Pressing and Drying

1. Shop-floor view: why drying is the "power hog"

The wet sheet leaving the couch has only 18%–22% dryness, meaning about 4–5 tons of water are still wrapped around every 1 ton of absolutely dry fiber. The press section must squeeze out another chunk of this water mechanically as much as possible, then hand it to the drying section — dozens of large dryer cylinders fed with steam — to evaporate the remaining water. Across the whole process, the energy to evaporate water far exceeds mechanical dewatering: there is an on-site rule of thumb, "one kWh of mechanical dewatering is worth several kilograms of steam in drying" — precisely because of this, every small increase in press-exit dryness visibly thins the drying section's steam bill.

Spreading out the machine's total energy input, the drying section typically accounts for 60%–70%, truly the main battlefield of energy consumption; the press section itself consumes little electricity, but it is the drying section's "front-end load reducer," so its status is amplified. This episode's logic also follows this energy chain: first how the press section squeezes water out with line load, the difference between plain roll and shoe press, then the contact-drying principle and three-stage curve of the drying section, and finally an energy tally and mass balance that works out that golden rule "every 1% rise in press-exit dryness ≈ saves 4–5% of drying steam" for you.

A concept to establish in advance: the press still removes mainly free water, while the drying section removes free water early and only bound water later. The further drying goes, the harder and more energy-intensive it becomes, so any water that can be shifted forward to the press must not be left to the dryer cylinders. This is also the fundamental driving force for modern paper machines to widely adopt the shoe press, pushing press-exit dryness from the plain roll's 38%–46% to 45%–52% — those extra few percentage points, multiplied by the whole machine's output, are the difference of thousands of tons of steam per year.

2. The press section: the last chance for mechanical dewatering

The core of the press section is a set of nips. In plain-roll pressing, the upper and lower rolls form a narrow nip at the contact line, and the sheet passes through sandwiched between compressible press felts, with water squeezed out by mechanical pressure. The nip line load (force per unit width) is typically in the 50–250 kN/m range — printing papers are on the lower-middle side, board and high-basis-weight products can reach the upper limit. The higher the line load, the greater the dewatering, but the peak pressure in the nip more easily exceeds the sheet's tolerance limit, triggering "crushing."

The nip structure can be imagined as a "sandwich": upper roll + felt + wet sheet + felt + lower roll. The felt plays multiple roles here — it transmits pressure evenly to the sheet, absorbs and conducts away the squeezed-out water, and is also responsible for smoothly carrying the sheet out of the nip. The felt is a compressible porous body, continuously saturated with water during running and then evacuated by suction; its air permeability and water permeability directly determine dewatering ability and life. Felt fouling (fine fractions, resin, filler clogging the pores) is the number-one cause of declining press efficiency, so cleaning is a daily O&M priority: continuous high-pressure water rinsing + periodic chemical washing (detergent soak/spray) to keep pores open.

"Crushing" is the fault the press section most needs to avoid: when the nip peak pressure is too high, or the drainage channel (felt + vacuum) cannot carry the water away in time, water is pressed back into the sheet, forming water lines or spots inside the sheet; in severe cases the sheet structure is crushed and strength is irreversibly lost. Control measures include reasonable line-load selection, ensuring felt vacuum suction, and using a shoe press to turn "high peak, short contact" into "low peak, long contact," avoiding crushing mechanistically.

3. Shoe-press details: how long contact dewaters "gently"

The reason the shoe press breaks through the plain roll's dryness ceiling is that it changes the nip's "line contact" into "surface contact." The plain-roll nip is only a few millimeters wide, the sheet stays inside for only milliseconds, dewatering time is short, and to remove more water one can only pile up line load and raise peak pressure — and that is exactly the breeding ground for crushing. The shoe press uses a fixed (or oscillating) shoe element, paired with a special high-pressure press belt, to form a 250–300 mm wide nip with the opposing roll; contact time is stretched to about 10× that of the plain roll, so water can be conducted away by the felt more abundantly without raising peak pressure.

The longer contact time brings another benefit: more uniform dewatering. At places where the sheet is thick-thin uneven, the long nip gives more thorough pressure equalization, and the cross-machine dryness distribution is flatter. This is also why the shoe press is especially welcome on high-basis-weight board and on printing-paper lines pursuing high dryness to reduce drying energy. The cost is a more complex shoe-press system with higher investment and maintenance costs, and higher reliability requirements on the felt, press belt and hydraulic system — on-site it is often placed in the second-to-last nip (the side closer to the drying section) to maximize the benefit of exit dryness.

Here we must correct a common misconception: pressing is not "the harder the better." Piling up line load too high may slightly raise short-term press-exit dryness, but the sheet is pressed denser, bulk drops, and stiffness and absorbency suffer, while crush risk rises sharply. On-site press adjustment is finding balance among "press-exit dryness, sheet bulk and strength, felt life, crush risk," not blindly adding pressure. The value of the shoe press is precisely that it bypasses the old path of "pressure for dryness" and decouples these contradictions by "time for dryness."

