Where peak electric demand charges are high or rising, using a natural gas-driven compressor instead of an electric motor could significantly cut operating costs. In many cases, the savings is sufficient to justify adding such a machine to an existing compressor bank, or even replacing existing older electric drive compressors.

When compressed air is part of a manufacturing process, having a natural gas-driven unit “in the loop” provides the same kind of security as a backup generator at a lower installed cost and without many of the headaches (e.g., permits) that are attached to on-site generation. Recent rolling blackouts and spiking power prices add new reasons for considering this option.

Compressed Air Is Everywhere

Often called the “fourth utility,” compressed air is an integral part of many varied building operations. Many manufacturing plants use compressed air in a variety of applications to operate equipment, move materials, and in chemical processes. And while many large institutions have moved over to direct digital controls (ddc), a remarkably large number of them maintain pneumatic systems to control air.

Nearly all such compressed air systems utilize electric motors as their source of drive-power, but the rising price of power has pushed some plant operators to look for other ways to compress air. Now in their tenth year of production, natural gas-driven air compressors have become a viable alternative. According to the Cogeneration Coalition of America, the number of installed gas-driven air compressors has risen by over 25% in the last year. Natural gas engine-driven air compressors are now available from 20 to 900 hp, and typically operate at variable speeds ranging from 1,500 to 3,600 rpm.

But Why not an Electric Motor?

Four reasons are generally given for considering a natural gas-driven air compressor:

  • Depending on the relative costs of power and natural gas (or propane), the cost of compressing air may be significantly lower using natural gas instead of electricity.
  • The typical gas-driven compressor employs a variable-speed drive, allowing more efficient load following than a continuous speed electric unit whose output is instead throttled; most gas drives maintain their high efficiency quite well over a 2:1 turndown ratio.
  • In industrial applications, recovery of the engine’s waste heat for plant operations is also an option.
  • Depending on its design, the unit may run even during a power outage (though some low-voltage engine controls typically require battery backup to sustain operation).

As usual, some engineering and integration with existing systems is essential to handle exhaust, noise, fire safety, etc., but the paybacks can sometimes be major. A recent installation at the Kraft Foods plant in Champaign, IL was expected to save about $225,000 in reduced energy and maintenance costs but, after only one year of operation, total savings were even higher.

Maintenance Is Important

Like all combustion-based drives, maintenance can be crucial. While most electric drive compressors require little attention, natural gas-driven units require regular replacement of major components every few years. Unless a company’s maintenance personnel are experienced in such matters, it may make sense to outsource the maintenance to an experienced contractor. Several years ago, another facility in Canada (Nestle’s Foods) found that the additional maintenance required by its gas drives was an unacceptable burden, stating that occasional breakdowns cost more in lost product than was saved in energy.

Gas Compressor Benefits

One of the best ways to squeeze value out of this option is not by replacing an existing electric compressor, but rather by adding a gas-driven unit that feeds into the same compressed air header as the electric unit. Where electricity prices are volatile, being able to switch from electricity to natural gas horsepower creates a “dual fueled” compressed air station that runs cheaper than either an all-electric or all natural gas-driven system.

Who Offers Such Equipment?

Several manufacturers and the Gas Research Institute (GRI) are happy to provide information on these still somewhat specialized pieces of equipment. Only a few firms, such as the Quincy division of Coltec Industries and Grimmerschmidt, presently offer these systems as skid-mounted, off-the-shelf products, but the changing energy landscape is providing new impetus for an expanded market for them. Several other air compressor manufacturers (such as Gardner-Denver, Ingersoll-Rand, and Joy) have also, at a customer’s request, supplied their equipment with gas engine drives. Check out the websites of compressor manufacturers and GRI for more information.

Being able to save 40% (or more) on the cost of providing compressed air in a factory, lab, or central plant may also be a good way to earn next year’s bonus! ES