Sand Filter Flow Rates: Max, Min, and Optimal

[spanman]

I'm trying to determine the optimal settings for both economy and effective filtration with my variable-speed pump.

My sand filter specs show:

• filtration area of 1.92 sq. ft.
• filtration gpm/sq. ft. = 20 gpm
• filtration flow rate of 38 gpm (i.e., 20 x 1.92)

Questions:

1. Should I consider 38 gpm the maximum advisable flow rate for this filter?
2. Should I assume that flow rates below 38 gpm could result in better filtration?
3. Is there a minimum flow rate below which filtration would be ineffective? (There's no such number in the specs.)
4. Can I determine an optimal filtration flow rate, somewhere between that high of 38 gpm and a minimum effective flow?

 

[JamesW]

In my opinion, the maximum flow rate for a sand filter should be about 15 gpm per square foot.

Backwash should be 15 gpm per square foot.

There is no minimum flow rate.

I would run the variable speed pump at the lowest speed that provides good skimmer action.

Some equipment requires specific flow rates, such as heaters.

 

[mas985]

Questions:

1. Should I consider 38 gpm the maximum advisable flow rate for this filter? YES. However, if you are asking at what flow rate a sand filter will be damaged, that is probably north of 25 GPM/sq-ft.
2. Should I assume that flow rates below 38 gpm could result in better filtration? YES
3. Is there a minimum flow rate below which filtration would be ineffective? (There's no such number in the specs.) 0 GPM
4. Can I determine an optimal filtration flow rate, somewhere between that high of 38 gpm and a minimum effective flow? What do you mean by optimal?

A filter's main purpose is to trap debris that gets by the skimmer and pump basket. It will pretty much do that at nearly all flow rates below the rating. Some tiny debris can be pushed through with too much flow rate but that is usually only an issue with dead algae and maybe pollen. Most pool debris is large enough to be caught by the filter regardless of the flow rate.

But there is an optimal flow rate for your Intelliflo for energy efficiency. This would be the lowest RPM where water still travels over the weir door in the skimmer so you still get skimming action at the lowest possible flow rate. With a single skimmer, this would be around 15-20 GPM. But usually, there is no need to go much below about 800-1000 RPM because the power draw of the pump starts to level out at that point. This operating point would also be a good thing for the filter as it is low enough flow rate to trap nearly all debris.

 

[spanman]

A filter's main purpose is to trap debris that gets by the skimmer and pump basket. It will pretty much do that at nearly all flow rates below the rating....But there is an optimal flow rate for your Intelliflo for energy efficiency. This would be the lowest RPM where water still travels over the weir door in the skimmer so you still get skimming action at the lowest possible flow rate. With a single skimmer, this would be around 15-20 GPM.

Much appreciated, Mark. Thanks for addressing this and my other questions. I'll keep an eye on the skimmer at various pump RPMs to find the lowest rate that shows positive flow over the skimmer weir door.