4. The drying section: multi-cylinder contact drying

After pressing, the sheet dryness reaches 45%–52%, but it is still far from finished paper — final moisture is usually pressed down to 4.5%–6.5%. The remaining water is mainly evaporated in the drying section. The mainstream method is multi-cylinder drying: a row of cast-iron dryer cylinders of 1.5–1.8 m diameter, internally fed with 0.3–1.0 MPa saturated steam, the sheet clinging to the cylinder surface is heated, and the moisture vaporizes at the surface and is carried away by high-speed hot air (hood). The printing-paper drying section often has 40–70 dryer cylinders, arranged in upper and lower tiers along the machine, with the sheet guided between cylinders by a single fabric (dryer fabric) and pressed to the cylinder surface.

Why so many dryer cylinders? Because drying is a surface-limited process: evaporation only happens at the interface between the sheet and the hot air; the heat and area the cylinder surface provides determine the evaporation capacity. Cylinder count, steam pressure and machine speed jointly determine drying capacity. The benefit of a single-tier (single-row dryer fabric) configuration is that the sheet's machine-direction (MD) shrinkage during drying is more controllable; combined with creping (tissue) or tension control, it can adjust the sheet's stretch and flatness. The drying section also often has a cooling cylinder, which lowers the sheet temperature and stabilizes moisture before reeling, facilitating subsequent reeling and finishing.

Mechanistically, cylinder drying has three stages (see the curve in the next section): heating-up stage, constant-rate stage, falling-rate stage. In the heating-up stage the sheet is rapidly heated to near its wet-bulb temperature; in the constant-rate stage surface moisture is sufficient, evaporation rate is basically constant, controlled by external heat and mass transfer; in the falling-rate stage the internal moisture diffusion in the sheet cannot keep up with surface evaporation, and the evaporation rate decreases as moisture content drops — here it is bound, tighter water being removed, the hardest and most energy-intensive. The constant-rate evaporation rate is typically 15–30 kg/m²·h, and the critical moisture content (the turning point into the falling-rate stage) is around 30%–40% dryness.

5. The energy tally: why press dryness is worth its weight in gold

Now let us work out the accounts clearly. Evaporating 1 kg of water requires about 3.0–3.2 MJ of steam heat (depending on steam parameters and cylinder thermal efficiency); and the drying section consumes 60%–70% of the whole machine's energy input. This means: any water removed extra in the press section saves the drying section's most expensive evaporation energy. Numerically, the industry rule of thumb is — every 1 percentage point rise in press-exit dryness saves about 4%–5% of drying steam consumption. The leverage is so large that a single line can save considerable fuel and carbon emissions in a year.

Why can "1% dryness" move "4–5% steam"? Because drying is the single largest energy-consuming stage of the whole line, and evaporation is the lowest-efficiency, highest-cost link within it. Squeezing water out of the sheet (mechanical) is far cheaper than steaming it out (thermal) — their unit energy costs differ by orders of magnitude. So raising press-exit dryness is equivalent to directly cutting a chunk of high-cost evaporation, and the steam-saving ratio naturally far exceeds the dryness-improvement ratio. This also explains why shoe presses, wide nips, and even double-shoe-press configurations are almost standard on new high-speed lines.

With the formula above you can directly see the press's role. Suppose absolutely dry output is 1000 kg; if press-exit dryness C_in rises from 0.46 to 0.52, the water entering the drying section drops from about 1174 kg to about 923 kg — about 250 kg less water to evaporate, at 3.0–3.2 MJ/kg that is about 750–800 MJ of steam-heat savings, and scaled to full-day and full-year output it becomes a huge number. This is exactly the origin of the earlier "every 1% rise in press-exit dryness ≈ saves 4–5% drying steam": it is essentially the slope manifestation of this mass balance.

6. Special case and wrap-up: the Yankee cylinder and moisture control

Not all paper follows the "multi-cylinder" route. Tissue uses a completely different drying method: a single huge Yankee cylinder, 4.5–6 m in diameter, with the sheet clinging as an extremely thin layer to the cylinder surface, and a high-speed hot-air hood outside blowing the vapor away. Because the paper layer is extremely thin with a huge specific surface area, the Yankee's drying rate can reach 100–200 kg/m²·h, far above the multi-cylinder constant-rate level; tissue also relies on the cylinder-surface creping blade to crepe, giving a soft fluffy feel — this is the key process feature distinguishing it from ordinary paper.

Whatever the drying method, the final task is to control sheet moisture to the target range. Moisture of the sheet before reeling (or before calendering) is usually targeted at 4.5%–6.5%; too high easily molds during storage and bubbles in printing, too low makes the paper brittle, highly static and wastes energy. Cross-machine moisture uniformity is equally important; cross-direction moisture deviation is usually required within ±0.5%, achieved by zonal hood control in the drying section, per-cylinder steam and hood-airflow cross-machine adjustment.

