Why Talk About Dryers?
April 1, 2009
Many manufacturers consider drying a necessary, but simple, task. However, in the current economic climate, it is important to examine every part of the production system in depth. After all, not every product dries the same. Years ago, we were taught that to build a dryer all one needed was a source of heat and a box to contain the product. Later, our understanding of drying became more sophisticated as we learned that some products warped or cracked easily, while others could withstand abuse without damage.
For example, most manufacturers using plaster molds know that if they heat those molds above 125°F during drying (or any time, really) they will start to recrystallize. Others have not known or might have forgotten that any clay-based ceramic will lose its plasticity if heated above 120°F. This may not matter to some manufacturers, but if product is reclaimed after drying, it could be very important.
Material ConsiderationsSome thin products can be dried very quickly with no harm. Other products might be able to dry quickly but will warp if not treated properly. For the purposes of this discussion, we will consider the following materials: plaster, cement, ceramic fiber, clay-based ceramics and non-traditional ceramics.
As mentioned previously, temperature must be considered when drying plaster. However, plaster does not have as much of a tendency to crack as traditional ceramics. Since plaster molds are porous, the water can move from the interior rather well. In order to dry more quickly, it is important to keep the surface from getting too dry. The moisture in the center of the mold will migrate to the surface faster if there is a moist connection to the surface.
The fastest drying is therefore not accomplished by zipping the water off the surface. It is better to remove the water as quickly as possible without allowing the surface to dry. By the time the surface dries, the center will also be dry (or nearly so). This is an important consideration because the mold cannot be subjected to high temperatures (due to the possibility of recrystallization).
Cement does not dry in the traditional way; it is actually cured. The reason it is being considered here is because the curing process is very similar to an effective drying process-humidity must be kept quite high. Temperature is less of a concern because the product will, in many cases, generate enough heat on its own. However, in large spaces, such as in a block plant, it is sometimes necessary to supplement the heat with an outside source. In every case where outside heat is provided, it should be low-humidity heat.
When curing cement, a source of electric heat can be used with a humidity generator. Curing is often done in a large room so that heat and humidity sources are located some distance from part of the load. In these cases, it is wise to provide a source that can project the heat and humidity evenly, and with some force, so it is experienced consistently by the ware throughout the room. Since the curing process needs to be kept gentle, it is typically better to avoid large, high-velocity air flows. Humidity control is essential, however.
Ceramic fiber boards and preforms are not as sensitive to warping as clay-based ceramics, but that does not mean they are totally immune. As with other drying situations, attention to relative humidity and temperature is required. As long as the air is applied evenly, a higher-velocity air can be used. This is helpful because many fiber products include a large amount of water that needs to be removed.
If humidity control is properly used, a portion of the circulating air is removed from the chamber with each pass, which expels the removed water. The circulating air humidity is controlled by adding fresh, dry air at the same rate at which wet air is removed. The percentage of air removed varies throughout the drying cycle. As the air becomes more dry after passing over the ware (due the amount of water already having been removed), less is replaced with fresh air.
During some seasons of the year, the climate is such that a certain amount of heat must be applied to the incoming air to keep the relative humidity within the control range. Although this sounds like a problem, it can be controlled automatically with the right equipment.
Virtually everything mentioned previously also applies to clay-based ceramics. The material’s surface should not be allowed to dry out prematurely, and the humidity level should be high in the beginning and lower after shrinkage has stopped. The moving air should also be relatively slow and applied evenly over the ware so that the outside edges are dried at the same rate. The thicker the product, the more gently the moist air must travel over the ware.
It is also important to bear in mind that a dense load is much harder to dry because the air reaching the inner pieces is usually filled with the moisture it has gathered over the first pieces encountered. It is always better to try to break up the load into sections where the air is applied and withdrawn over short spaces. In that manner, even very large areas can be treated as if they were very small. Figure 1 shows a larger dryer that has been broken into sections. The division of product does not interfere with the setting or the size of the dryer.
In terms of drying, there is very little difference between traditional and non-traditional ceramics. In either case, the point at which the body stops shrinking is the point at which faster drying is safer. The difference is that this point can be achieved sooner in some non-traditional ceramics. However, the need for uniformity and humidity control is just as vital.
