Improve Efficiency, Save Money
By Mike Fowler
Aquatic facility managers have learned over the past few years that going "green" can significantly lower operating costs. Pumps and filters are among the items most scrutinized due to the vast amount of energy they consume. To reduce costs associated with water circulation and filtration, it is important to understand why pumps and filters consume large amounts of energy and what options are available to lower consumption.
The type of filter used (cartridge, sand or diatomaceous earth) can have a significant impact on energy consumption, as each places different levels of resistance on the circulation system.
Resistance is related to energy efficiency because of its impact on water flow. Of the three filter types, cartridge filtration offers the least resistance, partially due to the absence of valves. Both sand and D.E. filters require multi-port valves to perform routine backwash procedures. These valves create so much resistance to flow that the California Energy Code (Title 24), The Energy Efficiency Standards for Residential and Nonresidential Buildings, has banned 1.5-inch multi-port valves. As a result, newer backwash valves have been designed to lessen flow resistance. On a related note, backwashing consumes water, so using a filter that does not require this type of maintenance will conserve both water and chemicals.
Although sand and D.E. filters function more effectively as dirt accumulates, a dirty filter can increase the pump's workload. In fact, the difference between a clean and dirty filter can nearly double the pump's energy consumption. If friction loss can be decreased in the facility's plumbing design and equipment sizing, less horsepower is required to achieve the desired flow rate. While the type of filter installed is often a personal preference, the energy savings realized when using an oversized cartridge filter to reduce overall system head loss is well documented.
Finally, by simply keeping the pool's filter clean and skimmer basket free of leaves and other debris, energy savings can be quite significant.
Proper pump selection (sizing) and optimal flow rates are additional ways to increase energy savings. Affinity laws indicate the power demanded by a pump is proportional to the cube of the flow rate. For example, if the pump's flow rate is doubled, then its power demand is increased by a factor of eight. Therefore, it is important to utilize the smallest pump capable of turning over the pool water in an acceptable amount of time.
During pump selection, the facility's auxiliary features (spray pads, fountains and waterfalls) should also be considered, as it is common for these to use the pool's main pump. Some building codes, however, require the use of a multi-speed pump, or in some cases, a separate pump for each auxiliary pool load. Pumps for aquatic facilities are oversized by design, sometimes more than 20 to 40 percent bigger than needed. This happens because many architects and engineers look at what is required, then pick the next size up to be sure the pump can handle the job.
Here are some tips to help select the proper pump:
- Determine flow rate in gallons per minute (GPM).
- Calculate total dynamic head (TDH) (the pressure head difference between the inlet and outlet of the pump) to account for friction loss. Adding 20 ft/head for a dirty filter is optional.
- Refer to the pump's performance curve.
- Locate pump horsepower required by plotting GPM vs. TDH. (If plotted point falls between two pump sizes, select the next larger pump size.)
- Do not oversize the pump. If the preferred pump does not provide a proper fit, consider a different model.
- Verify the selected filter can handle the system's flow rate and be sure the minimum backwash flow rates can be achieved.
Speed Is a Factor
Historically, pool pumps with induction motors, which only operate at one or two speeds, have drawn more energy than is required to circulate pool water. These units must operate at high speed in order to perform the pump's most demanding jobs (running a waterfall or pool cleaner). However, it takes far less power to simply keep water filtered—a difference single-speed pumps cannot address.
Where variable speed pumps (VSPs) differ is in their ability to be programmed to operate at set speeds to deliver the correct flow rate for each task they perform. This reduces energy consumption and increases savings.
They also can be programmed to achieve turnover times of exactly six hours, even if the filter is dirty. This allows motor speed, power and energy to be reduced during times when the filters are clean, instead of sizing the pump to assume worst-case operating conditions.
Some VSPs have built-in, constant-flow software, which maximizes the advantages these pumps have to offer, as it automatically adjusts speed to deliver the required flow rate for each programmed task. For instance, to achieve the maximum laminar height of 7 feet at a distance of 8 feet, a water supply of 10 GPM with 16 ft of head (measured at the laminar) is required to produce a smooth 6 feet of arc water. As the filter accumulates dirt, however, the pump will sense resistance in the circulation system and automatically ramp up its speed to continually provide the proper flow rate the water feature requires. With other pump types, the water feature will gradually throw a shorter arc of water as the filter gets dirtier.
No matter what type of pump is being used, a slower pump speed saves energy. It also dramatically reduces noise levels as well as wear and tear on the other pool equipment the water flows through.