Energy savings are often disguised as problems

Carefully managing compressed air can save money. Compressed air is often overlooked in energy studies because many people do not fully understand compressed air equipment, their own system, or what it costs to produce compressed air power.

By Scott Van Ormer and Don Van Ormer, senior application engineers, Air Power USA

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Air amplifiers, or inducers, will reduce compressed air flow, where a mechanical method, such as a wiper, cannot replace blow-off. Air amplifiers use “venturi” action to pull in ambient air and mix it directly into the compressed air stream. The ratio of ambient air drawn in can be substantial. Amplification ratios up to 25:1 are possible. Using 10 cfm of compressed air can produce up to 250 cfm of blow-off air. This is a savings of 15 cfm of compressed air per ¼-in. blow-off.

Although air-operated diaphragm pumps are not very energy efficient, they tolerate aggressive conditions well and are not seriously damaged if run dry. Also, they are not a serious hazard in intrinsically safe environments. If an electric pump — diaphragm, centrifugal or progressive cavity — can be made to work, it will usually be cost-effective. When an air-diaphragm pump is the still the best choice, reduce the utility costs by either shutting off the air if the pump is not needed, or lowering the supply pressure. Generally, when a pump must operate for long periods, an electric pump will save money over an air-diaphragm pump (Table 1).

Air hoists and air vibrators are sometimes chosen when electricity is not available or where the area is intrinsically safe. This safety issue is generally not a concern with pumps and agitators where isolation is easier. Hoists and vibrators pose other concerns. Air hoists used only occasionally have a high incidence of unnoticed air leaks. High pressure air is required by air vibrators to be effective; economically, they are often a poor choice — except where small size is needed.

Sparging with air may be necessary if the product is sensitive, e.g., friable, or where mixing mechanically is not economical or practical. One example where mechanical mixing might be uneconomical would be where a tank is very long, requiring an agitator with a long shaft. Another would be where air is available but electricity is not. If mixing by sparging is the best solution, doing so at the lowest possible pressure reduces the cost (Table 1). Use low-pressure air supplied by a blower, if available. Use a step-down regulator on compressed air, if only high-pressure air is available.

Not recovering heat

Compressing a gas generates a lot of heat. However, 85% to 90% of the motor horsepower used to run a rotary screw air compressor can be recovered in the form of heated air or water. Similar recovery, in the form of heated water, is possible with water-cooled compressors. It takes 8 hp of electrical energy to produce 1 hp of air power. That means that a portion of the 7 hp of unused energy is available as heat. Recouped heat can be used in many ways: 1) space heaters; 2) heating process water; 3) boiler make-up; 4) and running a “heat of compression” desiccant dryer. Figure 5 depicts a typical reclamation system.

Figure 5. Cooling needed for air compressors can be turned into heating savings.

Figure 5. Cooling needed for air compressors can be turned into heating savings.

Fix it when it breaks

This seems to be the modus operandi with utilities. Too often maintenance on compressed air is viewed as an expense when it is really an opportunity for savings. Maintenance on the supply side has a direct impact on plant energy use.

On a recent audit, we found a compressed air system sorely in need of basic maintenance (Figure 6). Dirty inlet filters lowered the inlet pressure from an expected 14.2 psia to 9.5 psia. This reduced the air production from each rotary screw compressor from 725 scfm to 501 scfm — a loss of 31%. All three compressors were run at full load to supply the 1,400 scfm demand. Correcting this problem saved $45,000/year in electricity.

Figure 6.  Dirty inlet piping costs $45,000/year.

Figure 6. Dirty inlet piping costs $45,000/year. (Click to enlarge.)

Audits also are useful in identifying residual design errors. Figure 6 shows two typical errors. First, a riser after the dryer could collect condensate, which can cause a drag on gas flow. Normally, dips in piping are allowable but should include a receiver and condensate trap; a riser at the dryer discharge is an exception. The second error is the use of tees instead of wyes. Another potential savings overlooked is using outside air (cold air is denser than heated air).

A high pressure drop in filters and separators is a problem we see a lot. If you knew what power costs per psig, then you would probably re-evaluate your schedule for changing separators and filters. A typical pressure drop for a new oil separator is 2-3 psig. After two years, the pressure drop may be 10-12 psig. With the cost of $200/year per 1 psig ($0.06/kWh), and assuming replacement elements cost $550/each, it is prudent to change the elements somewhere between 3-4 psig.

Measurement

If you don’t measure it, you can’t manage it! Do you have a material balance for compressed air? Are the minimum flow and minimum pressure documented for all of your equipment? Are you monitoring your compressor cycle times and power usage? Is anyone keeping track of tie-ins for new equipment or do you have a single 1-in. pipe feeding five packaging machines when a separate line should have been built for all five machines? Do you have flow meters installed at key points in your piping network — if so, when were they last calibrated? Most of the plants we have audited have flow meters on the main air supply but few in the production areas, with calibration every 10 years, whether they need it or not! As for pressure, we generally find that no test points or gages are installed in critical interconnecting pipes.

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