Effects of jet impingement on bel baie machines

It is generally accepted that retention, wire mark and fines distribution on Bel Baies are primarily affected by the positive drainage pulses over the shoe.

Early Bel Baie II's were typically operated with the jet impinging on the conveying fabric, well ahead of the first deflector. In this situation, the wire mark in the sheet was almost solely due to the conveying fabric and almost no mark from the backing fabric was observed. Typically, fabric selection for this condition would be fine mesh single layer on the conveying side to optimize retention and wire mark and coarser mesh on the backing side for drainage and stiffness.

In recent years, jet impingement practices on newer Bel Baie II's and III's have changed. It is now quite common to observe wire mark from the backing fabric as well as the conveying and in some cases, almost exclusively from the backing fabric. These observations are consistent with the tendencies for some newer Bel Baies to be run with the jet landing more equally between fabrics or directly into the backing fabric. There is also some evidence that fines distribution and two-sidedness may be affected by impingement geometry.

Importance of Impingement on Sheet Quality and Fabric Selection

Figure 1 -- This is a scale illustration of jet impingement on an actual Bel Baie II. It is typical of a more conventional impingement condition on a Bel Baie in which the jet lands on the conveying fabric well ahead of the first deflector. Because of the jet thickness, the impingement is unaffected by the backing fabric which does not contact the web until the middle of the first deflector. This example shows that the impingement should really be regarded as single wire machine impingement.

Figure 2 -- The data shows pitot tube measurements of pressure drop across the landing area of a jet on a single layer fabric. Note that the peak pressure at the first point of impact is proportional to Vj2 sin ß, where Vj is the jet velocity and ß, the angle of impingement. This observation, which our studies indicate is applicable to a wide range of impingement conditions and fabric designs, explains why jet impingement can be so important to wire mark and fines distributions. Although the impingement angles on Bel Baies are typically low (~5°), speeds have increased dramatically over the years. Today, 4000 fpm is not uncommon.

As the values in Table 1 illustrate, the peak pressures at impingement can become quite large at these high speeds and in fact, exceed the T/R pulses over the shoe.1, 2 It should also be noted that an impingement angle of 4°, which is not uncommon on many machines, would double the values in the table at any given speed.

Figure 3 -- The graphs show the effect of landing distance ahead of the lead forming board blade on initial drainage. At 4° the impingement drainage increases by about 10 IGMP/ft. for each additional 1" of landing distance. The drainage is also observed to approximately double for each 2° increase in impingement angle.

Effects of Impingement on Wire Mark, Fines Distribution and Two-Sidedness

Figure 4 -- This photo shows impingement on the backing fabric of a Bel Baie II. Note that the jet impinges at a point that would probably not be subjected to the suction of the existing breast roll nip. Also, the two fabrics are clearly well separated over the first deflector.

Backing	Conveying

Figure 5 -- These split sheet photos are of a sheet made on the Bel Baie shown in Figure 4. Although some mark is present from both fabrics, the backing is definitely stronger. Both fabrics were triple layer.

Total	Backing	Conveying

Figure 6 -- This series of photos was taken of a sheet from a Bel Baie II with jet impingement on the conveying fabric well ahead of the first deflector. Note the dominance of the conveying mark from the double layer fabric.

Conveying	Backing

Figure 7 -- These split sheet photos are of a sheet from a Bel Baie with conveying fabric impingement. The fabrics involved were triple layer.

Figure 8 -- Two scanning electron microscope photographs of the conveying and backing side of the sheet in Figure 7 show a definite difference. The conveying side is more porous and has fewer fines. Although pressing may have some effect on the difference in porosity, it would not alter the fines content.

Figure 9 -- The data on ash level and fiber length distribution through quarter sections of the sheet indicate a small degree of two-sidedness. The lower ash and fines present on the backing side are consistent with impingement on this particular machine which favors the backing fabric. It is noteworthy that the general variations in ash and short fiber contents behave very similarly in the quarter-sections of the sheet. This is likely due to the fact that the retention of both components is dependent on the retention of the long fiber fraction of the furnish. Both fabrics in this instance were triple layer.

Summary

It is apparent that jet impingement is an important consideration on Bel Baies, particularly with respect to wire mark. Invariably, the jet lands preferentially on only one fabric. The peak drainage pressure can become very high as impingement angle and machine speed increase. Thus, the first part of the sheet to form can experience very high forces that tend to embed fiber in the fabric. As well, a significant amount of drainage can occur due to single fabric impingement alone before the second fabric has any influence on dewatering. Thus, the fabric receiving the jet should be one with high fiber support (FSI)3 and excellent wire mark characteristics.The same impingement forces that influence wire mark also have an effect on fines distribution and two-sidedness. These will be optimized by high retention fabrics, such as triple layer or extra support double layer. In fact, it is apparent that these types of fabrics seem to be increasingly common on both positions of the fastest Bel Baies.

1. Sims, D.N. "Twin Wire Forming Developments 1986". Notes, CPPA TECH 86 (Mont Gabriel, Que.), April 20-25, 1986.
2. Brauns, R., "Wet End Developments". Notes, CPPA TECH 86 (Mont Gabriel, Que.), April 20-25, 1986.
3. Beran, R.L., "The Evaluation and Selection of Forming Fabrics", TAPPI, 62(4), p. 39 (1979).