The Drying Game
The process of drying plastic resin, the most energy-intensive auxiliary operation, offers tantalizing opportunities for molders to tweak their equipment and operations, whittle away energy consumptio...
The process of drying plastic resin, the most energy-intensive auxiliary operation, offers tantalizing opportunities for molders to tweak their equipment and operations, whittle away energy consumption and save money.
But, as with any technology, the devil is not only in the details, but in attitude. The most energy-efficient dryer in the world is squandered unless it is run correctly.
THEORY AND PRACTICE
The first hurdle molders face in specifying an energy-efficient drying system is how to best evaluate energy consumption. The standard method of calculating a dryer’s energy consumption is energy used per mass of material processed, for example kWh/kg. While the figure provides a useful benchmark of energy use, it can also be misleading, according to Jamie Jamison, drying product manager with Pittsburgh, Pa.-based Conair.
In practice, the energy efficiency of a drying system is the product of a multitude of external, operational variables, including type of resin, amount of regrind, ambient conditions, machinery maintenance and others. As well, energy efficiency is irrelevant if a drying system is not performing its primary duty: delivering properly dried material to the throat of a molding machine.
Airflow, Jamison said, is one of the keys to effective drying. The smaller particles sizes of regrind and fines especially can restrict airflow and inhibit drying. Most resin suppliers recommend between 0.6 and 1.0 cu. ft/min for each lb/hr of material to be dried, and this air volume should be supplied at a pressure of around one psi. A dryer that incorporates a smaller blower, Jamison explained, may consume less energy but fail to properly dry material, especially in the summer months.
Some drying systems are manufactured with temperature sensors near the dryer, which assumes there will be no heat loss between the dryer and the hopper. If, however, air ducts are improperly insulated there will be a heat loss. An operator may be seeing a reading of 250 Fahrenheit (F) but the actual temperature in the hopper may be 225F. Since it takes less energy to reach 225F, a molder may assume his system is efficient when it is merely drying improperly. A rundown of Jamison’s suggestions for energy efficient drying is given in Table 1.
For processors keen on squeezing down energy costs, the five ideas can be summarized as two rules of thumb: Don’t assume anything, and don’t let the quest for energy efficiency blind you to flawed equipment.
Aside from differences in the components specified by manufacturers for their dryers, another factor that directly affects dryer efficiency is the type of dryer. The four most widely used types of drying systems are hot air, compressed air, desiccant and vacuum. Table 2 provides a comparison of the energy consumption required to dry one kg of polycarbonate (PC).
Of the four types, compressed air is least efficient and is generally used only in small volume and niche applications. The remaining three types of systems consume comparable energy on an absolute basis; however, vacuum-type systems provide a distinct advantage with a much shorter drying time, 40 minutes versus 2.5 to 4 hours.
The significantly shorter drying time is achieved by using a vacuum to literally “pull” moisture out of the resin. Vacuum dryers, such as Aston, Pa.-based Maguire Products Inc.’s Low Pressure Dryer, initially heat the resin to a range of 160 to 240F for about 20 minutes, before applying a vacuum for another 20 minutes. The dried resin is then pneumatically conveyed to the hopper mounted on the machine. Maguire’s system is designed to carry out the heating, vacuum and conveying stages in three separate chambers mounted on an indexing carousel. According to Maguire, the energy-related savings of the dryer are as high as 80 per cent, compared to other types of dryers.
While allowing that vacuum drying systems offer significant reductions in drying time, one equipment supplier cautioned that the downside to vacuum drying is the higher up-front cost, as well as its design as a “batch” process. Once the dried resin is conveyed to the hopper it has to be used quickly or it will reabsorb water. In the case of a hygroscopic material such as PC, moisture re-absorption can happen quite quickly. Using the material quickly may mean dedicating a molding machine to a certain job, a luxury many molders cannot afford.
Hot air drying is usually an option for materials that only require surface drying. For hygroscopic materials, the widely-used desiccant dryer offers the processor a sort of middle ground in terms of operating costs and shop-floor flexibility, and many manufacturers have incorporated unique energy savings features into their equipment lines.
Plainwell, Mich.-based Motan Inc.’s Luxor Central twin-bed desiccant drying system features a patented energy recovery technology, called ETA, which uses the heated return air to preheat incoming process air. The energy is recovered from the return air directly at the bin via an optional, integrated heat exchanger. Tests have shown ETA can save as much as 42 per cent on energy costs for a high-throughput (500 lb/hr) low-moisture application such as polyethylene terephthalate (PET).
One downside to desiccant dryers is that the drying medium, usually a molecular sieve of alkalai alumino-silicate, must be purged of water or regenerated. Regenerating the desiccant takes a combination of time and energy.
