Industrial Utility Efficiency

# Plastics Plant Saves 55% in Energy with VSD Air Compressor and Sequencer

A Canadian plastics product plant has upgraded their system of older modulating air compressors to a newer system using a variable speed compressor and a compressor-sequencing controller. This article discusses their challenging experience.

### Molded and Blow Molded Plastics Application

The plant produces both molded and blow molded plastic parts on a 5 day per week, three shift schedule. Production and maintenance sometimes occurs on weekends, occasionally requiring the air compressors to run on a 24 x 7 basis so the practice was to leave the compressed air system always pressurized. The system consisted of three modulating lubricated screw compressors one sized at 150 hp and the others 125 hp (3 units), each controlled with their local compressor controllers. The system also had non-cycling refrigerated air dryers dedicated to each compressor. The flow profile had a fairly constant base component, but with occasional high variability depending on what production machinery was running.

### Initial Assessment of Compressed Air System

The local power utility was called in to do a scoping study of the system. Data loggers were placed on the 100 psi system, showing the compressed air system was running inefficiently due to modulating compressor control. In modulating control, the inlet valve of the compressors chokes off the compressor inlet air, reducing the compressor output in response to the system pressure change. As can be seen in Figure 1 when this modulation happens there is a reduction in compressor amps as the pressure rises. The other smaller 125 hp compressor amps rises at times of increasing pressure, and falls at times when the pressure reduces. This is not modulation but shows the compressor is in drawdown, at maximum capacity, inlet valve wide open.

Around the center of the chart, corresponding to a weekend, the amps of the 125 hp start to go down as the pressure rises. This is a sign the inlet valve is closing under light loading, creating a worst-case scenario, two modulating compressors sharing the load. Figure 2 shows a photograph of the inlet valve of this type of compressor in fully closed position.

In addition, you can see a third compressor with much lower amps. This compressor is running unloaded through the first third of the profile and was manually turned off part way in. You can see the unit continues to draw amps even though the motor is not running. This is due to installed power factor correction capacitors consuming amps (not power) when they are energized.

A profile like this presented a problem in estimating the flow based on amps (no flow meter was installed at the time) because a reduction in amps can mean two different things, either full load in draw down or a compressor in modulation at part load. Care had to be taken in simulating a flow profile in this case so estimated savings could be calculated accurately.

### Proper Installation & Sizing of VSD Air Compressors – Webinar Recording

Download the slides and watch the recording of the FREE webcast to learn:

• How to properly install and size VSD air compressors
• The effects of sizing and installation on energy savings and system reliability
• Tools to analyze a compressed air system’s load profile and kW, ensuring optimal sizing and configuration

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### Subsequent Compressed Air Data Logging Finds Problem

A number of years later the power utility approved another data logging by the compressed air supplier to check if the savings in the original verification were sustained. The assessment found most of the compressed air production equipment was operating adequately, but unfortunately, the compressor controller had malfunctioned, unknown to the plant operators. This caused one of the compressors to continue to run over the weekend in load/unload mode, even though there was an efficient two-stage compressor available. The plant had no way of monitoring the efficiency of the compressed air system, so energy efficiency had inadvertently slipped. The plant realized this after discussion with the utility and corrected the situation.

### Conclusion

This project shows the kind of savings that can be gained by not only replacing compressors, but also correcting the control through use of an intelligent sequencer and added storage capacity. Additional measures like efficient air dryers and flow control made for extra savings on top of what the efficient production equipment could deliver. Of course, a substantial savings was gained by simply turning off the compressors during non-production hours. Despite good control being in place, the extra logging showed if the system efficiency is not monitored, the operation of the system could become less efficient as time goes on.

##### Figure 8: Second logging years later showed problems. Click here to enlarge.

For more information contact Ron Marshall, Marshall Compressed Air Consulting, tel: 204-806-2085, email: ronm@mts.net.