How variable frequency drives for air compressors help reduce energy consumption

In today's business environment, companies need to minimise operating costs in order to remain profitable and competitive. In any production plant, energy bills can be very high due to heavy equipment that consumes a lot of energy. Therefore, solutions based on new advanced technologies must be sought that can offer not only high performance and reliability, but also energy efficiency and lower operating costs.

In these cases, one of the measures that can be taken is the installation of frequency converters for air compressors or the adaptation of existing air compressors with VFDs. The VFD frequency converter is an electronic module used to change the speed of the motor by varying the frequency applied to it. As motor speed is directly proportional to frequency, changes in frequency directly affect motor speed and the amount of energy consumed. Frequency converters can transform air compressors, which were traditionally high energy consumers, into energy-efficient equipment. Up to 85% of the total life cycle cost of an air compressor comes from its energy consumption. Once equipped with these variable speed drives, the energy cost of an average compressor is reduced by 18%, so compressors installed with VFDs help to reduce energy consumption.

How does the frequency converter work to control compressors?

The volume of compressed air required for an application varies depending on the different stages of production and the associated demand from other equipment connected to the compressor. In terms of compressors, there is a base load, which is the minimum demand over a period of time, and a surge load, which are peaks in demand during a cycle. A frequency converter for compressors is useful in managing energy consumption during the surge load.

In the case of a fixed-speed compressor, the air supplied (also known as free air delivery or FAD) is 100% during machine loading. If demand is lower than what is supplied, the pressure increases to the unload set point and the compressor unloads. During unloading, the compressor does not supply air, but continues to consume approximately 25-30% of the energy.

Even when there is no air supply from the compressor, the application continues to consume air and the pressure in the system drops to the load set point. At this load set point, the compressor begins to load again and produces 100% of air. This load-unload cycle will repeat depending on the air demand of the application.

In the case of a VFD-driven machine, the FAD is directly proportional to the motor speed. The frequency converter alters the speed of the compressor motor and therefore controls the FAD supplied. During the set load, the VFD supplies the variable air demand required for the exact time, without altering the pressure, which translates into lower energy consumption.

Energy savings through the use of frequency converters in compressed air systems

• Energy savings of 25% thanks to the elimination of the discharge cycle

  In general, compressors operate in load-unload mode. When there is demand, the compressor loads and allows the pressure to rise, and unloads once it reaches the discharge pressure set point. The compressor cannot be turned off, as the motors are not designed to be turned on and off frequently. During the unload cycle, the intake valve closes and there is no air flow, but the motor continues to run at rated speed. The compressor consumes between 25 and 30% of the energy. With the use of variable frequency drives for compression equipment, there is no unload cycle and air continues to be produced at the required pressure.

• Energy savings of 7% thanks to reduced operating pressure

  If an application requires an operating pressure of 6 bar, the fixed-speed machine operates with load and unload set points of 6 and 7 bar, respectively. To compress 1 bar more, approximately 7 per cent additional energy is consumed, which is wasteful. In a compressor driven by variable frequency drives, the pressure is maintained constantly and this loss is eliminated.

• 3% energy savings thanks to reduced leaks and pressure drops

  Quantifying and minimising industrial compressed air leaks is a priority.. Air leaks and pressure drops are inevitable in any compressed air system. For every 1 bar increase in compressed air pressure, there is a 3% increase in leaks and pressure drops in the compressed air circuit, which requires more energy. Since a fixed-speed machine requires 1 bar more than the desired pressure, it is bound to lose energy for this reason. In compressors driven by Variable Frequency Drives, the pressure remains constant throughout the cycle and losses due to leaks caused by the increase in pressure are eliminated.

Frequency converters for compression equipment are a highly effective way to save energy during compressed air generation. If the requirement is 900 cfm, the base load will be shared by the 400 cfm load-unload compressor and the adjustment load will be shared by the 500 cfm VFD compressor. With such convincing results, the demand for retrofitting with VFDs continues to grow.

Therefore, as the speed of the air compressor motor is proportional to the actual power consumed by the compressor, the installation of frequency converters for air compressors can control the motor speed (output power) in real time without changing the torque of the air compressor motor through the precise coordination of electrical control and frequency control in the air compressor system. This not only enables companies to be more efficient and save on energy consumption, but it has also been proven that investing in frequency converters for air compressors translates into a secure return on investment within a few years.

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