Sand Filter Sizing

[Schnozz]

Can someone tell me how to convert/calculate/translate inches of Sand Filter to SqFt of cleaning?


For example...How many SqFt is there in a 23" sand filter? Is there a formula for that?

Thanks, Bob

 

[denniswiseman]

D x D x .7854 divided by 144 equals 2.88 square feet



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[Schnozz]

Sorry, what's the first D and the second D? Is that Diameter Squared?

 

[denniswiseman]

Diameter of the area in inches


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[Schnozz]

Ta-Da!! Thanks. That will come in handy....

 

[denniswiseman]

Bear in mind that is the surface area at the top


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[Schnozz]

So, in the "Hydraulics 101" sticky, there is a table that lists filter sizes of the various types of pool filters (cartridge/sand/DE) available. For Sand filters, the sq-ft sizes listed in the table is the surface area of the sand filter.
If I understand correctly, when a sand filter is advertised as 2.8 sq-ft in size, that has reference to sq-ft of surface area?
For example, the sq-ft area of a 23" sand filter is calculated as follows...
23 x 23 x .7845 divided by 144? Or about 2.8 sq-ft?
Now, can flow in GPH/GPM be calculated from this info or does flow rate come from the manufacturer?

 

[mas985]

The manufacture flow rates usually comes from the NSF 50 APSP-15 specifications (link is in sticky). Some are recommendations while others absolute maximums. For sand filters, maximum is usually 20 GPM/sq-ft while the NSF recommendations are 15 GPM/sq-ft. So slightly below maximum. However, I have seen some manufactures use 25 GPM/sq-ft but that is pretty high and likely to result in very poor filtering.

or example, the sq-ft area of a 23" sand filter is calculated as follows...
23 x 23 x .7845 divided by 144? Or about 2.8 sq-ft?

Correct.

 

[JamesW]

Some manufacturers will specify 20 gpm/sqft for residential and 15 gpm/sqft for commercial.

 

[singingpond]

D x D x .7854 divided by 144 equals 2.88 square feet

Was scratching my head for a few moments about those constants -- pleased to realize that it is just standard formula for area of a circle after all (pi * r * r) to get area in square inches, and then divide by the 144 square inches in a square foot. The .7854 looked mysterious, but it's just pi/4, to make up for the fact that the formula squares the diameter rather than the radius of the circle.

I learned something new in this thread -- i.e. that the square feet of a sand filter is based on surface area at the top; always wondered where that number came from.

That brings up a question, though -- some sand filter tanks have a much more spherical shape, rather than the typical mainly-cylindrical shape. It would seem that the spherical ones could claim the same top surface area as a cylinder (depending on how high the sand is in the tank), but actually have less filtration medium because of the shape? Is that taken into account anywhere in ratings of sand filters?

 

[JamesW]

I would think that the manufacturer would give the actual sand surface area when the specified amount of sand was addded. I think that most rounded tanks fill to the largest diameter and don't fill much to where the diameter begins to get smaller.

 

[mas985]

The Tagelus 60D filter is a 24" filter with 3.14 sq-ft of area according to Pentair specs and the outside maximum diameter follows this. So I think James is correct. Triton and Sand Dollar also follow this largest measurement. Less tank wall thickness.

 

[singingpond]

I had in mind this shape of tank, for example:

Waterway Clearwater 22 Swimming Pool Sand Filter Has Durable Roto Molded Tank | eBay

This is so round that the sand volume below the top surface of the sand must be less, as compared to a mainly-cylindrical tank with the same top surface area. Wouldn't a lesser volume of sand lead to less filtration capability, even with the same top surface area?

 

[JamesW]

Sand surface area and depth are relevant considerations. Good point about checking both for comparison. I suspect that a spherical tank might compromise on surface area or depth compared to a tank with a cylindrical shape in the middle.

 

[Schnozz]

You guys are awesome. I am at times, awed by the depth of knowledge available through this forum. I cannot tell you how much I appreciate everyone taking the time to mull these questions over in their minds and sharing their thoughts with us layman on the forum. It's probably more information than I will ever remember. Clearly you guys are head and shoulders above me in this area, but I am positive that when I make a purchase to upgrade my set up, I have been given a wealth of experience and knowledge to get me through it. I am convinced that this information will assist me to make the correct pump/filter choice for my little Intex above ground pool. At the very least, I am able to understand the manufacturers spec sheets much better. You would think by the responses in this thread to my initial question that You were helping me plan an Olympic sized public pool. It just goes to show why the forum is here....TO HELP ALL POOL OWNERS. I am thoroughly enjoying the responses in this thread. Thanks again......

 

[Schnozz]

I'm sorry to be a pest with my questions, I apologize if I have too many threads running at the same time on similar topics. Please feel free to shut all but one down and combine them all into one or tell me to get lost....LOL...I'm starting to annoy myself.....

