Filtration rate vs turnover flow

Problem set: 6 worked problems

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Watch for Filtration rate is flow per square foot of filter, a different number from the turnover flow through the whole pool. Each filter type has its own maximum: high-rate sand near 15 gpm per square foot, diatomaceous earth near 2, cartridge near 0.375. Checking a number against the wrong filter type is the classic miss. Verify limits against local code and the Model Aquatic Health Code.

1. A system moves 300 gpm through a 15 sq ft high-rate sand filter. What is the filtration rate, and is it within the roughly 15 gpm per sq ft sand limit?

   • A 0.05 gpm per sq ft (fine)
   • B 4,500 gpm per sq ft (over)
   • C 15 gpm per sq ft (at the limit)
   • D 20 gpm per sq ft (over the ~15 limit)
   Show the worked solution

   Correct answer: D. 20 gpm per sq ft (over the ~15 limit)

   Filtration rate = flow / area = 300 / 15 = 20 gpm per sq ft, which is above the roughly 15 gpm per sq ft high-rate sand maximum, so the filter is undersized. Multiplying flow by area (4,500) instead of dividing is the setup error.

   Source: Public relationship: filtration rate (gpm per sq ft) = system flow (gpm) / filter surface area (sq ft)

2. How much high-rate sand filter area does a 450 gpm system need at the roughly 15 gpm per sq ft maximum?

   • A 6,750 sq ft
   • B 45 sq ft
   • C 0.03 sq ft
   • D 30 sq ft
   Show the worked solution

   Correct answer: D. 30 sq ft

   Required area = flow / max rate = 450 / 15 = 30 sq ft. Multiplying 450 by 15 (6,750) instead of dividing is the common inversion.

   Source: Public relationship: required filter area = flow / maximum filtration rate for the filter type

3. A 200 gpm system uses a diatomaceous-earth (DE) filter with a maximum of about 2 gpm per sq ft. How much filter area does it need?

   • A 13 sq ft
   • B 400 sq ft
   • C 50 sq ft
   • D 100 sq ft
   Show the worked solution

   Correct answer: D. 100 sq ft

   Required area = 200 / 2 = 100 sq ft. Using the sand limit of 15 by mistake gives 200 / 15 = 13 sq ft and undersizes the DE filter almost eight-fold, the wrong-filter-type error.

   Source: Public relationship: required area = flow / maximum rate; DE maximum near 2 gpm per sq ft

4. How does filtration rate differ from turnover flow?

   • A They are two names for the same number
   • B Turnover flow is measured per square foot of filter
   • C Filtration rate is always larger than turnover flow
   • D Filtration rate is flow per square foot of filter; turnover flow is total flow through the whole pool
   Show the worked solution

   Correct answer: D. Filtration rate is flow per square foot of filter; turnover flow is total flow through the whole pool

   Turnover flow is the total gallons per minute the system moves to circulate the whole pool; filtration rate takes that flow and divides by the filter's surface area, giving gallons per minute through each square foot of media. They answer different questions.

   Source: Public definitions: turnover flow (whole pool) vs filtration rate (per sq ft of filter)

5. A cartridge filter with 80 sq ft of media passes 60 gpm. What is the filtration rate, against the roughly 0.375 gpm per sq ft cartridge limit?

   • A 0.375 gpm per sq ft (at the limit)
   • B 1.33 gpm per sq ft (fine)
   • C 4,800 gpm per sq ft (over)
   • D 0.75 gpm per sq ft (about double the ~0.375 limit)
   Show the worked solution

   Correct answer: D. 0.75 gpm per sq ft (about double the ~0.375 limit)

   60 / 80 = 0.75 gpm per sq ft, roughly double the 0.375 cartridge maximum, so this cartridge is undersized for the flow. Cartridge and DE limits are far lower than sand, which is why the filter type must be matched to the number.

   Source: Public relationship: filtration rate = flow / area; cartridge maximum near 0.375 gpm per sq ft

6. A 24 sq ft high-rate sand filter passes 300 gpm. Is it within the roughly 15 gpm per sq ft sand limit?

   • A 20 gpm per sq ft, over the limit
   • B 12.5 gpm per sq ft, over the limit
   • C 0.08 gpm per sq ft, fine
   • D 12.5 gpm per sq ft, within the ~15 sand limit
   Show the worked solution

   Correct answer: D. 12.5 gpm per sq ft, within the ~15 sand limit

   300 / 24 = 12.5 gpm per sq ft, which is below the roughly 15 gpm per sq ft high-rate sand maximum, so this filter is adequately sized. Dividing area by flow (0.08) inverts the ratio.

   Source: Public relationship: filtration rate = flow / area; high-rate sand maximum near 15 gpm per sq ft