They don’t make ’em like that anymore

Centrifugal compressor designs have improved greatly in the past 30 years.

By Hank van Ormer

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Modern industrial centrifugal compressors represent an energy-efficient, effective supply of compressed air. Their advanced designs and manufacturing capabilities offer energy savings opportunities to those who understand how to select and install them. The compressor’s operating characteristics, which some might call “quirks,” represent opportunities to the savvy plant professional.

In a recent compressed air review at a large paper mill, we investigated six 500-hp, three-stage centrifugal compressors, all of same model by the same manufacturer and built at separate times during a 30-year interval. The operating curves for these units revealed differing specific power ratings and operating characteristics at 100% full load discharge pressure (Table 1). Much of this improvement in the later versions comes from machining and production processes that work with microns of tolerance instead of the mils used in the past. That capability frees engineers to follow visions that weren’t practical a few years ago. Also, improvements and new technology in electronics, controls, main drivers, inlet guide vanes and the like contribute to overall improvements in basic performance and flexibility.

Table 1
The improvements in similar-sized standard units offer almost a 15% improvement in specific power from 1970 to 2008, with an accompanying improvement in calculated effective turndown from 15% to 30% or more. About 3% of this comes from basic improvements in standard motor efficiency. If the older motors have been rewound or just lost efficiency because of the condition of the breaker, line or insulation, the overall improvement in specific power might well be higher; perhaps as high as 20% or more. The motor performance should be part of the overall evaluation. (Input kW = power to drive the centrifugal compressor’s motors at rated flow/pressure.)

If you bought your centrifugal compressors in the 1970s through the 1990s, it would be prudent to review the basic performance improvements that are available now. An important factor to consider is the net reduction in electric energy operating costs that your operation will enjoy if you upgrade your hardware. Depending on your application, the dollars can be significant.

Any plant suspecting an opportunity should look carefully at what’s available. Several years ago, a large bottling plant in the southeastern Unites States replaced its 1970-vintage high-pressure (100 psig) and low-pressure (55 psig) centrifugal compressors with units built in 2005. The resulting specific power improvement along with a 25% increase in turndown capability yielded a $600,000 per year electrical energy cost savings ($0.05 kWh, 8,760 hrs per year). The new units cost $600,000 and the total installed cost was $1.2 million, which meant there was a two-year simple payback. Utility incentives further reduced this payback period. This was an effective project spawned from a comprehensive, full-plant compressed air audit.

Turndown and electrical energy

The industrial centrifugal compressor is a dynamic unit that uses a rapidly rotating impeller to accelerate airflow (Figure 1), which then passes through a diffuser section that converts velocity head into pressure head through flow resistance. In the dynamic, or mass-flow compressor, like the centrifugal unit, the power to compress the air is a function of the weight of the air, the flow, volume, temperature, and the head or pressure.

Figure 1
The impeller is the heart of the centrifugal compressor.

The impeller’s design and speed establishes the energy imparted to a pound of air as it passes through the impeller. That energy is independent of inlet temperature, pressure, throttling and other variables. A centrifugal compressor, therefore, delivers a pound of air with a constant expenditure of energy, winter or summer. The volume of inlet air to be compressed varies with the inlet pressure and temperature.

If more compressed air is produced than is needed, the centrifugal compressor must unload (deliver less air) to avoid overpressure. A centrifugal compressor has a maximum pressure it can achieve under specific inlet conditions before the air flow reverses and surges, which triggers a compressor shutdown to avoid damage. This is an oversimplified description of surge; however, each unit has a rise-to-surge limit or maximum pressure. Turndown is the fraction of full-load flow the compressor can handle without experiencing surge. For example, 15% turndown means the unit must run at no less than 85% capacity flow to avoid surging.

Remember that the surge point varies with inlet conditions (Figure 2). Air density increases at colder temperatures and higher inlet pressure, reducing the volume of inlet air that reaches the maximum mass flow rate. To hold a constant target discharge pressure, the inlet air flow must decrease to avoid “running out on the curve” too far and reaching the stonewall area of potentially unstable operation. The opposite occurs at higher temperature and lower inlet air pressure. The centrifugal compressor can respond to a varying demand efficiently only within its turndown range. Beyond full turndown, the unit responds in one of two ways. Either it blows off excess air and delivers less air but with no reduction in power consumption, or the inlet valve closes partially and operates with blow-off or recirculation at a 25% to 35% power draw with no flow to the plant. This type of capacity control is storage-dependent.

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