On this curve for 3/4 HP, 1 HP & 1 1/2 HP Optiflo Pumps from Pentair, can someone tell me from where on the curve do you determine the proper Pump GPM for selecting a Sand Filter?
I understand, Head Loss needs to be calculated first (distance, elbows, elevation changes, plumbing size, etc. etc., etc.). But where do you start on the chart? Lets say for example, I'm considering the 1 HP pump (2-speed). Do I start at about 80 GPM (The greatest GPM flow of the 1 HP pump per the chart.) and deduct the head loss from there and use that GPM figure to select a Sand filter or do I select a Sand Filter based on 80 GPM? I'm sure its not that easy, or is it? One poster has said that calculating Head Loss is not a trivial thing, but a complicated endeavor. I do understand that, but I need a place to start...I don't mean to be difficult.

Again, my apologies, I just want to better understand the relationship between Pumps and Filters when I upgrade. The same way you all have taught us all about pool chemistry (for example, the relationship between CYA and Chlorine.). Those basic chemistry concepts are lost on most Pool Builders/Pool Stores/Pool Owners, but not lost on us. I would like that measure of clarity regarding Pump and Filter selection.

Thanks again

 

[pooldv]

Ok, just so you know, this forum is here for you to ask questions. Questions don't annoy us. If people don't ask questions then we have to go brush our pools or something.

You want a filter that is rated for the highest point on the curve. You just don't want to exceed the max flow rate. All filters will filter more efficiently at lower flow rates, like on low speed for a 2 speed pump.

 

[mas985]

You could use the right most point on the head curve but that is unrealistic because that assumes almost no head loss from the plumbing.

Without a detailed hydraulic analysis, the best you can do is to use one of the standard plumbing curves to represent your plumbing. Generally I use Curve A for above ground plumbing. You can plot this curve over the pump's head curve and you can get an approximate flow rate:

Plumbing Curve A - Head Loss (ft) = 0.0167 * GPM ^2

If you want to be conservative, assume plumbing Curve C which has lower head loss and results in higher flow rates.

Plumbing Curve C - Head Loss (ft) = 0.0082 * GPM ^2

However, to save you time, you can look up the flow rates from this table:

Pump Database - Google Sheets

The 1 HP Optiflo on Curve A is 48 GPM and Curve C is 63 GPM.

 

[Schnozz]

Pooldv...STOP BRUSHING YOUR POOL..(LOL)...Can you identify the "highest point" on the curve for the 3/4 HP Optiflo Pump on the chart above for me? Just so I see what you mean by "highest point".

Mas: So for the Optiflo Pumps from Pentair (lines 229 - 232) It looks as though it's basically the same motor. Even the RPMs are the same High Speed 3450/Low Speed 1725. I assume the flow rate is raised with the pump (Excuse the nomenclature if not correct) because the RPM's remain constant for each motor (3/4 HP, 1 HP, 1 1/2 HP)? Correct?

If I understand correctly, for the 3/4 HP Pentair Optiflo Pump/Motor (Line 229), one should use a 3.9 sq-ft Sand Filter (26" - 27") for optimum performance? For the 1 HP Pump/Motor (Line 231) one should use a 4.1 sq-ft Sand Filter (28"-29")? In the matched sets from Pentair (or any other manufacturer), the sand filter is never that big for a 3/4 HP or 1 HP pump/motor. Any ideas why?

For now, for my needs, the 3/4 HP Pump/Motor with a 26" Sand Filter will do the job if the equipment is 30' from the pool. assuming 1 1/2" Hard Plumbing.
If however, in the future I decide to add Solar on the Patio roof (another 10 feet above the ground) I may be better off going with the 1 HP Pump/Motor and the 28" Sand Filter with 1 1/2" hard Plumbing.

Is my thinking and assumptions correct?

The spread sheet was a huge help. I don't really want to do the math or necessarily understand it all. I just wanted to better understand the Pump/Motor Size to Filter size relationship better. Your table does it all. It is your table, right? Nice work whoever took the time....

I think I'm good for now unless my brain kicks in again....

I can't thank you both enough..You're always quick to respond...

 

[Schnozz]

Mas, Pooldv:
Speaking of filters too small for a 2-Speed, 1 HP Pump/Motor.....

For example this Pentair Combination, PNSD0035DE2160, has a 1 HP 2-speed Pump and, according to the Specs, that combination comes with a 1.4 sq-ft or 16" sand filter. The table states that this combination has a flow rate 35 GPM. The footnote says that that 35 GPM is based on the Filter Size not the pump/motor size. Isn't that backwards? Isn't that too much pump for a 16" Sand Filter. I believe the table in Hydraulics 101 and your spread sheet bases the Filter Size on the Pump/Motor Size.

Here is the link to the Specs for this Sand Dollar Combination..

http://www.pentairpool.com/products...dollar-aboveground-sand-filter-system-148.htm



Thanks