PPP: Why Do I Need A Dryer?
Potters often ask me, “Why do I need a dryer?” or “Why can’t I just use a drying room?” I am not surprised by the questions since drying is almost never mentioned in art classes and is only partially covered in ceramic engineering courses. There are few if any books on drying. Only recently have we been hearing much about the subject.
Potters often think that they do not need a dryer because their ware seems to dry very easily without a commercial dryer. Why invest hard-earned money in a piece of equipment when the air inside the manufacturing facility can do the job much less expensively? Look more closely at your operations, however, and you will see that a number of benefits can be gained from investing in a commercial dryer.
The Benefits of a DryerOne benefit to be gained is space. If you are drying your ware on shelves placed throughout the shop, you’re using a lot of space for drying. That space is expensive and may be needed for other operations as your company grows and expands. Shelves are also expensive and require a lot of labor to move around.
Even if space isn’t an issue for your company, you could undoubtedly benefit from a reduction in product losses. You know that you could be taking home a lot more money if you had no scrap ware. But that doesn’t appear to be a realistic goal. For many potters, warps and cracks are an inherent and accepted part of the drying process—but they don’t have to be. Using a commercial dryer can drastically reduce product losses due to these problems by eliminating the uneven and incomplete drying that causes them. In some cases, loss reductions have been as high as 50-60% compared to air drying.
Eliminating drying defects becomes even more important for the potter making unique or complicated pieces. For example, a 6-ft-high statue, which sells for a significant price, can cost you weeks or months of labor if it fails in drying. A good humidity dryer will allow the piece to be dried evenly without damage in a relatively short time, and will prevent the agony of having to remake the entire statue.
A commercial dryer also provides the ability to change the drying conditions based on a particular clay mixture. Almost everyone has experienced having to change a favorite mix that “won’t dry.” As it turns out, that mix will dry fine in a dryer, where the drying conditions are controlled. You shouldn’t have to change your creation just because adverse conditions outside prevent the ware from drying properly. With the proper humidity and a controlled atmosphere, those pieces will dry evenly and efficiently.
A commercial dryer is also much faster than the methods traditionally used by potters for drying. A conventional humidity dryer, for example, will dry ware completely in 8 to 24 hours, depending on the product’s shape and body mix. For instance, a piece 6 in. in diameter by 6 in. long might typically be covered with a wet towel and stored on a shelf in a drying room for the several weeks required to dry the product. Using a dryer can cut the drying time to three to seven days (depending upon the mix).
People who make tile often dry the tile in a sandwich of plasterboard. This process not only takes up a lot of labor time, but it can also result in latent stresses that show up in firing. The same tile could be placed on a rack, rolled into a humidity dryer and dried overnight. The next morning, bone-dry tiles could be rolled out of the dryer. One tile maker in Michigan, for example, extrudes his tile body then dries the wet, cut pieces in a humidity dryer. The 4 x 4 in. tiles dry easily overnight, while the 12 x 12 tiles are dry in just 24 hours.
Dryers cost money—but they also save money, making it possible for you to recoup your investment. Say you are making product that costs you $200,000 per year to produce. That includes the cost of the body mix, the required labor, the space required for making and drying the product, and the labor required to dry and move the product. If you lose 10% of those pieces, the losses are costing you $20,000 per year. A conventional dryer would cost just a little more than that, and would enable to you to increase your production efficiency, ensure faster deliveries, conserve space in your shop and maximize your profits.
Why Not Just Use A Drying Room?A drying room is generally unsatisfactory because it is so difficult to control all of the major variables: air velocity, temperature and relative humidity. If you do not have control, you do not have a proper dryer.
I have built drying rooms in emergency situations. I once built a four-stage dryer using four offices in an unused section of the manufacturing building. Each room was set to a different temperature and humidity level, and the ware was rolled into the room in carts. The carts were moved each day until they progressed to the final room, where they stayed several days until dry.
Using this drying method, RAM™-pressed 12 x 12 x 3 in. tiles went from 80% loss to less than 20% loss. But this was not a perfect solution. Drying the parts still took about a week, and neither the temperature nor humidity in each room could be accurately controlled. It was also nearly impossible to control the air velocity throughout each room.
Features of a Successful DryerA good dryer will be the shape of a relatively narrow room so that the air does not have far to travel. The most successful dryers have false walls, where the air enters on the side and passes into the chamber through holes or slots and is removed to another plenum behind a false wall on the other side with similar holes or slots.
A successful dryer will have a means of heating the air, but only to the extent necessary, and it will not introduce any outside air except under controlled circumstances. It will expel the wet air, but also only under controlled circumstances. It will have controls to maintain the temperature and relative humidity and to change both of those parameters as time passes according to a profile developed for the particular ware being dried. A good, modern dryer will also allow the operator to store several drying profiles and select the one required for the ware being dried at that time.
As a general rule, dryer temperatures can be relatively low in many applications. While high-speed industrial dryers often operate at much higher temperatures (e.g., 180-190¿F for sanitaryware and 250¿F or more for some structural clay products), many potters do not allow their dryers to rise above 120¿F—the temperature above which clay starts to permanently lose plasticity, even if rewetted. Below this temperature, clay can bounce back and be reworked rather well. Additionally, molds will last much longer if they are dried below 120¿F.
Although these important features can be incorporated into a “room,” it is often much easier and more cost-effective simply to buy a commercially available dryer.
We all have the same goal: to make drying as inexpensive (including the cost of the dryer) and efficient as possible. With this goal in mind, air drying is not a viable option. Instead, consider installing a commercial dryer. In the long run, a commercial dryer will save you product losses, time and money—making it easier for you to run a profitable business.