Compressed Air System / Operational Excellence / Energy Management

Get active about monitoring your compressed air system efficiency

Can you answer the question, how efficient is your system? If not, here's how.

By Ron Marshall, Certified Engineering Technologist

When it comes to compressed air efficiency, there are usually more questions than answers. Compressed air systems consume a big chunk of the electrical energy in a typical industrial facility, yet often they receive very little attention, unless something goes wrong. This article discusses six questions you need to answer regularly to keep your compressed air system running efficiently and trouble-free.

To answer these questions, you’ll need to use some type of measurement system to monitor your compressed air system. Best-in-class compressed air systems have permanently installed measurement systems (see Figure 1) that can help you know all there is to know about your compressed air system efficiency with a few clicks of your mouse

How efficient is your system?

Efficiency is a measure of how well a system is producing output as compared with the input. With compressed air, there’s an energy input to the compressors and dryers and a flow output at a specific pressure coming out the discharge pipe. A common way to measure compressed air energy efficiency is to measure the amount of power that goes into a compressed air system per given flow of compressed air out. This is called specific power and is usually expressed as kilowatts per 100 cubic feet.

Equipment manufacturers publish air-compressor and sometimes air-dryer power ratings in a format specified by the Compressed Air and Gas Institute (CAGI, www.cagi.org). This format helps buyers of compressed air components make better choices in the efficiency of the equipment they purchase. Calculating the specific power level of your system and comparing this with the equipment ratings can also help you make better choices on how to operate your system..

Can you answer the question, “How efficient is my system?” Most system operators can’t, so have a look at your compressors to find out why not. The search for energy-related metering devices usually ends in failure. Energy or flow metering is not typically something that comes with an air compressor or dryer; it has to be added by the system owner..

As an example of the usefulness of measurement, a food products manufacturer recently had its system power and flow measured by an auditor who placed temporary metering equipment on the compressed air system. The power input to the air compressors was measured at 16.4 kW average, and the flow output was found to be 13 cfm. The rated specific power of the air compressor at its best efficiency point is 20 kW/cfm, but if we do some calculations we can see this the actual system specific power calculates 126 kW/100 cfm, more than 6 times the rating.

Something was definitely wrong with this system, but the system owner was unaware until some measurements were done. At first the cause of this problem was thought to be the compressor, but it was not; the real cause of this problem is discussed at the end of the article.

How stable is your pressure?

If asked to choose between good system efficiency and stable pressure, most system operators would choose pressure. This is the compressed air production equipment’s job: to produce a constant uninterrupted supply of clean, dry compressed air at an adequate pressure. But how stable is your pressure? Do you know? Most system operators can’t answer this question and simply use a tried-and-true low pressure detection method – the phone.

When their customers experience low pressure, the customers call and complain, and then the operator goes to the compressor room to jack up the pressure, and the system is left to run at this higher pressure permanently. This increase in pressure makes all the running compressors use 1% more power for every 2 psi increase in discharge pressure. It also increases the flow, because higher pressure makes any unregulated uses of compressed air consume more. This increases the compressor power consumption even more.

In best-in-class systems, the operators don’t have to choose between good efficiency and stable pressure. Operators will know immediately if there are pressure issues, and will be able to detect when and why they have occurred. To do this, some pressure monitors must be installed at various important locations on the system. These monitors should be recorded or logged at intervals fast enough so that system problems can be detected and corrected, without jacking up the system pressure.

In our previous example, the food processing company was experiencing transient pressure problems and had already turned up the compressor as high as they dared, yet the pressure problems were still occurring (see Figure 2). Placing pressure monitors at the compressor discharge, after the air dryer, and at the main compressor room outlet to the plant showed that this plant was indeed experiencing pressure problems, but it was not the fault of the compressor. Again, the operator was unaware of this issue, which was causing intermittent production issues that were being blamed on the production machines.

Are the compressors operating correctly?

Every compressed air system has a control strategy to ensure the delivery of a constant supply of compressed air at the proper required pressure. With single compressor systems the strategy is usually very simple, but with systems of multiple compressors the strategy can be quite complex. The goal of any strategy should also be to deliver this adequate pressure in the most efficient way possible; this usually means turning down compressor power in some way, by making sure the system compressors are running with minimal unloaded power consumption and are turned off and on at the required times.

It is very difficult to assess whether a control system is doing its job without measuring the system. But if pressure, compressor power (or amps), and flow are captured over time, it is quite easy to detect the response of the control system to changes in flow and pressure. Using this data, long-term system operation can be tracked, with notations made for pressure problems, compressor operational problems, or even problems with peak system air demands causing low pressure.

In another example, a grain processor purchased an expensive compressor control system from the company’s compressor supplier to help maintain good system efficiency. The plant engineer trusted that this system, designed and supplied by a large multinational compressor company, would keep his system operating at optimum efficiency, and therefore no efficiency measurements were taken after the installation.

Years later, a compressed air auditor measured the system and found that the system was running two compressors during light system loading rather than one. Because of the choice of compressor sizes at this site, the supplied control system was not compatible and was malfunctioning. This caused poor system pressure regulation – more than two times the normal energy consumption, as well as extra maintenance and repair costs thanks to the extra compressor run time hours.
The measurement also showed a problem with a desiccant air dryer that was consuming thousands of dollars in wasted compressed air loss. The site had called in the auditor to figure out what size of air compressor they needed to purchase because they thought they were running out of capacity. It turned out that no additional compressor was required, and they were able to turn one compressor off.

