Even when there is no air delivery from the compressor, the application still consumes air and the pressure in the system drops to the load set point. At this load set point, the compressor starts loading again and produces 100 percent air. This load-unload cycle will be repeated based on the application’s air demand.
In the case of a VFD-powered machine, the FAD is directly proportional to the motor speed. The VFD alters the compressor’s motor speed and thereby controls the FAD delivered. During trim load, the VFD delivers the required variable air demand for the exact length of time, without altering the pressure, which results in lower power consumption.
For example, at 100 percent speed, the compressor delivers 100 percent FAD and consumes 103 percent power (3% loss). Or at 30 percent speed, it delivers 25-27 percent FAD (3-5% loss) and consumes 33 percent power. At lower speeds, the FAD is lower due to losses; approximately 30 percent is the speed limit for a compressor.
Energy savings – three different opportunities
1. 25 percent energy saving due to the elimination of the unload cycle: In general, compressors work in the load-unload mode. When there is demand, compressor loads and allows the pressure to rise, and unloads once it reaches the unload pressure set point. The compressor cannot be turned off since the motors are not designed to frequent switching on and off. During the unload cycle, the intake valve closes and there is no air flow, but the motor continues to run at the rated speed. The compressor consumes 25-30 percent of power. With VFD, there is no unload cycle, and air continues to be produced at the required pressure.
2. 5 percent energy saving due to reduction in operating pressure: If an application requires an operating pressure of 100 psi, the fixed speed machine operates on load-unload set points which are 100 and 125 psi, respectively. To compress an extra 10 psi, approximately 5 percent of additional energy is spent, which is a waste. In a VFD-powered compressor, the pressure is maintained constantly throughout and this loss is eliminated.
3. 3 percent energy saving due to leakage and pressure drop reduction: Air leakages and pressure drops are inevitable in any compressed air system. For every 10 psi increase in compressed air pressure, there is a corresponding 3 percent higher rate of leakage and pressure drop in the compressed air circuit, which demands extra power. Since a fixed speed machine requires 10 psi extra over the desired pressure, it is bound to lose energy on this account. In VFD-powered compressors, the pressure is maintained constant throughout the cycle and leakage losses due to increase in pressure are eliminated.
VFD compressors are highly effective ways to save energy during a compressor application. If the requirement is 900 cfm, the base load will be shared by load-unload compressor of 400 cfm and the trim load will be shared by the 500 cfm VFD compressor. With such compelling results, the demand for VFD retrofitting is steadily going up.
How do I know if my air compressor has energy saving potential through a VFD?
In every compressor there is an hour meter, which shows the run hour, load and unload hours. If the unload hours are 15-20 percent of total run hours, there is a potential to save energy through a VFD.
How does one confirm the base load and trim load requirements?
The best method to ascertain the base load and trim load requirements is to conduct an energy audit that will accurately measure the base and trim load demand.
What will happen if we install a VFD for base load requirements?
If you want to install a VFD for the base load alone, there will not be any energy saving since the base load does not go to an unload cycle. A VFD-powered machine will consume 103 percent power at full load (3% VFD losses). Hence there will not be any savings.