Air system & ventilation: turning "invisible air" into parameters

The dryer section turns the water in the sheet into steam and discharges it, but steam does not fly away by itself — it relies on air as the carrier. Each kg of water from liquid to vapor absorbs about 2,260 kJ of latent heat (supplied by dryer steam), and carrying away this steam requires a large amount of unsaturated air. So the fundamental task of the ventilation system is one sentence: with the right amount of air, promptly and orderly discharge the evaporated moist air, while recovering as much heat as possible.

The dryer section turns the water in the sheet into steam and discharges it, but the steam won't leave on its own — it relies on air as the carrier. Each kilogram of water absorbs about 2,260 kJ of latent heat of vaporization going from liquid to vapor (supplied by the dryer-cylinder steam), and removing that vapor requires a large volume of unsaturated air. So the ventilation system's fundamental task boils down to one sentence: with just the right air volume, promptly and orderly exhaust the evaporated moist air, while recovering as much heat as possible.

The cost of unbalanced airflow is two-sided: insufficient supply → exhaust humidity approaches saturation, dryer heat transfer worsens, sheet re-wets; excessive supply → heat escapes with exhaust, energy use spikes, sheet over-dries and wrinkles. So ventilation is not "just turn it on" but "run it balanced". A graphic-paper machine's ventilation volume is often tens of thousands of cubic meters per minute, and fan power is a significant share of total utility-system power; adjusting a little airflow is real electricity cost.

II. Hood supply/exhaust balance

The hood is a semi-enclosed "box", kept at slight negative pressure inside (generally -5 to -20 Pa), so moist air leaves only through the exhaust and not through gaps into the workshop. The two levers of balance: supply airflow determines drying capacity (supply temperature × airflow = heat brought in), exhaust airflow determines moisture-removal capacity. Exhaust slightly exceeds supply for negative pressure inside.

Each 10 °C rise in supply temperature significantly increases sensible heat brought in, supporting a higher evaporation rate — but the upper limit of supply temperature is constrained by hood material and paper grade, and too high a temperature over-dries and wrinkles the sheet surface. So the adjustment order is: first set supply temperature (process-given), then use airflow to adjust evaporation, finally use exhaust to hold negative pressure and wet-bulb. The three are coupled; adjusting one alone destabilizes the other two.

Balance is judged by three numbers: hood static pressure (slight negative), exhaust wet-bulb temperature / humidity ratio, and the volume-flow difference between supply and exhaust. Typical imbalance: steam escaping at hood doors (insufficient exhaust, positive pressure) → workshop condensation, equipment corrosion; or excessive negative pressure inside → large cold-air infiltration through gaps, thermal efficiency drops.

III. Pocket ventilation

When the sheet wraps the dryer cylinder, the "pocket" area between sheet and cylinder surface (pocket) most easily accumulates moisture — steam condenses and re-wets there, a high-incidence zone for uneven drying and paper defects. Pocket ventilation uses a row of small nozzles to blow hot air into the pocket the instant the sheet leaves the cylinder, "dispersing" the moist air and directing it to exhaust.

The pocket-ventilation air temperature, airflow, and nozzle angle directly determine cross-machine moisture uniformity. Clogging or wrong angle shows as increased cross-machine moisture 2σ and local over-drying/wrinkling. Maintenance focus: nozzles must not clog (paper dust + scale), ducts must not leak, air temperature must be stable. In design, pocket air temperature is generally slightly higher than hood supply, and velocity must be enough to "sweep" the trapped moist air out of the pocket without disturbing or breaking the sheet.

IV. Exhaust heat recovery

Exhaust is high-temperature and high-humidity; discharging it directly is a huge waste. The heat recovery unit (plate or rotary type) uses exhaust waste heat to preheat fresh air, raising supply temperature by 20–40 °C, equivalent to saving the corresponding steam. Recovery efficiency is generally 50%–70%, but has two enemies: fouling (water and fiber dust in exhaust plastering the heat-transfer surface) and air leakage (fresh air contaminated by exhaust).

Rough accounting: a machine with 100,000 m³/h exhaust, 40 °C above fresh air, 60% recovery means recovering about 100,000×1.2×40×0.6 ≈ 2.88 million kJ sensible heat per hour, a substantial steam equivalent. So each 1 mm of fouling on the recovery unit cuts efficiency sharply — far costlier than replacing a heat-transfer plate.

To judge whether the heat recovery unit is healthy: compare the temperature difference between "fresh-air outlet temperature" and "exhaust inlet temperature" against the design value; periodically check heat-transfer surface cleanliness and seals. For the rotary type also watch the rotor seal wear — leakage makes fresh air smelly and damp.

V. Mist and Dust removal

The dryer section has two troublesome by-products: Mist (fine water droplets, from evaporation and condensation) and Dust (paper lint and filler dust, from sheet friction with doctor blades/dryer cylinders). Mist condenses into water drops on the hood roof and beams, dripping onto the sheet as the "water-drop spot" defect; Dust accumulates on equipment and electrical cabinets, a fire hazard and root cause of falling insulation.

