INJECT AC: Metering system for feeding
liquids under pressure
by Ulrich Helbing.
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Oerlikon Barmag is
an established mechanical engineering company in the manmade
fiber industry and can look back over decades of development
and construction of highly productive and innovative
spinning lines and texturing machines. In addition to
spinning and texturing systems, the core competencies
include the manufacture of components for manmade fiber
production such as extruders, winders, pumps and godets.
Oerlikon Barmag consistently extended its product range to
polycondensation systems and components for chemical and
industrial applications. |
1. Overview
Manufacturing products from polymer melts
(e.g. spun textile manmade fibers) frequently requires
low-viscosity additives to be added to the melt under high
pressure. Adding these additives must be carried out with the
utmost precision, as even the smallest deviations can change the
product quality. For this reason, only metering systems can be
used that operate with total accuracy even at high pressures and
low viscosities. Many types of pumps – such as hose pumps,
diaphragm pumps and frequently also single gear pumps – must be
ruled out from the outset. Although other types of pumps –
including high-pressure piston pumps – can deal with the high
pressures, their feed is not uniform but pulsating.
It is for these very reasons that the
“INJECT AC” was developed. This system meters low viscosity
media under pressure with an accuracy of 100%, without the
occurrence of feed flow pulsations.
2. Sample applications
2.1. Feeding spinning plant systems with
extruders
In spinning plant systems, additives – such
as UV stabilizers, flame retardants, liquid dyes, etc. –
frequently have to be metered into the melt (e.g. PETP). This is
possible by adding the liquid without pressure to the extruder’s
hopper (additive 1). However, this can worsen the extruder’s
feeding behavior. For this reason, the additives are frequently
added under pressure downstream to the extruder screw.
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Figure 1: Feed
options for additives. |
The INJECT AC system is the ideal solution
for feeding in this zone. The first pressure feed method is
adding the product directly to the extruder between the extruder
screw and mixer (additive 2). However, the prerequisite is that
the extruder is equipped with a mixer capable of mixing in the
additive with sufficient efficiency. The second option is to
feed the product to separately driven dynamic mixers directly
installed in the melt pipe.
The mixers are available for all relevant
throughputs (FLUMIX AC up to approx. 200 kg/h). The third
variant for adding the additive to the melt is to feed it
directly into a spinning pump with an integrated dynamic mixer (PROMIX
AC or PROMIX VS pump). This option provides maximum flexibility
as it offers the possibility of injecting a different additive
for each spinning position.
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Figure 2:
INJECT AC (compact type). |
2.2 Feeding into cellulose spinning systems
When manufacturing certain cellulose
fibers, a low-viscosity additive-suspension is added to the
high-viscosity spinning solution. To this end, metering the
suspension must be carried out under pressure and at a
temperature. And the INJECT AC system is also the perfect
solution for ensuring absolutely precise feeding of the
suspension. The heating of the metering unit is carried out
using a liquid heat carrier medium, which is simultaneously used
for heating other components within the cellulose spinning
system.
2.3 Feeding PIB
In certain film systems, the melt for
manufacturing particularly elastic products, polyisobutene (PIB)
has to be added under pressure in a precisely defi ned quantity.
To be able to remove PIB from the tank and then convey it, it
has to be heated to approx. 100°C, which considerably reduces
the viscosity. The electrically-heated version of the INJECT AC
system is the perfect solution for this application.
3. Specifications and operating data for
the INJECT AC metering system as compact type
INJECT AC systems are autonomous “Plug and
Play” units that comprise a metering unit and a control box. All
electrical connections between the control box and the metering
unit are pluggable, so that the initial installation, as well as
subsequent re-installations, can be carried out with the minimum
of effort.
The INJECT AC metering units as compact
type fundamentally comprise the following components:
- Booster and metering pumps.
- Two geared motors with digital
speed measuring and integrated frequency inverter.
- Two pressure sensors for
measuring the pressures upstream and downstream to the metering
pump.
- Pump block for mounting the
pumps and connecting the liquid pipes.
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Figure 3:
Controlling of the INJECT AC unit. |
The following optional components are
available for metering heated media:
- Pipework for heating using
liquid heat carrier.
- Electrical heating.
- Heat-insulation for all
product-conveying components.
- Special pressure sensors for
temperatures in excess of 100°C.
The control box has the following main
features:
- All electrical connections to
the metering unit are pluggable.
- All settings are carried out
using a touch screen.
- All important parameters are
displayed on the touch screen.
- All important control and
adjustment functions are included.
- The control unit can be
connected to the customer’s overall system via a Profi bus
interface.
- Optionally, the control box can
be equipped with controllers for several heat zones (for the
metering unit, for heated hoses, etc.).
Operation data for the INJECT AC units:
- Maximum permissible feed
pressure: 300 bar.
- Minimum viscosity: 100 mPas.
- Throughput range: 0.03 - 70
Liter/hour.
- Maximum operating temperature:
100°C (optionally up to 200°C).
- Deviating application data (e.g.
viscosities smaller than 100 mPas) are possible depending on the
application.
4. Function of the INJECT AC units
The heart of the INJECT AC metering unit is
two precision gear pumps. These are mounted onto a joint pump
block and hence connected in series. The first pump assumes the
pressure build-up, while the second pump meters; for this, each
pump is equipped with its own drive.
The speed of the second pump is determined
by the conveyed feed volume, which can be set at the control
box.
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Figure 4:
Controlling of the INJECT AC drives. |
The speed of the first pump is set via a
complex control algorithm so that the pressure between the two
pumps is always exactly as high as the feed pressure (p1 = p2,
∆p = 0 bar). Even in the event of fast pressure changes in the
pipe downstream to the second pump (feed pressure), the pressure
between the pumps follows without significant delay and without
oscillations.
As a result of the above-described control,
the second pump always operates with a differential pressure of
0 bar and hence meters with a volumetric efficiency of 100%.
This means that there is an absolute proportionality between the
prescribed speed of the metering pump and its throughput of
additives.
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Figure 5:
Throughput of the INJECT AC metering unit. |
The high level of precision with which the
pumps meter and the use of speed-controlled motors enable an
extremely high speed setting range of almost 1:100.
Hence, a very wide throughput spectrum can
be covered with only one pump size (e.g. 0.2 cc/revolution). It
goes without saying that there is a large range of pump types
with various throughput capacities (cc/revolution) available.
These pumps all have the same hole pattern for bolt connections
and are therefore exchangeable.
5. Summary
The INJECT AC metering system was developed
to feed liquid low-viscosity additives under pressure. It is
characterized by the following properties:
- 100% metering accuracy by using
a booster pump and a metering pump.
- Feed pressures of up to 300 bar
are possible.
- Viscosities from 100 mPas are
possible. Additives with lower viscosities can also be processed
depending on the application.
- Very large throughput spectrum
of between 0.03 and 70 liters/hour.
- Heating by means of liquid heat
carrier or using electrical heaters is possible.
- INJECT AC is supplied complete
with control box as a “Plug and Play” system.
- All electrical connections
between control box and metering unit are pluggable.
- Operation via a touch screen.
- The control system is equipped
with a Profi bus interface.
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