Material Preparation
The dew point is the temperature at which the moisture carried by air begins to condense, and it is a measure of how dry the air is: the lower the value, the drier the air. A dew point of −40 °C in the drying air means that air will not release water unless it is cooled to −40 °C. It should not be confused with the process temperature; air can be hot at 80 °C and dry at a −40 °C dew point at the same time.
The dew point measures the air, not the residual moisture of the granulate. Even if the dryer's air is at −40 °C, the granulate can still be damp if the time is insufficient or the hopper air path short-circuits. Residual moisture can only be measured from the material itself, with a moisture balance, Karl Fischer or the calcium hydride method.
The measuring point is the hopper inlet. As the desiccant ages, the dew point slowly rises; tracking its course makes it possible to plan a bed change before a failure occurs.
Because it is a measure of dryness expressed in units of temperature. If there is no condensation even when you cool the air down to −40 °C, it contains very little water. The dryer's process temperature (50–170 °C) is an entirely separate value.