A New View on When to Backwash
With variable-speed technology as a factor, the industry may need to rethink its approach to backwashing.
For decades, pool professionals have learned to backwash or clean the filter when the pressure reading increases 10 psi above starting or clean pressure. The growing use of variable-speed pump technology requires a change in this long-understood “rule.”
From an energy point of view, think about this: Increased pressure in the filter adds resistance to the system. (A 10 psi increase in the filter, say from 12 psi to 22 psi, adds 23.1 feet of head to the system.) Higher resistance means lower flow. When a pump is running at a constant speed this means less energy is consumed, but it also means the pump would need to run longer to achieve the same turnover. If the flow is held constant, it would take an additional 200 watts to maintain 60 gpm with a 5 psi increase in pressure and an additional 500 watts with a 10 psi increase in pressure.
So what does an additional 500 watts mean in dollars? Here’s the formula: 0.5kW x $0.15kWH x 12 hrs. per day x 365 days per year = $328 per year.
With commercial systems using 15hp and 20hp pumps, this can mean a lot of money. If the pump is running at a constant speed, this could mean that the excess filter pressure causes the pump to fall well below the required turnover rate. With a variable-speed pump set at one speed, the 10 psi increase could mean that the excess pressure puts the pump at a flow rate that does not satisfy a minimum flow rate for other system components, such as a heater or salt chlorine generator, leading to a cold pool, a green pool or an unhappy pool owner.
The speed of the water leaving the peripheral edge of the impeller is what creates pressure in a pump. When the speed of the pump is cut in half, the pump can only create one-quarter as much pressure. The affinity law states this: Speed is directly proportional to pressure squared. At lower speeds, the pump curve gets flatter. This means that a high head pump curve looks more like a medium or low head pump curve at lower speeds. So a small increase in pressure translates into a larger decrease in flow at lower pump speeds.
That’s good and bad on a practical level. It’s good because friction loss or resistance in a swimming pool also decreases according to the affinity law. If we decrease the flow in a pipe by ½, friction loss or resistance decreases to ¼. As an example, let’s say we had a pump running at 60 gpm and decreased the flow to 30 gpm. At 60 gpm, let’s say the filter pressure was 28 psi. At 30 gpm, the filter would read that the system is at one-quarter of the pressure or resistance or 7 psi.
Here is the bad news, and an example of why we can’t use the 10 psi number any more with variable-speed pumps because they are circulating the water at lower flow rates: At 60 gpm, a 10 psi increase in filter pressure (because the filter’s dirt load increased, meaning it is working properly) would take the system from 28 psi to 38 psi. That would be a 36 percent increase in pressure. At 30 gpm, a 10 psi increase in pressure (from 7 psi to 17 psi) before backwashing or cleaning the filter would be a 142 percent increase in pressure. And that could lead to a no-flow condition. Using the old rule of thumb and adjusting it to the lower flow rates and flatter curve characteristics would tell us that, at 30 gpm, the filter should be backwashed/cleaned before a 2 psi increase is incurred. This could keep us away from a no-flow situation.
Commercial and residential pools can realize huge electrical savings running the pump at a lower speed when the filter is clean. This can be done without the chance of reaching a no-flow or low-flow condition.
This is how it works: A flow meter can feed information to a variable-frequency drive, which in turn increases or decreases the pump speed to maintain a chosen flow rate throughout the filtration cycle. As the filter gets dirtier and the pressure in the system increases, the flow meter sees a decrease in flow and tells the drive to simply increase its speed to maintain the flow rate.
This not only saves energy but, from a clarity point of view, dirt load in the filter actually helps to filter out finer particles of dirt, so it’s OK — actually good — to allow dirt to accumulate inside the filter.
Just not too much dirt …