The possibility to make uniform tissue rolls, which do not create problems in the following processing stages and in the quality of the finished product, starts from paper formation and, more precisely, even more from the drying phase.
A new patented hood system focuses on a new patented hood system, which acts on average drying intensity transferred to the paper, thereby guaranteeing an even moisture profile.
The drying phase is one of the most important phases of the paper production cycle, as it is during drying that quality of the finished product and the efficiency of the process are actually put to the test. Furthermore, from the point of view of energy consumption, the energy consumed during web drying is one of the most sensitive issues for a paper mill, as shown by the fact that this stage is often subject to technical improvements to make more efficient and reduce consumption.
How to manage the moisture profile
One of the problems that can trouble tissue paper producers is the uneven moisture profile at the roller, which thus determines the irregularity of the reel. As explained by Valmet www.valmet.com, this problem might be caused by a number of factors, e.g. uneven drying, problems with the cloth or the dry stock profile in the outgoing area of the presses. The identification of said factors is not always easy, nor is it easy to intervene. On top of this, the adopted solutions sometimes do not completely solve the problem, especially in situations where the problem has various causes. Furthermore, a number of problems linked to process runnability or in the following stages of tissue paper working may occur, which can ultimately jeopardize the final quality of tissue paper.
In order to avoid these problems and, therefore, obtain a uniform moisture profile, paper mills have adopted a number of arrangements. Valmet’s technicians explain that two major common problems are normally faced: the application of profiling crescent headers in high-yield hoods, on the one end, and stronger drying of the paper, on the other.
The first method, which is normally adopted in more modern hoods with crescent headers normally placed at paper entry side, allows to modify the moisture profile by changing the speed of the flow of hot air in the drying area. This way, and especially if you work in the cross-direction of the paper, drying is more even and the defect can, at least partly, be corrected.
The second method, instead, provides for more intense drying of the paper, resulting with a flattening of the moisture profile, though with increased energy consumption. In order to have more water evaporation of the residual water present in the sheet of paper, it is indeed necessary to use more thermal energy. Technicians have calculated that this can lead to both increased consumption, especially of gas, sometimes by more than 15-20%, and of the raw materials used, with an increase even higher than 3% of the fibre used for any ton of finished product.
The patented solution
Valmet has, thus, decided to design and produce a new system, which could be more effective than the traditional methods used, on the one hand, and which could make it possible to overcome the limitations and disadvantages described above, on the other.
The starting point of the Finnish multinational has been a first solution, which was already known to tissue producers and consolidated on the market, i.e. the Advantage AirCap technology. The new Yankee Advantage AirCap Heli hood has been designed based on this technology. At the core of the system lies a new concept of profiling crescent headers, which has been envisaged to further improve the tissue paper drying process, and the hood helical design.
In particular, Valmet technicians explain that they worked on the geometry of the ribs and the blowing tubes of the hood, as traditional configurations do not provide for even drying in the cross-direction of paper and do not allow for advanced control of the profiling capacity. The result of these different geometry is an uneven heat exchange and, as a consequence, a less effective drying process. With the Advantage AirCap Heli configuration, instead, the ribs themselves are slightly slanted in cross direction, unlike the classical shape where they are in machine direction. All this provides for more drying uniformity and a more precise profiling capacity.
Furthermore, compared to standard hood design, the patented helical shape of the new hood provides for an impingement speed across the sheet when drying that is more gradual and can, above all be finely adjusted based on the moisture profile that is to be obtained on the sheet of paper.
All parameters for the use of the new Yankee hood can be optimized by means of a control system produced by Valmet for the constant monitoring of the moisture profile. This automated system provides for the constant analysis of all moisture data related to the humidity of the wire section derived from the QCS (quality control system). The information provided by the QCS makes it possible for the control system to automatically define the most correct drying impingement step to apply on the hood.
Valmet has pointed out that Advantage AirCap Heli can be equipped with this quality control system, however the QCS can be bought also at a later stage.
The right time to choose a new hood
We asked Paolo Della Negra, Sales Department Manager at Valmet (Gorizia), to talk about the reasons why a paper mill should consider installing the Advantage AirCap Heli.
