Industrial Utility Efficiency

# Air Compressor Control at Remote Mining Complex

A large mining complex in a remote northern region of the world invited a compressed air auditor in to assess the efficiency of a problematic system. Site personnel and their air compressor supplier were concerned a system in one of the buildings was not running optimally, and wanted to know what size of compressor to install in the facility. The auditor found significant savings in this target system, but even larger potential savings were found in other ancillary systems in the complex, as part of an extra investigation conducted while at the site. Overall, the potential energy savings total more than half of a million dollars, if all recommendations are implemented. Due to the remote location, this facility pays a significant cost per kWh, therefore, potential project paybacks are very attractive.

### Initial Findings at Rock Crushing Target System

A rock crushing facility, one of many separate systems on the mine site, has a compressed air system consisting of two air-cooled screw compressors, one 250 kW (325 hp) 7.4 bar rated lubricated fixed speed for a plant air system, and one 45 kW (60 hp) 7.5 bar rated oil free compressor for a separate instrument air system. Historical operating hours show, until a few years ago, the small 60 hp oil free compressor fed the complete plant most of the time. A dewpoint controlled heatless dryer rated at 297 cfm conditions the instrument air. Only the instrument air used in the plant is passed through the dryer, the general plant air is undried. The dryer has a general-purpose coalescing filter installed before the air dryer, and a particulate filter on the outlet. Check valves exist at the discharge of the air compressors and between the plant and instrument air systems to prevent wet air from contaminating the dry instrument air.

The compressors and dryer are located in the basement area near a crushed rock unloading area, with poor air quality and ventilation. As a result, there is a fair amount of dust on the compressed air equipment. The compressed air is directed through the plant processing area by a system of steel piping.

A large 3,800-gallon wet receiver tank is located in the compressor room at the discharge of the large compressor. A 1,060-gallon dry receiver is located at the outlet of the air dryer for this instrument air compressor. The condensate drains used in the facility are timer drains. Most of the pressure loss in the facility is across the drying and filtering system. Leakage detection was done with only a few small leaks noted, nothing significant was found, and as such not reported. The plant personnel are doing a good job finding and repairing leaks.

### Compressed Air System Baseline

The compressed air system’s electrical power consumption was monitored using data loggers connected to the compressors. System flow has been calculated using compressor rated flow (from compressor nameplate), and multiplying by the compressor duty cycle (no flow meter was installed). Pressure loggers measuring pressure gradient were located at the compressor discharge and after the air dryer and filters. The captured baseline is shown below:

Rock Crushing Facility Baseline

Based on the baseline energy consumption the site electrical cost, the typical annual operating cost of this system would be about \$291,000 per year. The readings and observations during the measurement period showed the air compressor(s) are producing compressed air at a poor efficiency of 28.1 kW/100 cfm (normal is about 20 kW/100 cfm). This poor specific power is caused by excessive unloaded run time of both compressors. The actual plant loading is only slightly above the capacity of the 45 kW compressor, yet the only other available unit is a 250 kW compressor. The 250 kW compressor spends most of its operating hours in the unloaded condition, because it is much too large for the average flow. Compressor control settings limitations, actual pressure bands and the presence of check valves makes this problem worse because the small compressor also runs partially loaded at the same time. During some unnecessary purge operations, problems with the uncontrolled dryer purge control consume additional compressed air. Poor compressor room ventilation, and the presence of excessive dust is also causing less than optimum ambient conditions, as well as compressor shutdowns. ### Air Compressor Master Controls to Prevent Control Gap – Webinar Recording Download the slides and watch the recording of the FREE webcast to learn: • The air compressors’ need to be matched to load effectively and efficiently. • The instability and/or inefficiently that can result if the range of variation cannot be matched. • The problem with control gap and what to do to avoid it. • How a glass factory used flow meters for insight into their compressed air consumption. • The master controller that was installed to save \$150,000 annually on their consumption.
• The importance of compressed air measurement.
• Understanding what the data entails and how to best utilize the data that is received.

Take me to the webinar

### Compressed Air System Operating Profile

The following graphic shows a profile of the compressed air crushing system operations over the last part of the measurement period. During this time the 250 kW compressor was manually shut down between production cycles to save energy. Although this is an excellent practice, there are multiple periods of time where the plant flow exceeded the capacity of the small compressor. This causes the pressure to fall to low levels, because the 250 kW unit is not in automatic and could not start to support the pressure. During this time, the small compressor was operating quite efficiently, because a fixed speed compressor should be running near full capacity.

When the two compressors were both running together the system efficiency fell to low levels. The existing control settings and the check valves caused both compressors to operate in part load condition at the same time. This caused significant periods of unload run time for both compressors.

