Unwinding the spin

A critical look at variable-speed-drive air compressors.

By Chris E. Beals

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During the past several years, the variable-speed-drive (VSD) compressor has become a frequent choice for new compressor purchases. The VSD compressor’s popularity is partly due to rising energy prices and its efficiency as a trim compressor.

Unfortunately, much of the VSD compressor’s popularity is a result of marketing spin. For example, sales personnel offer “free audits” during which they install a black box that might as well flash the phrase on the screen, “Buy a VSD compressor.” Often, the free audits exaggerate paybacks by including savings the end user can achieve without purchasing a VSD compressor. The marketers also fail to provide the necessary engineering details required to maximize the purported benefits. It’s time to eliminate the spin and point out both the limitations and engineering required if you are to derive the benefits of a VSD compressor.

The spin

A trim compressor is a cycling compressor that matches supply capacity to fluctuating system demand, while a base-load compressor is a constant-speed compressor operating at its maximum efficiency.

Over much of its capacity range, the VSD compressor is the most efficient part-load rotary screw compressor. On the other hand, the VSD compressor isn’t a panacea for all your compressed air system problems, as the spin doctors would like you to believe. Some of what they say about VSD compressors includes:

Figure 1. The spin doctor will show percent power as a function of percent capacity, which masks the efficiency of the constant-speed compressor.
Figure 1. The spin doctor will show percent power as a function of percent capacity, which masks the efficiency of the constant-speed compressor.
  • Compressor efficiency: “You get maximum efficiency while peaking at 100%, or only operating at the average, 50% to 70% of capacity.” In graphical form, the spin doctors always show a power consumption comparison of operating modes in a graph of percent power versus percent capacity (Figure 1).
  • Energy savings: One ad claims, “Cuts energy costs up to 50%,” while another ad by the same manufacturer says, “40% reduction in energy costs possible.” Another states “Energy savings up to 35%.” Yet another manufacturer says, “Slash power use resulting in a life-cycle cost savings of 22%.”
  • Power factor: The spinners claim, “No penalties for spikes or low power factor.”
  • Free energy analysis: They want us to believe that a free energy analysis is the same as a compressed air system audit.
  • Turndown: Spinners want us to believe that every VSD compressor has a turndown of 80% or more.
  • Nameplate horsepower: The claimed horsepower often isn’t the required horsepower.
  • Constant pressure: One manufacturer claims, “Line pressures held within 1 psi,” while another says, “Line pressures held within 1.5 psi.” Still another states, “High process stability.” This leads compressor sales personnel to make the statement, “If you install a VSD compressor, you won’t need a pressure/flow controller.”
  • Storage: Because the VSD compressor responds to system events with online horsepower, the spinners tell us, “A large receiver isn’t required.”
    While, in a limited perspective, one can interpret these claims as technically true, the spin doctors have gone for the maximum rotational speed. From this point forward, however, the discussion about VSD compressors enters a No Spin Zone, where we peel off the spin to leave only the facts.

Compressor efficiency

Figure 2. To take out the spin in Figure 1, look at percent power as a function of volumetric flow.
Figure 2. To take out the spin in Figure 1, look at percent power as a function of volumetric flow.

VSD compressor literature uses the percent power versus percent capacity graph to show how much more efficient the VSD compressor is than a constant-speed compressor operating in either modulation or load/unload mode. The VSD and VSD II curves (Figure 1) represent efficiency curves from two manufacturers. The problem with this graph is that it hides the benefits of constant-speed compressors.

For example, at full load, a VSD compressor isn’t as efficient as the comparable load/unload compressor from the same manufacturer because the variable-frequency drive increases power draw by 2% to 4%. To remove the spin, simply plot the graph using actual air capacity instead of percent capacity. As you approach 100% power, the constant-speed compressor is more efficient than the VSD compressor (Figure2). In addition, constant-speed compressors generally have a higher capacity. This figure shows that the constant-speed compressor delivers 10% or 45 cfm more capacity at full load than the VSD compressor, while the VSD II compressor delivers 4 cfm less.


Figure 3. Efficiency as a function of volumetric flow.
Figure 3. Efficiency as a function of volumetric flow.

Graphing compressor efficiency (input kW/100 cfm) versus cfm (Figure 3) properly portrays the efficiency differences. The load/unload efficiencies assume 10 gal./cfm of storage, based upon the trim compressor’s capacity. The graph shows that as the load on the compressor decreases, the VSD compressor efficiency is relatively flat between approximately 46% and 78% of its capacity, while the load/unload efficiency decreases rather quickly. Furthermore, below 15% to 20% capacity, the VSD compressor’s efficiency follows a downward trend, while the load/unload compressor’s efficiency, because of unloaded horsepower, remains at a higher level.

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