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| Plastic Processing |
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L/H Distribution Head
The patented geometry is very simple, yet L/H units are highly successful and applied world-wide to many in-line mixing problems in pipe sizes from 1" through 24".
The L/H unit is used to introduce a very small amount of a low viscosity additive into a much higher viscosity melt stream.
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It consists of a cylindrical block of metal having a pair of identical holes bored parallel to the axis. These holes are just slightly smaller in diameter than the main pipe diameter D. Tapered inlet and outlets are provided. The low viscosity additive is introduced to the structure at the point where the two holes are substantially tangent. As the high viscosity material exits the two holes, natural hydraulic forces cause the material to occupy the entire pipe cross section. These forces are generated by downstream forming dies, static mixers, screen changers, etc. Each exiting stream is forced to form into a "D" shape to fill the pipe. The additive entering between the two flows has no option but to form into a thin sheet across the pipe diameter.
It can be shown that if "F" is the additive fraction where F = 0.01 for 1% then the A/t value for the L/H unit is 8/p 2F. In other words if F = 0.01 or 1%, then A/t = 81. This is clearly a big improvement over the value of p we get for a simple T inlet.
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 |
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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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