#Energy efficiency
Part 2: Applications with energy saving potential
What applications can benefit from variable frequency control the most? To help understand it we use some fundamental equations (don’t worry, there will be no high mathematics!).
- Mechanical power Pmech is the product of torque T and angular velocity w. The angular velocity is directly linked with speed n.
- Applications like fans, pumps and many compressors are square (quadratic) torque types of load. This means that the load torque is proportional to the square of the speed (Tload ~n^2).
- Combining both relations above the mechanical power is proportional to the third power of speed (Pmech ~n^3). Therefore, operating at 70% rated speed means 49% required torque (0.7^2) and 34.3% power (0.7^3). The saving potential is over 60%!
Of course, the system will likely not operate all the time at 70% speed, but knowing the typical operating profile one can easily calculate what are the yearly energy savings and what energy cost reduction can be expected.
Now coming back to the original question: Which applications have the largest energy saving potential? Mainly those with square torque profile (torque vs. speed curve). The wider the speed range, the more energy can be saved. If the current flow control method is throttling or bypassing, then the saving potential is huge and payback time will be very short. If there is more sophisticated flow control already in place (e.g. pitch control), then the savings will be somewhat smaller. Main VFD manufacturers have sophisticated tools that can accurately predict energy and cost savings, reduced carbon footprint, payback time etc (e.g. MVD Pump & Fan Save tool).
Summary
Applications with quadratic torque profile (load torque increases quadratically with speed) contain large potential of energy and cost savings. There are lot of tools available to quantify these savings. One of them is MVD Pump & Fan Save. More details about the calculations can be found in [1].
References
[1] How to calculate energy and cost savings, https://mb-drive-services.com/how-to-calc-energy-and-cost-savings/
