|Saving Energy in Dryers, Material Handling, and Granulators|
|by Doug Brewster, The Conair Group, Inc.|
|Energy Savings Summer 2010|
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.
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.
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.
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