The low-down on dryer airflow

Drying, it might be said, is the poor stepchild of the molding process. The fact is, the world of drying resin is a fascinating one, deserving of at least half the attention you pay to the press itself. As Conair's product manager for dehumidifying dryers Pete Stoughton says, "If you don't properly dry the material, you're dead in the water. You might as well turn off the lights and go home."

Take, for instance, the issue of dryer airflow. This is the rate at which dry, warm air passes through the hopper to dry the pellets inside. As simple as it may seem, the concept is actually more complicated than it sounds, and one on which many dryer manufacturers don't fully agree. While such a friendly debate can be healthy for any industry, molders should be particularly attentive as it may make them more savvy when it comes to purchasing a drying system. To sort out the details of this debate, IMM spoke with several of the molding industry's manufacturers of resin drying systems to get their perspectives and philosophies.

The Basics

Before launching into an explanation of dryer airflow, some basics of the drying process itself might be helpful. The drying process has four basic parameters that help produce dry, warm pellets. The first, and most important, is temperature. The temperature controls the rate at which the resin pellets come to equilibrium with the surrounding air. It's generally accepted that the higher the temperature, the faster the polymer dries. There are, of course, limits, as some materials can be overdried, damaging the resin or causing premature melting.

Second on the list is dewpoint, a measure of humidity in the drying air. It tells you how well the dryer is taking moisture out of the air. It dictates how readily water vapor in and around the polymer will vaporize into the ambient air.

The third parameter is drying time, sometimes referred to as residence time. This is the time during which the polymer is exposed to the drying air at the prescribed temperature and dewpoint. Because plastic pellets do not conduct heat well, sufficient time must be allowed for each pellet to reach temperature. Once properly heated, moisture is released to migrate to the surface of the pellet where the warm, dry air removes it from the hopper.

The fourth parameter is airflow. While most molders are generally familiar with temperature, dewpoint, and drying time, the function of airflow is often misunderstood. Simply stated, airflow is the rate at which a volume of air passes through the hopper. It brings heated air from the dryer into the hopper and takes moisture out of the hopper. This number is expressed in cubic feet/minute, referred to in shorthand as "cfm." Airflow is usually calculated relative to resin throughput (lb/hour), but airflow is not dependent on resin throughput.

The Airflow Predicament

The healthy debate among some dryer manufacturers centers on this question: What's the best airflow rate to use? Traditionally-and for many manufacturers this still holds true-the airflow benchmark has been 1 cfm per pound of polymer processed per hour. For instance, a press that consumes 50 lb/hour of resin would require a dryer that can move 50 cfm of air through the hopper. This 1:1 ratio is handy, easy to calculate, and backloaded with historical precedent and tradition. Some manufacturers, however, contend that 1 cfm is energy-wasting overkill, that dryer technology has progressed such that airflow less than 1 cfm is adequate.

Chuck Thiele, president of Motan (Plainwell, MI), is one of those people. "If you go back 25 years, everything was done at 1 cfm. But that's history," he says. "To stay at 1 cfm is to say that the technology of drying hasn't improved at all. Every supplier has improved his bed switching valve seal designs, temperature controls, drying hopper design, and dewpoint. Additionally, there are considerably better air distribution systems. These advances must result in an improvement in overall efficiency or they would not have been made." Thiele says Motan builds its drying systems based strictly on the type of resin being used and the rate at which it is consumed. Based on materials data collected by Motan, he says that often a material can be dried well at .5 to 1 cfm, with some higher exceptions for extremely hygroscopic polymers.

Conair's Stoughton agrees. "Airflow," he says, "is just a means to transfer heat and a means to move moisture." He believes airflow should be "based purely on that material at that temperature at the required residence time." For Conair systems, Stoughton says airflow through the hopper averages out to .7 to .8 cfm/lb. He adds that a well-designed and insulated hopper with insulated hoses could approach .5 cfm/lb, depending on the material. Stoughton contends that good, well-maintained insulation is critical to reducing airflow.

On the other side of the fence is a range of companies, including AEC/Whitlock, Novatec, Comet, Thoreson McCosh, Dri-Air, and Universal Dynamics among others. Although the philosophy of each company varies slightly, many give the same explanation as Joseph Dziedzic, manager of product and system development for AEC's Whitlock Product Group (Wood Dale, IL). "Material tests have shown that dedicated processes can work well with dryers supplying less than 1 cfm/lb of material to the process," he says. "However, if you provide a dryer that is rated to supply 1 cfm/lb of material, then you have more versatility." That versatility, OEMs say, allows molders to adequately dry many of the most commonly used resins.

Tom Rajkovich, president of Comet Automation (Dayton, OH) is a follower of the 1-cfm philosophy and points out that a new dryer in fact can dry well at less than 1 cfm. But over time, he notes, a dryer is notoriously ignored when it comes to maintenance. Eventually seals and parts fail, undermining the efficiency of the dryer. Then, that 1 cfm of airflow is needed.

Jerry Muntz, vice president at Thoreson McCosh (Troy, MI), says that although airflow needs vary with the material, ultimately he likes to deliver a system that he is certain will always dry adequately. "Our philosophy is that we should eliminate variables for our customers," he says. "Using that 1-cfm guideline eliminates that variable."

Jim Smith, national sales manager at Novatec (Baltimore), points out how complicated the picture becomes when regrind flake and exotic engineering materials are thrown into the mix. Regrind flake, irregularly shaped, often takes longer to dry than virgin pellets. Engineering materials, on the other hand, may require a combination of greater airflow and increased residence time. "With all the engineering materials around, it's hard even for us to keep up on all of them."

