Dryers are used to dry hygroscopic materials such as PA (Nylon), PC and PET. Drying these materials is necessary to maintain certain properties. Moisture creates air bubbles in the product. This is because water boils at 100 degrees and this creates vapor which is trapped inside the product. A dryer works with a dew point. A dew point is expressed in degrees. The lower the dew point, the lower the humidity. The low humidity is necessary so that the hygroscopic material can return the moisture to the air. This requires a dew point of at least -18 ° C. A frequently asked question is whether a dew point of -40 ° C is twice as good as a dew point of -20 ° C. The answer to this is short and simple, no it is not.
Relationship between dew point and humidity
The relationship between dew point and air humidity is not linear. Richard Mollier, a German professor of applied physics and mechanics, was a pioneer in the research of thermodynamics, in particular water, steam and humid air. He also investigated the relationship between dew point and humidity and presented it in the graph below.
Dew point less than -20°C not needed
What is clear from this graph is that a dew point of -40 ° C is not twice as good as a dew point of -20 ° C. The absolute zero of the moisture content starts at -273 ° C. The gradient is almost linear up to -20 ° C, the curve starts from -20 ° C (see black line Mollier diagram). At -20 ° C, the moisture content is about 0.75 g / kg, which means that each degree of temperature difference corresponds to a difference in moisture content of about 0.003 g / kg. The moisture content at -40 ° C is approximately 0.69 g / kg. The difference is only 0.06 g / kg. For many materials it is not necessary to dry with a dew point below -20 ° C. It takes a lot of energy to get a lower dew point but ultimately has a minimal impact on the end product.
Different types of dryers
Various dryers for materials such as PET, PA or PC are available on the market. The most commonly used dryers are central dryers, compressed air dryers and honeycumb dryers. Central dryers are dryers in which a drying unit is linked to one or more hoppers. The drying unit has two or sometimes three drying towers with drying material. The dryer starts drying at a set dew point value, often well below -20 ° C, and then slowly rises to a set dew point that is often above -20 ° C. The dew point gradient is therefore not constant and a lot of energy is consumed without any added value. An example is outlined in the graph below:
The honeycumb dryers work with a rotating disc, regenerating, cooling and absorbing moisture at the same time.
Because these processes run simultaneously, a very constant dew point is created and regeneration can be carried out at a much lower temperature. The dryers therefore consume a lot less energy. This type of dryer also has a drying unit per hopper so that contamination no longer occurs. In addition, these dryers are easier to move because it only requires power.
The big disadvantage of dryers with drying towers is that material is sometimes treated differently than other times. After all, the method of treatment depends on when it ends up in the hopper. Another important drawback is that contamination occurs, because the air is often divided over several hoppers, which means that dust particles become mixed together. The drying material can also cause contamination. The dust can also cause smearing in the dryer. When the drying unit needs maintenance or has already come to a standstill, all hoppers no longer work because they depend on the same drying unit. Finally, energy is lost during the transport of the dry air from the drying unit to the drying hopper and it takes a lot of energy to regenerate the drying material.
In addition to the dryers with drying towers, there are also compressed air dryers. These dryers have a very constant dew point and these dryers are especially suitable for small amounts of drying. The disadvantage of compressed air dryers, however, is that they must have compressed air. When compressed air is not “over” it takes a lot of energy and therefore money to create compressed air.