Approach flow system

High shear forces are prevelant in the approach flow system to the paper machine (i.e. as the fibre suspension approaches the point of deposition on the wire), and these have a large impact upon the efficiency of retention aids (Figure 7.8). A study of the effect of shear can often be helpful in establishing the mechanism of retention. Bridging flocculation is irreversibly sensitive to shear (i.e. when the shear forces are removed the suspension does not reflocculate) whereas charge neutralisation is reversibly sensitive to shear. 

For multi-parametric determinations, successive measurements under different conditions should be made and the data treated using specific chemometric approaches. Flow systems provide an excellent tool for accomplishing this task (3.3.3). 

Stock preparation is followed by the approach flow system connecting stock preparation with the paper machine. Its main tasks are... 

The approach flow system ends at the distributor of the headbox. 

Paper broken during the manufacturing process has to be recycled and fibers are recovered from the white water of the paper machine in a saveall. White water is fed back from the paper machine to the approach flow system and stock preparation. Fresh water is supplied to the paper machine. 

Broke is an important stock source and occurs on a continuous basis as trims from the wire as wet broke and from the winders as dry broke. It can also occasionally occur as reel slab-offs, in the finishing room or during breaks in the paper machine or coating equipment. Usually all broke is fed back to the process in the approach flow system. Broke treatment starts with slushing in different pulpers. These pulpers are installed under the machine and dimensioned according to the... 

Paper machine (PM) loop including the approach flow system and the white water systems I (WW I) and II (WW II)... 

Stock usually the best dewatering stock component used at the paper machine is selected in order to Hmit the size of the disc filter. In addition to cloudy and clear filtrate, disc filter save-aU applications also produce a superclear filtrate with very low solids content This superclear filtrate is used as a fresh water substitute for spray apphcations in the paper machine. The clear filtrate is stored in a buffer tank. The cloudy filtrate is usually fed directly back to the inlet of the disc filter, the used sweetener, including the recovered stock, is fed back to the thick stock in the approach flow system. DAF (dissolved air flotation) type save-alls are used in older machines and nowadays when a certain degree of ash and fines removal from the process is demanded. In this case, the sludge of the DAF is rejected. The advantages of a disc filter save-all compared to a DAF save-all are higher filtrate quahty, no chemical consumption and less space requirements (see Sections 4.2.62 and 4.2.9). 

The objective of the process on the paper machine is to produce a continuous paper web of the required quality, uniform in machine (MD) and cross machine (CD) directions. The process equipment consists of the approach flow system, the headbox and the wire section, the press section, the dryer section, often a size press, sometimes a coating station, a calender and a pope reeler. After the web has been reeled up this full width mother reel is cut into smaller rolls with widths and diameters according to the customers requirements. The rolls are then wrapped and made ready for shipment... 

Changing the pH value of the suspension in the approach flow system from add to neutral, allowing the application of calcium carbonate fillers and ensuring more durable papers... 

Fig. 6.27 Approach flow system for a modern high speed paper machine (source Voith).

Fig. 6.28 Metering and mixing in the approach flow system [2] (source Voith).

Automation by fast, stable and accurate control loops for consistency, flow, pressure and level is elementary for providing the necessary constancy in the approach flow system. Variations in pressure for example will mainly directly influence the MD profile whilst stock consistency deviations will affect both CD and MD basis weight profiles. In addition the retention on the paper machine and the chemical conditions of the water systems must be kept constant. Here a retention control loop is standard in many appHcations keeping the white water consistency constant by adapting the quantity of retention agent added. Controls for filler, color, air content, cationic demand or zeta potential are also available today. Combining the different controls to a total control concept will lead from purely functional controls to control systems which also address quality and production issues. 

The suspension is dehvered from the approach flow system through piping that has a circular cross section. The flow must be turned and distributed uniformly and with great accuracy across the full width of the headbox. It must then flow out of a narrow slit (normally called a slice, 6-25 mm in height, and more in certain applications) that can be more than 10 m wide. In modem headboxes, this uni-... 

In the wire section, a fiber web is formed out of the suspension supplied by the headbox. The kind and quality of suspension delivery from the headbox to the vdre has a strong impact on the quality of the paper web formed. Therefore headbox and tvire section - together -with the approach flow system - have always to be regarded as one unit. The main objectives of the vdre section are ... 

Drainage is opposed by the resistance to filtration, which depends on the degree of beating, chemical treatment, and type of stock, as well as on the amount of fines and fillers present. The dry content after the wire section in most cases is about 18-20%. The water removed in the filtration process (white water) carries away fibers, fines and fillers. The percentage of solids of the suspension retained on the wire, also called retention, can be increased by the addition of retention aids. The white water is reused to dilute the thick stock in the stock approach flow system. Figure 6.42 shows the filler distribution in the z-direction (across the web thickness) for dewatering the stock to only one side and symmetrically to both sides. 

A. Gommel, Advanced Wet End Process -A New Engineering Concept for Approach Flow Systems, Proceedings 28 EUCEPA Conference, Lisbon, 2003, Session 3, p.l26. 

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