Steadily rising energy costs are a major burden for pool owners.
In response, the use of variable frequency drives (VFD) has become commonplace. However, this approach by pool designers is too often a “monkey see, monkey do” reaction; if everyone else is doing it, they should also be using them in their own designs. If there was a better understanding of how we save energy with the use of VFDs, we wouldn’t be able to walk into many of these new equipment rooms and find that the pumps are operating at 100 percent speed. That means that the owners of that facility, instead of saving money, are paying higher electrical costs due to 2- to 3 percent power production losses inherent in VFD circuitry.
Although VFDs can provide substantial savings in operational costs, in order for them to be effective, there are certain guidelines that must be followed.
VFD design mistakes
A pump operating with all valves in its attached piping wide open will be working against the least resistance to flow possible in that particular piping scheme. If the resultant flow exceeds the required flowrate, the pump rotational speed can be reduced to the point where minimum required flow is achieved. At the same time, that lower flow velocity reduces the friction losses through the filter, hair strainer and all of the associated valves and piping. This results in very low static pressure in the circulation piping; as low as 5 psi or less.
There are many side-stream requirements in pool systems that can be affected by such low pressure in the main piping. Heaters or heat exchangers, chemical sample streams for test probes, heat recovery systems and, possibly, supplemental disinfection systems, such as ozone or UV, may all need to be combined with small horsepower side-stream booster pumps.
If instead, the operator closes a bypass valve to force water to that particular component, he or she has just increased the work demanded of the circulation pump. The result is lost savings. Enough restricted valves can eventually force the pump to operate at 100 percent speed — all savings lost.
Another important factor is proper pump selection. Good design dictates that a pump be selected so that it will be non-overloading across the entire curve. In other words, its performance curve won’t ever cross the horsepower curve of the associated motor, even at the end of the performance curve. For instance, if that results in the selection of a 20hp motor, the designer should require that the pump be supplied with the largest diameter impeller that will still be non-overloading. He also may find lower pump speeds will still provide the minimum flow requirement.
We have experienced total loss of VFD savings when UV cells, with their stainless-steel-cloth glass capture baskets, are added to a circulation system. In new designs, the engineer needs to examine the need for a larger horsepower pump to overcome the pressure loss through this type of basket.
The designer could also opt for the use of ozone. Ozone systems already employ a side-stream booster pump as a necessary component.
If regenerative media filtration is considered as a way to reduce water and sewer costs and to reduce the cost of heating replacement water up to pool operating temperature, the performance characteristic of the design chosen can greatly affect VFD savings. If automatic regeneration of these media filters, several times a day, is not part of its normal operational parameters, pressure loss through the filter will quickly spike and the pump will eventually be forced to run at 100 percent speed, attempting to achieve minimum required flow.
This is not meant to discourage the use of regenerative media filters. On the contrary, that type of filtration delivers the highest level of filtration efficacy available. This fact comes in to play when we discuss the negative impact of reduced flow during off-hours.
Proper use of the off-hours of any pool facility is essential to maintaining a healthy pool. The filtration and chemical treatment systems are taxed during daytime heavy usage hours. Overnight, the system can catch up because there isn’t any additional chlorine demand being placed on the pool by swimmers.
During a typical off-hours period, a pool system can achieve one and a half turnovers while no new demand is placed on it. That’s a big chunk of the four turnovers per day required. During those one and a half turnovers, if DE filtration or ozone or UV systems are in place, they are effectively removing or destroying pathogens.
In the pool industry, the time it takes to achieve four to five turnovers is considered the minimum number of hours required to move, theoretically, every molecule of pool water through the filtration and sanitation systems (see illustration). That, of course, assumes that there is a well-designed circulation system in place. Reduced flow at night increases the number of hours required for four or five turnovers. Lengthening that time frame is not a good idea.
We have recently been told that the same health department that balked at using VFDs 18 years ago is now approving flow reductions at night in some cases. Many operators in Florida and California are completely shutting down their pumps during non-use hours.
In Wisconsin, some companies are recommending VFDs and a reduction of flow at night to reduce operating costs. Whether owners in those states are acting with the blessing of their local jurisdiction, or on their own, our opinion is that it is not a wise decision.
We would hate to see this practice become commonplace. VFDs must be implemented properly to result in real savings. Use DE or supplemental disinfection systems but don’t lengthen the time it takes for pool water to pass through these enhancements. Turnovers per day are just as important as, or even more important, than removal/neutralization capabilities of the system components.