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



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)


Variable-speed drive:

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


= 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)


= 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.

Variable-speed drives also can pull down to very low loads (15 percent to 25 percent) rather efficiently and then shut off and restart more times per hour than a similar constant-speed induction motor. Certain types of drives have a greater ability for unlimited starts and stops per hour than others. When reviewing various options for a specific load profile, the plant should not overlook this very important feature ," it can and will save significant energy dollars under certain conditions.

Almost all manufactures offer unlimited starts and stops per hour. The plant should review this start and stop performance with careful detail to its application. At one end of the spectrum are DC motor drives that have virtually no heat buildup, while some other types have a 20-second (sec.) or more ramp-up time.

A lubricant-cooled unit with a 20-sec. ramp-up time must have enough effective storage to allow it to fully unload, blow-down if required, coast down and restart to full flow ," without collapsing system pressure if it shuts off.

Table 2 compares two typical variable-speed systems.

Variable-speed drives with lubricant-free or lubricant-cooled rotary screw air ends, when properly applied, can bring about great energy ," and cost ," savings. In addition, they:

Provide excellent trim capability when required.

Deliver an effective steady pressure of 1 psig to 2 psig operating band within their turn-down range.

Start and stop the motor effectively more often when required.

It is critical for the plant to accurately ascertain the demand before considering the cost-effectiveness of any new compressors ," whether the compressors involve a variable-speed or another type of drive. Only then can the plant accurately review the projected input kW and the overall impact on the facility's electrical energy costs.

Other articles and publications detail how to perform a complete air system review. The chemical plant can do this itself, or can ask the variable-speed-drive sales personnel to do it. Independent consultants also are available. Plant operators should not make major decisions unguided, potentially missing "golden opportunities."

A plant example

One processing plant has a 1,526-cubic-foot-per-minute (cfm) compressor and a 1,026-cfm, 200-hp compressor. Both are fixed-speed, lubricant-free two-stage rotary screw compressors. The 1,526-cfm model runs base load, and the 1,026-cfm, 200-hp model is used as the trim machine.

The demand profile is between 2,000 cfm and 2,500 cfm, with an average flow of 2,300 cfm to all three production shifts (6,240 hours per year) and about 900 cfm for limited production and maintenance during weekends and holidays (2,520 hours per year). The plant has large air receivers and piping for effective storage.

A sales presentation on a 1,526-cfm class variable-speed-drive two-stage lubricant-free rotary screw compressor pointed out that at 50 percent load (750 cfm), the compressor was at only 50 percent of full-load power, while the conventional units with effective two-step unloading would consume approximately 60 percent of full-load power. According to the sales presentation, a new variable-speed-drive 1,526-cfm trim unit in the system could slash electrical energy costs by as much as 40 percent.

The data sheets supplied by the salesperson included BHP (shaft horsepower) ME and full-load amps, but not PF or input kW. The manufacturer then supplied input kW data at various loads to allow an accurate comparison. The plant was able to fill out a load profile/operating cost estimate sheet. See Tables 3, 4 and 5.

For the plant example, the total estimated annual electrical energy cost with the existing 1,526-cfm baseload unit and 1,026-cfm trim unit is $142,784. The total estimated annual electrical energy cost with the existing 1,526-cfm baseload unit and "new" 350-hp class variable-speed drive unit is $149,616.

In this particular case, the existing 200-hp two-step control unit acting as the trim unit has an estimated $6,832 lower electrical energy cost (at 0.05 kWh) than an alternate variable-speed-driven lubricant-free rotary screw compressor. However, in another profile with other equipment ," or even the same equipment ," the variable-speed-driven unit might offer significant savings and quick payback. (For example, a measured lower load at longer hours; lubricant-cooled rotary screws with a wider efficient speed range, etc.)

The key point is that plants should NOT simply assume the variable-speed drive will be the silver bullet that will reduce compressed air energy costs. It is important for plant operators to:

Obtain all the important data, particularly input kW, for all units at critical percent-of-load points. Plants can and should evaluate existing equipment.

Identify and optimize demand or load profile over all pertinent conditions ," production, nonproduction, etc. Assign hours per year to each of these conditions to evaluate the true operating cost. The plant example provided here is somewhat simplistic in that the production process does not vary much. However, the example is based on an actual processing plant's logged data.

Identify blended electrical power costs in $0.0/kWh, but do not lose sight of the effect on demand charge if a smaller or variable-speed drive can trim and shut off a larger unit.

Prepare an operating cost comparison.

Optimize the system. Is air on necessary on weekends and holidays? How much air is essential? How much air is being lost to leaks?



The variable-speed drive, particularly in rotary screws, is an excellent tool for optimizing the electrical energy cost to drive chemical plant air compressors. However, it is only one of many available tools. Like other tools, it must be implemented correctly and at the right time to avoid misapplications and missed opportunities. Its capabilities and drawbacks must be clearly understood.

Because most compressor manufacturers now offer excellent "factory packages," these manufacturers have the tendency to oversell the variable-speed drive. Remember, it is not a cure-all for every application and might use more electrical energy than other options, depending on the facility's unique requirements.

The purchase of a new air compressor as a base or a trim unit is a major capital expenditure; therefore, plant decision-makers should evaluate their options carefully. In all likelihood, the plant will spend about the same amount on electrical energy per year to run the unit as it initially paid for it. A mistake or misapplication in this area not only has the potential to waste capital money with little or no offsetting gain, but also could bring with it continuing higher operating costs each year over the life of the compressor. CP

Hank van Ormer is president of ," and Don and Scott van Ormer are senior consultants with ," Air Power U.S.A. Inc., Pickerington, Ohio. They can be reached at (740) 862-4112.

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