It is also important to treat very large pieces gently, meaning the surface should not be allowed to completely dry until enough time has passed to allow moisture from the center of the body to migrate to the surface. It is not easy to find the point at which shrinkage stops or the point at which the moisture has migrated sufficiently to start harder drying. The best results are obtained with a little experimentation; the results can usually be applied to different sizes fairly easily.
EnergyThe cost of energy has caused a lot of manufacturers to rethink drying temperatures. When manufacturers use waste heat from other processes as the source for drying energy, the temperatures tend to be relatively low due to both the constraints of the temperature in the heat source and the need to prevent ware loss. Although this process has proven to be very effective, a lot of manufacturers think it is too slow.
In order to understand the process, we need to examine what makes drying take place. Drying is a complicated process with several influencing factors. For the purposes of this article, we will concentrate on the “pool equation.” The speed with which water will leave any surface (whether a pool of water, a wet ceramic or even a piece of paper) can be defined by the following equation:
Lbs/sq ft/hour = .192 k(1-V/250)(w0 - wRH)
where k is a factor that relates to the product being dried; V is the velocity of perpendicular air in feet per minute; w0 is the partial pressure of water in air at saturation at a given temperature, in inches mercury; and wRH is the partial pressure of water in air at the given relative humidity (%) at the same temperature as above, in inches mercury.
The equation shows that, while increasing air velocity has an effect on drying, reducing the relative humidity of the air has a much greater impact. Temperature’s role increases the effect of low relative humidity.
For most clay-based ceramics, the effect of lowering the relative humidity will yield a very strong drying rate without damaging the product. On the other had, raising the temperature can cause the product to dry too quickly on the exposed surfaces, leading to cracking and/or warping. It is therefore better to dry with a fixed air velocity and vary the relative humidity. The temperature can be raised later in the drying cycle, after the shrinkage water has been removed. Products with a higher resistance to warping can be made to dry more quickly through the use of high-velocity air at a fixed temperature, along with a control mechanism for relative humidity.
Drying TimeA ceramic material can typically be dried more quickly if the process begins with a higher relative humidity in order to retard the drying rate. Drying too quickly can cause the product to warp or crack. In addition, as previously mentioned, if drying is so fast that the film of water on the surface of the ware is removed, it will actually slow down the transport of water from the center to the surface.
It is important to try to find the right balance, to dry as quickly as possible without removing that surface water or causing a defect. Drying a little more slowly in the beginning actually allows that crucial center-of-load water to get out faster. In the long run, it enables manufacturers to speed up drying overall-with less energy and fewer losses-to increase productivity and reduce costs.
HandlingManufacturers often have a tendency to place more load on moveable carts. The problem with this practice is that too much load in every direction causes the moving air to take longer to get to different parts of the load, which in turn causes a slower drying cycle or increased losses.
Loads should be arranged so the air travels only a short distance and reaches all of the ware evenly. It is quite possible to arrange the load in the dryer the same way it is arranged in the kiln, because the drying air affects the speed and uniformity of the operation just as the hot kiln gases do. Once this is understood, it is possible to achieve excellent results.
LossesMany smaller manufacturers do not appreciate the value of losses the same way that larger companies do. The aggregate loss of only 2% of fired product has been enough to justify the cost of replacing the equipment involved for a large manufacturer due to the large annual cost. Therefore, even in drying, the cost of losses should not be ignored. Fortunately, the best drying practices involve less heat and horsepower while improving productivity and reducing losses.
In many cases, significant changes can be made with a relatively small effort. For example, one manufacturer rolled their ceramic into thin pieces that were then hand-cut. The company found that they could not transport the pieces from the cutting board because they were still wet and very flimsy. The company also lost up to 80% of the pieces on days with bad weather or through warping due to the speed of the dryer. Once the pieces were placed into a controlled chamber (where the air velocity was low, the temperature was only about 80 or 90°F, and the outside air was conditioned before moving across the pieces), the losses dropped below 10%.
Controlled DryingMany manufacturers find that it is possible to take pieces that experience a loss rate and actually get them to dry more quickly with drastically lower losses once they are placed into a closed chamber with controlled air velocity, temperature and relative humidity. In every case, it’s the combination of control and separation from the outside world that makes the difference.
For more information regarding drying, contact Ceramic Services, Inc., 1060 Park Ave., Bensalem, PA 19020; (215) 245-4040; fax (215) 638-1812; e-mail firstname.lastname@example.org; or visit www.kilnman.com.