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| Plastic Processing |
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H/L Distribution Head
The high viscosity material is introduced in the form of an annulus. The annulus diameter is usually about 2/3 of the main pipe inside diameter. The annulus thickness is governed by the relative flow rates of the main flow to the additive flow rate, and contact area between the two components is developed on both side of the annulus.
The A/t value for this device is 3.6/F. If our additive flow rate is 1% of the main flow, then the A/t value is 360. This is more than 100 times the value that a simple T input for the additive would provide.
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VISCOSITY AND PRESSURE DROP CONSIDERATIONS
Most polymeric materials are shear sensitive. That is to say, their viscosity is dependent on the shear rate to which they are exposed. Usually an increased shear rate produces a lowering of viscosity and is referred to as shear thinning. Viscosity versus shear rate data is usually provided by the polymer manufacturer in the form of a graph on log/log paper. We list below the shear rate and pressure drop formula for each of the devices we have described and others together with the units involved.
Main pipe inside diameter = D inches
Main product flow rate = Q gpm
Shear rate = g sec-1
Viscosity = m cP
Pressure drop = dP psi
Axial length = L inches
Additive fraction = F
Number of mixing elements = n |
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| Device |
A/t Value |
Shear rate |
Pressure drop |
Comments |
| Open pipe |
---------- |
39.2Q/D3 |
22.73x10-6Qm L/D4 |
Hagen-Poiseuille Equation |
| Helix |
np |
96.0Q/D3 |
205x10-6Qm n/D3 |
Helix L/D ratio = 1.5:1 |
| L/H unit |
8/p2 F |
215Q/D3 |
5.9x10-4Qm /D3 |
Each hole dia. = 0.45D |
| H/L unit |
3.6/F |
81.7Q/Dt2 |
22.1Qm L/Dt3 |
Where annulus dia.= 2D/3 |
| SIDD |
288/p2 F |
593Q/D3 |
12.6x10-4 Qm /D3 |
Each hole dia. = 0.3D |
| PARAFLO |
8pN/F |
Contact factory for shear rate and pressure drop calculations. |
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