How do preconditioners

manage product levels?

The product level, and indirectly the retention time, is primarily controlled through the pitch of the paddles in reference to their rotation. In most traditionally designed preconditioners, the typical mass flow relationship will, and must, hold true: What goes in must come out.

Paddle pitch impacts performance

For example, let’s use a traditional double shaft preconditioner with all the paddles set to neutral (see Figure 11). This setup will most likely give you poor control of your retention time. In this configuration the preconditioner is set up as an extremely dynamic mixer that “fluffs” the product.

As the paddles close to the inlet strike the product, particles are forced into the air and suspended for a short _{Flow direction} period. While suspended, new product is being fed into the inlet, pushing suspended Figure 10 Illustrations of paddle pitch. product toward the discharge of the preconditioner. This example shows that even a system with all paddles set to neutral will convey the product.

A forward pitch to the paddles offers good conveyance in the forward

Neutral pitch

0°15°

45°

90°

-45° -15°

Forward pitch

0°15° 45°

90°

-45° -15°

Reverse pitch

0°15° 45°

90°

-45° -15°

direction toward the discharge of the preconditioner. Typically, if any forward pitch is used, the paddle is set to 45 degrees, even though other angles can be used.

To reduce forward conveyance, a reverse pitch is used, which improves product retention. As with the forward pitch the paddle is set to 45 degrees with a reverse pitch, even though other angles can be used. Barring further research, it can be assumed that if an angle of less than or greater than 45 degrees is used, the conveying force will be decreased or increased proportionally when compared to 45 degrees. See Figure 10 for further explanation.

Specific configurations

Figure 11 represents a DCC configured with all the paddles set to neutral

Figure 11 This illustration represents a DCC configured with all the paddles set to neutral. With this setup, a retention time of 20 to 35 seconds is expected at a dead-stop weight of 50 to 60 lbs.

September 2007 Extru-Technician 8