The air temperature also plays a role—a higher air temperature typically lowers the absolute humidity. In many successful dryers, the air temperature is held to a maximum of 90∞F in the winter and 100∞F in the summer. However, in some applications, the temperature must be higher to drive the relative humidity down at the end of the drying cycle or to penetrate a heavy load. In general, the temperature should be as low as possible (generally about 20∞ above room temperature) to avoid wasting energy while still permitting efficient, uniform drying.
The temperature of the part can also affect drying by changing the surface tension of the water within the part. Higher temperatures reduce the water’s surface tension, enabling the internal or residual water to move more easily through the small pores inside the ware. Although this can be very important in some cases (e.g., very thick pieces with tightly compacted bodies made through a wet process), it is not a factor in most applications.
Heating, ventilating and air conditioning (HVAC) engineers have developed equations to help determine the rate of evaporation in a pool room, and these same equations can be used to determine the maximum rate at which water can be removed from a ceramic product. (See the “Calculating Drying Time in a Fixed Air Dryer” sidebar.) Clearly, water should not be evaporated from a ceramic product at its maximum rate to avoid damaging the piece. But knowing the outer limits of drying can help you choose the right dryer and avoid using too much energy for extra heat or higher fan speeds.
Many different types of successful dryers exist, and there is no one-size-fits-all solution. However, by understanding how the drying process works, you can draw some general conclusions about the type of drying system to use for a given application. For parts that do not shrink much while drying or are physically strong, high-velocity air can be very useful. However, be sure that the air is directed uniformly throughout the dryer or you will not get even drying.
For parts that have high clay content or otherwise must be treated gently, the low-temperature, high-humidity, low-air-velocity approach is generally preferred. Some brick dryers use moving fans to generate a relatively uniform distribution of high-velocity air over the ware. The brick are strong and can take the force of the air without cracking or breaking, but the removal of water from the surface makes drying the interior more difficult.
Maintaining a wet surface helps create a water “chain” inside the part, and this chain helps the water migrate from the inside of the product to the surface through osmosis. With high temperatures and a dry surface, the water must evaporate inside the piece before it can reach the surface. For this reason, high-humidity, low-velocity dryers can often provide much faster drying.
Water removed = .192 *k* ((1+ Velocity (ft/min))/200) * (w0 – wRH)
where k represents a constant that varies with the product and the system, w0 is the amount of water in the air when it is saturated at the given air temperature, and wRH is the amount of water in the air at the given temperature and the percent relative humidity
If you assume that the constant (k) doesn’t vary in a particular dryer when drying the same materials of similar shape, and that the velocity is also fixed, then the variation in drying is directly proportional to the formula (w0 – wRH). If you know the temperature and relative humidity, you can calculate a number that gives the relative amount of drying time using the same formula.
For example, suppose you have a velocity of 100 ft per minute, a constant (k) of 1 and a water saturation level (w0) of 3. At 50 percent relative humidity (wRH =1.5), the water removal rate would be 0.432 lbs of water per sq ft of surface area per hour. If you have to remove a total of 2 lbs of water per sq ft, the total drying time might be 2/0.432, or 4.6 hours.
If, however, the relative humidity changed to 70 percent, then the amount of rater removed per square foot per hour would be 0.2592, and the relative drying time would increase to 7.7 hours.
A major china manufacturer processes bowls with an automatic wet processing system, similar to jiggering. The bowls, which contain a high percentage of clay, were previously dried in six hours using several large box dryers. The company wanted to add a continuous dryer to its operation to reduce handling requirements (and thereby labor costs), but it had only 15 ft of space available in its plant. Installing a new dryer that would adequately dry the bowls in such a small amount of space would require the company to increase the speed and efficiency of its drying operation.
The company chose a tightly controlled, high-humidity, low-velocity air dryer to achieve its goals. The bowls are set on the tunnel kiln cars and are automatically sent to the 15-ft-long dryer, where their moisture content drops from 17 percent to the required 3 or 4 percent in just three hours. The kiln cars then automatically move to the tunnel kiln without intervention by an operator.
Reducing Product Losses
A manufacturer of grinding wheels uses several large, room-type dryers that were converted from ovens. The wheels contain very little clay and only about 5 percent binder in the mix. For the most part the company's dryers worked well, but some of the particularly large wheels (36-in.-diameter by 6-in.-thick) often cracked during drying or firing, leading to high product losses.
The company converted its drying ovens to a tightly controlled, high-humidity, low-velocity design and was able to significantly reduce its losses. The savings from higher recovery rates paid for the conversion costs in a very short period of time.
Improving Speed in a Periodic Dryer
A manufacturer of high-tech ceramic products extrudes its greenware in 4-in.-diameter by 24-in.-long “logs,” which must be thoroughly dried before they can be machined into the final components. The ceramic body is composed of about 10 percent clay. The plant’s previous drying method was to set the logs on trays to air dry in a drying room, but this process took about six weeks—or longer, depending on the temperature and humidity in the outdoor environment—for the pieces to be dry enough to machine.
The company recently installed a cart-style air dryer that dries the logs in just three days. The logs are still placed on trays, but these trays are now positioned on a moveable rack that is rolled into the dryer. The dryer is sealed from the outside and follows a computer-controlled program for temperature and relative humidity. The program has been adjusted to optimize drying for even the largest logs and now dries the same way every time, regardless of outside climate conditions.
In addition to improving the company’s drying operation, the new dryer has also reduced storage requirements, allowing the rack area previously used for drying to be used for expanded manufacturing equipment.