Graphic paper · Dryer section & finishing: keeping the drying stable and the reeling even

I. What the dryer section of graphic paper does

The dryer section of graphic paper (wood-free, coated, writing & printing paper) is a "multi-cylinder drying chamber": dozens of dryer cylinders (diameter often around 1.5 m) in series, with the sheet snaking back and forth between cylinder faces and dry fabrics, using steam latent heat to dry moisture from about 60% (entering dryer) to 93%–95% (leaving dryer). It directly corresponds to the "after press → after dryer" segment of the dryness ladder in M04: about 40%–45% dryness after press, about 94% after dryer, these 50 percentage points of moisture are all evaporated by the dryer section. Each kg of water evaporated consumes about 2.5–3.0 kg steam (depending on heat recovery), so the dryer section is the paper machine's largest energy consumer.

Graphic paper is extremely sensitive to "two-sidedness": the front side (against the cylinder face) is smooth and glossy, the back side (against the dry-fabric face) is rough. This requires coordinated adjustment of dry-fabric tension, cylinder-face condition, and size press/calender. In O&M, the dryer section must not only dry the water but simultaneously get three things right — "machine-direction moisture uniformity, cross-machine moisture uniformity, controllable two-sidedness" — if any one fails, the calender and the print shop will come calling.

II. Dryer groups and drying-curve interpretation

Dryer groups are divided into three functional sections: first dryer (low inlet dryness, fast evaporation, high cylinder temperature), after dryer (near target dryness, needs two-sidedness and wrinkle control), post-size-press dryer (re-drying after sizing, stable temperature to prevent adhesive migration). The drying curve is the moisture/temperature distribution along the cylinder order. Interpretation principle: the early slope should be steep (fast evaporation); if it is flat early, it is mostly insufficient steam pressure or condensate blockage; if it still drops steeply late, there is surplus heat and steam can be cut to save energy.

Cylinder temperature generally rises then levels off along the path: steam-in temperature is limited by steam pressure; the condensate-out temperature and cylinder-wall temperature determine heat transfer. An often-overlooked point: dryer surface temperature ≠ steam temperature; in between lies the cylinder-wall thermal resistance and condensate film — poor condensate drainage immediately drops the cylinder-face temperature by 10–20 °C and the evaporation capacity with it. So when the drying curve is bad, the first reaction is not to add steam but to check the condensate system.

III. Condensate system: siphon, rotary joint, vacuum breaker

The dryer cylinder is a heat exchanger with "steam in, condensate out". Condensate is drawn from inside the cylinder by a siphon and discharged through the rotary joint. The siphon-to-wall clearance and insertion depth directly determine drainage capacity and residual water-film thickness — too large a clearance slows drainage, leaves more water in the cylinder, worse heat transfer; too small easily scrapes the cylinder. The rotary joint is a dynamic seal; steam leakage is a high-frequency fault — check the seal ring and bearing first for leaks.

Another key device is the vacuum breaker: on shutdown or slowdown a negative pressure may form inside the cylinder that "holds" the condensate and prevents discharge; the vacuum breaker admits air when cylinder pressure drops below a value, breaking the vacuum so condensate drains by gravity. Maintenance: periodically re-check siphon clearance against the manufacturer's curve, monitor rotary-joint leakage trend, include vacuum-breaker actuation test in shutdown inspection. Only with a healthy condensate system is the drying curve stable.

IV. Size press and calender

Graphic paper's two-sidedness is solved by sizing: the size press thinly coats starch or surface-sizing solution on both sides, raising surface strength and water resistance; coated paper (art paper) adds a coating layer after sizing. Size amount is controlled by metering rod/blade/film transfer; unevenness directly shows as print dusting and poor surface strength. The calender uses multiple hard nip rolls to flatten the sheet, raise gloss, and control thickness — nip line pressure, roll temperature, and speed determine smoothness and density.

