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VARIABLE FREQUENCY DRIVE - The ultimate solution to energy saving?

The use of variable frequency drives (VFDs) to reduce energy consumption has been resorted to in controlling the operation of motor-driven equipment, like pumps, compressors, and even in process control like conveyor system.

Though it cannot be denied that VFDs can produce substantial reduction in energy consumption, improper design or application will not produce the most effective result. VFDs reduce energy consumption simply by reducing speed of rotation to balance output against demand which is not possible to achieve with fixed-speed motors. Careful consideration must be directed on the effect of speed reduction to the over-all efficiency of the system.

For this discussion, I will present the use of VFD in centrifugal and turbine type water pumps.

Pumps are designed to perform best, meaning with maximum efficiency, at the designed speed, capacity and total dynamic head. This is also referred to as the Best Efficiency Point (BEP). In actual applications, the performance in field conditions may deviate from the designed operating parameters. The deviation from the BEP affects the power consumption of the motor-driven pump. The reduced speed to match demand and supply results in lower power consumption but the new operating point will also result in reduced efficiency.

Careful analysis of the operating condition must be made wherein some modifications may be necessary to maximize efficiency.

Let us take a look at the performance curve of a centrifugal pump shown.

1.      The best efficiency point (BEP) at 1750 RPM, full diameter impeller, is at 525 gpm, 102 feet TDH, 79% pump efficiency.

2.      Affinity laws for ratio of diameter and ratio of speed, will result to the same pattern shown in the performance curves.

3.      For constant pressure application (BLUE arrow), reduction in demand to 400 gpm will reduce the pump efficiency to approximately 76%.

4.      If the control system can follow the system curve (GREEN arrow), pump efficiency at 400 gpm with lower head of 98 feet can achieve an efficiency of about 76.5%.

However, if intermittent system demand drops lower to 40% of designed maximum capacity, it might be prudent to use two (2) pumps with lower capacity, say 350 gpm at 102 feet that can supply the system requirement with a wider range of variable demand.

Other possible strategies are:

DUPLEX PUMP SYSTEM – Use two pumps of same rating if variable system demand does not fall below 60% maximum expected demand. They can run alternately which can insure 100% demand can be supplied even if one (1) pump fails.

TRIPLEX PUMP SYSTEM – Three (3) pumps are installed but only two (2) pumps are needed to supply the system demand. One (1) pump serves as standby in the event of any pump failure.

QUADRUPLEX PUMP SYSTEM – Three (3) pumps are needed to optimize                system performance with one (1) unit as standby pump.

FINANCIAL CONSIDERATION:

Choosing the best strategy must be based on its financial impact on the business operation. Oftentimes, selection among bids to supply the equipment is based on purchase price which eventually result to high maintenance and operating cost due to lower efficiency.

It is best to make a LIFE-CYCLE COST ANALYSIS to determine which option will give the equipment user the highest RETURN ON INVESTMENT (ROI).   

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