energy/water transfer is critical for your process, the single shaft is not your solution.

In Figure 3, it becomes apparent why the single shaft would not excel in the areas of retention time, energy transfer and/or water absorption. At high shaft speeds (picture on left of Figure 3), the product is kept to the interior walls of the cylinder, prohibiting a decent transfer of energy or water.

Conversely, at low speeds (picture on right of Figure 3), the product is piled across the bottom of the cylinder, which also will prohibit a good transfer of energy and/ or water. The single shaft performs well at low levels of required cook and moisture, especially when considering total cost of ownership.

Traditional double shaft

The double shaft cylinder comes in a couple different

Figure 3 With a single shaft preconditioner running at high shaft speeds, left, product is kept to the interior walls of the cylinder. At low speeds, right, product is piled across the bottom of the cylinder. Both situations prohibit a good transfer of energy and/or water.

Figure 4

Double shaft preconditioners come in traditional and nontraditional varieties.

varieties, as you can see in Figure 4.

The standard two-stage double shaft is simply a single shaft stacked on top of another single shaft. As shown in Figure 5, the two stages use two extreme ranges of operation on a standalone single shaft. A high-speed,

Figure 5 The two stages use two extreme ranges of operation on a standalone single shaft.

Table 1

Guidelines: single shaft vs. dual shaft preconditioners

Use a single shaft Use a dual shaft preconditioner when: preconditioner when:

Low cook is required

Long retention time is not required

Higher levels of cook are required

Longer retention times are required

Minimal liquid/slurry addition is involved

Liquid slurry additions are involved

High degree of mixing is required

September 2007 Extru-Technician 5