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By John Watt
Why filter? When it comes to keeping pool water clean and clear, the pool industry knows filtration is imperative. Chemicals help keep water clean, but even chemical manufacturers agree if the filter is not working properly, chemicals alone cannot provide clean and clear water. That said, it is important to get back to basics to truly understand how sand, diatomaceous earth (DE), and cartridge filters work.
However, to truly understand filter operation, pool professionals must know how the pump feeds water into these devices. Understanding the relationship between the pump and filter gets to the heart of obtaining clean and clear water. Pool professionals who recognize this relationship can then incorporate filter cleaning, chemicals, variable-frequency drives (VFDs) and automatic controllers to obtain pool water nirvana.
Pump and filter sizing
Filtration systems, regardless of the media being used, are designed to filter water within a particular flow range. One of the most common mistakes service techs see in the field is a filtration system equipped with an oversized pump. This is problematic because a pool pump that pushes water through a filter too fast impedes its ability to perform properly. The faster water is moved through filtration media, the worse it performs. The slower the velocity, the better a filter will perform at separating debris from the water. Therefore, it is extremely important the filter equipment is properly sized for the required pump flow to achieve efficient water filtration.
The current standards for filtration include:
- High-rate sand filtration is 56.7 litres per minute (lpm) (15 gallons per minute [gpm] per square foot of surface area;
- DE is 3.78 to 7.5 lpm (1 to 2 gpm) per square foot of surface area; and
- Cartridge filtration is 1.4 lpm (0.375 gpm) per square foot of surface area.
To size a filtration system correctly, it is necessary to determine the required turnover rate for the pool as well as verify the proper filtration rate. A typical commercial pool containing 567,812 L (150,000 gal) needs a six-hour turnover rate and would therefore require a 1575 lpm (416 gpm) flow rate. The challenge occurs when factoring in clean and dirty flowrates. When a filter is dirty, the resistance to flow goes up and the flowrate drops. When the filter is clean, the resistance to flow goes down and, in turn, the flowrate increases.
For example, a pump that will provide 1041 lpm (275 gpm) at 25.3 metre of head (83 feet head) and 1514 lpm (400 gpm) at 18.3 metre of head (60 feet head) (23 total dynamic head [TDH] change between clean and dirty filter mode). That said, the filtration will now have to handle 1514 lpm (400 gpm) even if the required flowrate for turnover is only 1041 lpm (275 gpm).
Three things need to be taken into consideration when looking at the system.
- Clean filter flowrate: Making sure the system will handle the extra flow when the filter is clean.
- Dirty filter flowrate: Making sure the minimum turnover flowrate is met when the filter is dirty.
- Backwash flowrate: Ensuring the flowrate meets the manufacturer’s requirements for sand and DE filtration where backwashing is applicable. (This pertains to filter, pump, and backwash plumbing size).