“If you want to know why your family does something differently than other families, ask your grandparents.” I heard that statement quite a few years ago, and it was related to the family turkey. One Christmas, a woman who was hosting her first family Christmas celebration asked her mother why she made an extra cut off the top of the turkey before putting it into the roasting pan. “I don’t know, my mother always did it,” was the reply. When grandma arrived for the Christmas meal, her granddaughter asked her why she had always done it that way. The response was, “The pan was too small to fit the whole bird. I had to trim away a little bit of it so it would fit.”

Is that how your converting application is these days? Are you using technology “because that’s how we’ve always done it”? Worse yet, are you using methods that create waste and inefficiency because simply because you’ve never thought there might be a more efficient, less wasteful way?

Enlarge this picture Figure 2. The chart illustrates the difference between the ability of a flotation dryer and a roll-support dryer to impart heat energy into the web. In this example, the web is being dried in a 20' long dryer at a speed of 300 ft/min, using a supply temperature of 350oF (177oC), and a nozzle velocity of 8,500 ft/min.

Flotation dryers have been around for many years and have been used in many different industries. From printing to adhesive tapes, pressure-sensitive label stock to reverse-osmosis filtration media, the industries using flotation dryers can be quite extensive. There are many advantages to using this technology, yet there are many converters who are still using roll-support dryers. In many cases, the sole reason for this has been history. Another common reason is related to a bad experience with an older flotation dryer. But the reasons to change to a flotation dryer are many.

In a flotation dryer, the web floats through the dryer on a heated cushion of air that is delivered from opposing air bars (figure 1). Several things occur to the web as it passes through the air-bar array. The first, and most obvious, is that the heat emitted from the air bars is transmitted to the web itself. Initially, this is done to increase the web temperature so the evaporation process can begin. As the web reaches the process temperature and the solvent begins to evaporate, the direct air impingement on the web works to scrub away the boundary layer of air on the web surface. This air also serves to carry away the solvent and enables the drying process to continue quite rapidly.

Another benefit of the opposing air bars is the form that the web takes: a sinusoidal web path. This is an important feature as it creates additional stiffness in the web. For example, if you take a regular sheet of paper, you will see that there is very little dimensional stability -- you can hold the paper by each end, but the unsupported sides will droop down. However, if you put a slight bend into the web and create a sine wave with the paper, this “droop” goes away. The web becomes more rigid. This added stability is beneficial with substrates that may curl or flutter during the drying process.

Noncontacting flotation drying enables the printer or converter to print or coat both sides of the web simultaneously. Because two-sided processing eliminates a coat/dry/coat/dry process, it can save time, energy and material as well as reduce scrap.

The noncontact nature of flotation drying also reduces the amount of cleaning required. If rolls are used, they can become dirty or accumulate coatings easily, which can lead to defects in the web. Because the web does not touch the air bars in a flotation dryer, cleaning can be performed less frequently.

Flotation drying particularly lends itself to use with substrates used in many of today’s applications. Materials and products such as specialty films and nonwovens that did not exist 20 years ago are now commonplace. If a substrate is susceptible to scratching and defects, noncontact drying via a flotation unit makes sense to avoid defects.

Enlarge this picture Figure 3. In a poorly designed flotation dryer, suction can cause the air and even the web to shift to one side. Good dryer and exhaust design can mitigate this effect.

Flotation dryers also may be able to impart more heat energy into the web than roll-support dryers. Because both sides of the web are heated simultaneously in the flotation dryer, the web temperature can increase more rapidly than with the single-sided heating of a roll-support dryer. This can result in a faster drying rate, as noted by the web temperature curve illustrated in figure 2. In this example, the web is being dried in a 20' long dryer at a speed of 300 ft/min, using a supply temperature of 350^oF (177^oC), and a nozzle velocity of 8,500 ft/min.

When the web enters the dryer, the temperature of the material begins to rise. In this case, the web temperature is approximately 10^oF (5.5^oC) higher in the flotation dryer than the roll-support dryer as the drying curve levels off. The rapid rise in temperature just past the halfway point in the dryer zone indicates that most of the solvent has been removed from the coating, and the evaporative cooling that was keeping the web at a constant temperature is no longer taking place. This indicates that the web is nearly dry -- in this case, just over 95 percent.

With the roll-support example dryer in the example, the drying curve levels off at essentially the same time as the flotation dryer. However, the single-sided heating from the roll-support dryer produces in a lower rate of heat transfer. As a result, the web material just starts to increase in temperature (reaches 95 percent dry) as it comes to the end of the dryer zone, later than it would in the example flotation dryer.

Putting It to Work

Enlarge this picture Figure 4. A well-designed flotation dryer with properly designed exhaust ductwork ensures that the flow of the air being exhausted is perpendicular to the web itself, minimizing the risk of web shift.

One concern sometimes raised about flotation dryers is proper web control. Some ask, without any rolls contacting the web, what keeps the web from weaving or shifting to one side of the dryer when it is inside the dryer zones? The answer to this question is a simple one: good design.

In situations where the web shifts, it typically shifts towards the gear side, where the exhaust fan is located. Air is a medium that will take the path of least resistance, and if there is more suction on one side, the air will naturally flow in that direction (figure 3). As the air is pulled across the dryer box, it tends to also pull the web toward that side of the dryer. In addition to problems with web handling, a web shift such as this also can lead to uneven drying. Because the air is pulled from one side toward the other, the web actually is exposed to more heating energy on the gear side of the web. This uneven drying can lead to costly product defects, or it may result in operating the process at a lower production rate to offset this problem. Effective dryer design can mitigate these potential outcomes.

A properly designed exhaust will ensure that the flow of the air being exhausted will be more perpendicular to the web itself. By doing this, web weave or shift can be avoided, and even drying will take place. Figure 4 illustrates how a properly designed exhaust ductwork will accomplish these results.

When considering a new production line, keep an open mind with your drying choice. If you have been using flotation, make sure the air bars being used suit your specific needs. If you have not tried flotation drying, it may be worthwhile for your application. Flotation drying can provide many returns to your process: noncontact drying; the ability to run different substrates without concerns of scratching; higher efficiencies; minimal maintenance; and process flexibility through the use of different types of air bars. If your process uses a roll-support dryer simply because that’s how “it’s always been done,” maybe it is time to take a new look at your pan and how you cook your turkey.