Sizing and calendering are "detail amplifiers": a little cross-machine non-uniformity from earlier drying is amplified here into gloss stripes. Maintenance points: crown and covering condition of the size/metering rolls, surface hardness and bearing clearance of calender rolls, closed-loop control of nip line pressure (echoing M07 hydraulic loading). Once a calender roll has chatter marks or scratches, the whole sheet width is damaged — this is graphic paper's most expensive "single flaw".

V. Dry-end defects: wrinkling, water-drop spots, poor gloss

Dry-end defects are directly tied to graphic paper's "face", and mostly have stable cause-effect: wrinkling (machine-direction folds) comes from cross-machine moisture variation or wrong dry-fabric tension, dirty cylinder faces; water-drop spots (periodic water dots) almost always come from M08's hood-roof condensation or supply air carrying water, not the water system; poor gloss/smoothness comes from calender-roll condition or uneven sizing; excess two-sidedness comes from dry-fabric tension and cylinder-face distribution. These look random on the sheet, but the root cause is often the "slow drift" of drying and ventilation.

The core of this table is "check the chain first, then touch the source": the root cause of dry-end defects is often not the dry end itself but M08 ventilation, M07 hydraulic loading, even M03 wet end — troubleshoot by priority to avoid adjusting the stock formulation right away.

VI. Rewinder: unwind / slitting / rewind three stages

Finishing of graphic paper starts with rewinding. The rewinder has three stages: unwind (parent roll) sets the reference tension, slitter uses circular knives to slit the full width into customer widths, rewind (core) winds the slit paper into a tight, square-edged finished roll. The tension and speed ratio of the three stages must match, otherwise "chrysanthemum" (uneven edges), uneven tightness, and tail folds appear. The slitter blade edge and overlap determine cut quality — dull blades give fuzzy edges, too much interference breaks blades.

VII. Threading and web-break handling

Dryer-section threading is the process of leading the sheet tail from the press exit through dozens of dryer cylinders to the reel; modern machines mostly use automated threading rope/air duct. Threading failure is a major start-up loss. After a web break: first stabilize the equipment by M09's auto-slowdown sequence, then clear broke and re-thread. Dry-end breaks mostly come from uneven wet-end feed, cross-machine moisture variation, sudden dry-fabric tension change, or cylinder-sheet buildup — not "the dryer section misbehaving" but a lagging manifestation of upstream.

O&M must build a discipline: record for every break "which section it broke in + the ventilation/hydraulic/drive parameters at the time". Frequent at a single point is an equipment problem (a cylinder doctor, a dry-fabric segment); frequent across the whole width is an upstream (wet-end/stock) problem. Correlating break location with parameters is ten times stronger than "adjust by impression".

VIII. Dry-end energy and steam-water balance

Dryer-section steam consumption is 60%–70% of graphic paper's energy per ton. Quantify: evaporating 1 kg water theoretically needs about 2.26 MJ latent heat, actual steam use is 2.5–3.0 kg steam/kg water (including heat loss). M04 gave the rule: each 1 percentage-point rise in outgoing dryness cuts steam use 4%–5% — so the press (M02/M07) removing one more point of water means the dryer section burns far less steam. The dryer section's own energy saving relies on: condensate waste-heat recovery, M08 exhaust heat recovery, cylinder-group insulation, and reasonable steam reduction (when the curve's late section has surplus).

The other side of the steam-water balance is white water: the dryer section produces no white water, but broke from sizing/calendering and wash water all enter the white-water system (M03/M13). Keeping one "ledger" of energy and water balance lets you know how much electricity/coal cost each 1% steam saving corresponds to, and persuades management to invest in M08's heat-recovery retrofit.

