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Vacuum loaders used to load materials into a drying hopper typically employ what is called a gravity-discharge valve. The discharge valve employs a flapper that hangs freely beneath the vacuum loader. When the material level within the drying hopper falls below the vacuum loader’s discharge valve flapper, it allows a path for ambient air to enter the top of the drying hopper due to the suction of the return air circuit (Figure 1).

Also, at this point the level switch built into the discharge valve closes and calls for more material to be loaded into the drying hopper. Next, a vacuum is applied to the loader body which pulls the gravity discharge valve into the closed position. (Figure 2) The vacuum loader will pull material from the source of supply on a timed cycle, which is manually adjusted for the optimum amount of time determined by the machine operator.

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When the vacuum loader’s timer completes its cycle and the vacuum is shut off, gravity takes over to allow the material within the loader body to force the flapper discharge valve to open and discharge the material into the drying hopper. (Figure 3).

During the time shown in Figures 1 and 3, the suction of the dryer’s return air circuit will be drawing some ambient air along with ambient moisture into the “closed loop circuit” via the dryer’s return air circuit.

In many cases, this small amount of ambient air entering the “closed loop air circuit” will not cause a problem due to several reasons such as:

1. Dehumidifying polymer dryers are sized to perform well during the hot, humid summer months of June through September, when ambient dew point levels often reach as high as 60° to 80° F, and coast along with little effort during the relatively cool dry months from October through May.

2. Some polymers such as ABS, acrylics and some others are mildly hygroscopic and easily dried.

3. Some polymers such as DuPont’s Zytel (nylon) are more forgiving of moisture content and can be successfully molded with a moisture content of up to 0.20% or 2,000 ppm, according to DuPont’s Zytel molding guide.

However, some polymers are much more sensitive to moisture during processing. Polymers such as PET, PBT and most other polyesters (along with polycarbonate and some polyurethanes) can cause endless problems if not dried to extremely low residual moisture levels during the hot, humid summer months.

So how do we contend with a low volume of wet ambient air leaking into our drying system when processing extremely sensitive polymers during the hot, humid summer months?

The answer may be a compressed-air-operated discharge valve that provides a positive seal when it closes and ensures that no wet ambient air seeps into the drying hopper (Figure 4, right).

Another option is a custom-made transition hopper between the drying hopper and the vacuum loader, which will be filled with material and thereby block the path of ambient air entering the drying hopper through the vacuum loader (Figure 4, left).

ABOUT THE AUTHOR: Pete Stoughton has more than 40 years’ experience with dehumidifying dryers and PET crystallizing systems. He has held various technical positions with the Conair Group, including technical services manager, product manager and ����ý unit manager. He now has his own consulting firm, Polymer Drying Services. Contact: 814-516-3838; pstoughton@comcast.net; .

Struggling With Undried Material at the Hopper Bottom? 

Consider A Cone Dryer Design

Do you dry moisture-sensitive hygroscopic polymers? Do you also do short runs, changing materials frequently? Do you struggle with drying the material at the very bottom of the drying hopper when you first start to run a new material immediately after a material change?  

The lack of drying in the lower portion of the drying hopper during static pre-drying is a problem experienced by many molders and extruders that frequently change the type of materials they are processing.

When a drying hopper is first filled with wet material and pre-dried prior to starting your injection molding machine/extruder, there is a small quantity of material in the very bottom of the drying hopper that has not exposed to the drying air (Figure 1). 

FIG 1

The drying air enters the lower section of the drying hopper through the material spreader cone and is immediately drawn to the drying hopper air outlet located at the top of the hopper. The drying air is not forced to the very bottom of the drying hopper, and therefore a relatively small quantity of material at the bottom of the hopper is not dried during static pre-drying. The size of this “dead spot” can vary greatly depending on the size and design of the drying hopper. Larger drying hoppers will obviously have a larger dead spot.

To date, the most common solution to this problem has been to drain the undried material from the bottom of the hopper after static pre-drying is complete, being careful to drain all the material that was not exposed to the drying air during static pre-drying.  

A solution that will provide complete pre-drying is a feature called a Cone Dryer (Figure 2).

FIG 2

The Cone Dryer connects the return air circuit of the dryer (suction) through a hand-operated valve to a custom-designed hopper material discharge assembly. This special discharge assembly features a perforated screen, which contains the material and directs it through the discharge assembly into the machine throat. The hand-operated valve is turned to a partially open position during pre-drying.  

During pre-drying, a small portion (approximately 10% to 20%) of the hot, dehumidified drying air entering the drying hopper is drawn downward through the hopper cone section and into the material discharge assembly. The drying air exits the discharge assembly through the perforated screen and is directed through the open hand-operated valve to the dryer’s return air circuit. Once pre-drying is completed the hand-operated valve can be closed, thereby redirecting all of the drying air upward through the hopper as shown in Figure 2. 

Unfortunately, Cone Drying assemblies are not being manufactured as a standard product by any of the dehumidifying dryer manufacturers that I know of. 

Therefore, processors will have to work with a local fabrication shop to have a Cone Dryer assembly custom-made to fit your particular drying hopper and machine throat.