Forming and Dewatering on the Wire

1. Shop-floor view: the wire section is the sheet's "birth certificate"

Standing beside the wire section of this papermaking line, you see a thin stock at only a few thousandths concentration jetted from the headbox, landing on a wire (bronze wire or polyester forming fabric) racing by at a dozen to dozens of meters per second. In a few seconds it must do something that seems impossible: turn a suspension close to clear water into a continuous, uniform, strong wet sheet. The faster the line runs, the less time the wire section has — this is exactly the most severe test that modern papermaking's "high-speed" imposes on the wire section.

The fundamental task of the wire section can be summed up in two sentences. First, lay the fibers, fillers and fine fractions onto the wire in the designed structure — this is "formation." Second, while forming, remove as much water as possible so the wet sheet gains the minimum strength to be led to the press section — this is "dewatering." These two things constrain each other: the faster the dewatering, the less time fibers have to spread evenly, so formation uniformity may worsen; the more formation pursues uniformity, the gentler the dewatering rhythm must be. Much of the whole paper machine's design is finding balance between the two.

From a material perspective, the wire section is where the whole line consumes and removes the most water. For every 1 ton of paper produced, the thin stock entering the wire section carries about 100 tons of water. The wire section must remove the vast majority of these 100 tons — empirically over 95% of total dewatering happens at the wire section, and only the small remaining amount is handed to the press and drying. So the wire section's dewatering efficiency directly determines the load on downstream presses and dryer cylinders, and directly determines the whole machine's energy level.

This episode follows the real workshop process sequence: first the headbox that "balances" the stock flow, then the sequence of dewatering elements laid along the wire, then how dryness evolves along the wire length, then a comparison of the two mainstream forming methods — fourdrinier and twin-wire — and finally retention rate and white-water circulation, the core of operating economy. Each section is accompanied by figures and parameters for on-site reference.

2. The headbox: tuning the stock flow into a "uniform sheet"

The headbox is one of the most precise components of the entire paper machine. Its job is not simply to "pour" stock onto the wire, but to take the stock from the fan pump at 0.3%–1.2% consistency (typical for printing papers) and organize it into a thin layer that is highly uniform across both the cross-machine and machine directions, with stable velocity and no streaks or flocs, then jet it onto the wire at a controlled angle and speed. One could say the sheet's uniformity is half decided inside the headbox.

Modern paper machines widely adopt the hydraulic headbox. The stock first enters a pressure-stabilized rectangular tapered manifold, which distributes it evenly across the width to dozens or even over a hundred branch tubes; after the branch tubes is a bundle of small tubes called "turbulence-attenuator tubes" (diffuser/turbulence generator), whose job is to break the large-scale turbulence from pumping into a large number of micro-scale, high-frequency small eddies. These small eddies decay rapidly before jetting, just enough to break up fiber flocs without leaving visible streaks after landing on the wire — this layer of "decayed turbulence" is the key to uniformity.

The ratio of jet velocity to wire velocity is called the jet/wire ratio, usually controlled between 0.95 and 1.05 on-site. A ratio above 1 means the jet is faster than the wire, fibers are "stretched" and oriented, and the machine-direction (MD) strength of the sheet rises while cross-direction (CD) drops slightly; below 1 it reverses. Hidden in this seemingly tiny 0.1 interval is the most direct knob for adjusting the sheet's anisotropy (MD/CD ratio); on-site it is often locked in by fine-tuning the slice opening and wire speed.

For grades needing a layered structure (e.g. multi-ply board, high-grade printing papers with a top layer), a multi-ply headbox is used, commonly with three-ply forming: a fine top-layer stock ensures surface smoothness and printability, a coarse-fiber core provides thickness and stiffness, and the bottom layer balances back-side performance and cost. Multi-layer jetting requires each layer to overlap precisely near the impingement point; the layered flow channels and velocity matching of the slice are the difficulty, and the reason such machines are expensive.

3. The dewatering-element sequence: vacuum steps up along the wire

After the stock lands, water must be forced out step by step by a series of dewatering elements. Starting from the forming board, these elements proceed along the wire: forming board → foils / table-roll zone (with blade dewatering pulses) → table roll → low-vacuum suction box (5–15 kPa) → high-vacuum suction box (20–35 kPa) → couch roll (40–70 kPa). Each step's vacuum (or pulse intensity) is higher than the previous, and the dewatering intensity the sheet can withstand also rises.

The very front of the forming zone is the forming board, which sits behind the impingement point, providing the first support and removing the top-layer free water. Next are the foils (also called dewatering blades, suction blades), each a blade with an angle, typically 10°–15°; as the wire sweeps over the blade's upper edge, a negative-pressure pulse forms behind the blade, "sucking" water out of the wire holes. This pulse dewatering is an important source of fourdrinier formation uniformity — the pulse frequency and intensity are directly related to machine speed and blade angle; dense blade arrangement and large angle give strong dewatering but easily disturb formation.

