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MO's corner: IN THIS EDITION OF MO'S CORNER - OUR REGULAR FEATURE THAT AIMS TO ANSWER COMMONLY ASKED QUESTIONS ABOUT THE MANUFACTURE AND PROCESSING OF PLASTICS - WE LOOK AT THE RELATIONSHIP BETWEEN DEW POINT AND (RESIDUAL) MOISTURE.

Q: HOW IMPORTANT IS THE DEW POINT WHEN DRYING PLASTICS?

The meaning of 'dew point' has already been covered in previous editions of BP&R, however, owing to the many misconceptions associated with it, this month's column will take a more in-depth look.

A short recap: Four parameters are important when drying plastics: drying temperature, dry air flow, dew point temperature, and drying time. These four parameters need to be coordinated with each other, as each one influences the drying process and the result. This means that if one parameter is changed, then the rest also need to be adjusted in order to achieve the same degree of dryness.

Before we look at the dew point in detail, we must be aware that firstly, the higher the temperature of the air, the more water (vapour) can be absorbed by it; secondly, the maximum water content of the air therefore depends on the temperature; and, finally, the relationship of actual water in the air ([m.sub.H2O] in g/[m.sup.3]) to maximum possible water content at this temperature ([m.sub.max] (T)) is called relative moisture content:

F = [m.sub.H2O]/[m.sup.max.sub.H2O](T)

If the air cools, it can hold less water. This means that with a decreasing [m.sub.max] the relative moisture of the air increases (see above formula). If the relative moisture content reaches 100 percent due to further cooling, the air can no longer hold any water and water droplets condensate (dew, fog, rain). The temperature at which this occurs is called the dew point. At 100 percent air humidity, air temperature and dew point temperature have the same value. If you then reheat this air, its dew point--and therefore also the absolute moisture content--stays constant, the relative moisture content decreases.

The dew point, or to be more precise the dew point temperature, is therefore a way to measure the absolute moisture content of the air (g of water vapour per [m.sup.3] of air). The dew point temperature is specified in [degrees]C. It is important to bear in mind that the dew point temperature as a measure of air moisture content is not an actual temperature. Instead, it is the temperature at which the air would have a moisture saturation of 100 percent. Therefore, each dew point temperature also has a corresponding specific water content.

Everyone knows the phenomenon of cold glasses steaming up in warm rooms. We therefore also know, that the temperature of the glasses must in this case be equal to or below the dew point temperature of the air in the room. The water condensates on surfaces with temperatures which are below that of the dew point temperature (dew formation).

In summary, the lower the water vapour content of the air, the lower its dew point temperature. Or, the dryer the air, the lower the dew point, or better the dew point temperature. The following two values help to better understand the maximum possible absorption of water by air: for air with a temperature of 100[degrees]C it is 600g/m3, at 0[degrees]C only 4.8g/m3.

For hot air drying, the dew point varies depending on the current climate factors of the environment, whereas for dry air drying the dew point is maintained constantly within predefined boundaries independent of weather. However, it is important to remember that very low dew point temperatures require very large amounts of energy for air conditioning (drying of the air).

For the drying of moist hygroscopic plastics, a dew point temperature of approx. -20[degrees]C is completely sufficient. At a constant drying temperature, different dew point temperatures have been proven to have only little impact on the drying time, whereas they determine together with the drying time the minimal possible residual moisture content (balance between granule moisture and dry air). These relationships will be explained in more detail in the next issue of Mo's Corner.

FURTHER QUESTIONS?

EMAIL: ASKMO@MOSCORNER.DE
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Title Annotation:MACHINERY / NEWS
Comment:MO's corner: IN THIS EDITION OF MO'S CORNER - OUR REGULAR FEATURE THAT AIMS TO ANSWER COMMONLY ASKED QUESTIONS ABOUT THE MANUFACTURE AND PROCESSING OF PLASTICS - WE LOOK AT THE RELATIONSHIP BETWEEN DEW POINT AND (RESIDUAL) MOISTURE.(MACHINERY / NEWS)
Publication:British Plastics & Rubber
Date:May 1, 2017
Words:667
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