VFD... It No Longer Means "Very Funny Dude"

There's a lot of talk these days in pump circles regarding variable speed pumping -- the advantages, disadvantages, and justification. In the recent past, when pump professionals were asked if they specified or installed many small to medium variable frequency drive systems, they probably gave a "VFD" answer, something like "Very funny, dude!" This humorous, and very American response, was a reasonable one, considering variable frequency drive technology was difficult to understand, install, operate, and maintain relative to the benefits it offered. Engineers and maintenance personnel tended to avoid them because "...unless it's 20 hp or larger, it isn't worth the hassle!." Today, through improvements in technology and design, VFD no longer has the old connotation. VFDs (variable frequency drives) are generating significant interest through ease of use, reliability, system operation benefits, and substantial cost justification.

A 3-step procedure, playing on the variable frequency drive acronym, can be used to validate the cost justification of VFD-based booster pumps and systems:

• Validate the demand
• Figure the savings
• Determine the return

The figures displayed below are based upon the use of a 1.5 hp pump. The VFD-controlled and constant speed pumps are identical. Only the motors differ and a pressure regulator valve (PRV) is required for the constant speed pump to maintain constant output pressure.

Validate the Demand
Every pressure system has its unique demand profile. This profile determines the potential energy savings available from a variable speed system. When demand drops, the VFD pump slows down as it maintains constant pressure, rather than "choking down" the pressure with a PRV. The result is significant energy savings over PRV systems. To determine the energy savings potential for a given system, demand changes are plotted for an average 24 hour period (Figure 1). The flow profile shown here is typical for a multi-family dwelling structure. N/A>Click here for Figure 1

Determining average hourly demand is normally accomplished by one of two methods. If historic flow demand records are available for an identical or very similar application, projected estimates are quickly determined and very accurate. Standard estimating tables and formulas are also available to predict demand given the number of water demand units, for instance, in a typical building, sinks and toilets. Specifying engineers most commonly use this method for new construction and retrofit when demand records are not available.

Once the hourly flow profile is plotted, it may be summarized in the form of a histogram to make energy savings calculations simple and accurate (Figure 2). N/A>FIGURE 2 N/A>

Figure the Savings
Now you're ready to calculate the savings. This is a comparison of energy and dollar savings between a constant speed pump with a PRV and a VFD pump. Several factors are required in addition to the demand. Figure 3 groups the required data on to a spreadsheet.

The TDH (total dynamic head) column lists the maximum value which the pump is required to produce at each demand point. The pump discharge set-point will be higher than the TDH if the pump inlet has a gauge pressure higher than zero, or will be lower than the TDH if the pump is in a suction lift condition. Some installations will have different discharge set-points for different times of the day, when the inlet pressure may fluctuate. In those cases, different maximum TDH values would be listed. For this example, a constant set-point and inlet pressure are assumed.

Columns 2 and 3 list the gallons per minute (gpm) profile values from the demand histogram (Figure 2). Column 4 calculates the operating time percentage for each period. This data is common to both VFD and constant speed pumps.

Columns 5 and 6 list the motor input power in kilowatts (kw) and total power consumption in kilowatt-hours (kwhr) for the VFD pump. Motor input power is what the user pays for -- wire-to-water - and includes motor, VFD, and pump losses. The same information is listed for the constant speed pump in columns 7 and 8, which includes motor, PRV, and pump losses. Determining the motor input power for various flow points can be tedious. Done manually, for each flow point you must divide the pump end horsepower requirement by the pump end efficiency, divide that result by the motor efficiency for that load point, and for a VFD-pump, divide by the VFD efficiency if the VFD is not integrated into the motor. Finally, multiply by 0.746 to convert horsepower to kilowatts. These values are available from pump, motor and VFD manufacturers for their specific models.

The easier method is to obtain the information from pump manufacturers who provide motor input power curves for their pumps with and without a VFD. Also ask your pump manufacturer if a software program which calculates the flow profile, savings, and return is available. It will make the process much easier.

The bottom portion of Figure 3 summarizes the average daily power for each pump design, multiplied by the number of annual operating days and the electrical energy cost at the job site. This produces total annual energy operating costs for the VFD and constant speed pumps. The VFD pump in this example reduced the annual energy cost by $386, a 37 percent energy savings. N/A>Click here for Figure 3

Determine the Return
Determining the return on your investment requires a comparison of the two installed system costs. Figure 4 lists the end-user installed costs for the constant speed and VFD pumps. The additional cost for the VFD pump is compared to the annual savings to determine the pay-back period and the internal rate of return (IRR). N/A>Click here for Figure 4

Pay-back period is the amount of time required to recoup the additional investment for the VFD pump ($367) through energy savings, reduced maintenance costs, improved process cost reduction, and any other cost savings or additional revenue provided by the VFD pump. In this example, the pay-back period is 1 year.

Internal Rate of Return (IRR) is not used as frequently as pay-back period, but it is sometimes preferred by finance departments. IRR is the percent of return which an alternative investment must provide to equal the return of the VFD pump. For example, if the additional $367 in this example is not invested in the VFD pump system, it will be "invested" somewhere else, either as a liquid asset, other factory equipment, or some other investment. To equal the return of the additional $367 invested in the VFD pump, the alternative investment must return a minimum of 102 percent annually for the next 5 years. If the $367 is borrowed, the loan percentage would have to be 102 percent or greater to reject the VFD-pump decision. N/A>FIGURE 5

Reduced maintenance costs were not included in Figure 5. PRV maintenance for this pump capacity can run $100 annually or more in cleaning, parts replacement, and valve adjustment. The VFD system requires no periodic maintenance. Including the additional $100-per-year maintenance savings, the VFD-pump pay-back period drops to 0.8 year, and the IRR increases to 130 percent.

Why the Change?
Why are VFD-based pump systems under 20 hp recommended now when they were discouraged until recently? The reasons are new technology, economics, natural resources, and environment.

Significant improvements in reliability, size reduction, ease of installation, and hassle-free operation make it painless to specify VFD-pumps. Completely integrated VFD motors, which include the VFD and system controller, eliminate all wiring except the power supply and the sensor. This is an important and worthwhile improvement over separate component VFD and controller systems. Most pump manufacturers also offer the sensor in factory-pre-wired condition, and one company produces completely integrated pumps which include the pump, VFD, motor, tank, pressure sensor, and system controls, making installation much easier and quicker than constant speed systems.

Energy supplies are limited. Installing the small VFD-based pump used in this example saves enough energy annually to operate a 100 watt light bulb 24 hours a day for about 4.4 years. Add up the numerous possible applications of this technology and one can appreciate the significant reduction of consumption of natural resources, as well as of negative effects on the environment, that could be accomplished.

Suggest to a pump professional who has specified, installed, and operated today's fully integrated VFD-pumps that they should install constant speed pump systems whenever possible and you may now get the same "VFD" answer you used to get!

About the Author: Len Petersen is product manager for the Commercial Industrial Product Line at <%=company%>.