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). Cotton's nominal regain in the standard atmosphere (20 °C, 65% RH) is about 8.5%, and about 6–7% at 50% 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 relative humidity does not change, the equilibrium moisture content of the material does not change either in practice; the sorption isotherm depends only weakly on temperature. This is why relative humidity, not dew point, is the governing parameter in process control: when temperature drifts at a constant dew point, RH changes and the product's actual moisture content shifts with it. This is why textile and printing plants hold temperature and relative humidity together.
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: 960 kg/day × 220 working days × 2.8 USD/kg = 591,360 USD/year revenue loss • Additionally 12-18% yarn breaks, static dust, machine downtime
Solution: Neptronic SKE4 resistive steam humidifier system (4 × SKE4-90, 90 kg/h units), ~360 kg/h total steam capacity for the 25,000 m² shop. The shop is held at 22°C / 65% RH (dew point ≈ 15°C); with temperature and relative humidity controlled together, 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: 591,360 − 95,000 = 496,360 USD • Payback: <5 months
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 RH control tracks regain reliably. 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; setpoints should be reviewed more often against outdoor changes during this period.
Target: material moisture should be held within ±0.5 percentage points; this requires dew-point control accuracy of ±0.8°C.
NKT Application Link
For this application NKT – Nem Kontrol Teknolojileri supplies TFT, Neptronic and Microwell equipment. It supports unit selection with capacity calculation and technology matching based on space conditions, target temperature and RH (or dew point) and moisture load, and provides technical service and spare parts. Design, installation and commissioning remain within the scope of the facility's designer and contractor.