As manufacturing has became more intensely competitive, plastics processors are
looking at auxiliary equipment that can reduce energy consumption, make better
use of raw materials, reclaim scrap, and improve process yield. Today, these
initiatives take on new significance as “sustainability” becomes the watchword
for consumers and manufacturers alike.
Not all auxiliaries are power hungry. Feeders and blenders, for instance, use
very little power, so saving energy does not become a critical issue. Other
equipment, however, provides ample opportunity for processors to reduce costs
even while increasing productivity.
DRYING
A perfect example of how auxiliary equipment can contribute to economic
and ecological sustainability is found in resin dryers. In many applications,
particularly those involving materials that dry at high temperatures, drying can
be among the most energy-intensive processes. In PET preform molding, for
instance, drying accounts for 25 percent of total process energy consumption,
second only to the molding machines themselves. Fortunately, it also is an area
where significant savings can be achieved.
For high–volume operations, two-stage drying systems actually recycle hot air to
preheat material entering the drying hopper. Some of the return air exiting the
hopper is channeled through a filter and then reheated before it is
re-introduced through an inlet cone located in the middle of the hopper. Because
it still contains some moisture from its first pass through the hopper, it
cannot do the drying job by itself, but it can economically heat new material
entering the hopper and begin the drying process. The two-stage system saves
energy by eliminating the need to remove heat in the return-air heat exchanger
and by not having to add as much heat to the air for its next trip through the
hopper. Simpler single-stage systems also have been introduced for lower-volume
applications requiring highdrying temperatures, applications in which throughput
varies, and where electrical costs are high.
A key element in both single- and two-stage systems is a control system which
monitors the temperature of the material in the hopper and the return air dew
point. A variable-frequency blower drive dynamically adjusts airflow to maintain
optimum drying conditions without excessive energy input, regardless of
throughput changes, variations in material temperature, or ambient conditions.
This control system allows processors to gradually reduce energy input to an
absolute minimum while maintaining conditions necessary to ensure
finished-product quality. Single-stage or two-stage, less energy is used in
processing each pound of material and less scrap is produced so the drying
process is more sustainable.
Other points to remember about energy savings in drying include
· Pay attention to the moisture characteristics of the material you are drying.
Drying parameters set by resin companies usually assume a certain initial
moisture level. If the material is stored improperly and moisture levels get too
high, it can take additional energy to achieve the desired moisture content. A
modest investment in proper storage and handling can lead to big energy savings
at the dryer.
· Adding dew point monitors and/or dew point control can dramatically improve
the performance of older dryers. By providing a digital readout of the drying
air dew point, a dew point monitor allows the operator to confirm that the air
being delivered to the drying hopper has been properly conditioned. Dew point
control goes a step further, allowing operators to specify a certain dew point
value. The control locks onto this setting and automatically adjusts various
dryer functions to hold the set value, virtually eliminating wasteful overdrying.
· If you are looking for added drying capacity, avoid the trap of buying
“inexpensive” used dryers. Modern, compact desiccant wheel dryers have fewer
parts, less structural mass, and smaller desiccant volumes, so they take less
energy – more than 35 percent less – to operate.
MATERIAL HANDLING
In materials conveying, there are many ways to improve energy
efficiency, and perhaps the biggest benefit can be realized by using a central
conveying system with vacuum pumps, piping, and other equipment to supply
materials to the process equipment in lieu of small, self-contained loaders that
are notoriously power hungry.
Once you’ve decided on a central vacuum system, you may want to consider
specifying high-efficiency motors on the vacuum pumps. Built to the NEMA Premium
Energy Efficiency Motor Standard, the motors typically cost 10 percent to 15
percent more than conventional energy-efficient motors and tend to be more
cost-effective when annual operation exceeds 2,000 hours, where utility rates
are high, or where electric utility motor rebates or other conservation
incentives are available. Actual savings are dependent upon operating profile,
duty cycle, efficiency gain, and dollar savings, as indicated in the table
above.
Regardless of what type of motor you specify on your vacuum pumps, consider
adding an idle-mode valve or vacuum-relief valve so that the pumps won’t need to
shut down completely between loading cycles so you save energy, as shown in the
graph below.
When a pump motor is turned on or starts up, it draws eight to ten times more
current than the normal full-load amperage (FLA) required to keep it running. By
allowing the pump to continue running between loading cycles, you eliminate the
high inrush current demand reducing energy costs; reduce wear and tear on the
starter, motor, and blower (which extends service life); and improve
instantaneous conveying rate by eliminating start-up time.
GRANULATION
In general, granulators use a lot of horsepower, which tends to make them
energy-hungry machines. They do tough work, however, and the tendency has been
for processors to over-power their granulators rather than seek to minimize
energy consumption. That’s because concern about work-flow disruption and
maintenance costs associated with a granulator jam has tended to outweigh the
potential benefit of reduced power consumption. Still, as energy costs rise and
environmental awareness builds, many processors are looking for ways to save.
Perhaps the best way to do that is with a solid-state motor controller, which,
when installed on a standard granulator, provides a soft start and reduces
inrush current up to 70 percent. Energy savings of up to 50 percent are
possible. The system operates like cruise control on an automobile. As scrap is
loaded (either batch- or conveyor-fed), the controller ramps up the power
applied to the drive motor to handle the increased load. As scrap clears the
cutting chamber and load decreases, the power supplied is reduced – all without
affecting motor speed.
Regardless of whether you use a motor controller or not, there are a few simple
maintenance tips that, if followed, can ensure your granulator operates
efficiently:
· Keep your blades sharp and properly adjusted, and make sure screens are not
worn. Your granulator will cut more efficiently with less noise and fewer fines.
· Don’t overfeed. Forcing too much scrap into the cutting chamber will lead to
wasteful spikes in power consumption and could cause a jam.
· Make sure to empty the granulator catch bin regularly or install an automatic
evacuation. If regrind cannot flow cleanly through the screen, the granulator
will have to work harder, generating more heat and fines, and also wasting
energy.
As customer service manager at Conair, Doug Brewster heads a staff that includes
project engineering, project management, and field service personnel. He joined
Conair in 1987 as a systems engineer and has held a series of progressively more
responsible positions, including bulk-system product manager, project manager,
national accounts manager, regional manager, and national sales manager. For
more information, call (724) 584-5425, email
[email protected], or visit
www.conairnet.com. |
