Taming multiple compressors

Capacity controls aren't all equal when the system is complex.

By Niff Ambrosino and Paul Shaw

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People on the plant floor know when not enough air compressors are operating. The demand-side system air pressure drops below what production equipment requires, which triggers phone calls and complaints. But, do people know when too many compressors are running and inflating your plant’s energy costs?

The Compressed Air Challenge, sponsored by the Department of Energy, has impressed upon us that compressed air is an expensive utility. Typically, only 15% of the input power to a plant’s air compressor system produces useful compressed air. The rest is wasted as heat and pressure drop. In addition, compressors running in a partially-loaded condition or, worse, totally unloaded for hours on end waste considerable energy and maintenance resources. Many compressors can consume as much as 70% of full-load power while delivering less than 30% of full-load capacity.

The paradox is that even compressors with energy-efficient part-load controls can be inefficient when operated together.

– Niff Ambrosino and Paul Shaw

The controls on individual compressors try to match the compressed air supply with the demand. If supply exceeds demand, system pressure rises and one (or more) compressors need to reduce output or risk exceeding system pressure limits. Rotary-screw, centrifugal and reciprocating compressors, each with varying individual compressor controls, can be efficient if applied and operated properly. The paradox is that even compressors with energy-efficient part-load controls can be inefficient when operated together or in concert with other capacity control types or air compressor brands. This is where modern multiple-compressor controllers can alleviate the part-load energy waste and the effects of wide swings in pressure on your compressed air system and production.

Many multiple compressor controllers available today use different logic to solve the same problem. Some controllers merely start and stop compressors based on system pressure; some rely on time of day to determine which compressors to run, while others will work only with specific compressor types or those from only one manufacturer. The most advanced controllers address all of these situations. Let’s examine the evolution and application of some of the more popular types.

Cascading may require a wide band

Figure 1. Cascading-pressure control operates multiple compressors in a predetermined manner.
Figure 1. Cascading-pressure control operates multiple compressors in a predetermined manner.

This form of multiple compressor control has been in use for a long time. Multiple compressors are controlled based solely on system pressure. As system pressure falls below a setpoint, additional air compressors are brought online in a predetermined sequence. In many cases, the compressor with the greatest horsepower is started first and subsequent compressors are selected in order of descending horsepower. If the system air pressure continues to fall to a lower pressure setpoint, the next pre-selected compressor comes online. This controller requires the plant to operate with a cascading pressure band (Figure 1).

For example, the lead compressor loads or turns on when system pressure falls to 100 psig. The next compressor in the sequence might turn on when system pressure falls to 95 psig and the one after that at 90 psig, and so on. As online capacity begins to exceed system demand, system pressure rises to allow the last compressor brought online to unload or turn off (in our example somewhere around 100 psig) The previous compressor brought online will require system pressure to rise to 105 psig before unloading or turning off. As the number of compressors increases, so does the pressure band and energy consumption.

In a nominal 100-psig system, a 2-psi increase in discharge pressure results in a 1% increase in input power to the compressor. In addition, higher system pressure increases the consumption attributed to all unregulated users and leaks. Often, systems with a cascade-type controller have insufficient storage capacity and require a wide pressure band. When the pressure differential between the compressors’ maximum allowable operating pressure and plant’s minimum required pressure is tight, too many compressors end up on line for the demand required, and the full benefit of individual compressor energy saving controls is lost.

Networking might not be the answer

Figure 2. Networked controls can maintain tighter control over pressure swings.
Figure 2. Networked controls can maintain tighter control over pressure swings.

Connecting a microprocessor controller to individual air compressors improves response time to system demand changes and allows for network-type sequencing of multiple, similarly equipped air compressors (Figure 2). Networking multiple compressors allows them to operate within a tighter control band. Typically, the control scheme has only one compressor operating at part-load while the other compressors run close to or at 100% capacity at the target pressure. The controller is connected via a communication cable. A network-type multiple compressor control system usually requires that compressors be of the same type from one manufacturer, are equipped with the same microprocessor controller and will only control air compressors. Because communication among the network elements requires cable, it’s often impractical to connect compressors located in remote areas of the plant.

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