Somewhat straddling the fence is Universal Dynamics (Woodbridge, VA). Ron Bankos, regional sales manager, says Universal Dynamics typically uses 1 cfm as a starting point, but rarely hits it because of unique customer needs. He points out that drying is a heavily material-dependent process; material type, rate of use, hopper size, hopper type, and other factors can have a big impact on the drying system design and airflow. In the end, though, he says, "Having more airflow in most cases will not hurt you."

At Dri-Air Industries (East Windsor, CT) president Charles Sears says he prefers the 1-cfm standard as it allows him to design and build a system that dries the largest percentage of materials that it might conceivably run. "I think we all try to make it easy for the customer to size a unit," he says.

Back at Motan, Thiele points out that while there may be disagreement as to the rate of airflow, most dryers on the market today do dry polymer adequately. "To be honest, a customer just wants a dryer that works," he says. "They couldn't give a flip about airflow."

The ultimate question, Thiele says, is this: Are molders willing to spend the money to heat what he contends is extra air? "The real issue here is energy, space, and capital investment," he says. "Dryers tend to suck up a lot of energy." Stoughton agrees. When you have two dryers side-by-side, drying equally well, he says, the question becomes one of energy use. Thiele cites his data that show that a dryer rated at .75 cfm/lb to dry polycarbonate at a rate of 200 lb/hour can save up to $750 a year in energy costs over a unit that is rated at 1 cfm/lb.

AEC/Whitlock's Dziedzic says energy use is more complicated than a difference in airflow. He contends that increased airflow does not necessarily lead to higher energy costs; other variables come into play that may cause energy costs to be higher or lower, despite airflow. "Energy costs," he says, "depend on the design and control of all of the components in the drying process, not just airflow."

Rating Airflow

All of this airflow talk is further complicated by this fact: Some manufacturers rate their dryers using free-flow air, while others rate them on airflow under load. The significance is that a dryer rated to move 200 cfm of unrestricted air, for example, will move less than that when pellets are in the hopper, restricting airflow. How much the airflow is reduced under load depends on the density of the resin, the type of blower used to move the air, and the design of the air distribution system.

Virgin material will cause the drying system to generate a pressure drop across the hopper of 6 to 10 inches of water column, says Stoughton. Regrind flake often presents a more dense geometry that can cause the drying system to generate a pressure drop across the hopper of up to 15 inches of column water. Stoughton also reports that Conair tests show fan-type centrifugal blowers are more sensitive to backpressure, while regenerative blowers are less sensitive. Air pressure is also lost to hoses, pipes, and valves between the dryer and the drying hopper.

Universal Dynamics and Novatec dryers are rated based on free-flow air, as are AEC/Whitlock's mid-range (90 to 225 cfm) dryers. Dri-Air, Motan, Conair, Thoreson McCosh, and Comet dryers, as well as AEC/Whitlock's line of small and large dryers, are rated using restricted airflow. By early 1998, Dziedzic says all of AEC/Whitlock's dryers will be rated under load.

Opinions as to the value of rating a dryer under load vary. Novatec's Smith says, "It's somewhat difficult to rate them under load because pellet configuration may change." Universal Dynamics' Bankos points out that a dryer can be used with differently sized hoppers, varying the throughput capability, making rating under load complicated. Although Dri-Air rates its dryers under load, Sears says, "There's really very little resistance by pellets in a hopper." Data from his tests show that 5 percent of total load backpressure is attributable to the resin itself, and that hoses, valves, and desiccant are bigger factors.

Motan's Thiele says, "If you rate your dryer at free-flow air, without a maximum restriction, you might have a 100-cfm dryer that only produces 90 cfm." AEC/Whitlock's Dziedzic says he thinks a rating under load provides a more accurate representation of the dryer's true capability.

What to Do, What to Do

As Thiele says, all any molder wants to do is dry pellets. While measuring temperature, dewpoint, and residence time is easily done, there is no inexpensive and simple way to measure airflow through the hopper. It's just not easy to know how much air is flowing through the pellets. The tendency of most molders is to assume that the dryer is doing the job. Usually it's not until molded parts start to show the symptoms of too much or too little moisture that the drying system is graced with attention-usually negative. Thoreson McCosh's Muntz says ideally a dryer would have continuous moisture analysis online. But right now, such a system is not feasible, practical, or affordable.

The next best thing, say Muntz and Conair's Stoughton, is a little vigilance and attentiveness. Both recommend monitoring the temperature profile through the hopper. Airflow that is too low carries less heat into the hopper and produces a higher gradient-that is, higher temperatures where the hot air enters the hopper, but cooler temperatures where the air exits the hopper. Adequate airflow carries more heat into the hopper and produces a lower temperature gradient, or more uniform temperatures throughout the hopper. Temperature sensors in the vertical space of the hopper, Muntz and Stoughton say, provide the best practical measure of airflow.

AEC/Whitlock's Dziedzic warns molders, however, that "it's more than just airflow that governs that temperature gradient." He points out that the design of the dryer and hopper play a big role in determining how air is distributed around and through the pellets. Ultimately, a well-dried resin is one that spends the proper amount of time at the prescribed temperature and dewpoint. Stoughton recommends that molders pay particular attention to residence time. "Residence is not the time the material spends in the hopper," he explains. "It is the time spent in the hopper at the correct temperature and dewpoint."

In the end, says Dziedzic, the ultimate goal of the drying process is to remove moisture from the resin. Airflow, he adds, is the mechanism that does this most effectively. The higher the airflow, the more efficient the drying process.

Although some engineers are more dryer savvy, almost all of the manufacturers contacted by IMM say that molders, in general, don't know enough about the drying process to ask good questions when it comes to buying a drying system. Many of the OEMs say they spend at least some time with each potential customer in order to do some form of dryer education. They all look forward to the day when resin drying is no longer perceived as just one more necessary evil.