Making the Most of Filters

Making the Most of Filters Image

Wednesday, October 20, 2021

Pool & Spa News | John Watt | June 2021

When it comes to keeping pool water cleaner and clearer, the filter is key. Even chemical manufacturers agree that chemicals can’t provide cleaner and clearer water without a properly functioning filter.

In fact, with sanitizers in high demand and low supply this season, it has become more important than ever to maximize the abilities of filters, whether sand, DE or cartridge. Understanding the relationship between the pump and filter gets to the heart of maintaining clean and clear water. Pool professionals who understand this relationship can then incorporate filter cleaning, chemicals, variable frequency drives and automatic controllers to achieve pool water perfection.

Water Flow

Filtration systems of all media are designed to work within a specified water-flow range. But of all the filtration-related mistakes that service techs see in the field, oversized pumps are one of the most common.

Why is this a problem? A pump that pushes water through the filter too quickly impedes the filter’s ability to do its job correctly. The faster the water moves through the media, the worse job it does. The slower the velocity, the better job the filter does separating debris from the water. It is therefore extremely important to properly size the filtration system to the required pump flow to achieve efficient water filtration.

An oversized filter with a lower-horsepower pump can cause other problems. A typical example of inadequate backwash time or flow rate will show up when the system is put back into filtration mode. When a sand filter is not properly backwashed, a small amount of debris, or silt, will come out of the returns for the first few minutes after returning the system to normal filtration mode. Some of the silt will also remain in the filtration system, working its way deeper into the sand bed. This can lead to costly service, repairs or even premature replacement of the filter.

To avoid this mistake, comply with the current standards for filtration:

• High rate sand filtration: Size for 15 gallons per minute (gpm) per square foot of filtration surface area.

• DE: Allow 1 to 2 gpm per square foot of surface area.

• Cartridge: Limit flow to 0.375 gpm per square foot of surface area.

To size a filtration system correctly, professionals must determine the pool’s required turnover rate, as well as verify the proper filtration rate. For example, a commercial pool of 150,000 gallons needs a six-hour turnover rate and would therefore require a flow rate of 416 gallons per minute.

The challenge comes in accommodating both clean and dirty flow rates. When the filter is dirty, resistance to flow goes up and flow rate drops. For a clean filter, the resistance to flow goes down and the flow rate goes up. Take, for instance, a pump that generates 275 gpm at 83 feet of head and 400 gpm at 60 feet of head, accounting for a 23 change in total dynamic head between clean and dirty filter mode. The filtration will need to handle 400 gpm, even if the needed flow rate for turnover is only 275 gpm.

When sizing the system, consider these three issues:

• Clean filter flow rate: The system must handle the extra flow when the filter is clean.

• Dirty filter flow rate: The minimum turnover flow rate must be met when the filter is dirty.

• Backwash flow rate: Flow rates should meet the manufacturer’s requirement for sand and DE filtration where backwashing is applicable. This pertains to filter size, pump size and backwash plumbing.

Working on existing systems

To properly size a replacement filter for an existing pool, you’ll need to assess the system’s parameters and capabilities. Follow these steps:

1. Backwash the filter as prescribed by manufacturer

2. Attach vacuum and pressure gauges to the pump

3. Convert vacuum and pressure readings to total dynamic head (TDH), using this equation:

vacuum x 1.13 pressure x 2.31

4. Apply the TDH to the manufacturer’s curve for the existing pump. This will give you the flow rate at its maximum with a clean filter. Make sure that the maximum flow rate does not exceed the maximum flow rate for your filtration system.

5. To determine the flow rate with a dirty filter, add 23 feet of head to your TDH. Make sure this does not drop below the minimum flow rate to meet your turnover time, as well as the minimum required flow for other equipment such as heaters, skimmers, sanitizing system, etc.

If the filter you’ve chosen doesn’t fit, you have three options:

• Change the pump to one that better fits the flow requirements.

• Add a variable-frequency drive with a flow control. This will allow you to maintain a constant flow rate by compensating for TDH changes during the clean-to-dirty-filter cycle. As an added benefit, this option could potentially pay for itself over time in energy savings.

• Add filtration to the system by installing an additional filter or a larger one.

Filter Pressure

Once the filter gets dirty, it must be cleaned to continue operating properly.

Filter pressure serves as one of the best indicators that a filter needs cleaning. A change in pressure differential, measured in pounds per square inch (psi), helps service professionals determine if a filter is dirty or needs backwashing.

It is helpful to keep a log of each pool’s filter pressure. Many service technicians do this at pool opening and leave the information posted near the filter in the pump room, or they log it into the customer’s pool chart. One of the most important figures to note is the initial psi on the gauge at the top of the filter tank.

