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Gaining a real understanding of the drying process requires careful observation and measurement of the effects of drying on a ceramic body.
Understanding the drying process can help manufacturers solve drying problems and develop faster drying schedules. But since each ceramic body dries differently, gaining a real understanding of drying requires both observation and measurement of the effects of the drying process on a given ceramic body.
As an extruded or cast ceramic body with 15 to 20% water dries, the body loses water and shrinks. It is relatively easy to measure the dimensions of the shape with a ruler and to measure its weight with a scale. These manual measurements can be plotted as a function of time. The resulting curve would show that the body ceases shrinking at some point while it continues to lose weight. Eventually the body ceases losing weight and is dry.
A simple drying test can be performed by placing a wet sample into a heated chamber and measuring its dimensions and weight at certain intervals as the temperature of the chamber is changed to simulate a production drying schedule. Every time a data point is taken, the sample must be removed from the dryer, measured and returned to the dryer. Figure 1 is a graph of such a test using a wet, extruded structural clay brick. Note how the shrinkage progresses quickly and ceases while the weight continues to drop, as predicted by drying theory.
Figure 2 is another simple drying test of another wet, extruded, structural clay brick. This time the initial chamber temperature was higher to accelerate the schedule, and the brick began cracking almost immediately.
From drying theory, we know that the temperature, the percent relative humidity and the air velocity (speed and direction) all influence the rate of drying. These conditions are present in every drying chamher, and should be recorded every time the dimensions and weight are measured. However, not every heated chamber can monitor the percent relative humidity and air velocity, so these critically important factors may not be known.
The ability to change and control the factors that influence drying rates is helpful in studying, characterizing and optimizing drying cycles. Many conventional heating chambers or dryers cannot alter or control...