Definition
The mass of water that hygroscopic materials (natural fibres, paper, wood, food) absorb at a given ambient relative humidity, expressed as a percentage of dry mass (% w/w). ASHRAE reference Table 1 lists cotton at 18.5% regain at 50% RH and 25.8% at 90% RH; this is the primary reference for set-point selection in the textile, printing, food and furniture industries.
Detailed Explanation
Hygroscopic regain is the mass of moisture a material absorbs from the air at a given temperature and relative humidity, expressed as a ratio of dry mass. Formal definition: R = (mwet − mdry) / mdry × 100. This is a dry-basis figure; it must not be confused with wet-basis moisture content commonly used in food science.
Cotton, wool, silk, viscose, paper, wood, leather, tobacco and many food matrices are typical hygroscopic materials. Each has its own sorption isotherm (BET or GAB model); at the same 50% RH wool gains ~14%, cotton ~7-8%, paper ~7%, MDF furniture board ~9%.
As long as ambient dew point does not change, the equilibrium moisture content of the material does not change either — which is why dew-point setpoints are preferred over RH in process control. Temperature drift skews the RH reading, but the product's actual moisture content tracks the ambient dew point. This principle is the foundation of the NKT - Climate Track control logic NKT delivers to textile and printing customers.
Why It Matters
Hygroscopic regain is a sector-economic parameter for two reasons:
1. Weighing and sales — cotton yarn, wool roving, paper rolls and tobacco leaf are sold by mass. Under standard "commercial regain" conditions the official weight of the product is priced. ISO 139 defines the "standard atmosphere" for cotton as 20°C ± 2°C, 65% ± 4 RH; under this condition cotton's nominal regain is 8.5%. A producer delivering at 5% moisture loses 3.5% of weight = direct revenue loss.
2. Process quality — low-moisture yarn breaks, accumulates static and produces fibre dust on the machine. High-moisture yarn sticks, dyeing shifts and microbiological risk rises. Outside the optimum band, production efficiency drops 10-25%.
The same applies to printing paper: paper below 50% RH curls in offset printing and registration drifts; above 60% RH paper softens and ink-drying time extends. ASHRAE Table 4 specifies 22°C, 45-50% RH for paper storage and 22°C, 55-60% RH for the press room.
Practical Example
A Bursa cotton spinning mill produces 24 t/day. The target commercial regain is 8.5% (ISO 139). The shop drops to 38% RH in summer and 22% RH in winter due to heating.
Current state (uncontrolled): • Summer: product delivered at 5.2% moisture → 3.3-point deficit = 24 × 0.033 = 792 kg/day weight loss • Winter: product delivered at 3.8% moisture → 4.7-point deficit = 1,128 kg/day weight loss • Average: 950 kg/day × 220 working days × 2.8 USD/kg = 585,200 USD/year revenue loss • Additionally 12-18% yarn breaks, static dust, machine downtime
Solution: NKT-supplied SKE4 resistive steam humidifier system (4 × 80 kg/h units), ~320 kg/h total steam capacity for the 25,000 m² shop. The dew point is held steady at 13°C — even if temperature varies between 22-28°C, cotton equilibrium moisture stays in the 8.3-8.7% band.
ROI: • System capex: ~180,000 USD • Annual energy + maintenance: ~95,000 USD • Annual gain: 585,200 − 95,000 = 490,200 USD • Payback: <5 months
NKT engineering performs a 4-week sorption-isotherm measurement before the project; the setpoint strategy is delivered as an engineering report.
Engineering Note
Five engineering-critical points in regain control:
• Sorption hysteresis — adsorption (gaining moisture from dry) and desorption (losing moisture from wet) curves differ. At the same 50% RH, cotton equilibrating from dry may reach 7% regain, while equilibrating from wet may stop at 8%. This gap must inform setpoint planning. • Temperature dependence — the sorption isotherm is weakly temperature-dependent (~0.5% regain shift per 10°C), which is why dew-point control is more reliable. With temperature held in the 18-30°C band, the RH-regain relationship is practically constant. • Equilibration time — a large product mass (e.g. cotton bale) reaches equilibrium moisture in 24-72 hours. It must be held in a conditioning room before production. • Local microclimate — a large bale moved by crane equilibrates with the shop ambient, but the bale core stays 3-5°C cooler; the interior dew point stays elevated and mould risk appears. A multi-point sensor network is needed. • Seasonal transition — outdoor variability is highest in March and October; adaptive-PID systems such as NKT - Climate Track pre-tune setpoints with a 24-h forward forecast.
Target: material moisture should be held within ±0.5 percentage points; this requires dew-point control accuracy of ±0.8°C.
NKT Application Link
NKT delivers turnkey humidification solutions for textile, printing, paper, tobacco and furniture plants requiring hygroscopic regain control:
1. Steam humidification — Neptronic SKE4 resistive steam systems (5-80 kg/h models), compatible with DI/RO/mains water, ±1% RH precision. Hygienic and lime-independent. 2. Adiabatic (ultrasonic or high-pressure water mist) — Neptronic SKG4 and SKH3 series, cooling + humidification combined, low energy (~50 W/kg steam). 3. NKT - Climate Track control logic — sorption-isotherm-based setpoint strategy, dew-point PID, 24-h forward forecast, BMS/SCADA integration. 4. Multi-point monitoring — T/RH/dewpoint measurement at 12-32 points across the shop, regain-map reporting.
After commissioning, the NKT engineering team monitors sorption behaviour for 6 weeks and tunes setpoints against measured product regain. Typical outcome: regain CV (coefficient of variation) drops from 15% to 3%, yarn breaks reduce by 30-50%.