Water turnover - how long?

[AnnaK]

Maybe this is a dumb question but I have to ask: How do you figure out how long it takes for one complete turnover of your water?

I see a lot of comments saying 4 hrs, 6 hrs, 8 hrs etc. How do you folks calculate that?

I have an approximately 12,000 gallon pool and a 2-speed Pentair pump, 1.5 HP, which I run mostly on low unless I'm vacuuming, backwashing, or mixing. The white papers are minimal and I cannot find a thing which would help me figure this out.

Any tips?

 

[duraleigh]

Anna,

Somewhere in your pump info or on the manufacturer's website, you should find a pump curve. It'll tell you how many gpm your pump produces given the resistance (head) in your pool. Use 45 for the head as a good average. Once you know that, and know how many gallons are in your pool, it'll be pretty easy to figure how long it'll take that pump to "turn over" the total gallons in your pool.

 

[Rangeball]

You calculate your dymanic head loss, look at the flow chart for your pump, follow the grid to where your head loss intersects pump flow, which tells you how many gallons per minute you are getting.

Then, you dividide the total gallons in your pool by the gallons per minute of your pump, then divide this answer by 60, as there are 60 minutes in an hour. This gives you the number of hours it takes your pump to provide 1 complete turnover of your pool water.

At low speed, the flow rate is 1/2 of the high speed gpm, so it takes twice as long to get one turnover. However, you use only about 1/3 the electricity at low speed, so you are getting the same work done over a longer time for less electrical cost.

As for figuring out your dynamic head loss, hopefully Mas will be along soon, or else you can read his "are you losing your head" sticky in the pool equipment forum.

 

[AnnaK]

Thanks, Dave. I'll go to Pentair and look around.

 

[AnnaK]

Oh wow! I had missed mas's sticky. Should have looked there before I asked.

I checked the Pentair site and found this chart: Dynamo Curve but I'm not really sure I know how to read it If I assume a head of 45 then line C (for the 1.5 hp) says it moves 30 g.p.m. Am I doing this right? If so, and using rangeball's calculation primer, I'd have a total turnover in 6.7 hours.

[Correction: Since the pump runs at low speed I'm only turning over 1/2 of the volume]

I run my pump 6 hrs at night. Not a bad guess for not knowing what I'm doing (and really still don't)

Love this forum!

Anna

 

[Rangeball]

Anna, that's the problem with assuming a head loss, with some pumps it doesn't make much difference, with others, like the dynamo, it makes more.

The 1/2 flow on low speed is a general rule. I recently email Pentair regarding the dynamo 2 speed (I have the dynamo 1 sp on my IG pool), and was told they don't have flow rate info for low speed for it, so I'm not really sure if the general rule is accurate here.

If you are running low speed, you might want to bump the run time up, for not much added electrical costs. But if it were me, unless I knew my head loss and flow rate for certain, I'd go by how the water looks. If it starts appearing dingy, I'd run the filter more.

Sorry if this muddies things up. Accurately calculating head loss is one of the hardest things there seems to be. Mark is working on a calculator for it (I believe), and I hope it's all nailed down before I need a new pump

 

[mas985]

For most 2 speed pumps, low speed is 1/2 the RPMs of high speed. Therefore, according to pump affinity laws, the flow rate should be approximately 1/2 of high speed.

Also, you can also estimate head loss from filter PSI measurements. This will get you a little closer than a guess. With 1 1/2" plumbing on the suction side, head loss is usually around 4 x PSI.

 

[AnnaK]

This is getting way over my . . . uh . . . head. The water looks great, undefinable substances are being caught in the skimmer sock, the filter gets backwashed every 4 weeks whether it needs it (PSI increase) or not.

I'm going to turn this over to my husband for reading and pondering. I did fine in biochem but physics escaped me

Thank you very much for your input! You've fairly well blown my mind, o yay, and I'm in way deeper than I care to be. And yet, I am so glad I asked

Anna