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
|
|
| 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. |
|
