A topic of much conversation in the industry lately has been energy efficiency in swimming pool systems.
The focus of these conversations is usually the pump, but filters play an important role as well. Before we can discuss why filter selection affects energy consumption, it’s important to understand the role of resistance in the system.
Water encounters resistance as it moves through the skimmer, fittings, pipe, pump, valves, filter, check valves and heater. The sum of these resistances is measured in feet of head, which is the equivalent column of water that would cause the same resistance.
Consider that 1 psi = 2.31 feet of head. If a filter gauge reads 23 psi, then it can be calculated that 23 X 2.31 = 53.13 feet of head. This means that the resistance to push water through the filter and back to the pool is equivalent to a pump raising a column of water up 53.13 feet, regardless of the size of the column.
The effect of lowering resistance may depend on the pump. Reducing the head or resistance in a system with a constant flow pump (one that varies the speed to maintain a constant flow) will allow the pump to run at a lower speed, which reduces the electricity consumed.
However, reducing the resistance in a system with a standard induction motor pump will cause the pump to move more water, and it will consume more electricity. With an induction motor, you have to reduce the pump’s runtime to save money and use a flow meter to ensure the water quality isn’t compromised.
Also, in low head systems, induction motor pumps (which have been traditionally designed for systems that operate in the 40- to 60 feet of head range) could run at the end of their pump curves over their maximum amperage rating.
Using a constant flow pump is not the only way to improve efficiency with lower resistance. Different kinds of filters add different amounts of resistance to the system, so filter selection between cartridge, DE and sand plays an important role.
Cartridge filtration offers the least amount of resistance to flow. In the 30 gpm range, a cartridge filter creates about 0.7 to 2 feet of head. At 60 gpm, a cartridge filter sized for that flow capacity creates 2-5 feet of head. Also, cartridges conserve water and chemicals by not requiring the backwashing of other types of filters.
Another advantage of cartridge filters is that they offer more square footage of filtration area. Some have upwards of 400 square feet of media, as compared with DE filters, which max out in the 100-square-foot range, and sand, which offers between 3 and 7 square feet. A larger area for the water to pass through means it takes less energy to move the water through it. Also, the resistance to flow increases at a slower rate because it takes more dirt to reach an accumulation level that reduces flow.
However, cartridges don’t guarantee the most energy efficient equipment pad. Anyone who has ever cleaned a filter knows that when the pleats are packed tight in the cartridge, they often will stick together, and some pleats will have dirt on them and appear to be filtering while other pleats are completely clean.
DE and sand
Most DE and all sand filters add resistance to a system by requiring backwashing, typically by using a valve. However, not all backwash valves are created equal. Multiport valves create much resistance to flow; in fact, Title 24 has banned 1.5-inch multiport valves. Slide valves, also called push-pull valves, add less resistance than multiport valves because the water passes through them more directly. Other kinds of backwash valve styles have been designed to add even less resistance to the system by using larger openings for the water.
The reliance of some Diatomaceous Earth (DE) filters on backwash valves to occasionally remove used DE from the grids is one reason DE filters are not always the most energy efficient. Of course, DE is a great filter media when it comes to fine particle removal, but it takes more pressure to move water through the DE than other media.
In terms of resistance, a DE filter without a multiport valve creates around 1.5 to 2 feet of head at 30 gpm; add about 3 feet for a 2-inch multiport and about 7 feet of head for a 1.5-inch valve. At 60 gpm, a DE filter creates about 5-6 feet of head, plus about 12.5 feet for a 2-inch multiport valve, and 18 feet for a 1.5-inch valve.
Some builders choose to install DE filters without multiport valves to reduce resistance, which requires that the DE filter be taken apart to remove the used DE. These filters may present a good alternative for customers who want the polish of DE with the energy efficiency and maintenance ease of cartridge.
Of the three filter designs, sand adds the most resistance to the system. Most residential applications include a multiport valve, and moving that water down through those particles of sand requires a great deal of pressure.
For the sake of comparison, a large residential sand filter (3.4 sq. ft. of filtration area) without a multiport valve creates around 3- to 4 feet of head at 30 gpm and add about 3 feet with a 2-inch multiport valve. At 60 gpm, the same sand filter adds about 11 feet of head, plus 12.5 feet for the 2-inch multiport valve. See the table on page 30 for how the three filter types compare on this important detail.
However, sand is the preferred method of filtration for many pool service professionals. Sand filters are the quickest and easiest to clean during routine maintenance.
Weighing your options
If the most important criteria in your filter selection is the resistance generated by the filter, then it is clear that the best choice is a large cartridge, with a very close second being a DE filter without a multiport valve.
However, it is important to recognize that some customers will prioritize the easy maintenance of sand filtration. And some will want the polished water you get with DE and the ease of maintenance you get with the multiport valve for backwashing.
As with any equipment choice, the important thing about filter selection is to understand the pros and cons of each option and give your customers the best advice possible.