«The biggest advantage,» explains Della Negra «is especially for those paper mills wishing to replace their hood because it has reached the end of its lifecycle, or because their existing hood is damaged, or when the hood has the common problem of being too distant from the Yankee dryer, with this hampering the drying capacity and efficiency of the whole system. The new Heli hood adds to the traditional Valmet hood all the advantages of having a completely flat drying profile if crescent headers are all open and, above all, the capacity to better correct the moisture profile if problems of this kind occur,» whereby this kind of problems, explains Della Negra, «are very common in paper mills». The new solution basically offers «the master paper maker an additional tool to improve the final quality of his product».
The application case in France
A significant number of sales has so far been registered of the new hood, and a successful installation was done three years ago at a French paper mill. The company chose to install Advantage AirCap Heli to meet two specific needs, i.e. increase energy efficiency in the drying area on the one hand and improve the hood profiling capacity. The method used so far to manage the moisture profile in the exit area of the machine consisted in the traditional overdrying of the paper, which was carried out with aim of obtaining an even profile, as was said. This solution required higher energy consumption for the drying stage and more raw materials used and resulted in unsatisfactory parameters for the paper mill. According to Valmet technicians, especially the 2sigma value, i.e. the standard deviation that as to be as low as possible, for the moisture profile at the roller was much higher than a value that could have been considered acceptable for the paper mill, as it was, indeed, equal to 1.39. Furthermore, the overdrying of the paper had led to force the machine to be run with an average dry stock value at the roller of 96.3%, while the corresponding value would have been equal to 94% without overdrying. Although production remained stable, Valmet calculate how, if you consider a cost for gas of 0.24 euro per Nm3, this solution implied an increase in costs of 165 thousand euro every year for gas. As regards the costs for raw materials, Valmet had calculated that, starting from a cost of the fibre between 600 and 700 euro per ton and considering an increase in consumption of about 790 tons every year due to overdrying, the increase in costs ranged between 470 and 550 thousand euro every year.
The installation of the Advantage AirCap Heli has enabled the French paper mill to improve in all these aspects.
The performance analyses carried out after the installation of the new hood have, thus, proven how the paper mill has managed to improve all the critical elements highlighted before the application. For example, the 2sigma value of the moisture profile at the roller has decreased to 0.79, thereby registering even lower values than the ones forecast by the Valmet technicians, who had forecast about 1.00. Similarly, the conditions to force the work of the drying area have turned out to be no longer necessary, thereby allowing to reduce both gas consumptions and the quantity of raw materials used in production. All of the above has occurred with a better hood profiling.
The example of the French paper mill is an interesting example of application. Yet, we wondered what cases are the most appropriate for a tissue paper mill to consider choosing this technology. Also, this time, we asked this question to Mr. Della Negra. «Advantage AirCap Heli is always proposed as an option to the conventional hood, » explains Della Negra. «The first installation was performed in 2017. Since then, we have produced about 40 hoods, 13 of which with Heli configuration. Given the low cost difference for its production, especially in case of a complete reconstruction of the hood and the air system, and given the excellent results and feedback from the customers who adopted it, we are currently considering the idea of having Advantage AirCap Heli as our standard option for all our products».
The advantages of research
Research and information exchange are at the basis of the development of new technologies and solutions. This approach, which Valmet has always adopted, has proven valid also in the case of the new Advantage AirCap Heli.
In particular, constant dialogue with the companies using the technology of the Finnish multinational and the exchange of data and information, also through monitoring and the search on the field of machines working parameters, are as essential for the improvement of the adopted solutions, as are investments in R&D, which nonetheless constitute the basis of all group activities.
Valmet’s Italian subsidiary has currently adopted over 500 Yankee hoods worldwide. About one hundred of these are Advantage AirCap models and as many as thirteen are the sales of the new Advantage AirCap Heli.
At Valmet it is always said that the installation of Advantage AirCap Heli can help improve the moisture profile of the web and enhance the efficiency of the drying process, thereby facilitating the following converting stage and providing for a higher quality of final products at the same time.