### Compressor Control Problem

Analysis of the captured data shows, in normal operation, the plant compressed air demand is only slightly above the capacity of the 45 kW instrument air compressor (240 cfm). Peak demands during the measurement period were as high as 500 cfm. When the compressed air demand increases over the capacity of this small compressor, the only option to maintain adequate pressure is to start the large compressor and run it lightly loaded. Unfortunately, when the large compressor runs, the check valves block the actual system pressure from reaching the small compressor control. This causes the small compressor to keep running inefficiently by loading and unloading at the same time. Essentially, a large 1,600 cfm compressor is started to feed a 100 to 200 cfm shortage of compressed air.

The table below shows the operation profile and the compressor pressure setpoints.

Compressor Setpoints

These settings make the large compressor the lead unit whenever it is active.

With these setpoints, the small compressor should normally turn completely off when the large compressor runs, minimizing its wasted unloaded run power. However, due to the check valves at the compressor discharge and before the plant air wet tank, the small compressor cannot “see” the true system pressure when it unloads. Leakage in the compressor supply lines (the compressor condensate drain leaks) between the check valves, causes the pressure to fall at a faster rate than the actual plant air receiver pressure, as the air drains out of the lines. This causes the small compressor to reload at its 90 psi setpoint before the large compressor gets a chance to load at 94 psi. A special test was done, bypassing the check valves and tying the two compressors to the same header by opening a crossover line. When this was done the small compressor timed out and shut off, saving power.

It is very inefficient to run two compressors in load/unload mode at the same time to produce the air only one compressor could produce. In this mine’s case, it is also very inefficient to run the large compressor, rated at about 1,600 cfm, solely to feed about 370 cfm of average load. The unloaded run power of the large compressor (49 kW when no compressed air is being produced) is actually higher than the fully loaded kW of the small compressor (43 kW).

Normally, if the small compressor was in good condition, the recommendation would be to simply add a second 60 hp compressor to work with the existing unit. However, the existing compressor is in poor condition, so it would be best to replace it with a new compressor with a larger capacity and VSD control.

Some increased efficiency could be gained by controlling the system pressure to slightly lower levels of 90 psi. VSD control above minimum speed would maintain a constant plant pressure, reducing the compressor power, and slightly reducing the plant demand.

### Potential Energy Conservation Measures

Some specific potential opportunities are as follows:

Recommendations:

• Replacement of the 60 hp (45 kW) instrument air compressor with a new 110 kW (150 hp) VSD controlled compressor would save 336,600 kWh, annually worth \$107,700 per year in operating costs (37%). • Remove the check valves from the compressor discharge and from before the plant air receiver. Run the system with the main crossover valve open to tie all compressor discharges together. • Set the new VSD compressor to 90 psi (current average 96 psi) target pressure. This would save 13,200 kWh and \$4,200 in operating costs.
• Replace air powered vibrators with electric for \$21,500 annual savings. • Repair faulty air dryer dew point control or replace air dryer with purgeless design, for \$18,000 annual saving.
• Upgrade system filters to mist eliminator or oversized design.
• Replace timer drains with more efficient airless drains.
• Upgrade ventilation to ensure compressors remain clean.

### Extra Savings Summary

An example of the savings that can be gained is shown in the following profile of the PH1 starting system compressor operation (Figure 4). It can be seen, other than a few cycles of extended unloaded run time, the compressor loads and then completely shuts off, reducing the wasteful run time. This is the reason this system has significantly lower costs than all the others. The occasional periods of unloaded run time could be eliminated by simply widening the compressor pressure band or adding storage receiver capacity. As an example, the PH2 starting system is running at less than 10% capacity, yet consuming about 70% of full load power (Figure 5).

##### Figure 6: PH2 modulating compressor consumes constant power even at light loads. Click here to enlarge.

Basic energy conservation measures recommended in these systems:

• Installation of properly sized VSD compressors in each large system, 90 kW and above.
• Operation in start/stop mode with large storage for smaller systems.
• Repair or recalibration of faulty air dryer dew point controls.
• Better operation of internal refrigerated dryers so they shut off with compressors.
• Replacement of timer and manual drains with airless style.

A summary table of the estimated potential savings is as follows:

### Conclusion

About \\$580,000 total, in potential energy savings were identified in only a few days of study. This is just as effective in generating profits as finding a large chunk of a valuable resource, like gold or diamonds. If implemented, the energy conservation measures generate these savings year after year. This is a good example of what can happen when a system is assessed with instrumentation. Plans at this site are to have a closer look at the mine air, and some other additional systems, to see if more savings can be gained.

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