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.
Saving Energy in 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% of total process energy consumption, second only to the molding machines themselves. Fortunately, it is also 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 have also been introduced for lower-volume applications requiring high drying 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 or 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.
More Energy-Saving Tips for Resin Dryers
Properly Store & Handle Per Your Material’s Moisture Characteristics
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.
Eliminate Over-drying of Resin
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 over-drying.
Use Energy-Saving Designs
If you are looking for added drying capacity, avoid the trap of buying “inexpensive” used dryers. Modern, compact desiccant wheel dryers, like the Conair Carousel™ Plus dryers, have fewer parts, less structural mass and smaller desiccant volumes, so it take less energy – more than 35% less – to operate them.
Saving Energy in 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% to 15% more than conventional energy-efficient motors and they 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, and efficiency gain, and dollar savings, as indicated in the table below.
|Annual Savings from Specifying NEMA Premium Motors|
|Full-load Motor Efficiency (%)||Annual Savings from Use of a NEMA Premium Motor|
|Horsepower||Energy Efficient Motor||NEMA Premium Efficiency Motor||Annual Energfy Savings, kWh||Dollar Savings $/year|
Note: Based on purchase of a 1,800 rpm totally enclosed fan-cooled motor with 8,760 hours per year of operation, 75% load, and an electrical rate of $0.05/kWh.
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.
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; you reduce wear and tear on the starter, motor, blower, extending service life; and you improve instantaneous conveying rate by eliminating start-up time.
Reducing Energy Use in Material 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 overpower their granulators rather than to 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%.
Energy savings of up to 50% 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.
The graph below illustrates the potential power reduction on a 7.5-hp (5.5-kW) granulator operating at 480 volts.
Additional Granulation Energy-Saving Tips
Regardless whether you use a motor controller or not, there are a few simple maintenance tips that, if followed, can ensure your granulator operates efficiently:
Granulator Blade Maintenance
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.
Proper Scrap Feeding
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 wasting energy.