Fluff pulp leaves the mill at a controlled moisture. It arrives at your warehouse at whatever moisture the journey decided. In tropical and subtropical climates, that journey can add two full points.
Why humidity matters more than heat
Pulp is hygroscopic. The fibre wall absorbs and releases water until it reaches equilibrium with the surrounding air. At 50% relative humidity, bleached kraft pulp equilibrates around 7%. At 85% RH, the same pulp equilibrates near 11%.
Temperature matters mainly because warm air holds more moisture. A warehouse at 35 °C and 80% RH contains far more water than one at 25 °C and 80% RH, and the pulp will find it.
Five practices that work
- Pallet clearance. Keep bales at least 100 mm off the floor and 300 mm from walls. Concrete floors wick moisture upward and the bottom layer is always the first to fail.
- First in, first out. Moisture equilibration is a function of time. A bale stored for six months is not the same material as one stored for six weeks.
- Wrap integrity checks on arrival. A torn wrap turns slow equilibration into a local wet patch that will jam the hammer mill. Reject or repack damaged bales immediately.
- Ventilation over cooling. Moving air removes moisture; cold air alone does not. A simple extraction fan running during daylight hours does more than air conditioning.
- Do not stack against a sun-facing wall. The thermal gradient drives moisture migration inside the stack, concentrating water in the cooler bales.
What to measure and how often
A hygrometer in the storage area costs very little and tells you whether the environment is drifting. Log RH daily. When it stays above 75% for more than a week, expect delivered moisture to rise by roughly half a point.
Sample core moisture on one bale per pallet on arrival. Trend it by supplier lot. If you see a lot drifting consistently, that is a conversation with the mill about wrap specification, not a warehouse problem.
The cost of getting it wrong
Two points of excess moisture on a 500 tonne shipment is 10 tonnes of water bought at pulp prices. It also means higher defibration energy, more dust and a slower line for the entire batch. Storage discipline is the cheapest quality control you have.