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Debunking the VSD compressor myth

With an increasing number of variable speed drive (VSD) compressors being employed to meet compressed air needs, a common misconception has arisen — that a VSD will always be more efficient than an on/off motor.

With many industrial facilities operating around the clock, plant-wide efficiency is a paramount concern. In recent years, the quest to address efficiency in compressed air systems has seen an increase in the use of variable speed drive (VSD) compressors. Together with this, however, has come the often incorrect assumption that a VSD will always provide a more energy-efficient compressed air solution than an on/off motor.

This misconception is born from the belief that a variable speed rotary screw air-end displays the same performance characteristics as a variable speed fan. However there is a fundamental difference between the two — the efficiency of a rotary screw air-end is affected by changes in the speed of the apex of the rotary screw (tip), while the efficiency of a rotating fan blade remains essentially constant, regardless of the rotational speed of the fan (see Figure 1).

This difference stands to reason, given that the rotary screw air-end is a far more complex device than a simple fan. A rotary screw air-end transforms the volume, temperature and pressure of incoming air, whereas a fan merely moves the air from one side of the fan blade to the other.

Pushing the efficiency envelope

VSD compressors, by design, have an optimum tip speed ‘envelope’ within which they operate most efficiently. The optimum efficiency ‘envelope’ is usually bound by tip speeds of 20 m/s (at the low end) and 40 m/s (at the high end). A tip speed of less than 20 m/s results in air escaping back through the tip, while tip speeds greater than 40 m/s creates turbulent and inefficient compression.

Unfortunately, VSD compressors are designed to span the entire tip speed range corresponding to 0 to 100 per cent compressor load. As a result, VSD compressors cannot operate within this optimum tip speed ‘envelope’ across the entire capacity range of the compressor. Manufacturers of VSD compressors predominantly size the air-end to operate with an optimum tip speed corresponding to 70 per cent load. When compressor loads reach above or below 85 per cent, the tip speed is, in turn, adjusted above or below the optimum, and the VSD compressor’s efficiency drops away.

As a result, compressors employing VSDs are more efficient than conventional on/off motor-driven compressors through approximately 50 per cent of their capacity range — outside this ‘50 per cent’ envelope they are far less efficient. The manufacturer determines where this ‘50 per cent’ envelope lies. If the compressed air load regularly occurs outside this range, then other compressor options would be best suited to the application in place of a VSD compressor.

A variable output compressor (VOC) represents an ideal solution to maximising the efficiency of compressed air usage on the plant floor. The VOC is able to address changes in compressed air demand without changing the tip speed, making it one of the most energy-efficient compressed air solutions available. By controlling the delivery of air to the rotary screw air-end, the effective rotor length and compression volume is modified, enabling the delivery of compressed air at the selected capacity and pressure (see Figure 2). The VOC operates at an optimum tip speed across the entire load range (0-100 per cent) and does not change throughout the output capacity range.

The compressor bottom line

Before spending capital on a VSD compressor, careful analysis of the compressed air requirements of the application is crucial. As many applications require varying quantities of compressed air at different pressures, and at intermittent times, the resulting fluctuation in compressor load profile can complicate the task of selecting an appropriate compressor system.

A rotary screw air-end has an optimum tip speed envelop that can be breached by a VSD. When compressor loads frequently reach levels that demand rotor speeds outside of the optimum tip speed envelop efficiency decreases and energy and money is wasted. VSD compressors are therefore only a more efficient compressor option for certain applications. Conventional on/off load compressors are often still the more cost-effective solution.

*Dino Alessio

Marketing manager

Champion Compressors Pty Ltd

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