TPGOA Technical Insights
Hot-air dryer is recommended for drying polyolefins at temperatures in the range of 150°F to 180°F; a drying hopper should be sized for 2 hours of drying time. This type of drying system will provide a stable incoming material temperature and lower the surface moisture of the pellets.
The principle of operation of a hot-air dryer is simple: hot air will absorb and hold more water than cool air; for example, ambient air at 75°F with a dewpoint temperature of 60°F has a relative humidity of 59.62%, meaning it has absorbed over half of the water that it can absorb at that temperature. If that air is then heated to a temperature above 75°F, the relative humidity of the air will decrease, and the drying air then will be able to absorb more water.
A hot-air dryer will dry polyolefins down to 0.1% moisture, and often much lower, which will satisfy the vast majority of polyolefin processors. However, if your application requires an extraordinary low final moisture level, a dehumidifying dryer may be required.
A dehumidifying dryer is a hot-air dryer on steroids. It first passes the drying air through a bed of molecular-sieve desiccant, which removes water from the drying air and lowers the air’s dewpoint. The dehumidified air is then heated, which lowers the air’s relative humidity. A dehumidifying dryer can easily reduce the surface moisture of polyolefin pellets to 0.005% or lower.
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