the startup of the new profiler, the 2-sigma moisture variation has been reduced by about 25% compared with the previous profiler. The display shows how the reel moisture profile is controlled

Delivering steam efficiently

The key to the new profiler's efficiency lies in how it delivers the steam jet so it condenses in the sheet and therefore transfers a maximum amount of energy.

The main objective of a steam profiler is to transfer heat from steam to the paper web in order to increase its temperature and increase dryness after pressing. At a higher temperature the viscosity of the water in the web is lower; thus, it can be removed more easily by fabrics and suction elements. This main principle has always been the same for all steamboxes. The only design factor that differentiates one steam profiler technology from another is the efficiency with which it achieves the main objective of increasing sheet temperature so that dewatering is improved. If the efficiency of a steam profiler is good, most of the energy available in the steam is transferred to the web, the sheet temperature in the z- direction (thickness) of the sheet will increase accordingly and there should be no steam spillage.

This high heat transfer efficiency is achieved by designing the equipment to produce the optimum steam velocity to get full penetration in the sheet and optimum steam temperature so that most of the steam to condenses within the web. Steam gives up the most energy (its latent heat) when it changes from vapor to liquid phase and most of this condensation is within the web. Thus the highest sheet temperature is achieved with the minimum possible steam consumption in the steam profiler. The steam velocity vector is mainly a function of the steam pressure, internal and diffuser plate design of the steam profiler, 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.

Similarly for steam temperature, 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 superheat of the steam delivered by the profiler is defined by the temperature above the steam saturation point. This required degree of superheat is based on the dynamics of the installation location. If this temperature is too high the steam will not condense effectively in the sheet, and will condense after it has passed through. If the superheat temperature is too low the steam will start to condense in the approach piping, steambox, or in the air before it reaches the web. Thus the steam profiler must deliver the steam at the temperature where the heat transfer efficiency will be optimum. 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. For this reason the steam delivery system on PM1 was rebuilt to Metso's specifications.

The steam profiler is positioned on the press suction roll of the triple-nip press between the first and second nips

Optimized for Ponderay machine

The moisture profiling is achieved by creating a differential temperature profile across the paper web. As a result, the paper machine dewatering elements provide a varying water removal in the cross-direction. When this cross-direction water removal is controlled automatically the reel moisture profile is corrected. The inherently high heat transfer efficiency of the profiler helps to achieve a CD actuator-zone response, which has high correction capability as well as a small response width for finer control. This control range and resolution determines the CD moisture profile improvement potential of a steam profiler. In addition to higher heat transfer efficiency, IQSteamPro also does not have a pre-heat zone, which allows it to use the entire amplitude of the actuator response for profile correction.

Considering all of the above, IQSteamPro was dimensioned to be able to deliver the right velocity and temperature range for this application at Ponderay. The final optimum steambox operating parameters were determined using Metso's proprietary steambox optimizing process. As a result of this optimization, the newsprint machine at Ponderay is getting the optimum steam profiler performance that's possible within the laws of physics.

Before the evaluation period, the new steam delivery system was hooked up to the previous steambox. This was done to make sure the new profiler delivered the results as expected and that the efficiency improvement due to the steam profiler itself was proven. Once the new profiler was commissioned the guarantees were fully met and the results were excellent.

Bernard Kessler, paper machine manager, Ponderay Newsprint, Usk , WA;Ankur Gupta, product manager, Metso, Atlanta, GA