Methods of Addressing a Fluctuation in Air Demand
Over the years, compressor manufacturers have developed a number of different types of control strategies. Controls such as start/stop and load/unload respond to reductions in air demand, increasing compressor discharge pressure by turning the compressor off or unloading it so that it does not deliver air for periods of time. Modulating inlet and multi-step controls allow the compressor to operate at part-load and deliver a reduced amount of air during periods of reduced demand.
Start/stop is the simplest control available and can be applied to either reciprocating or rotary screw compressors. The motor driving the compressor is turned on or off in response to the discharge pressure of the machine. Typically, a simple pressure switch provides the motor start/stop signal. Its advantage is that power is used only while the compressor is running, but this is off-set by having to compress to a higher receiver pressure to allow air to be drawn from the receiver while the compressor is stopped.
Load/unload control, also known as constant speed control, allows the motor to run continuously, but unloads the compressor when the discharge pressure is adequate. Compressor manufacturers use different strategies for unloading a compressor, but in most cases, an unloaded rotary screw compressor will consume 15-35 percent of full-load horsepower while delivering no useful work. As a result, some load/unload control schemes can be inefficient.
Inlet valve modulation (often just referred to as modulation) throttles off the air inlet to the compressor as pressure rises above the set-point pressure. This causes the compressor to draw in less air, matching compressor capacity with air usage for relatively steady pressure control. This causes part load performance to be very poor (a machine that modulates to 0 capacity still uses about 70% of its full load power). The modulating compressor can typically regulate discharge pressure with no or minimal air receiver storage
Some modulation controlled machines may be adjusted to fully unload or “blow down” if capacity reduces to a certain level, such as 40%. This saves some energy, but requires the use of air storage receivers to meet demand when in the fully unloaded state.
A variable speed drive (VSD) controls the frequency supplied to the drive motor on the air compressor in response to real-time demand on the system. When demand is low, the motor will run at a reduced speed, requiring less energy to operate. When demand for compressed air is higher, the VSD will automatically increase the frequency to the air compressor motor to meet CFM demands.
The biggest advantage of a VSD air compressor is energy savings. A fixed speed compressor always draws the amount of power needed to meet the maximum compressed air demand. VSD will ramp down RPM, and therefore power consumption, when demand for compressed air is lower
Multiple Compressor Control
Systems with multiple compressors use more sophisticated controls to orchestrate compressor operation and air delivery to the system. Master controllers, like those of AIRMATICS™ link your system together to form a chain of communication that makes decisions to stop/start, load/unload, modulate, vary displacement, and vary speed. Usually, one compressor assumes the lead role with the others being subordinate to the commands from this compressor. System master controls coordinate all of the functions necessary to optimize compressed air as a utility. System master controls have many functional capabilities, including the ability to monitor and control all components in the system, as well as trending data to enhance maintenance functions and minimize costs of operation. Other system controllers, such as pressure/flow controllers, can also substantially improve the performance of some systems.
If you have questions or concerns about how to better manage your air system in order to meet the ever fluctuating air demand, please reach out to us. We have branches throughout Michigan, Ohio and Indiana, to better serve Grand Rapids, Lansing, Detroit, Toledo, Indianapolis, and Fort Wayne.