Load Cells Increase Accuracy of Vacuum Dryer

February 27, 2015

3 Min Read
Load Cells Increase Accuracy of Vacuum Dryer
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Maguire Products has incorporated load cell technology into its patented VBD vacuum resin dryer, increasing its accuracy and reliability and providing plastics processors with new capabilities for controlling and documenting their use of material.
The VBD dryer now uses load cells for continuous control of the resin level in two of the three vessels in the system - the vacuum chamber and the retention hopper. The load cells operate on a loss-in-weight basis, providing real-time data on material throughput that is unaffected by variations in bulk density. They achieve accuracy of +/- 0.5%.
“The use of load cells in the vacuum and retention vessels provides 100% reliability and is superior to alternatives such as capacitive proximity sensors and paddle wheel sensors,” said Mike Gera, senior dryer product manager. “Because of their accuracy, they provide molders and extrusion processors with new tools for increasing the efficiency and versatility of their operations.”
Gera cited these capabilities made possible by the use of load cells:
•    Enhanced control over material throughput. Load cells enable the microprocessor in the controller to monitor, display, and log throughput in real time. As the first batch of material is conveyed away from the dryer to the processing machine, a throughput rate is calculated. This rate is updated once per minute. Material usage documentation is available via USB port, making possible proactive monitoring of inventory. Load cells also enable the microprocessor to run a resettable flow totalizer, which is displayed on the screen and entered in the event log.

•    Energy savings. Improved control over throughput rate makes it possible to set the rate to precisely match that of the process, enhancing energy efficiency and avoiding an excess of dried material left over at the end of a run.

•    Option to run batches. While the VBD dryer provides a continuous flow to accommodate the throughput of a molding or extrusion process, load cells enable the dryer to run batches.  For example, with Batch Mode selected in the controller, the operator is prompted to enter a batch size—for example 1000 lb. The VBD will run 1000 lb and then automatically shut down. The shutdown is anticipated so that the vacuum chamber and retention hopper are empty, which is desirable at the completion of a batch run.
The cost savings achieved with vacuum drying in comparison with standard desiccant dryers result from a reduction in energy consumption of more than 60%; elimination of the need to regenerate desiccant; and reduction in startup time as a result of a drying cycle that is only one-sixth as long. In addition, resin is exposed to elevated temperature for as much as 80% less time than with the desiccant dryer, which means less processed-in stress.   
The VBD system eliminates most moving parts, using gravity to move material through vertically arranged stages of the drying process. It is typically a continuous batching operation carried out in three heavily insulated, stainless steel-lined vessels arranged vertically. High-speed slide-gate valves for dispensing material from one vessel to another have an accuracy per dispense of +/- 4 g and are of the same design widely employed in other Maguire equipment. The system uses ambient air that is circulated by a blower whose variable-frequency drive saves energy.
Maguire Products Inc., Aston, PA 610-459-4300 www.maguire.com

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