Regeneration involves heating the bed with hot dry air, then cooling it down to prepare it for the next cycle. According to Carl Litherland, national sales manager with Motan, one advantage that a twin-bed desiccant dryer has over wheel or rotary desiccant dryers is a smaller molecular sieve, which requires less heat to purge the water. This not only saves energy directly, but also reduces exposure to severe heat-cool cycles, prolonging the life of the desiccant bed. After regeneration, the bed can be allowed to cool over time, rather than cooling with chilled return air. “Our tests have shown that bed dryers use 50 per cent less energy for regeneration than wheel dryers,” Litherland said.
And the PCT2 line of twin-bed desiccant dryers from Universal Dynamics Inc., headquartered in Woodbridge, Va., has a patented design that diverts a portion of the dried return air ahead of the blower to cool the bed, rather than cooling with ambient air. This prevents loading the desiccant with moisture before it is put into use and reduces the total amount of energy needed to regenerate by about 33 per cent, according to the company’s vice-president of engineering, Bob Crawford.
Suppliers of rotating bed dryers point to the efficiency of having one bin or canister drying, one regenerating and one cooling (the usual configuration) in a continuous flow process. Some manufacturers have addressed the energy inefficiencies associated with desiccant regeneration, cool down and general operation by integrating various energy-saving features.
One large source of inefficiency is the way dryers are used under production conditions. “Dryers are designed to process a certain amount of pounds per hour,” Jerry Muntz, of Troy, Mich.-based Thorson McCosh Inc., said. “In practice operators usually under-use a dryer’s rated throughput capacity.” To prevent wasting energy as a result of over-heating, Thorson McCosh has equipped its rotating desiccant bed dryers with a Delta T option. The Delta T feature compares the exhaust air temperature, typically cooler, with the inlet air temperature of the resin-holding hopper. When a molding machine is using less material than the rated throughput for the dryer, the hopper begins to stabilize at the air inlet temperature. The heat is turned off until the pre-set temperature differential (usually in the range of 5 to 10) is reached, when it is turned on again.
Current cutting-edge research on both the materials and equipment side is focused on simplifyi
ng the drying process. “The dream of every molder is to not have to dry in the first place,” Motan’s Litherland said. In the meantime, though, the best hedge against wasteful drying practices is a good game plan. “At the end of the day, you’ve got to stay realistic,” Litherland advised. “A molder has to make the right choices based on the needs of his operation.”
Michael R. LeGault is a former editor of Canadian Plastics magazine
TABLE 1. FIVE SIMPLE IDEAS FOR ENERGY EFFICIENT DRYING*
1. Ensure adequate airflow through the hopper
2. Keep up with maintenance
3. Avoid unnecessary heat losses
4. Deliver proper temperature to material in the hopper
5. Don’t sacrifice drying effectiveness just to save energy
*Courtesy of Conair
TABLE 2. COMPARISON OF DRYING SYSTEMS*
|System||Energy Consumption (1 kg of PC)||Drying time|
|Hot air||58 Wh/kg||4 h|
|Compressed||Air 261 Wh/kg||3 h|
|Vacuum||61 Wh/kg||0.66 h|
|Desiccant||64 Wh/kg||2.5 h|
*Courtesy of Motan Inc.
Nucon Wittmann Inc., of Markham, Ont., has extended its Drymax series of compact dryers with the new Drymax ES-40 model, a one-tower dryer that is best-suited for economical, energy-efficient drying of small material throughputs. The ES-40 can provide a constant dew point of -35 Celsius (C) — (-29F) — and throughputs of about 17 kg/hr (37.4 lb/hr) depending on material type. The dry air generator and drying hopper are mounted together on a compact cart with coasters for shop-floor flexibility.
Baltimore, Md.-based Novatec Inc. has received a license from Stricker IRD Patent GbR to produce a new type of dryer/crystallizer for PET that uses far less energy than a conventional dryer system. The IRD dryer consists of a large, horizontal, stainless steel cylinder. As the cylinder is rotated, a helical arrangement of flights on the inside walls of the cylinder transport PET through the length of the drum, exposing the surfaces of the pellets and/or regrind to a bank of infrared heaters. Moisture removed from the core of the material is carried away by ambient air. According to Novatec, tests show that the crystallizing and drying process can use up to 65 per cent less energy than conventional systems.
And the HiCore desiccant systems from AEC Inc., of Wood Dale, Ill., are composed of heating elements inside the hollow core of the desiccant tanks. During drying, air from the blower enters the canister from outside the cylinder. As the air moves towards the core, its pressure and velocity increase, aiding moisture adsorption. The design lowers energy costs by 10 to 15 per cent and reduces regeneration time to less than one hour, AEC said.