New Twists in Drying

Dryer design has been a hotbed of activity in recent years, but by far the most original twist on drying plastic pellets comes from Maguire Products Inc.

While conventional dryers merely heat pellets and keep them hot as moisture works its way to the surface, Maguire’s Low Pressure Dryer (LPD) uses vacuum to actively draw moisture from within the pellets. As a result, the LPD dryer reduces the time needed to properly dry 100 lb. of resin from as much as four hours to only 40 minutes, says Stephen B. Maguire, president. In addition, energy consumption is cut by 75 percent compared with conventional twin-tower desiccant style dryers.

The LPD uses no desiccant, eliminates prolonged start-up procedures and makes it possible to change colors without downtime.

The first LPD model is a beside-the-machine unit sized for processing lines running up to 45 kg/hr (100 lb./hr.). Future models will include a smaller capacity unit for machine mounting and a larger unit for processing operations with higher throughput.

Maguire notes that vacuum drying is already employed in the manufacture of plastic products and ground coffee. “We transformed this batch process for high-volume bulk operations into one that is virtually continuous, and can be carried out with equipment compact enough for beside-the-press or machine mounting.”

ADDING VACUUM SPEEDS THE PROCESS

The LPD heats most resins to temperatures in the range of 71 to 105 C (160 to 220 F) before drawing the vacuum. This excites the molecules in pellets and facilitates the movement of moisture to the surface; then the vacuum goes to work.

In normal operation, the dryer carries out three procedures at the same time:

– Resin is loaded into a vacuum chamber from the surge hopper and is heated by hot air for about twenty minutes.

– A second vacuum chamber containing a batch already up to temperature is sealed at top and bottom, a vacuum is drawn for 20 minutes, and moisture is evacuated to ambient air.

– While resin is being loaded into the first vacuum chamber, dried resin in a third sealed chamber is pneumatically conveyed to the receiver mounted on the processing machine.

TWEAKING THE TRADITIONAL

Dual-bed desiccant dryers are the workhorses of the plastics industry, well-known, reliable, versatile, but not without drawbacks. Manufacturers that employ this technology continue to refine it, making significant gains in energy efficiency and process stability.

Universal Dynamics’ pulse cooling technology stabilizes the temperature and dew point spikes associated with switchover of desiccant beds through a proprietary method of controlling the valves. The technology is available on a variety of Una-Dyn dryers. Most recently, Una-Dyn introduced the PCT2 dryer, which incorporates pulse cooling and has a reduced footprint and full cabinet enclosure. It is available in five different sizes with capacity from 300 to 1500 lb./hr.

Motan’s ETA-process achieves maximum utilization of the heat generated during high temperature drying and consequently reduces energy consumption. The ETA-process heat recovery system uses the return heat from the drying bin to preheat the process air coming from the dryer. An additional benefit is that the air temperature drops significantly as it passes through the ETA-process heat exchanger, so less chilled water is consumed by the after-cooler to cool the air.

The ETA-process is incorporated on Motan’s Luxor line of central drying systems. The Luxor bins have been redesigned with an easily-accessible console face where the air flow balancing meter, ETA-process energy efficiency gauge and booster heater temperature controls are located. The large access door for clean out has been moved to the front of the hopper and the full-length vertical sight glass retained. The bins are available in capacities ranging from 100 to 3500 lb.

Dri-Air’s new drying control monitors the temperatures inside the drying beds to optimize both the heating and cooling of the desiccant during regeneration. This avoids temperature spikes that can result in surface melting of the resin.

The company has expanded its product line to include two new high-capacity, multiple-bed dryers capable of processing 400 and 500 lb./hr. The HP4-X/400 and HP4-X/500 models are extremely efficient, typically using 50 percent less energy than conventional two-bed and rotary bed dryers.

By locating the regeneration heaters within the desiccant towers, Dri-Air keeps heat loss to a minimum and uses less air for regeneration. The use of dry process air for regeneration also adds to the operating efficiency.

COMPRESSED AIR: SIMPLE BUT LIMITED

The appeal of compressed air dryers is that no desiccant is needed, the temperature swings characteristic of heat-regenerated desiccant bed dryer systems are avoided, and there are no moving parts, no seals and no bearings to wear.

But the limiting factor of all compressed air dryers is throughput. Cactus Machinery Inc. tops the category with a 150 lb./hr. unit. The dryers have been redesigned to achieve this higher capacity, and have a number of user-friendly features. For ease of cleaning, the top of the hopper can simply swing out of the way.

Higher capacity models are forthcoming for Una-Dyn’s Auto-dry hopper dryer as well. The unit is currently available with 20, 40 and 60 lb. hoppers; a 120 lb. hopper is planned. Current sizes are suitable for about six to 24 lb./hr. throughput.

Technical sales representative Mark Milligan explains that Auto-dry employs a solid cone hopper design which avoids the possibility of clogging.

Introduced earlier this year, the NovaDrier puts a bit of a twist on the compressed air drying principle. In Novatec’s design, the compressed air is passed over a coil of absorptive media, moves into a heater, is mixed with return air from the hopper, and then is fed back into the hopper. NovaDrier consumes less than half the quantity of air of other compressed air units, reports Novatec. Initial models have 5 to 120 lb./hr. capacities.

ROTOR AND CAROUSEL DESIGNS ELIMINATE SPIKES

“The desiccant wheel or rotor technology has the advantage over loose granular or traditional resin dryers because a greater velocity of air can be passed through a smaller desiccant media with less heat carryover. This generally results in a smaller and lighter dehumidifier package and virtually eliminates dew point spiking,” explains Graydon Griesse, vice-president, Bry-Air Systems.

In Bry-Air’s rotor-technology dryers, the hot, dry regenerated desiccant rotates back into the process air stream where some of the process air cools the desiccant so it can begin adsorption again.

Bry-Air’s FD series handles 15 to 600 lb./hr. throughput while the FDL series is suitable for 700 to 6000 lb./hr. As an added feature, Bry-Air will begin offering true dew point control as an option, allowing operators to set and maintain the desired dew point.

Conair’s CD and CS series dryers use a patented carousel that is said to eliminate dew point and temperature spikes. Multiple desiccant tanks present dry desiccant to the material drying circuit more frequently. For energy savings, the carousel models use dehumidified air to cool the regenerated desiccant cartridges. The CS models handle throughput from 100 to 750 lb./hr. while CD units range from 450 to 6000 lb./hr.

Cutaway schematic illustrates the

40-minute drying cycle of Maguire’s Low

Pressure Dryer.

Material spends 20

minutes at the first

station being heated

(far right), 20 minutes in the second under

vacuum to draw out

moisture (far left), and is pneumatically conveyed from the third station.