Backwash Rate = cross-sectional surface area X flow rate per Sq. Ft. of media
Most "filter" medias have a specified Backwash rate of 8-10 gpm/sq. ft.
That is not the specification for backwash rate for the diameter tank size.
You have to do some "MATH" to get the DLFC ( backwash flow ) rate in GPM.
The surface area in the tank that has to be "RAISED" by the water
is less than a "SQ.FT" in the tank.
is less than a "SQ.FT" in the tank.
This is called the Tank Cross-Sectional Area
and
type ( weight ) of the MEDIA to determine the Backwash Rate required.
I do understand how you would be confused by the way manufacturers specify the product backwash requirements.
I had to learn what they really meant myself.
For a 10" tank using Katalox Light as the media,
At a manufacturer-specified requirement of 8 to 10 GPM/sq. ft.,
a standard 10-inch diameter tank requires a backwash flow rate between 4.36 GPM and 5.45 GPM to properly expand and clean the filter media.
Therefore, a standard system supplying 5 or 7 GPM is entirely adequate.
a standard 10-inch diameter tank requires a backwash flow rate between 4.36 GPM and 5.45 GPM to properly expand and clean the filter media.
Therefore, a standard system supplying 5 or 7 GPM is entirely adequate.
1. Calculate Tank Cross-Sectional Area
Backwash requirements are determined by the surface area of the top of the media bed, dictated entirely by the tank's inner diameter. [1, 2]
- A 10" X 54" tank has an outer diameter of 10 inches, which equates to an inner diameter of approximately 10 inches (0.833 feet).
- The radius (r) is half of the diameter:
r=0.833 ft divided by 2 = 0.4167 ft
Using the circle AREA formula:
A=pi times r squared
A=pi times r squared
In this case = pi ( 3.14 ) divided by 0.4167 ft squared = approx 0.5454sq.ft.
2. Determine Required Backwash Flow Rate
To find the actual gallons per minute (GPM) required, multiply the surface area by the media specifications: [1]
- Minimum requirement (8 GPM/sq. ft.):
Minimum Flow Rate = 0.5454sq. ft. times 8 GPM sq.ft.= 4.36 GPM - Maximum requirement (10 GPM sq. ft.):
Maximum Flow Rate=0.5454 sq.\ ft. times 10 GPM sq.ft. = 5.45 GPM
Therefore the commonly used (10" x 54") tank requires a backwash flow rate of (4.4 to 5.5 GPM ) based on a media specification of 8 to 10 GPM/sq. ft.
Tank Surface Area: A 12-inch diameter tank has a radius of 0.5 feet.
Using the formula for the area of a circle , the cross-sectional surface area is 0.785 square feet.
Minimum Flow Rate (8 GPM/sq. ft.): Would be 0.785 sq.ft. x 8 GPM = 6.28 GPM
Maximum Flow Rate (10 GPM/sq. ft.): Would be 0.785 sq.ft. x 10 GPM = 7.85 GPM
For a 12" tank using filter media ( NOT softening resins ).
Using the same formulas, we see 6.3 gpm to 7.9
We would used 7.0 gpm, since this is the maximum backwash flow rate for a Fleck 5600 Filter Backwash ( or AIO ) valve head.
If you need a higher backwash
( because your are using a larger tank with more media ),
you'd need a different model valve ( 2510 or 2850 ).
( because your are using a larger tank with more media ),
you'd need a different model valve ( 2510 or 2850 ).
Water Temperature Variations:
While 8–10 gpm/sq. ft. is the benchmark, water temperature affects fluid density.
If you are dealing with exceptionally cold well water (e.g., around 50°F to 55°F), the water is denser and the media lifts more easily, allowing you to stay safely on the lower end of that range.
Warmer water may require closer to 10–12 gpm/sq. ft. to achieve the same 30% bed expansion.
While 8–10 gpm/sq. ft. is the benchmark, water temperature affects fluid density.
If you are dealing with exceptionally cold well water (e.g., around 50°F to 55°F), the water is denser and the media lifts more easily, allowing you to stay safely on the lower end of that range.
Warmer water may require closer to 10–12 gpm/sq. ft. to achieve the same 30% bed expansion.
Generally this will calculate out to an additional 20% of backwash flow rate required, if the water is warm ( 70 F and above ).
