Finding the right match

To the select the most energy-efficient capacity control system, you need to determine your compressor type and the plant’s load cycle

By Niff Ambrosino

PlantServices.com

Compressed air systems hold one of the keys to greater productivity, efficiency and profitability. This is the fifth in a series of articles that introduces some key points of Compressed Air Challenge training.

The greatest expense associated with compressed air is the cost of energy. Compressors operate most efficiently when fully loaded. However, most compressed air systems allow compressors to operate partially loaded.

Compressor capacity control systems regulate air output to match system demand and are one of the most important factors in determining overall system efficiency. To select the most energy-efficient capacity control system, you need to determine the compressor type and the plant’s load cycle.

Start/stop

The most basic form of compressor capacity control is start/stop. It requires compressing air and storing it in an air receiver. When pressure reaches an upper pre-set limit (cut-out), the air compressor turns off and no further air is compressed. The stored air supplies demand.

When pressure drops to a predetermined low setting (cut-in), the compressor starts and the cycle is repeated. The cut-out pressure is typically set 25% above the cut-in pressure. This can be an efficient way to control a compressor, because power is used only while air is being compressed. However, using compressed air at a pressure above the required minimum increases power consumption, thereby partially offsetting  any gains. Therefore, many systems will, or should, use a pressure regulator to reduce line pressures to the required minimum.  Note that every 2 psi increase in discharge pressure, requires an additional one percent in energy. 

Start/stop controls are found on reciprocating compressors ranging from less than one hp to 30 hp. System storage capacity must be considered to provide adequate time between motor starts. Started too frequently, an electric motor can fail prematurely.

Constant speed or load/unload

With constant speed, or load/unload, control, system pressure is set at a high point (cut-out) and a low point (cut-in). However, rather than turning off when system pressure reaches the cut-out point (typically 10 percent above the cut-in setting), the compressor motor continues to run while the compressor operates in an “unloaded” mode. When unloaded, compression is interrupted and the compressor’s internal pressure drops.

Typically, constant speed controls are found on reciprocating and rotary screw compressors, and on some centrifugal compressors. Unloaded input power requirements on reciprocating compressors is 8 to 12% of full-load requirements. On rotary screw units, the unloaded requirement is approximately 15 to 25% of full load power. Constant speed or load/unload controls are used in applications where loads are continuous and a start/stop control would cause premature motor failure.

As with start/stop control,  over-pressurization is required to maintain minimum plant operating pressure. When running in the unloaded mode, power is still consumed, but  the compressor is not delivering any useful work.

In the load/unload mode, adequate storage is critical for part-load efficiency. Figure 1 illustrates the effect of receiver size on a lubricated rotary screw compressor with load/unload controls .

Dual control

Figure 1.

Dual control allows for manual selection of either start/stop or constant speed control. A typical application would have the compressor operating at a constant speed during production when demand for compressed air is continuous and in the start-stop mode during low demand, non-production periods. Dual control is found on reciprocating compressors and smaller rotary screw units.

Auto-dual control

Auto-dual control automatically selects either constant speed or start/stop control. With autodual control, a reciprocating compressor runs in a constant speed mode. When it unloads, a timing relay is activated. If system pressure doesn’t fall below the cut-in set point within a specified time period, the compressor shuts off. When system pressure eventually falls to the cut-in set point, the compressor restarts and the control process resumes. On a rotary screw compressor with load/unload control, the control process is identical to that for a reciprocating unit. When inlet modulation control is used, the rotary compressor will typically modulate (throttle back) to approximately 40% of full capacity before completely unloading (blowing down). At this point, the timing relay is activated, and assuming system pressure does not fall below the cut-in point within the specified period of time, the compressor will turn off. Overall rotary screw part-load efficiency will be directly affected by system storage capacity when operating in the load/unload mode.

Modulating control

Modulating (inlet throttling) control is found on rotary screw and centrifugal compressors.  Modulation allows for stepless output control to meet varying demand. At less than full capacity, the inlet valve throttles back, thereby matching system demand by restricting airflow into the compressor. Because the inlet valve remains wide open (fully loaded) up to the modulation set point, the minimum allowable system pressure and the control set point can be the same.

Rotary screw. On a rotary screw compressor, modulation is an inefficient way to control capacity, because it affects the inlet pressure of air being compressed. Throttling the inlet valve creates  a vacuum at the rotor’s inlet. This reduces the absolute pressure at the start of the compression cycle, thereby increasing the overall compression ratio. Brake hp is dependent on the volume of air compressed and the compression ratio.