Need help calculating total head to size pump

[Heffspool]

New to the forum but I've been browsing here for a while . I'm going to be finally heating my pool after three years of a chilly pool. Due to my unique system figured it was time to join and ask the pros!

I'm installing a 150,000 btu heat exchanger, the problem is it's 200' from the pool. Not only that it's also 24' vertically below the pool. The specs for the heat exchanger call for 85gpm at the unit and max of 72 psi. I was thinking I would install a 120 gpm cartridge filter. I am going to run 3" pvc down to the heat exchanger and 3" back to the pool. The company that manufactures the heat exchanger told me to add 10 feet of head
For the unit.

My questions, how much head do I add for the filter, the 400' of 3" pvc, and how do I account for the elevation change?

The elevation change is what I'm least sure of. The pool pumps the water down to the heat exchanger but then back up to the pool for a net elevation change of 0. So do I just ignore this?

Also, 24' of elevation drop at the heat exchanger will increase my pressure by 24x.44=10.56psi, this won't put me anywhere near max psi of 72, right?

Thanks in advance! Glad to be a part of the forum.

Heff

 

[JasonLion]

You can use a TDH calculator, like this one to answer some of these questions.

The manufactures head estimate for the heat exchanger assumes some flow rate. Since it required 85 GPM, I would assume that the 10 feet of head is at a flow rate just a little higher than that. Did they also give you a maximum flow rate?

When calculating TDH for an elevation change, you care about the starting height and the ending height. Elevation changes in-between don't count (they both add and subtract, having no net effect). When calculating pressure at the heat exchanger you do count the elevation change as you have done.

By they by, with pipe runs that long, electricity to run the pump starts to become worth considering as part of your total heating cost. With 3" pipe it should still be a secondary effect, but still worth taking note of.

 

[Heffspool]

Thanks for the help with that.

The maximum flow rate for the heat exchanger is 99gpm. I've been a little curious as to what happens if I go above that. But assuming I size pump and filter to pump <99gpm I will only achieve that when the filter is brand new.

Also, so are you saying that 10 feet of head would be at 99gpm?

 

[JasonLion]

They apparently didn't say what flow rate the 10 feet of head is correct for. That creates some uncertainty. My guess is that it applies around 85 GPM, and is higher than that at 99 GPM.

They have a fairly narrow allowed flow rate range. It is going to be a little tricky to hit that with any certainty.

Metal heat exchangers can have the metal be slowly eroded away if the flow rate is too high.

 

[mas985]

Do the specs say the minimum flow rate is 85 GPM or the nominal flow rate is 85 GPM. Most heat exchangers can deal with lower flow rates than that and I can't believe they would only allow for a window of 14 GPM.

What pump model do you currently have?

 

[Heffspool]

The min gpm is 60. I am currently trying to size the pump right now along with a filter. I might step down to a smaller size heat exchanger but the distance is still the same.

I was mainly concerned about the fact the heat exchanger was 24' below the pump and uncertain how that affected head.

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The existing pump and filter is too small, so I was going to add another skimmer and supply to the pool for this system.

 

[Heffspool]

I'm also curious how much head should be added for the filter rated at 120gpm if it's say running at 85 gpm?

 

[mas985]

The existing pump and filter is too small, so I was going to add another skimmer and supply to the pool for this system.

So how does another skimmer help?

But those items are not in your sig so what is your current filter and pump?

 

[Heffspool]

Maybe skimmer is not the right terminology? I plan on adding a whole new pump and filter and leave the existing one still hooked up for when I'm not heating. The existing pump is a 3/4hp jandy. Not sure the filter size (not at home) but it's a pretty small cartridge filter.

I was going to add another (skimmer?) to the pool to suck water to my new pump filter heater setup and add a new supply hole into the pool for this as well.

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And I was thinking about getting a new/used120gpm cartridge filter for this new line and curious how much head I should add if I'm shooting for a target gpm of 85gpm after the long run of pipe, heat exchanger and new filter.

 

[mas985]

Well 120 sq-ft is pretty small cartridge to me. It might be ok for a 10k pool but I would put at least a 250 sq-ft on a 20k pool. I have a 420 sq-ft on mine and only have to clean it once a year.

But if you are getting a new filter and pump anyway, why bother with the second skimmer and setup. You can put in a bypass for the heater. Just get the right pump and filter and you should be good to go. But to do that, I need to know how far you plan on putting the pump and filter in relation to the heater and pool. Is the equipment going to stay close to the pool and just the heater is going to be 200' away?

 

[Heffspool]

Ya I was looking at a hayward 325 sq ft cartridge rated at 122gpm.

The pump and filter will be at the pool, the heat exchanger is 200' away.

I was thinking I would leave the old pump to save on electricity when I'm not heating because it seems like I'm going to need a 1.5hp (or bigger?) pump for the long run. And I don't really want a variable speed.

However if you think it's best I could eliminate the old setup. It would definitely be an easier install.

 

[mas985]

If you mean a 1.5 hp AG pump, perhaps but a 1.5 IG pump would be way too big. They are not the same.

Also have you considered a two speed pump? You could run on low when not running the heater.

 

[Heffspool]

Using the tdh calculator at 85gpm with 400' of 3" line adds 7' of tdh, the heat exchanger adds 10'?, the pump is about 3' below the discharge of the pool, what does the filter add to tdh?

Using TDH can I then look at a pump curve to choose which pump gives me 85gpm at that given head?

A two speed pump sounds like a good idea.

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Also, the 20+ bell fittings to go 400' add head as well.

 

[JasonLion]

Also, the 20+ bell fittings to go 400' add head as well.

You want to use schedule 40 slip fittings, which have a negligible effect on TDH and can be ignored.

 

[mas985]

Did you include all valves and fittings and eyeball diameters and skimmer port sizes, etc.? Don't forget that head loss is dependent on flow rate and pump flow rate is dependent on head loss so you have to solve two simultaneous equations to solve for the operating point of the pump. What you are trying to do is not trivial and could lead you to an incorrect conclusion. Have you calculated head loss in pool plumbing before?

But why are you targeting 85 GPM? Given your location, I would think that you would want the lowest GPM possible and still stay above 60 GPM. Remember that power is a function of GPM^3 so you will use close to 3x the energy at 85 GPM then you would at 60 GPM. Something to think about.

I have a fairly detailed head loss model that I can use but you need to give me every detail of your plumbing, including all pipe sizes/lengths, pipe fittings, valve types and locations, skimmer type and port sizes, return eyeball sizes and number of returns. Also, every filter has different head loss characteristics so you need to decide on manufacture and size. A schematic drawing would be ideal.

 

[Heffspool]

This IS my first rodeo... Haha.

Ok sounds like I need to do a little more planning and work on some schematics. I really appreciate all your input it has given me quite a lot of good info.

Existing skimmer is the one that came with the /pool and has a 1 1/2" port and unless I add or replace is my only suction side port. Return eyeball is like 1 1/4" with 1 1/2" fitting and is also the only return.

I'll try to pick out some equipment and report back . Thanks a million.

 

[mas985]

A couple of other data might be useful.

Current filter pressure

Heater make/model#