The table roll was the standard on early fourdriniers, relying on centrifugal force to throw off water, but at high speed the thrown water easily splashes back and contaminates the sheet; modern high-speed lines mostly replace it with foils or keep only a few. Entering the suction-box zone, dewatering shifts from "passive pulse" to "active suction": the low-vacuum suction box at 5–15 kPa does preliminary suction and further conditions the wet sheet, the high-vacuum box at 20–35 kPa pushes dryness close to couch level. The final couch roll at 40–70 kPa is both a dewatering element and the turning roll that "lifts" the formed wet sheet off the wire and leads it to the press section.

4. How dryness evolves along the wire length: one curve clarifies the dewatering rhythm

Plotting the wet sheet's dryness (ratio of absolutely dry substance to wet weight) along the wire length as a curve is the most intuitive way to diagnose wire-section operation. The curve as a whole is "S"-shaped: in the first few meters after landing, the forming board and foils' pulses rapidly remove surface free water, raising dryness from 1%–2% to around 5%; in the mid vacuum-box zone the slope steepens and dryness is pushed past 10% into 12%–18%; in the last stretch near the couch, the dewatering rate naturally slows and finally stops near 18%–22% — beyond that the wire section can hardly raise it further, and the press must take over.

Here is a key understanding: the wire section removes "free water," the press also removes mostly free water, and only the drying section mainly tackles water more tightly bound to fibers. So every 1 percentage point more of free water removed by the wire section and press means the drying section evaporates that much less water, and evaporation's energy cost far exceeds mechanical dewatering's — which is why raising the wire-section and press-exit dryness is the first lever for energy saving across the machine.

Experience tally: per 1 ton of paper, the thin stock at the wire inlet carries about 100 tons of water, while after the couch the wet sheet holds only about 4–5 tons of water; the other 95+ tons of water are all removed at the wire section. The vast majority of this 95 tons is removed by extremely low-energy gravity + vacuum dewatering; only those 4–5 tons after the couch enter the high-energy press and dryer-cylinder stage. The wire section's "cost-performance" is therefore extremely high, and any retrofit that improves wire-section dewatering efficiency usually has a short payback period.

5. Fourdrinier vs. gap former: single-side vs. synchronized two-side dewatering

The traditional fourdrinier has only one wire surface (facing down or up) that supports and dewaters; the side of the wet sheet against the wire (wire side) and the exposed top side (top side) experience different dewatering conditions — the wire side drains smoothly and fibers and fillers are more easily carried toward the wire side, so the two sides differ in smoothness, filler distribution and pore structure; this is the "two-sidedness." For most grades, two-sidedness must be minimized, otherwise printing shows uneven ink uptake on the two sides.

Twin-wire forming (gap former) uses two wires to sandwich the stock flow and dewater from both sides simultaneously. Simultaneous two-side drainage brings three direct benefits: first, two-sidedness is markedly reduced, with better sheet uniformity and surface consistency; second, dewatering efficiency is high and the drainage path is short, so it supports higher machine speeds — modern gap formers are designed for speeds generally above 1500 m/min, far above fourdriniers; third, gap formers discharge white water at higher concentration with a relatively smaller white-water circulation, favoring closed-loop circulation and water saving.

The two forming methods are not a replacement relationship but a division of labor by grade. The fourdrinier is flexible to retrofit and adapts to a wide range of grades, and still exists in large numbers in medium-speed and printing-paper/board applications; the gap former dominates in high-speed newsprint, LWC/SC magazine papers, and high-grade thin papers where two-sidedness and speed are demanding. It should be noted that the gap former's "high speed" also brings new challenges: the residence time in the forming zone is compressed to seconds or even shorter, pushing the precision requirements on headbox uniformity and turbulence attenuation to the limit.

6. Retention and white-water circulation: the wire section's operating economy

The wire section must not only dewater but also "retain" what should stay. First-pass retention (FSR) is the proportion of solids intercepted at the first pass through the wire section, usually controlled at 50%–70% on-site. Too low retention means large amounts of fiber and filler pass straight through the wire holes and are lost as white water, wasting raw material and increasing downstream water-treatment load; but higher is not always better either — too high often accompanies excessive filler retention and changes in sheet uniformity and bulk, requiring a trade-off against the grade target.

