In the industrial compressor market, most variable-speed drives (VSDs) are found on rotary-screw and rotary-vane compressors; however, VSDs can do well on reciprocating units and VSD centrifugal compressors running on magnetic bearings, which can be an excellent trim compressor under the proper conditions.

These units have been available on the aftermarket for decades. Like so many other products, when VSDs were introduced as factory packages on rotary compressors, most manufacturers saw them as an instant fix for numerous air system inefficiencies. Many plants bought this configuration, even if no real need existed and even if there were other options offering more efficiency for less capital cost. Some thought that if one was good, two were better. VSD units purchased as trim units installed with a central control system couldn’t always maintain proper alignment.

The VSD is the most power-efficient unloading control for lubricant-cooled rotary screw compressors operating from 75% to 80% load down to the lowest at which it can operate. The most power-effective approach from 75% to 100% load is the variable-displacement concept, which usually unloads the compressor in four steps of 12.5%, down to 50% and then defaults to base control below that.

With two-stage, oil-free, rotary screw compressors, the VSD often isn’t significantly more efficient than the standard constant-speed two-step control. However, it’s easy to install and avoids short cycling.

As is the case with every important decision, pay attention to detail when selecting a VSD. Know the demand profile. Know how each VSD unit on your compressors will match up with your profile. A VSD of any design will use from additional power draw compared to operating at constant speed.

Consider rotary-screw compressors operating 8,000 hours/year and producing 490 acfm at 100 psig using 110 bhp on a 460 V/60 Hz/3 Ph circuit, where power costs $0.06/kWh.

Constant-speed drive:
133 A
Power factor = 0.91
Mechanical efficiency = 0.923 x pf = 0.84
Efficiency = 3.684 cfm/A
Power draw = (133 x 460 x 1.732 x 0.84)/1,000 = 89.0 kW
Annual cost = 89 kW x $0.06/kWh x 8,000 hr = $42,720/yr

Variable-speed drive:
130 A
Power factor = 0.98
Mechanical efficiency = 0.958 x pf = 0.944
Efficiency = 3.769 cfm/A
Power draw = (130 x 460 x 1.732 x 0.944)/1,000 = 97.8 kW
Annual cost = 97.8 kW x $0.06/kWh x 8,000 hr = $46,931/yr

Variable-speed drive is 9% less efficient than a constant-speed drive at full load for the same flow and pressure: (46,931-42,720)/46,931 = 0.0897.

Compressor manufacturers use several types of VSD units. These include:

• Variable frequency drive (VFD)
• Switched reluctance drive (SRD)
• Permanent magnet.

VSD compressors don’t exhibit the same performance. Know what you’re reviewing. Get a complete performance curve of scfm/input kW throughout the operating band. This is standard manufacturer operating data. Most systems need only one VSD trim unit. Some situations might be able to use multiple drives more successfully, but be sure to review the operating data in detail. Many factory VSD packages limit the top speed to fit the discharge pressure you select. The higher the pressure setting, the slower will be the full-load speed and the delivered air flow. If you find you can run at a lower pressure, often the electronic controller can be reset to a lower pressure, faster speed and more air.

How does the VSD fit into this system control scenario? The VSD can hold a discharge pressure within 1 psig to 2 psig, so you usually don’t need a flow controller and VSD for trim to hold a steady discharge pressure on the same system. Even with a VSD control, we generally recommend storage to smooth out the flows and give enough time for proper switching.

Central control systems

There are few systems with three or more units that can’t benefit from an effective central air management system. Modern systems sense, or calculate, the coming change before it occurs and quickly take appropriate action, sometimes even before evidence appears in the system.

To make it work, you’ve got to know what you’re doing now that creates poor efficiency, what you want the central air management to accomplish, the recoverable energy cost and how much you must invest to achieve the proper return on investment. Many suppliers of central control systems also are compressor manufacturers, independent, HVAC control companies and the like. Each offers an “intelligent” microprocessor that minimizes energy cost and maximum energy savings.

Early systems often were called alternators or sequencers. The basic protocol was pressure-based as typified by the cascade-type control system (Figure 1).

Figure 1. In cascade control, four compressors, each with a 10-psig operating band, are programmed slightly differently to achieve smooth operation of the overall system. As system pressure deviates from the desired 85-psig minimum to 110-psig maximum, on-board capacity controls load and unload the units.

Modern communication equipment eliminated this pressure-based system in favor of a single target set point pressure that is read from a stable source. Now, we can run the same four units, but over only a 10-psig operating band (85 psig to 95 psig) most of the time. Because this system is pressure-based, nothing happens until system pressure changes.