IX. Maintenance points and criteria

• Daily: spot-check the drying curve; check exhaust wet-bulb and in-hood static pressure (M08); IR point-check of cylinder-surface temperature; and inspect reel tension and slitter condition.
• Weekly: observe syphon drainage; log rotary-joint leakage; re-verify dryer-fabric tension; visually inspect calender-roll surface.
• Monthly: trend dryer-cylinder bearing temperature and vibration; test vacuum-breaker valve actuation; inspect and replace slitter-blade edges; check size-press roll crown.
• Quarterly: re-measure condensate-system efficiency; inspect dryer-internal fouling/scaling (grade-dependent); test calender-roll hardness.
• Annually: pressure-test dryer cylinders (statutory inspection of pressure parts); replace dryer fabrics; regrind or re-cover calender rolls.
• Special programs: trend dry-end defect statistics; map web-break location distribution; run a monthly two-sidedness review.

X. Interfaces with the Common Base / Part B

The graphic-paper dry end is the "integrated exam hall" for everything covered earlier: M04's dryness ladder and OEE turn here into steam consumption and capacity; M07's hydraulic loading shows up in calender linear pressure and size-press roll loading; M08's ventilation directly determines the drying curve and sheet defects; M09's drive speed chain runs through unwind — slitting — reeling. So Part C is not about "an isolated dry end," but about "how the graphic-paper grade grows on the horizontal base already erected" — which is also the fundamental reason Part B precedes Part C.

Another interface is the CBM of M01: dryer-cylinder bearings, rotary joints, and calender rolls are all objects that can carry sensors for condition monitoring. Feeding cylinder temperature, bearing vibration, and joint leakage into a trending system enables early warning before "web break / cylinder burst." Graphic-paper machines are high-value with costly downtime, so CBM's ROI is most pronounced on them.

XI. Typical Operating-Condition Snapshot

The "typical operating-condition snapshot" that pairs with the graphic-paper dry end should include, under three conditions — full load / 50% / no load — the average cylinder-group temperature, inlet/outlet dryness, steam consumption (kg steam/kg paper), exhaust humidity, reel tension and slitter life, and web-break frequency. When an anomaly occurs later, comparing against the snapshot immediately separates "systematic degradation" from "normal fluctuation due to condition switching." Collecting this snapshot costs almost nothing, yet turns "this dryer section seems less energetic than before" into quantifiable, attributable data — together with the equipment, it forms the graphic-paper machine's "birth record."

A concrete suggestion: turn the four charts — drying curve, exhaust wet-bulb, cylinder temperature, reel tension — into a single dry-end dashboard (echoing M04's KPI thinking), and review it for 5 minutes before each shift. Most of graphic paper's unplanned stoppages show early signs on this dashboard 2–4 hours before they happen — provided someone actually watches it.

XII. Quick-Reference Mnemonics for This Episode

• The drying curve is an electrocardiogram: steep drop at the front, gentle leveling at the rear; the whole curve shifting up = heat shortage, a flat front = water blockage or insufficient steam.
• Cylinder temperature ≠ steam temperature; a condensate film lies between — poor drainage drops the cylinder surface by 10–20 °C.
• Set syphon clearance per the curve, watch rotary joints for leakage, and test the vacuum-breaker valve at shutdown — manage these three and the curve stays stable.
• For dry-end sheet defects, check the chain first: wrinkling → ventilation, water-drop marks → hood roof, gloss → calender, two-sidedness → dryer fabric.
• Match the three-stage reel tension to prevent the chrysanthemum-petal (rosette) end-face defect; a dull slitter gives burrs, excessive interference shatters the blade.
• One more percentage point of water removed at the press means a lot less steam burned in drying — energy savings live in the wet end, not the dryer.

