10 Things to Know About Variable Speed Pumps

Tuesday, January 23, 2018

Aquatics International | October 2017

Variable speed pump (VSP) technology has made quite an impact on the pool/spa and aquatics industries in the last few years, and there are no signs that their popularity will end anytime soon. Looking at the enormous benefits VSPs provide to the bigger picture of swimming pool management, it is no wonder that VSPs are often the leading choice over standard single-speed pumps for dealers and consumers alike. Below are 10 great reasons you should consider this intelligent equipment choice when selecting your next filtration pump.

1. VSPs cost less to operate.

Variable-speed pumps get their name from the fact that they operate at different speeds, measured in rotations per minute (RPMs). They can be adjusted up to 3,450 RPMs to suit the specific function.

VSPs use less energy when their speeds are reduced. This is based on a known set of engineering principles called the Pump Affinity Laws. These teach that when the speed of a pump is reduced, the power consumption is decreased by the cube root of the reduction in speed. So if the speed is reduced by half, the power consumption is only th of the higher speed. This implies that electricity consumption also is reduced by a factor of eight.

This has serious energy-savings implications.

But the flow rate at the slower speed is lower than at the higher speed, so the pool must be operated for a longer period of time to process the same volume of water through the filter. Another Pump Affinity Law teaches that when the pump speed is reduced by half, the flow rate is only reduced by half. So if speed is cut by 50%, the pump at the slower speed can run twice as long, and the power consumption would still be only ¼th that of the higher speed. This is a 75% energy savings.

So the financial savings can be significant.At high speed, a pool pump can easily use 4,000 kilowatt-hours of electricity per year in the sunbelt. If the pool owner were paying $0.25 per kW-hr, they could save $750 per year by halving the pump speed.

Interested readers should consult energy-savings calculators provided online by many pump manufacturers to run alternative scenarios.

VSPs have higher price tags than single-speed pumps but often pay for themselves through energy savings in as little as one or two seasons.

This ability to significantly reduce energy consumption is why VSPs qualify for the federal Energy Star program.

2. VSPs lower hydrodynamic pressure (or energy) loss through the plumbing.

As water flows through pool plumbing, it rubs against the inside of the pipe. So bends, turns and even lengthy straight sections cause friction loss, resulting in energy consumption. Friction loss increases as the water moves faster.

It is known that friction loss decreases with the square of the water-flow rate. So reducing the flow rate by a factor of two lowers friction loss by a factor of four. This directly corresponds to the energy savings described above.

3. VSPs improve filtration.

Irrespective of the type of filter used on a pool (e.g. sand, cartridge or D.E.), filtration performance improves when the velocity of water moving through the filter is reduced. When water passes through the filter media more slowly, there is an increased opportunity for particle removal by mechanical blockage or chemical adhesion, so filtration may even take less time. At high flow rates, particles can be forced though the media, causing the formation of voids, or channels, and leading to lower filtration efficiency.

And at lower flow rates, the pressure drop, or head loss, created by the filter goes down. Taken together, the lower pressure drop and improved filter efficiency can mean less energy is used for filtration.

4. VSPs improve the effectiveness of each pool turnover.

The turnover rate of a pool is defined as the time required to filter the number of gallons that a pool holds. If the pool contains 20,000 gallons and the flow rate is 50 gallons per minute, one turnover takes 6 hours and 40 minutes.

But practically speaking, not all water molecules can pass through the filtration system during one turnover. This is because filtered water returns to the pool, where it mixes with the unfiltered water. The mixture of clean and dirty water then goes back to the filter. So in the time it takes to filter the equivalent volume of the entire pool, some molecules are processed multiple times while others do not go through the filter at all. This increases the time and number of turnovers needed for all water to be filtered. The mathematics is complicated, but suffice it to say that, for commercial pools, four turnovers per day are often required by local health codes to help ensure that the vast majority of the water has been filtered.

