Which Sand filter? (inground pool)

I am having a heck of a time finding an answer. I have a 13,500 gallon pebble tec freeform pool. I.A 763 surface area of 435square feet. 82 foot perimeter. I am going to replace the Jacuzzi cartridge filter with a sand filter( Hayward, Pentair,sta-rite...other suggestions appreciated), filled with one of the Zeotype medias. BUT I cant figure out what SIZE filter to buy.. 18/24/30? Is there somewhere to determine this online? I have been told three different things by different dealers.
Thanks Brandon
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Brandon wrote:

http://www.pested.msu.edu/BullSlideNews/bulletins/pdf/2621/E2621chap8.pdf
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Yes. And its a circultar solution because the filter head pressure affects how much water your pump can pump. Most pumps have appalingly flat curves, which means a slight change in head pressure can have a dramatic effect on flow rates.

Good question. The head pressure caused by the filter will increase as flow increases, but this isn't necessarily bad. If you pump more water through the filter than it is rated for, I believe filter performance (quality) will diminish. Another problem I have is that when backwashing, the pressure is really low so the pump flows even more than when filtering. I'm just on the edge of cavitating my pump when backwashing. If I bypass the filter directly to waste, the pump does cavitate so I can't do that. It is better to have a filter that slightly exceeds your pump capacity than the other way around.
Personally, I think it is better to have a pump running longer at low flow than a huge one that filters the whole pool in two hours. There are times when you want to run the pump all of the time (freezing conditions, algae blooms), and a giant one will cost you more to run during those times. It also takes time for the water to mix well and the things in it to stratify. This again makes long and slow better than short and fast.
Finally, realize that there is a speed limit in water pipes. If the pump intake from the pool skimmer is 1.5" PVC pipe, you shouldn't flow more than 60 GPM through that pipe. Otherwise, it will deteriorate faster due to water friction. For some reason, pressure pipes have a slightly higher speed limit than suction pipes, but for pools the suction pipe is usually the limiting factor.
-- Mark Kent, WA
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I am still at a bit of a loss on what head pressure is, and what changes it. I am assuming it is directly affected by flow rate through the filter. I.E when the sand is dirty, you head pressure increases, correct? So, when you backwash and the sand is clean, that is "normal" as read on the pressure gauge?
I ordered a Hayward 310T( I got it for 355 delivered, a heck of a deal considering everyone else I found was 400 plus freight), just because it is rated at 92 GPM. I am leaning towards a 1 hp Pentair whisperflo pump which will push at about 82GPM at 20 feet of head( I have 2" plumbing).
Any thoughts? Thanks for all the information you have offered. It is much appreciated. Brandon

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Head pressure is caused by the resistance to flow and is measured in feet of water or PSI. This is the pressure that must be overcome to move the water. Pipe length and water flow rate increase head pressure as they increase. Pipe diameter lowers head pressure as it increases. A filter has a given head pressure and the manufacturer will state what it is. This is the "clean" head loss. As the filter becomes dirty, head pressure will increase. For a sand filter, at 10 PSI more head than normal, it is time to backflush (so you want a pressure gauge on your filter).

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Glad you have 2" pipes, so you should be able to flow this much water. As I said before, I think a smaller pump run longer is better than a large one run faster. My pool is slightly larger than yours (17,000 gal) and I run a 3/4 HP pump at around 50 GPM (actually 30 to 60 GPM depending on head pressure). When the pump dies, I'll replace it with a 1/2 HP pump that has a better flow curve (almost always 50 GPM instead of only 50 GPM when the filter is clean).
That being said, what you propose should work, it will just cost more when you have a water problem to remedy because you'll need to run it 24 hours a day for up to a week.
-- Mark Kent, WA
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Mark or Sue writes:

Quite true. You will spend $1000s for electricity running this pump, it pays to get it optimized for power costs. Size it as small as possible to turn the water over during daylight hours.
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