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Tech Talk

Improve Your Plant’s Efficiency and Reduce Cost with Variable Speed Drives

by Andre L. Narbonne

Equipment driven by electric motors often operates at less than full capacity. The motor may turn on and off or there may be a clutch mechanism to disengage the motor to match the equipment’s power requirements. Either design wastes energy compared to a variable speed drive (VSD), which changes the frequency and voltage of the electric power that drives the motor so that the output of the motor matches the capacity requirement. This study shows the benefits of using a VSD oil-free air compressor compared to a standard oil-free air compressor and provides lessons that can be applied elsewhere in a production facility. For example, VSDs are seeing more application in centrifugal chillers (as opposed to inlet vane control), cooling towers (for fan speed control based on temperature), circulating pumps, and HVAC fans.

In this case study, clean air is produced by two oil-free rotary screw air compressors. One serves as the primary while the other is the backup. These roles are automatically switched weekly to balance the wear on the two machines and to ensure the backup is always operational.

In March, 2008 one of the oil-free screw-type air compressors experienced a problem. One of the air ends had seal leakage that required a major repair. Although the repair would be covered under warranty, we conducted a study to quantify the benefits of an oil-free VSD compressor. Compressed air is known to consume 15 percent of the average manufacturing plant’s electricity. This, and the recent increases in energy costs, suggested there could be a significant opportunity for savings.

An important consideration is the cost of electricity to power the 50 hp motor of the air compressor. At $0.13 per kWh electricity cost:

Electricity cost ($/yr)

  • = Motor Size (hp) ∙ 1/Power Factor or Efficiency ∙ Conversions (hr/yr) (kW/hp) ∙ Elec Cost ($/kW-hr)
  • = 50 hp ∙ 1/0.95 ∙ 8760 (hr/yr) ∙ 0.746 (kW/hp) ∙ $0.13 ($/kW-hr)
  • = $44,713 per year

The units in this example do not have Variable Speed Drives (VSD), but rather cycle between loaded and unloaded state, running at a constant speed. For example, the motor runs at full load 37 percent of the time, running unloaded the rest of the time. While the above calculation does (incorrectly) assume that the motor is run at full load all the time, the electricity cost is still substantial, since the motor still consumes 20 percent of the maximum electric power when unloaded.

VSD technology in air compressors can achieve cost savings in two ways:

  • Load Matching – Instead of cycling between a loaded and unloaded state, the machine varies its speed to the required load. When the motor is running unloaded it is consuming electricity but performing no useful work. The greater the time the motor load is below peak capacity, the greater the savings (see Fig. 1).
  • Instead of running in a wide pressure band (typically 10-20 psi) the VSD can maintain a steady output pressure. This reduces generation cost due to the greater energy required to generate air pressure above the set point and reduces parasitic losses due to greater leakage at higher pressures. A 10 psi reduction in output pressure reduces generation costs by about 5 percent.

The study was set to determine if buying a new VSD air compressor would be more cost effective than repairing the existing unit. The study involved using a clamp-on current meter and system pressure recorder to record load data on the system over one week. This data was then graphed and it was clear that the use pattern was, in fact, conducive to a VSD-type of unit (see Fig. 2).

Because a VSD machine can run at less than full capacity, it is able to turn down to deliver just the quantity of compressed air actually required without going through load and unload cycles. It is this cycling that causes energy waste. We used the collected data and AIRMaster+ software (the Department of Energy-approved calculation tool) to quantify the expected savings shown in Table 1.

Table 1 Image
Table 1. Expected Savings

Figure 1 Image
Figure 1. Power consumption of a VSD unit versus that of a Load/No-Load unit.

Figure 2 Image

We examined the cost savings from the decreased electricity usage of a VSD unit but this alone did not justify the expense of the new machine. Since electric utilities encourage the use of energy efficient equipment by offering incentives, we captured this benefit into our analysis. In addition we worked with the equipment supplier to “buy back” the existing (three-year old) machine for $10,000. We applied a Net Present Value (NPV) analysis to compare the value of buying a new VSD machine versus having the old machine fixed.

The results showed a return so favorable that even if the old machine had not broken we would still have been better off replacing it with a new machine.

Table 2 Image

Table 2. Net Present Value Analysis of repair or purchase new options.

The NPV analysis in Table 2 shows the advantage of buying the new machine versus repairing the old one. You may want to look for VSD applications in your own plant if you want to save money on ever-increasing electric costs.

Andre L. Narbonne, PE , Qualified AIRMaster+ Specialist, is a Sales Engineer with Ingersoll-Rand Industrial Technologies, Air Solutions Division. He has 10 years experience in compressed air systems providing a wide range of equipment and service solutions to help improve productivity, reliability and efficiency.

Page last updated: 5 March 2009