Choose a Winning Drive Strategy

Electric variable-speed drives on lubricant-cooled and lubricant-free rotary air compressors are not a cure-all for system woes -- chemical plants must implement them correctly and at the right time to achieve maximum energy savings

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In the 1990s, extra large storage with high-pressure and regulated flows often was the "silver bullet" for all compressed air system woes. The electric variable-speed drive on lubricant-free and lubricant-cooled rotary screws now is trying to take its place as the "silver bullet" for the new millennium.

Of course, no silver bullet exists for air system problems. However, like proper storage with controlled flow, the variable-speed drive ," particularly on today's rotary screws ," is an excellent tool to help increase system efficiency.

Electric motor variable-speed-driven compressors are not new and have been employed many times during the last 30 to 40 years. Most of these drivers historically were variable-frequency drive (VFD), and the units usually were built as specials by the manufacturer or packaged by specially trained and experienced organizations.

However, modern electronic control systems have improved control response time significantly and often compress the effective operating pressure band to a 1-pound-per-square-inch-gauge (psig) or 2-psig variance. New types of variable-speed drives have become available in addition to VFDs. VFDs themselves have become more efficient and responsive.

Today, almost all manufacturers offer some form of variable-speed drive as a "standard" product in their rotary screw compressor packaged lineups.

Reviewing the basics

A number of "basic truths" cover all variable-speed drives.

All types of electric variable-speed drives are less power efficient at full load than comparable constant-speed units. An electrical energy loss always is associated with variable-speed-drive equipment and will show up dramatically at higher load conditions.

For example, two 100-horsepower (hp) lubricant-cooled rotary screw compressors at full-load pressure, by the same manufacturer, can be compared ," one at a standard constant speed and the other at a variable speed.

Each unit delivers a rated 490 actual cubic feet per minute (acfm) at a 100-psig full load pressure and a power draw of 110 brake horsepower (BHP). The BHP is the power required at the compressor shaft to deliver full flow at full pressure. The scenario assumes the plant's electrical energy bill is based on input kilowatts (kW) to the electric motor (electric energy cost = kW x hours x rate per kilowatt-hour [kWh]).

The variable-speed-drive model draws 124 amps at 460 volts at full load. The motor has a 0.944 power factor (PF) and a 0.958 motor efficiency (ME) at full load. The constant-speed drive model draws 133 amps at 460 volts at full load and has a 0.84 PF and a 0.923 ME at full load.

The variable-speed-drive motor pulling 124 amps is more power efficient than the constant-speed motor pulling 133 amps ," right? Wrong. Consider:

Input kW = (amps)(volts)(1.732)(PF)

1,000

Variable-speed drive:

Input kW = (124)(460)(1.732)(0.944)

1,000

= 93.26 kW/$40,848/yr.@$0.05 kwh/8,760 hr.

Note: 1.732 is a constant in the input kW formula, converting the number for three-phase power.

Constant-speed drive:

Input kW = (133)(460)(1.732)(0.84)

1,000

= 89.0 kW/$39,986/yr.@$0.05 kwh/8,760 hr.

At full load, the constant-speed-driven unit is approximately 5 percent more efficient and costs $862 less per year to run at $0.05 kWh. The "break-even" point between similar-size constant-speed controls and variable-speed drives usually will be approximately 70 percent to 80 percent of load, depending on the true operating characteristic of each unit.

Therefore, variable-speed drives should not run at higher loads for a significant number of hours per year. They are effective "trim" units, not base load. Most plants will need only one appropriately sized variable-speed-driven unit as a trim compressor.

Most capacity control performance data are presented as a "percent of full-load power." Regardless of the type of capacity control, it is unwise to use "percent of load" to determine the best choice for the plant's operating environment. This rule of thumb is particularly true with variable-speed drives.

The "percent of load" should be translated into actual input kW throughout the operating load profile and applied to the number of hours of operation to determine the total kWh per year. See Table 1.

The calculations should compare some typical capacity controls at various loads. The manufacturer is capable of providing actual input kW. The conventional method of using BHP x 0.746 or 0.7457 and dividing by ME is misleading. It will miss the variable-speed-drive equipment loss and/or belt losses in belt-driven equipment.

Keep in mind that the variable-speed drive starts at a higher full load (kW) than a comparable constant-speed model. Accurate performance charts will show where the break-even point is and where the plant can start saving energy.

Applying variable-speed drives

Of all the capacity controls available on lubricant-free and lubricant-cooled rotary screw compressors, none are more power efficient in their effective turndown range than variable-speed drives. Variable-speed drives can operate at a partial load at basically a full-load efficiency.

This effective turndown range will vary with different manufacturers, different types of variable-speed drives and the performance profile of the rotary screw air end. For estimating purposes, the most effective turndown range usually will be from 40 percent to 80 percent of full flow. Lubricant-cooled rotary screws usually have a wider effective turndown range than lubricant-free versions. The plant should review the unit's specific performance data to verify this.

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