For maximum sustained press section efficiency, each felt should be run within a relatively narrow range of moisture content. Most sheet transfer and felt conditioning operations depend on the availability of adequate felt water while sheet quality and water removal efficiency considerations require felt moisture to be kept at a minimum. Some paper grades, press sections and felt designs are more forgiving than others but running in the optimum range is always desirable. The optimum level is unique to each position, grade and felt design, and the water balance of each position is also a function of these factors.

Water Balance

On the input side of the balance are the various showers and the existing sheet water. Typical flooding shower volumes for paper machines operating under pressure-controlled conditions are:

Pick-up position: 33x FPM/1000 gals/min/inch
Second position: 10x FPM/1000 gals/min/inch
Third position: .06x FPM/1000 gals/min/inch

High pressure showers operate at the .05 to .1 gallon/min/in level, depending on pressure, and lubricating showers add approximately .05 to 0.08 gallons/min/in, depending on speed. Typical exiting sheet water flows are listed in the table in Figure 1 for several paper grades. Using these guidelines, the second press felt of a 3000 fpm newsprint machine is estimated to receive 32.6 gsm of water from flooding showers, 13 gsm from high pressure showers and 9.8 gsm from lubricating showers in addition to 24 gsm of exiting sheet water, for a total of 79.4 gsm.

While actual shower volumes are seldom available on commercial paper machines, the total amount of entering felt water is well known on many positions through weir flow measurements. Results from one such position are shown in the graph of Figure 2. The graph plots steam consumption against total weir flow, for a narrow range of basis weights, monitored over a two-year period. Apparently, in this case, steam usage in the dryer section is unrelated to second press weir flow, suggesting that the influence of shower volumes far outweighed that of sheet dewatering. Clearly, weir flow readings cannot provide a reliable estimate of sheet removal performance unless shower volumes are also monitored or kept constant.

Suction Versus Pressure Dewatering

Weir flow measurements can also provide a basis for comparison of pressure versus suction felt dewatering on a press position. Felts are dewatered by press nips and suction boxes, and to a much smaller extent, by throw-off and evaporation around the felt run. The purpose of felt dewatering can be felt cleaning, felt drying or both. If felt cleaning is a purpose, the use of a press nip alone is usually inadequate. The uni-directional filtering of the exiting sheet water makes the press nip more of a felt filling than a felt cleaning device. In comparison, suction boxes can be very effective both as felt cleaning and felt dewatering devices. They produce a reverse flow of water exiting through the sheet side of the felt, picking up impurities deposited by sheet water. They can also dewater the felt sufficiently to allow dry nip operation or to avoid some types of marking. Most single layer and double layer felts compress to a small void volume in the nip extruding a relatively large amount of felt water into the surface voids of the press roll. Such felts reach the suction box relatively dry, showing little response to suction dewatering. Suction dewatering can be increased by increasing the entering felt moisture content such as through higher shower volumes, as illustrated in the graph of Figure 3. The result is a high cleaning flow but also a wetter felt exiting the suction box.

The graph in Figure 4 compares pressure against suction felt dewatering as a function of felt age. The data was obtained on the same position, using several felts of the same design, over a two-year period. Despite the scattering of data points, the trends are evident. While the total water flow was largely independent of felt age, new felts tended to dewater more effectively by suction than by pressure. Therefore, less water visible at the nip with a new felt is not necessarily a sign of poorer water removal performance.

The data plotted in Figure 5 was also taken on a commercial paper machine but with felts from several suppliers. Still the trend of decreasing felt moisture content and caliper with felt age is clear and the similarity of the two curves suggests an inter-relationship.

If the purpose includes drying up the felt in order to achieve dry nip operation or to control crushing or marking, a good approach is to use a relatively incompressible felt and a high capacity suction box. The nip is then used for its primary purpose of dewatering the sheet while all of the cleaning and most or all of the felt dewatering is left to the suction box. This separation of the sheet and felt dewatering functions provides the greatest flexibility to optimize sheet quality, machine runnability and press efficiency related factors.

Most presses operate under saturated nip conditions with felt moisture content closely related to press loading and suction felt dewatering used primarily as a cleaning device. Under these conditions, the cross directional felt moisture profile is often assumed to be the direct result of the pressure profile in the previous nip, higher moisture levels corresponding to lower pressures. This should always be confirmed by a secondary indicator as the felt moisture profile is also influenced by such other factors as the uniformity of showering, suction dewatering, incoming sheet moisture, entering basis weight, rewetting from roll surface voids, etc. In fact, an unsaturated previous nip can reverse the relationship between specific nip pressure and felt moisture content with higher nip pressures producing wetter areas in the felt.

Pickup and Blowing

With the exception of some tissue machines, most modern press sections use suction to transfer the sheet from the forming fabric to the pickup felt. However, following the transfer, most high speed machines depend on surface tension to hold the sheet to the pickup felt. This requires a certain minimum felt moisture content that is a function of the felt surface capillary size. The finer the felt surface, the less felt water is needed for adequate sheet adhesion. Blowing in freestanding presses is controlled by the same mechanism requiring a certain minimum felt moisture content depending on felt surface fineness, press geometry and other factors. Most blowing prone positions employ an inside suction turning roll or box, usually at a low vacuum level of 3-4" Hg. The function of the blow box is similar to that of the suction pickup roll. They are both there to initiate a close sheet-to-felt contact, later to be maintained by a thin layer of interface water. Should this film of water be disturbed, such as by excessive blowbox vacuum, blowing will increase in intensity. There is an optimum blowbox vacuum for each position and felt design at any point in the age or surface condition of the felt.

Felt Design

Felt stratification can reduce the amount of felt water needed by making use of capillary forces to hold a high concentration of felt water in the sheet contacting surface layer. The graph in Figure 7 displays the results of experimentation with batt stratification to produce a moisture gradient in the cross-section of a press felt. In this case, a felt body-to-surface decitex ratio of 2:5 produced a felt surface moisture content 1.65 times that of the body of the felt. These results indicate felt structure to be highly influential in determining the critical moisture level for a given position. For this reason, it is advisable to freeze felt design when setting standards of performance in the press section. Conversely, felt design should be freed when a change in press performance is desired.

The control of felt moisture content is of particular importance in double-felted nips where no suction is available to ensure proper sheet transfer. Press geometry, felt moisture and relative felt capillaries offer the only means of ensuring good runability.

Excessive Moisture

While running with felts of insufficient moisture content tends to produce runability problems, excessive felt moisture levels are likely to have an impact on sheet quality. Most shell, shadow or groove-marking problems are related to excessive felt moisture in the nip. In addition, excessive felt moisture can cause sheet crushing, wet end breaks and excessive rewetting as well as press vibration. Again, what is excessive on any given position is felt design-related and the limits of safety should be established accordingly.

In summary, felt moisture content has a significant influence on press performance and therefore it should be carefully controlled. The optimum operating range is paper grade, press section and felt design-related. Water balance studies offer an approach to water management but their weakness of usually unreliable shower volumes should be remembered. For the greatest flexibility in felt design and press section operation, the press nip should be used for sheet dewatering while felt dewatering should be done primarily at the suction box. This requires adequate suction dewatering capacity and multi-layer felts of good wear and compaction resistance.