Removal means: mist-collection on the hood roof, local negative-pressure extraction, periodic flushing; electrical cabinets and motors must be sealed against fiber dust. A hard rule on site: electrical cabinets and cable trays above the dryer section must be dust-proof and condensation-proof, otherwise insulation incidents recur after three years. Control cabinets at least IP54, critical spots use pressurized ventilation cabinets.

VI. Fan and valve inspection

The actuators of the ventilation system are fans and air dampers. Fan inspection: bearing temperature and vibration, belt/coupling, impeller dust (destroys dynamic balance), inlet/outlet flexible connection damage, VFD current. Damper inspection: actuator action, open-position feedback matches command, valve plate not stuck.

VII. Linkage between ventilation, paper defects, and energy use

Ventilation imbalance hurts both quality and energy, and is often misjudged as other problems: cross-machine moisture variation → blamed on headbox or calender; sheet wrinkling → blamed on cylinder face or doctor; high steam use → blamed on condensate system. Frontline judgment: first check exhaust wet-bulb and hood static pressure; only when these two are normal look elsewhere.

VIII. Maintenance points and criteria

• Daily: hood static pressure, exhaust wet-bulb/humidity ratio, supply-exhaust airflow difference — record these three each shift.
• Weekly: visually check pocket-ventilation nozzle blockage, check duct flexible-connection leaks.
• Monthly: fan bearing temp/vibration and current trend, damper position-feedback consistency.
• Quarterly: re-measure heat recovery efficiency, clean rotary/plate type.
• Yearly: clean dust and scale inside ducts, check cabinet dust/condensation protection (IP54 re-verification).
• Special: hood-roof insulation and condensate-drip check, prioritized when water-drop spots recur.

IX. Ventilation energy magnitude and benchmarks

The ventilation system is the second-largest energy block after the dryer section; fan power often accounts for 15%–25% of the paper machine's utility-system electricity. Quantify: exhaust 100,000 m³/h, 40 °C above fresh air, 60% recovery recovers about 2.88 million kJ sensible heat per hour — a substantial steam equivalent that lands directly on the steam and electricity bills. So ventilation is not an "ancillary system" but an energy battleground on par with the dryer section.

The cross-machine moisture vs steam-use linkage was given in M04: each 1 percentage-point rise in outgoing dryness cuts steam use 4%–5%. Ventilation imbalance invalidates the premise — when exhaust humidity ratio is high, more steam is needed to keep dryness, so steam use rises and capacity drops together. Therefore the ventilation "sweet spot" must be held: exhaust humidity ratio 0.18–0.22 kg/kg, hood static pressure -5 ~ -20 Pa, heat recovery efficiency ≥ 60%. Beyond 10% off the sweet spot, the energy bill reacts immediately.

X. Standard work for ventilation inspection

Turn inspection into an executable SOP, not "look when free": daily three numbers — hood static pressure, exhaust wet-bulb/humidity ratio, supply-exhaust airflow difference, logged each shift; weekly — visually check pocket-ventilation nozzle blockage, check duct flexible-connection leaks; monthly — fan bearing temp/vibration and current trend, damper position-feedback consistency; quarterly — re-measure heat recovery efficiency, clean rotary/plate type; yearly — clean dust and scale inside ducts, check cabinet dust/condensation protection (IP54 re-verification). Each item has criteria — not "inspected" but "is the reading in range".

The discipline of inspection matches the "trend discipline" of M04: readings must carry "previous value + rate of change". A single-shift change over 5% must be watched, three consecutive days of one-way change must be investigated, change with rising energy use must be scheduled for a window repair. The early signal of ventilation problems is almost always "slow drift" — humidity ratio rising 0.01 per month, static pressure drifting 2 Pa per month, only exploding into a paper defect after half a year, by then hard to trace. So the value of ventilation inspection is not "finding big faults" but "catching slow drift".

XI. Interface between ventilation and paper grades (preview of Part C)

The differences of the ventilation system across paper grades mainly come from the form of the dryer section, not the principle. Graphic paper is multi-cylinder drying, with sectional hoods and dense pocket ventilation, each section balanced independently; board has large basis weight and width, so total airflow and heat-recovery scale are larger, and the heat recovery unit is mandatory not optional; tissue has only one Yankee dryer, the hood is a high-temperature high-velocity hot-air hood, ventilation is almost the drying capacity itself, and the traditional "pocket" concept is replaced by the "creping-zone". Therefore the ventilation "sweet spot" parameters shift with grade, but the methodology of "balance, humidity control, recovery, mist/dust exhaust" is unchanged.