Stringing this episode together: the wire section (EP07) removes over 95% of the water yet consumes almost no energy; the press uses mechanical means to push dryness from 18%–22% to 45%–52%, the cheap-dewatering finale; the drying section uses expensive evaporation to remove the remaining water, eating 60–70% of the machine's energy. Every link of the energy chain answers the same question — how to drive water out of the sheet at the lowest cost. Understanding this chain is understanding why modern paper machines treat "raising press-exit dryness" as the top energy-saving handle.

Next episode we leave the "dewatering" main line and enter the world of sheet surface and finishing — sizing makes paper water-resistant, coating gives smoothness and printability, calendering sets gloss and density. That set of processes decides whether the paper can finally "run well on the press and look good in print," and is also the bridge connecting papermaking and printing (EP10). Pressing and drying gave us a dry, strong sheet; sizing, coating and finishing turn it into a "usable" sheet.

7. Drying ventilation and heat recovery: the overlooked energy-saving space

The drying section is not only dryer cylinders but also a huge ventilation and heat-exchange system. The water vapor evaporated from the sheet is collected by the hood and carried away by the exhaust fan — this humid-hot air carries a large amount of latent heat. An efficient drying section recovers this heat: using a heat-recovery exchanger to preheat the incoming air or process water with the exhaust's waste heat, or using a heat-pump approach to upgrade low-grade waste heat for use. The drying section's thermal efficiency (useful heat for evaporation divided by total steam heat supplied) is often only 60%–70% or even lower, meaning three to four tenths of steam energy is lost with exhaust, cylinder-shell heat dissipation and condensate — precisely the corner with the richest payback for energy-saving retrofits.

The condensate system is often underestimated. After the steam in the dryer cylinder releases heat it becomes condensate; if not drained in time it occupies cylinder space, lowers heat-transfer efficiency and causes uneven cylinder-surface temperature (corresponding to cross-machine moisture fluctuation). Modern lines install condensate flash tanks and recovery pumps, flashing and recovering each cylinder's condensate in stages, recovering both sensible heat and ensuring uniform cylinder temperature. The steam-side pressure reduction, steam-trap selection and condensate recovery are "invisible" yet critical operating details of the drying section; steam-trap failure often manifests as a single cylinder's temperature falling behind, with locally higher moisture in the corresponding web.

Returning to the overall trade-off of press and drying: this episode repeatedly stresses that water solvable by mechanical dewatering must not be handed to evaporation. So the priority of energy saving is clear — first put on a shoe press to raise dryness, then optimize drying-section ventilation and heat recovery, and only last raise steam parameters. For an already-built line, the combination of "press +1% plus drying recovery efficiency gain" is often more economical and lower-carbon than simply increasing boiler output, with a much shorter payback period.

It is worth emphasizing that heat-recovery retrofit often does not require touching core equipment, but only adding an exchanger between the hood exhaust and incoming air, or adding flash recovery to the condensate system; the construction window can be the annual overhaul, with little production impact. Precisely because the change is small and the benefit large, it becomes the preferred path for many old mills to "lower energy consumption without replacing the main machine," and also the easiest item in carbon-footprint accounting to capture emission reductions.

8. Press-section operation and break control

The stable operation of the press section directly determines the whole line's runnability. If the nip drains poorly or the felt deteriorates, the sheet easily produces press marks, crushing or breaks at the nip; one break means tens of seconds to several minutes of production interruption and broke. To judge press state on-site, the common method is observing the "nip water line": at the sheet edge exiting the nip, watch the water-mark position; the further back the water line, the more thorough the dewatering; combined with on-line dryness or moisture scanning, it can judge in real time whether the felt needs cleaning or replacement.

The felt's "breathing" cycle deserves a separate mention: during running the felt is saturated with water, evacuated by the vacuum box, then re-pressed and drained at the nip — a cycle. The vacuum-box vacuum and the hole-distribution on its face determine how fast it can carry water away; if vacuum is insufficient or holes clogged, the felt enters the nip carrying water, and dewatering efficiency and dryness drop directly. So the daily life of the press section is largely "serving the felt" — continuous high-pressure water rinsing, vacuum-box maintenance, periodic chemical washing, none dispensable; any slack link shows up in the press-exit dryness curve.

Looking at press and drying together, their common goal is only one: drive water out of the sheet at the lowest cost. The press is responsible for "cheaply" pushing dryness past 50%, the drying for "expensively" finishing to finished-paper moisture. Understanding this energy chain and their respective mechanisms, the operator facing a break or moisture fluctuation in the early morning knows which valve to turn, whether to check the felt or adjust the hood — exactly the on-site intuition this episode wants to deliver.

By the way, a common misconception: some think the higher the drying-section temperature the better. In fact, too-high cylinder and hood temperature aggravates surface crusting of the sheet, instead hindering internal water from diffusing out, and pointlessly raises energy consumption and fire risk; the reasonable approach is zonal control, gentle heating-up stage and full use of heat-transfer area in the constant-rate stage. Drying is not "burning fiercely" but "transferring evenly and draining smoothly." In addition, the cross-machine zonal steam-pressure adjustment of each drying stage is the direct means to control cross-machine moisture uniformity, forming a closed loop with the on-line moisture scan before reeling — this part will be detailed further in EP09 finishing and EP10 quality control.