When a pool has trouble with water turbidity or the heater cycling as it tries to maintain the proper water temperature, professionals can start their diagnosis by looking at the filter pressure reading. If, for example, the pressure reading at pool opening was 18 psi but then shows 30 psi when the trouble occurs, the filter may be dirty. Begin by backwashing to see if that fixes the problem. Restored flow allows the filter to function properly, which should clear the water. This might even fix the pool-heater problem. Sometimes a lack of flow through the heat exchanger allows the water to absorb too much heat and shut off on the high-limit safety circuit built into the heater.

However, a newer challenge has developed with the increasing popularity of variable-speed pumps. Before, the pump was either on or off, and service tech took the psi measurement when the pump was on. But VSPs do not run constantly at full speed, making it harder to obtain an accurate psi reading. The flow and pressure increase, but this doesn’t necessarily mean the filter is dirty. To overcome this issue, many service techs turn up the pump to the highest pre-set circulation speed — not the pump’s maximum speed, but the highest pre-set — and take a psi reading during weekly service. Those who don’t do a weekly service often ask customers to perform this task and tell them to call the service company if the psi increases by more than 10 psi.

Cleaning when there is a 10-psi increase in pressure is especially important with cartridge and DE filters, as it can prolong their lives. Following this procedure will help avoid compression and compaction of dirt and debris on the face of the grids or cartridges, increasing their life cycles and making them easier to clean.

Backwashing Sand Filters

Proper backwash flow rates: 15 to 20 gpm per square foot

Example: A 3.1-sq. ft. sand filter needs no less than 46.5 gpm and no more than 62 gpm.

Backwash time: This should not be determined by looking at the water flowing out of the backwash line. Sand filter backwashing should be done in 3 minutes. Set a stop-watch for a 3 minute backwash cycle — don’t guess.

With too much water flow: This could lift the sand bed and pass it to waste. Much worse, you could damage the laterals or filter tank from the sandblasting effect as water is introduced at the bottom of the tank at a high velocity.

Inadequate flow: This may not be able to completely remove the debris from the sand bed. The debris that remains will work its way deeper into the filter, settle to the bottom and form caliche, which often cannot be removed. In the worst cases, this will require the filter to be replaced.

Other issues to avoid: Sand filters can be backwashed too frequently in desert or dusty environments, where silt can pass completely through a clean filter. Customers may call, explaining that they have sand in their pool and that additional sand comes out of the returns when they vacuum. To diagnose this situation, brush the pool and notice if sand makes a pile or a cloud. A pile indicates there could be an issue with the filter. If it forms a cloud, it is not sand, but rather silt. This means the sand filter is too clean. Stop backwashing the filter, allow it to load up, and then begin to trap the silt into the filter. A clarifier or flocking agent can help.

Backwashing DE Filters

Backwash time: DE filters should be backwashed for 3 minutes. However, break down the cycle into a 1-minute backwash, then a 1-minute filtration for three separate cycles. (Make sure you shut the pump off each time you switch between backwash and filtration. This helps separate the DE coating from the grids so it can be removed.

When to backwash: Like any other filter media, backwash when a 10-psi increase over normal operating pressure is reached.

Note: Unlike sand filters, backwashing DE filters frequently does not affect the filtration rate.

Other maintenance: A DE filter should be torn down, cleaned and degreased at least twice a year. Body lotions and sunscreen collect on the grid material and cannot not be backwashed away, so a degreasing agent should be used. Avoid muriatic acid, as it will permanently lock oils and lotions into the grid material.

When recharging, use the amount of DE recommended by the manufacturer. Use a 5-gallon bucket, then add water and DE together to create a slurry. Slowly pour the slurry into the skimmer while the pump is running. This ensures that the grids or elements are properly coated from top to bottom.

Cleaning Cartridge Filters

Cartridge filters are not designed to withstand water flow in the reversed direction, therefore they are not backwashed but must be taken apart to clean. The process is the same as a DE filter teardown: Hosing surface debris off the cartridge is only the first step (In fact, if it is your only step, you might as well not bother).

Body oils and lotions are the number one contributor to a plugged cartridge filter, so removing them makes the difference between long-lasting cartridges or frequent replacements. The oily buildup on the surface plugs up the pores on the cartridge, restricting water flow, and it also creates a very sticky surface, which traps the dirt in place and very quickly plugs the filter up.

Many pool service pros now offer cartridge-filter cleaning services. They hose off the filter, soak it with muriatic acid and a filter-cleaning agent, then re-soak with a liquid chlorine bath to brighten and lighten the filter so it looks nicer when returned to the client. Finally, allow the filters to air dry completely. This way, the fibers can fluff back up and expand, making them more effective. If the fibers can be pushed down easily, it reduces the filter cycle.

If customers own two sets of cartridges, they can continue using their pool while the other set is cleaned. Many techs find it most effective to take the filter media when they close a pool for the season, clean it at their shop, then return the filters ready-to-use for spring re-opening. Because pools are typically at their dirtiest during spring opening season, those cartridges are filtering more debris than usual, so some pool service professionals change out the filters after the pool startup. Once the water is clear, they install new filters to provide more productive filtration.

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