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Low-pressure dryer

August 1, 2000

5 Min Read
Low-pressure dryer

npe12.jpgLow-pressure dryer -- Maguire Products has entered the resin drying market with a technology that pulls moisture out of resin and does not use the hot-air or desiccant drying methods used conventionally.

The Low Pressure Dryer (LPD) uses vacuum to draw moisture from the pellets. This, says Maguire President Stephen Maguire, reduces the time needed to properly dry resin from as much as 4 hr to 40 min and cuts energy consumption by up to 75%.

Maguire adds that with a capital investment comparable to or less than that of conventional dryers, the LPD system eliminates longer start-up procedures and makes it possible to change colors without downtime. Also eliminated are desiccant beds, which must be regenerated and periodically replaced.

The first LPD model is a beside-the-press unit sized for machines running up to 100 lb/hr. This includes small extruders and 60% of injection molding presses, says Maguire. It is priced at $12,500 and has a five-year warranty.

Future LPD models will include a smaller-capacity unit for machine mounting and a larger-capacity unit for operations where throughputs exceed 100 lb/hr (for example, injection molding of PET bottle preforms).

Because vacuum drying requires lower temperatures than conventional drying, it may provide advantages in applications involving heat-sensitive polymers or additives subject to volatilization. "Particularly promising are PET applications, since excess heat can cause a drop in intrinsic viscosity, which correlates with processability problems and end-product physical properties," adds Maguire.

To build the new dryers, Maguire has opened a 25,000 sq ft manufacturing facility near its headquarters in Aston, PA.

How does it work? As air pressure drops, water boils or volatilizes at lower and lower temperatures. While water boils at 212 F under normal sea-level atmospheric pressure of 29.92 in. Hg, the boiling point falls to only 133 F when pressure is reduced to 25 in.

"The LPD generates a very strong vacuum of up to 29 in.," says Maguire, noting that a perfect vacuum at sea-level is 29.92 in. (like atmospheric pressure, the counterbalancing strength of a vacuum is measured in terms of the height of a column of mercury that air pressure supports).

npe13.jpg"In practice, we heat most resins to temperatures in the range of 160 to 220 F before drawing the vacuum. This is enough heat to excite the molecules in pellets and facilitate movement of moisture to the surface; the vacuum does the real work, needing only about 20 min to reach required resin dryness."

Maguire compares this to conventional dryers that heat resin to temperatures in the range of 180 to 350 F and keep it there for up to 4 hr, adding to the heat history of the material.

The short drying time of the LPD is said to allow it to work with smaller batches, but while matching the throughput requirements of the extruder or molding machine. In turn, smaller batches are said to simplify morning start-ups and color changes.

"Because a conventional dryer takes up to 4 hr to evacuate moisture from the pellets, it must start with a 400-lb batch in order to supply a 100-lb/hr processing line," says Pat Smith, Maguire's VP of Marketing and Sales. "Since a completely dried batch has to be ready for Monday morning start-up, this means turning on the dryer 4 hr before the first shift begins. By comparison, the LPD takes about 40 min to dry its first batch--less than is normally required to bring a cold processing machine up to processing temperature."

So what enables the LPD to run small batches yet keep pace with the processing line? The entire LPD system is made up of eight basic elements, all of which take up the space of just the desiccant and heating unit of a conventional drying system:

1. A three-station indexing carousel with three removable cylindrical vacuum chambers, each of which holds up to 1 cu ft of resin, or about 35 lb

2. Two 1400 watt heaters and closed-loop hot air circulating system

3. A Venturi vacuum generator

4. A 1-hp blower that alternately circulates heated air through the resin and transfers dry resin to the processing machine

5. An optional see-through surge hopper for loading resin into the system

6. A Maguire Clear-Vu mini-receiver, with resin capacity of 1.1 lb, for loading dried resin into the processing machine

7. A controller that automates dryer operation

8. An enclosed cabinet

In operation, the LPD 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 20 min. A second vacuum chamber containing a batch already up to temperature is sealed at the top and bottom, a vacuum is drawn for 20 min and moisture is evacuated to ambient air. While resin is being loaded into the first chamber, dried resin in a third, sealed chamber is pneumatically conveyed to the receiver mounted on the processing machine.

"At any one time in this cycle, there is never more than 70 to 100 lb of material in the system, which is 75 to 83% less than in a conventional dryer," Smith says. "And because the material is distributed among three vacuum chambers, processors who don't plan ahead for color changes are never left with more than 35 lb of unused material. With a little planning, it is possible to change colors without wasting material or stopping production."

For an 'on-the-fly' color change, Smith says the operator switches the dryer control from the 'automatic' to the 'clean' mode about an hour ahead of time, cleans out the vacuum chambers, and loads new material. Extra vacuum chambers can be purchased to further streamline this procedure.

The controller is also said to be easy to use. Using supplier recommendations for specific resins, the operator enters drying parameters with simple thumbwheels. The software maintains temperature and vacuum strength within specified limits and stores settings for materials that will be run again.

As for cleanout, the vacuum chambers have lift-out handles and when returned to their stations, are self-aligning and lock into place.

Maguire Products
Aston, PA 

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