Improve compressor efficiency at the Dirty 30

Reduce wasted inefficiencies at the end of the line.

By Paul Shaw and Niff Ambrosino, Compressed Air Challenge

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In brief:

  • Air leaks are most likely to appear in the line connecting the distribution header to the tool using the compressed air.
  • Air lines should be sized for low pressure drop at peak flow rates, not at average flow rates.
  • Select full-port disconnects, FRLs, and line fittings to minimize pressure drops.

In recent years, manufacturing plants have recognized the importance of improving compressed air system efficiencies. Two key points for plant engineers and energy conservation champions to focus on are to eliminate leaks and to reduce air compressor operating pressure to the lowest set-point possible.

Leaks and pressure drops can occur anywhere, both on the supply side and demand side. These two wasteful energy consumers are typically most prevalent within the final 30 ft between the main distribution headers and the operating equipment, commonly referred to as the “Dirty 30.” This includes pipe runs and associated items such as valves, pressure regulators, filters, lubricators, quick disconnects, hoses, and flexible connections.

One common practice resulting in excessive pressure drop is the use of inadequate pipe size between the main distribution header and the production equipment. The pipe is undersized frequently because the installer or designer based the selection on the expected average compressed air demand only, but didn’t take into consideration the rate of flow. For example, let’s assume that a packaging machine (Figure 1) specification lists the compressed air requirements at 60 cfm. The pipe from the distribution header to the packaging machine is a 30-ft straight run. If we refer to a Friction Loss Through Pipe Chart, a 1-in. pipe might be considered adequate because it shows less than a 0.5 psi drop.

Figure 1: Pay attention to the sizing of components between the air header and the end uses.
Figure 1: Pay attention to the sizing of components between the air header and the end uses.

But this assumes the 60 cfm is consumed equally throughout the minute. If, however, the packaging machine requires 1-sec actuations, each consuming 10 cubic ft of air once every 10 sec at a rate of 6-cycles/min, the rate of flow becomes much higher than the average. Although the average flow is 60 cubic ft/min, the instantaneous rate of flow actually is 600 cfm during each 1-sec cycle. (10 cubic ft/sec x 60 sec/mine is 600 cubic ft/min). In this instance, the 1-in. pipe run would have a pressure drop exceeding 30 psi. One way to overcome this could be to set the compressor pressure higher to compensate, causing the entire system to consume more energy.

When operating positive displacement air compressors in the 100 psig range, every 2% decrease in operating pressure requires about 1% less input power. For example, reducing a 100-hp rotary-screw air compressor’s pressure set-point just 5 psi from 100 psig to 95 psig reduces the input power to compress air by 2.5%. At $0.10/kWh, this calculates to approximately $1,900/year in energy cost reduction. Calculated energy cost reduction is based on the compressor running at full capacity, operating 8,760 hours per year with a motor efficiency of 93.6%. In addition, a decrease in system operating pressure reduces compressed air consumption at unregulated end users.

The correct repair would be to increase the size of that connecting pipe. With these effects combined, the use of a 2-in. pipe in the packaging machine might have reduced the air compressor’s power requirements by as much as 20%.

Point-of-use filters, regulators, and lubricators (called FRLs) provide a clean, stable, and lubricated compressed air supply. The Compressed Air Challenge recommends the following when it comes to filters, regulators and lubricators:

  • Filter and dry the air only to the level required for each point of use or application.
  • Use low pressure drop, long-life filters.
  • Size filters, regulators, and lubricators based on the rate of flow.

You must understand the flow characteristics of air system devices, equipment, and tools and consider them in your compressed air supply system design and installation. In many cases, we find FRLs undersized for the maximum flow rate. Under-sizing any of these components, or sizing them solely on pipe-thread size of other connected components, will result in an increased pressure drop.

For example: the regulated pressure on an FRL system is set at 80 psig, However, the pressure drops to 50 psig or less when a downstream machine cycles and consumes compressed air (Figure 2). This usually results in the incorrect diagnosis of low air pressure in the mains. Often the compressor discharge pressure is raised to compensate for the FRL pressure differential. The correct repair would be to replace the FRL with properly sized components.

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