Let’s start with humidity: Humidity describes moisture or water vapor that is contained in air. The weight of water present in a cubic meter of air is known as its Specific Humidity.
Few people use that measure however. It’s much more useful to discuss Relative Humidity, which compares the actual moisture in the air to the total moisture capacity of the air (at a given temperature). As a fraction, it would look like this:
|Relative humidity =||
Specific humidity of air (actual moisture present)
Relative humidity is always specific to air temperature. And, since warmer air has a greater capacity for holding moisture than cooler air, 60°F air with a relative humidity of 70% is holding a lot more moisture than 0°F air with the same relative humidity.
Dew Point refers to the temperature at which air reaches 100% relative humidity, or moisture saturation. When air is at or below its dew point temperature, it can’t hold any more moisture, so any additional moisture condenses into water droplets. This is why dew often forms when humid air cools overnight.
In the world of resin drying, there’s a lot of discussion about the importance of low dew point air. For example, dryer makers like Conair may refer to -40°F dew point air used in drying processes. This means that, before it is exposed to heat, the air that will be used in a Conair drying circuit is dehumidified to an extremely low moisture level, comparable to what air at -40°F could hold. This is what is meant by “low dew point air.”
Low dew point air is created by passing moist air through a desiccant bed or wheel that adsorbs moisture. Then, this air is heated rapidly to the required drying temperature and blown through an airtight circuit into the bottom of the drying hopper. The combination of super-dry air and rapid heating in the dryer circuit creates an airflow into the hopper that is capable of adsorbing a lot of moisture. It is said to have a very low vapor pressure.
Resin pellets that contain moisture have a much higher vapor pressure than the heated drying air that is being pushed through the hopper. As the pellets warm up, the resulting imbalance in vapor pressure causes moisture within the pellets to migrate outward, seeking to create an equilibrium vapor pressure with the drying air surrounding them. As the drying process continues over time, more and more moisture is carried out and away from the resin pellets until they are dry enough for processing.