So how are pool water mixing and VSPs related and why is it important? In a pool with a higher flow rate, water is mixed more quickly than at a lower rate. So at the higher rate, less time elapses before the “cleaned” water will mix in and get to the skimmer to be filtered a second time, even before some water molecules were filtered once. At low flows, one can picture a slow-moving wall of clean water moving toward the skimmer, giving the filter system a chance to more thoroughly clean the water before too much mixing occurs. Less time is required to filter the whole pool, and the filter efficiency of each turnover is improved.

5. VSPs improve high-flow functions.

Not all VSP advantages spring from low flow rates. Some come with higher flow rates.

Heating represents one example. Heat transfer from the walls of the metallic tubes to the moving fluid inside improves as the flow rate increases. This occurs because of a thin layer, mere microns thick, of near-stagnant water at the wall of the tube. As the flow rate increases, the thickness of this “boundary layer” becomes thinner, so heat transfers more efficiently.

Suction cleaners also require higher flow rates. Insufficient flow through the skimmers leads to inadequate suction, and debris cannot be lifted through the hose. Furthermore, many suction pool cleaners utilize a diaphragm that rapidly opens and closes as water is drawn through a relatively narrow throat opening. Higher flow rates are typically required to generate the force needed for diaphragm operation.

Chemical feeders, too, benefit from higher flow rates, which encourage more rapid mixing and homogeneity of the added chemical with the water. In the case of chlorine or acid addition, for example, it is desirable to encourage dispersion of these chemicals quickly to create a uniform chlorine residual and pH.

To minimize time needed to operate the pump at high speeds, thereby reducing operating costs, the best choice is to conduct heating, chemical-feeding and suction-cleaner operations at the same time.

6. VSPs optimize waterfeature aesthetics.

Bubblers, laminar jets, waterfalls, spillways and other waterfeatures require different speeds to create the exact aesthetic desired. Imagine a water slide with little water flow: That would lead to a rather uncomfortable ride. Waterfeatures require that the user “dial-in” the speed that extracts the best performance. VSPs permit this seamless optimization of any type of waterfeature.

7. VSPs improve UV system performance.

Ultraviolet light disinfection systems have grown in popularity. Interestingly, UV disinfection performance has a logarithmic dependence on flow rate: As the flow rate decreases by a factor of two, performance increases by 100!

Let’s take the example of a UV system capable of inactivating 99% of microorganisms at 50 gpm. At half the flow rate, the system would inactivate 100 times that amount, or 99.99%.

We know that the energy or cost required to operate the pump at the lower flow rate will be much less than at the higher rate. So, even accounting for the longer running time for the UV system and its associated electricity consumption, the cost of operating the pool at the lower flow rate is less, and the bacteriological quality of the water is much improved.

8. VSPs are less noisy.

Sound reduction is a significant benefit of VSPs. When operated at lower speeds, pool pumps produce less noise. At full speed (the only option for single-speed pumps), many units emit noise levels of 65-90 decibels at about a 1-meter distance from the pump. By comparison, a whisper has a noise level of about 20 dB, whereas normal conversation is 60 dB, and a vacuum cleaner is 80 dB. A reduction of only 10 dB is associated with a 90% decrease in noise intensity. It has been observed that decibel levels can go down 30% or more as the pump’s impeller speed is reduced.

9. VSPs have a longer product life.

Pump motors don’t last forever, but running at lower speeds is less taxing on them, leading to increased operating life. Heat is the primary enemy of most motor components. Just as power and noise are greatly reduced at lower speeds, so too is heat generated by the motor, leading to considerably less stress on mechanical and electrical components.

10. Energy utility companies will pay you to buy one.

Pool pumps are notorious energy hogs: Often, they are the second largest consumer of electricity in a home. During the summer months, many utilities cannot supply the peak power demand. In an effort to reduce this demand, dozens of utilities across the U.S. offer rebates of up to $400 to entice consumers to switch to a variable speed pump for their pools. These rebates reduce the upfront cost of a VSP, resulting in an even faster payback on investment.

 

Conclusion

Variable speed pumps deliver a wide range of benefits compared to traditional single speed pumps. Significant energy savings and improved equipment performance are only some of the reasons to make the switch to VSP technology.

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