Energy conservation using VFDs in HVAC industry

It is estimated that nearly 60% of the electrical power generated (1)is used to power Fans and Pumps. Variable frequency drives (VFD's) are often recommended as a way to save electrical energy. The VFDs do not save energy in many cases. In fact they consume more energy than other solutions (2). Case study has shown that approx.75,000 kWh per year can be saved by using VFDs in a Hospital building with A.C. plant capacity of 900 Ton (600 Working+ 300 standby). All analysis should be on the basis of wire to water, i.e. the kW-hrs to movem3 of fluid. Not the energy shown on a pump curve or predicted by the laws of Affinity. Although tender documents may require such analysis, this rarely happens. The low cost tender is accepted and thus the most efficient plant is not always installed. Actual energy saving will vary greatly how the discharge pressure of the constant speed pump is controlled and how it is operated after the VFD is installed (3). In general energy savings are increased when bypass is minimized or eliminated. Pressure sensors for controls are located near the most remote end use, and the pressure control set point is minimized.