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Figure 3: A typical F(t) Curve for an Open System


Figure 4: Single shaft preconditioner


Figure 5: Twin shaft preconditioner and agitators


What Type and Configuration Should I Use? The preceding discussion gives a fundamental understanding of the technical requirements of preconditioning, which can then guide selection, configuration, and operation of preconditioning systems. Types of Preconditioner A large number of different types of preconditioner are available on the market. These can be classified into a number of categories, as summarised in Table 2.


Table 2: Classification of Preconditioners


Pressurised vs. Atmospheric Single Shaft vs. Twin Shaft “Open” Designs vs. “Weir” Designs Single Stage vs. Multi-Stage Vertical vs. Horizontal


To briefly define and compare these designs: In Open-type Preconditioners (no weir), the angle (pitch) of the


Pressurised vs Atmospheric – as discussed, temperature is a key parameter in the cooking reaction. The use of a positive pressure in the preconditioner allows the use of an elevated temperature (as high as 115 o


C) and thereby promotes an increased reaction rate i.e. more


“cook” for the same amount of time. The negative aspect of operation at an elevated pressure is the requirement of a far more complex sealing arrangement for the infeed and discharge. Pressure feeders or stuffers are also required to force the feed into the system. This higher complexity has limited their use.


Single- vs Twin- Shaft: Figures 4 & 5 show Single- and Twin-Shaft Preconditioners. Twin Shaft Preconditioners are claimed to provide enhanced mixing capability.


Open vs Weir Design Weir-type Preconditioners incorporate a weir just prior to the outlet to increase fill in the chamber. The residence time (for a particular throughput rate) is directly dependent upon the weir height. But it is important to have paddles positioned to effectively sweep just upstream of the weir to prevent “dead” zones.


PAGE 62 MARCH/APRIL 2017 FEED COMPOUNDER


paddles can be altered, with the aim of holding product back to achieve a higher fill in the chamber. General recommendations are: • Paddles beneath/near the inlet in forward pitch to keep infeed clear. • Last few paddles at discharge end forward pitch to positively move material to the outfeed • Just prior to this, one or two sets of paddles in reverse pitch to “heap” product in the middle of the chamber. • The remainder of the paddles at neutral pitch (flat to maximize mixing effectiveness).


The paddle configuration has a minimal impact upon the residence


time for weir type systems. But mixing behaviour is strongly influenced by both the paddle configuration and the paddle speed. For open type systems, however, the residence time is strongly influenced by both these parameters – increasing agitator speed will tend to improve mixing, but will also tend to decrease residence time.


Single Stage vs. Multi-Stage To avoid the compromise between mixing and hold time related to agitator speed, some manufacturers supply Multiple Stage


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