HOW IS SME MEASURED/CALCULATED Remember that SME is about the mechanical energy going into the product, not Total Energy – it is important to subtract the No-Load Energy. If the extruder is instrumented to provide a direct measure of Screw
Torque, then record the torque when no product is passing through, and Net Power Input is given by:
Pnet where,
= Torque under no load (Nm) N = Screw speed (rpm)
τ = Torque (Nm) τ0
If a torque measurement is not available, power can be calculated from amps drawn by the drive motor.
For AC drives the Net Power Input (kW) can be estimated via: Pnet
= √3 V [ (f η I ) - (f0 where,
f = Power Factor ( - ) V = Supply Voltage (V)
(Subscript 0
η = Efficiency (%) I = Current (Amps)
refers to no-load conditions)
For DC drives the Net Energy Input (kW) can be estimated via: Pnet
= V [ (η I ) - (η0 I0 ) ] / 1000(kW) The SME is then readily calculated as: η0 I0 ) ] / 1000 (kW) = 2 π (τ – τ0 ) N / 60000(kW)
SME = Pnet where, MTotal Note: MTotal = Total Throughput Rate (kg/hr). is the Total Melt Flow through the extruder – not after
drying – and is the flow rate of Powder, Water, Steam, Oil and any other Additives.
SOME GUIDELINES FOR SME IN AQUAFEED EXTRUSION A very broad range of Aquatic Feeds, with vastly different characteristics (both Floating and Sinking) for a large number of species are prepared via Extrusion Cooking using a diverse raw material base. It is therefore impossible to define a single target value for the magnitude of the SME (or any of the other energy inputs). A typical range for the Specific Energy Inputs can be presented, however (see Table 1).
Table 1: Specific Energy Inputs for the manufacture of Aquatic Feeds
Specific Convective Energy, SCE Specific Thermal Energy, STE Specific Mechanical Energy, SME Total Specific Energy, TSE
0.040 to 0.060 kWhr/kg -0.002 to 0.007 kWhr/kg 0.025 to 0.045 kWhr/kg 0.063 to 0.112 kWhr/kg
(This paper draws from a book by the same authors: “Food and Feed Extrusion Technology: An Applied Approach to Extrusion Theory”, ISBN 978-0-9945433-0-1. See
www.fie.com.au/books)
/ MTotal
(kWhr/kg)
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PAGE 32 MARCH/APRIL 2018 FEED COMPOUNDER
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