What’s causing problems?

If you’re carefully tracking your system, you’ll be tipped off to problems if there is a change in your specific power and/or a significant change in average flow. Often, system monitoring programs will have a dashboard of real-time energy performance indicators. In addition, the data may be calculated hourly, daily, weekly, or monthly for easy comparison. If any numbers change, it should trigger more investigation. 

Because data is being collected perhaps as frequently as every 10 seconds and sent to a database, it will be possible to go back when a problem emerges, review the system operation, and identify what went wrong. In some cases, you’ll even be able to identify a faulty piece of equipment.

What is my system waste?

System monitoring not only measures the efficiency of compressed air production equipment, but also it can often be used to measure system waste. Consider a shift-oriented plant that has no production on weekends. If the compressors run during that time, most of the flow will usually be related to leakage. Monitoring systems can measure the flow and power during these low-load periods at a specific time – say, Saturday at midnight. Then this level can be tracked, with a significant increase over time triggering leakage-reduction efforts.

Many monitoring systems can be programmed to do the calculations for you (see Figure 3) and to calculate the cost of compressed air based on the actual cost per kWh. System efficiency can be seen easily using graphical methods (see Figure 4). It is then possible to place on leakage flow a dollar value – something that plant managers understand much better than they do kilowatt hours.

Of course, if you implement changes to your system, you’ll want to know how much money you have saved. A good monitoring system will have captured the baseline energy, flow, and cost before and after your project, allowing you to make comparisons and show the actual savings. This goes a long way in persuading management to spend more dollars for additional system upgrades. It also makes power utilities happy when they can clearly see proof that the savings gained by a project justify the utility incentive dollars.

Sometimes as facilities grow and more equipment is installed, there comes a time when the system reaches its capacity. If you are constantly monitoring your system, you will be able to track flow as it increases, giving you fair warning that action to increase capacity is required. Having a good flow monitoring system will let you determine the correct size of equipment to install; this in turn will allow you to avoid costly oversizing that might make your system run with poor efficiency for years.

How can I monitor my system?

Years ago, the instruments required to monitor a compressed air system were expensive and hard to find. These days, a number of readily available, low-cost options can be installed on your equipment and connected to a plant monitoring system. Here are some instrumentation recommendations:

  • Pressure transducers should monitor the compressor discharge, the dryer discharge, and the pressure output of the compressor room for each compressor room area. This will allow you to track pressure differentials across system components and ensure the equipment is regulating the pressure correctly.
  • Amp/power transducers (power is definitely preferred) should be installed on all significant energy uses inside the compressor room. Monitors should definitely be placed on each compressor, and optionally on each air dryer. Sometimes fans and pumps are consuming power in the compressor room, so it is up to you whether you monitor these. It is most convenient if the power transducers record both kW and kWh so that long-term cost calculations can be done with a minimum of calculations.
  • Flow meters should be installed at important points in a system – definitely at the output of each compressor room. Sometimes it’s also convenient to install the meters at the discharge of each compressor (this is wet air). In this way, the performance of the compressors can be tracked individually and then compared with the power input. The most common flow meters used in the industry are thermal-mass-style; these are good only for dry air. However, pitot-tube and other meter styles can be used to measure wet air.
  • Dew point meters should be installed to monitor air quality. You should not rely totally on the meters installed on air dryers for this task.
  • Temperature can also be logged in the compressor room, as can (in some cases) the temperature of the compressors’ compressed air discharge, to ensure things are not operating abnormally.

Central control system reports

In many cases, a central compressor control system can not only orchestrate the efficient operation of your air compressors, but it also can monitor and track your system efficiency. Many of these controllers can provide a system performance dashboard on a web page, store system data, and generate reports to allow you to troubleshoot problems and monitor energy consumption. Most major compressor companies have a controller of this sort; there also are third-party companies offering systems that can control and monitor many different brands and types of compressors in the same system.

It is important to realize that some of these monitors calculate the flow and power only from assumed values, based on what the controller thinks the compressor is consuming for power or is producing for flow. The best system monitors measure actual power and flow, or even better, provide both calculated and measured values.

For example, at the beginning of this article, the food-processing plant system was consuming 126 kW per 100 cfm because of a faulty piece of equipment in the compressor room. The piece of equipment was an air dryer that was consuming excessive purge flow – about 70% of the compressed air produced due to a faulty control circuit.

If the monitoring system has been simply calculating flow based on compressor status, then the specific power would have shown as normal, and the monitoring system would have been incorrectly reporting much higher flow into the plant. But if a flow meter were used to monitor flow as an input to the calculations, then the system would have detected low output flow to the plant but an abnormally high compressor-specific power.

Conclusion

Compressed air system monitors provide answers to the important questions that you should be asking about system efficiency and reliability. These systems can open your eyes to system problems and help you provide solutions quickly. If you wish to learn more about compressed air efficiency, consider attending a Compressed Air Challenge seminar in your area. Visit www.compressedairchallenge.org for more information.