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?