XII. Graphic-Paper Dry-End Start-up and Shutdown Key Points

Start-up and shutdown of the graphic-paper dry end follow a sequence; scrambling it means "web breaks and wrinkling right at start-up." Start-up: first admit steam to warm the cylinders slowly (control the heating rate to avoid thermal shock — exceeding the cylinder body's thermal-stress limit cracks the surface glaze or damages the shell), with cylinder temperature climbing in gradient per the drying curve; simultaneously engage M08 ventilation (supply air before raising temperature, to prevent moisture buildup and rewetting inside the hood); build dryer-fabric tension in order; thread the sheet; finally raise machine speed. Shutdown reverses this: first slow down, clear broke on break, relieve pressure and cool down, stop ventilation, release dryer fabrics. The core discipline is "slow is fast" — heating/cooling rates are limited by the cylinder body; the few dozen minutes saved by rapid cooling/heating are repaid by one cylinder crack or rattle. After a major overhaul, the first start-up must record the drying curve, cylinder temperature, rotary-joint leakage, and reel tension into the "birth record," so later comparisons have a baseline.

XIII. Typical Graphic-Paper Failure Cases

Case 1: High steam consumption, slow late-stage drop of the drying curve. The first instinct "heat shortage" suggests simply adding steam, but checking M08 exhaust humidity showed normal, then checking condensate revealed an oversized syphon clearance with water retained in the cylinder; after fixing drainage, steam consumption dropped ~6%. Lesson: for an abnormal curve, first tell apart "not enough heat supplied" from "moisture not drained" — don't just add steam, because steam won't cure water blockage. Case 2: Cross-direction wrinkling and print-dusting complaints. Three rows of pocket-ventilation nozzles were found clogged and the CD 2σ widened; clearing the blockage restored it. Lesson: for dry-end sheet defects, check M08 ventilation first. Case 3: Full-width gloss streaks. Root cause was a calender-roll chatter mark; stopping for calender regrinding cost far more than routine surface checks. Lesson: calender-roll surface condition is an "expensive injury" — daily visual and tactile inspection is the cheapest. All three cases point to one statement: most graphic-paper dry-end failures root in the chain (ventilation / condensate / hydraulic loading), not in the dry end itself.

XIV. Graphic-Paper Two-Sidedness and Printability

Graphic paper ultimately goes to the printing press; "two-sidedness" (front/back difference) and "printability" are the real acceptance criteria. The front side contacts the cylinder — smooth and glossy; the back side contacts the dryer fabric — relatively rough. Excessive two-sidedness → print show-through and uneven color; insufficient surface strength → dusting/linting that fouls the plate. Adjustment relies on: dryer-fabric tension (controls the back side), cylinder-surface condition (controls front-side smoothness), and sizing/calendering (controls surface strength and gloss, see Episode 4). Printability also depends on smoothness, gloss, and opacity. O&M must bridge "paper-machine parameters" and "printer complaints" into one mapping table: when the printer reports dusting, first check size-addition and post-size drying temperature; for show-through, first check two-sidedness and density; for color difference, first check coating (M11) or sizing uniformity. This "machine → press" mapping is the core competence distinguishing graphic-paper O&M from other grades — what you manage is not paper, but the printer's yield of good product.

• Show-through / uneven color → check two-sidedness and density (dryer-fabric tension, cylinder surface).
• Dusting/linting fouling the plate → check size-addition, post-size drying temperature, and stock.
• Color difference → check sizing/coating uniformity before touching the headbox lip.

XV. Graphic-Paper O&M Ends at the Printing Plant

The endpoint of graphic-paper O&M is not "paper produced" but "the printer runs smoothly." Reading the printer's complaints of show-through, dusting, and color difference as "remote alarms" of paper-machine parameters wins you half the battle. One iron rule on site: upon a printing complaint, first pull the four charts — drying curve, post-size drying temperature, two-sidedness, coating solids — and eight out of ten times you'll find the counterpart on the machine, rather than bouncing it back to the stock recipe. This also echoes M01's "locate the level before discussing repair" — the place closest to the sheet is easiest to blame, yet the root cause often lies in the utility system farthest from the sheet. Whoever can translate a printing complaint into paper-machine parameters is the true expert of graphic-paper O&M.