White water is the mixture of water removed at the wire section and the lost solids, rich in fine fibers, fillers and additives. Modern paper machines widely practice white-water closed circulation: the clarified liquid from treating the white water discharged by the couch and suction boxes is reused to dilute stock, recovering fiber and filler while greatly reducing fresh-water consumption. The core equipment for white-water treatment is dissolved air flotation (DAF), which floats and recovers suspended solids (especially fiber and filler) from the white water; the clarified liquid circulates and the float returns to the system or to low-quality use.

Another easily overlooked set of parameters is the balance between turbulence and flocculation. The forming zone has continuous shear flow; the shear intensity determines how small the fiber flocs are broken to — too large flocs give poor sheet uniformity and cloudy patterns; too small flocs lower retention and raise white-water concentration. Directional shear (e.g. the MD stretching from jet/wire ratio deviating from 1) also amplifies the MD/CD property difference, making the sheet more anisotropic. So on-site formation tuning is often repeatedly finding the point among "shear intensity vs. retention vs. anisotropy."

It must be reminded that the more thoroughly white water is closed, the more easily dissolved and colloidal substances (DCS) accumulate in the system, possibly causing pitch trouble, foam and deposit problems. So closed circulation is not "the more closed the better," but setting a reasonable water-circuit hierarchy among water saving, recovery economy and maintainability. This is why when many old mills retrofit the wire section, they simultaneously add DAF and multi-stage white-water reuse, rather than simply closing the discharge.

Returning to the tally at the start of this episode: the wire section removes over 95% of total water, almost without energy (relative to drying). Jointly optimizing wire-section dewatering efficiency, first-pass retention and white-water closure is the foundation of this line's "low cost, high uniformity." Next episode we enter pressing and drying — that is the real energy battlefield, and the "amplifier" of the wire section's dewatering results.

7. Operating essentials: formation diagnosis and wire-section adjustment knobs

Whether the wire section is good ultimately comes down to formation. The simplest on-site diagnosis is to hold the sheet up to the light: a uniform one transmits light evenly, a poor one shows "cloudiness" (local thick-thin spots) or directional streaks. Cloudiness mostly comes from overly large flocs in the forming zone or insufficient shear; streaks are often tied to uneven headbox slice, improper foil arrangement or wire-speed fluctuation. Quantitatively, a formation-uniformity meter can measure local basis-weight variance, but most mills still rely on experienced visual inspection combined with on-line scanned basis-weight CV to control it.

The wire-section adjustment knobs are actually limited yet critical: first, the jet/wire ratio, adjusting fiber orientation and two-sidedness; second, the slice opening and impingement point, adjusting jet momentum and stability; third, the foils' angle, number and spacing, adjusting pulse-dewatering intensity and uniformity; fourth, the vacuum of each suction-box stage, adjusting dewatering rhythm and dryness distribution; fifth, the wire speed itself, adjusting forming-zone residence time. These knobs are coupled — for example, raising the foil angle strengthens dewatering but may disturb uniformity, so adjust in small steps and watch the scan trend.

Among common wire-section faults, web breaks trouble operators most. The wet sheet after the couch is inherently low in strength; if first-pass retention is too low making the wet sheet non-uniform, or there are un-refined flocs and foreign matter like plastic pieces in the stock, the break probability rises sharply. Foam and pitch trouble also accumulate at the wire section, producing pinholes and spots. Forming-fabric selection and maintenance are equally important: polyester forming fabrics differ by air permeability, seam type and wear layer; the tensioning and guiding system ensures stable running, and not replacing the fabric when its life ends directly drags down dewatering and uniformity.

One more on the wire-section/environment linkage: the higher the first-pass retention and the more thorough the white-water closure, the less fiber and filler are lost in discharge and the lower the end water-treatment load; but over-closure brings dissolved colloidal substance (DCS) accumulation, causing pitch trouble and foam. Holding retention steady in the reasonable 50%–70% range and pairing it with good DAF flotation is the pragmatic choice balancing cost and compliance; this trade-off is expanded in EP10's recovered-fiber and environment chapter.

Finally, a systems perspective: the wire section is not an isolated unit. The freeness, filler amount and retention aids of the stock from the headbox, the minimum exit-wire dryness the press can accept, and the cleanliness of the white-water system — all inversely constrain the wire section's operating window. Tuning the wire section as "the product of coordination across the whole line" stabilizes output better than single-point optimization, and better realizes the intended effect of every parameter listed in this episode.

One more point on wire speed and forming: the higher the speed, the shorter the forming-zone residence time, and the stricter the requirements on headbox uniformity and turbulence-attenuation precision; this is also why high-speed lines generally turn to gap formers and treat the headbox as "core know-how" to tune. If wire-section operators understand this residence-time constraint, they can more clearly judge which paper defects come from speed and which from elements or chemicals, rather than blindly adjusting vacuum at the first sign of trouble.