This also explains why Part B (utility systems) is placed before Part C (grade-specific volumes): first establish the cross-grade methods of hood balance, pocket ventilation, heat recovery, and mist/dust removal; when specific grades are later discussed, only the equipment form and parameters need replacing, not the principles. For O&M staff, ventilation is one of the few systems where "parameters change, logic stays" — when transferred across grades, the method migrates directly without relearning. A common cross-grade trap is "copying parameters": someone moved from graphic paper to tissue often applies the multi-cylinder hood's supply/exhaust balance experience directly to the Yankee hot-air hood, resulting in either over-dry brittleness or uneven creping.

The root cause is that the two hoods have opposite control goals: the multi-cylinder hood pursues "uniformly carry away moist air", while the Yankee hot-air hood pursues "precisely control cylinder-surface evaporation and coating". So when switching grades, ventilation parameters must be re-tuned; experience can only give direction, not numbers — which is also why Part C devotes a separate drying-and-ventilation section to each grade.

XII. Ventilation anomaly vs paper-defect mapping

The manifestation of ventilation imbalance on the sheet is not random but has a stable correspondence. Positive hood pressure lets moist air escape through hood-door gaps, causing workshop condensation and external equipment corrosion, while the sheet edge shows the "edge-wet" defect from local re-wetting; insufficient exhaust raises exhaust wet-bulb and pushes humidity ratio toward saturation, lowering dryer heat-transfer coefficient, showing as barely adequate dryness but clearly rising steam use; excessively high or uneven supply temperature directly shows as over-dry wrinkling and larger cross-machine moisture variation. Memorize the causal chain "ventilation parameter → paper defect", and at pre-shift gauge reading you can infer a class of problems from one number, instead of panicking only when the defect appears on the sheet.

Conversely, when a defect appears, don't touch the stock and doctor first; do a "ventilation-side triage" by the mapping table: cross-machine moisture variation → first check exhaust wet-bulb and supply-exhaust airflow difference; sheet wrinkling → first check if hood static pressure is too negative; water-drop spots → first check hood-roof insulation and supply-air atomization; abnormal steam use → first check exhaust humidity ratio. Only when triage is normal attribute to headbox, calender, cylinder face. This order avoids the common detour of "adjust the formulation as soon as a defect appears", saving much trial-and-error time and preventing mis-attributing ventilation problems to the process side and making things worse.

Print this mapping table and post it by the dryer-section console; it is more intuitive than any training. Emphasis: the correspondence between ventilation parameters and paper defects is not one-to-one but "prioritize checking" — after triage points to ventilation, if parameters are normal yet the defect persists, then move to the process side; never change the stock formulation or doctor angle before ventilation is cleared. Ventilation-side problems are usually far cheaper to fix than process-side ones; checking the low-cost direction first is field-engineering common sense and the pragmatic way to minimize downtime.

There is also a common cognitive bias on site worth calling out: once a paper defect appears, the first reaction is often to change process parameters, because the process is closest to the sheet and easiest to tweak. But statistics show a significant share of defect root causes lie in the utility systems — ventilation, hydraulics, drive — not the stock itself. The reason: utility-system parameters drift slowly, the defect is only a lagging manifestation, separated by several process steps, making it hard to directly link the two by eye. So establishing the troubleshooting discipline "check the utility systems first, then touch the process" is not dogma but putting the most likely, cheapest, and most often overlooked direction first. An experienced O&M team correlates ventilation parameters, hydraulic pressure, and drive speed ratio — data far from the sheet — with defect records for association analysis, rather than reacting passively only when the problem reaches the sheet. This front-loaded correlation is precisely the key to reducing unplanned downtime, and the fundamental reason the three Part-B lectures precede the Part-C grades.

XIII. Mnemonics for this lecture

The difficulty of ventilation is "invisibility" — where air enters, where it leaves, how much humidity — all rely on instruments. Compress the judgment logic into mnemonics and infer from readings on site:

• Hood must be slightly negative, -5 to -20 Pa; positive pressure steams out, excessive negative sucks cold air.
• Exhaust humidity-ratio sweet spot 0.18–0.22; above 0.25 steam use clearly rises.
• For defects first check the ventilation side: cross-machine variation → wet-bulb, wrinkling → negative pressure, water-drop spots → hood roof.
• Pocket nozzle blockage enlarges cross-machine 2σ — visually check weekly.
• Heat recovery efficiency ≥60%; 1 mm fouling cuts it sharply; cleaning is cheaper than replacing plates.
• Electrical cabinets above the dryer section dust/condensation-proof ≥IP54; insulation incidents three years later all stem from here.

One sentence summary: ventilation imbalance makes defects and energy use explode together, but fixing it is cheaper than touching the process — so "check the utility systems first, then the process" is discipline not dogma. Correlating ventilation parameters, hydraulic pressure, and drive speed ratio — data far from the sheet — with defect records is precisely the front-loaded approach to reducing unplanned downtime; the reason the three Part-B lectures precede the Part-C grades is rooted here.