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Unwinding the spin

Nov. 14, 2006
A critical look at variable-speed-drive air compressors.

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.
  • 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.

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.

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.

The second compressor efficiency graph (Figure 4) zooms in on the crossover point, above which the constant-speed compressor is more efficient. At full load, the constant-speed compressor is approximately 12% more efficient than the VSD compressor and approximately 5% more efficient than the VSD compressor operating at its most efficient operating point.

Energy savings

Figure 4. Efficiency as a function of volumetric flow (expanded detail).

Statements such as “cuts energy costs up to 50%,” “40% reduction in energy costs possible,” “energy savings up to 35%” and “slash power use and reduce life-cycle cost savings by 22%” lead plant personnel to believe that just by installing a VSD compressor, they can reduce their total annual energy cost by as much as 50%. The spin doctors reinforce this notion by offering a free energy analysis or audit. However, unless you operate only one compressor, a 50% reduction isn’t possible merely by installing a VSD compressor. To reduce the total annual energy cost in a multi-compressor system by these large percentages, the VSD compressor supplier also must configure the system to base load each compressor except the VSD unit. Of course, you can base load all of your compressors, except one, without installing a VSD compressor. Therefore, the only actual savings attributable to the VSD compressor is the difference between it and the operating mode for the constant-speed trim compressor. In fact, because it’s easy and inexpensive to convert most modulating compressors to load/unload, the savings attributable to the VSD compressor is only the difference between its annual energy cost and that of a load/unload compressor, not a modulating compressor.

Figure 5. Savings as a function of capacity.

Figure 5 shows the annual savings percentage available by installing a 125-hp VSD compressor versus a 125-hp constant-speed compressor. Note that it’s more expensive to operate a VSD compressor near full load. The graph shows that the VSD compressor won’t provide any savings unless the load/unload compressor operates below 83% of its capacity. In addition, you can achieve most of the savings shown in the lower capacity range by replacing the oversized constant-speed compressor with a smaller unit.

Generally, if you expect that the constant-speed trim compressor will operate above 80% of its capacity, it’s the more efficient choice. On the other hand, if the constant-speed trim compressor is operating below 80% of its capacity, then replacing it with a VSD compressor will provide additional savings. However, the question you must ask is “Will installing a VSD compressor generate enough savings to provide an acceptable payback?” This isn’t an easy question to answer.

For example, in a multi-compressor system, we want the VSD compressor to operate in the trim mode. When demand decreases, we must unload a base-load compressor rather than stopping the VSD compressor. To prevent the base-load compressors from short cycling, which can damage them, the VSD compressor’s turndown capacity must be equal to or greater than the capacity of the largest base-load compressor that will unload.

Therefore, given an 80% turndown, the VSD compressor must be a minimum of 1.25 times the size of the largest base-load compressor that will unload.

If the VSD compressor has only a 50% turndown, then it must be twice the size of the largest base-load compressor that will unload. To use a smaller VSD compressor and still prevent short cycling the base-load compressor, you must either provide sufficient storage or reduce the size of the base-load compressors. As you can see, the payback for a VSD compressor depends on its turndown and system storage volume. In addition to the VSD compressor cost, you must account for other costs such as line reactance and a remote transducer.

Two clients decided to install a constant-speed trim compressor rather than a VSD compressor because the simple payback exceeded a 24-month requirement. In one case, one compressor manufacturer quoted a $22,660 or 74% premium for a VSD compressor having the same horsepower rating as the constant-speed compressor, while another manufacturer wanted a $10,785 (30%) premium, but the constant-speed compressor cost $5,187 more. The second project required a VSD compressor larger than the constant-speed unit. The premium for this VSD compressor was $27,720, or 2.42 times the cost of the constant-speed compressor.

On the first project, we projected the VSD compressor to operate at 50% capacity and provide an annual savings of approximately $7,000, giving a simple payback of 47.5 months. On the second project, operating at 70% capacity, the VSD compressor had a simple payback of 83.1 months. Both projects involved retrofitting existing systems.
When we design new systems, we find we that installing appropriately sized constant-speed compressors provides the same or lower annual energy costs than a system containing a VSD compressor, and at a lower capital cost.

Chris E. Beals is president of Air System Management, Inc., in Denver, Colo. Contact him at [email protected] or 303-771-4839.

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