What is Hygroscopic material?
Most plastic materials, such as PA (Nylon), PC and PET are hygroscopic. They absorb moisture from ambient air and release moisture to dry air.
To release moisture back to the ambient air, the airflow around the grain must have a dew point of at least -15°C.
Process air with a dew point lower than -25°C is not needed to process hygroscopic material. It takes a lot of energy to achievelower dew points, but it does not affectthe final product.
Is a dew point of -40°C twice as good as -20°C?
The absolute zero point of moisture content begins at -273°C. Up to -20°C the gradient is approximately linear, at -20°C the curve begins (see black line in the Mollier diagram). At -20°C, the moisture content is about 0.75 grams of water per kg of air; this means that each degree of temperature drop drop is equal to a difference in moisture content of about 0.003 g/kg. The moisture content at -40°C is about 0.69 g/kg.
