Solar heaters - low flow

[dschembri]

Did you see any air coming out of the solar returns?

When I use the plumbing you have described and the pump model, I get a pressure of around 16.5 PSI for that configuration. When was the filter last backwashed?

I still think we are missing something here.


Unfortunately, that does not contain any head loss information.

I started with a backwash prior to posting this thread really, as it was my first thought so it has only been 3 days.

There was no air coming out from the solar return... there was yesterday but it all cleared when i ran it as described previously to put pressure on the solar return.

Could it be the fact that the angles in the plumbing are so close to each other maybe, potentially causing more friction than if they were spaced out? Specifically in the attached photo.

 

[dschembri]

Also, can you show a picture below the one showing all the valves and include the pump and filter as well.

Attached

 

[mas985]

I started with a backwash prior to posting this thread really, as it was my first thought so it has only been 3 days.

There was no air coming out from the solar return... there was yesterday but it all cleared when i ran it as described previously to put pressure on the solar return.

That implies that solar may be fully primed but difficult to confirm. In my head loss model, if the panels are fully primed, pressure drops from 16.5 PSI to 14 PSI and flow rates go up to 43 GPM. But that also means the pressure at the top of the panels is around -7 PSI (i.e. partial vacuum).

On the solar return, there is a fitting with a wire, is that a temperature sensor or something else?

Could it be the fact that the angles in the plumbing are so close to each other maybe, potentially causing more friction than if they were spaced out? Specifically in the attached photo.

That is a possibility but difficult to confirm. Is there a way you could measure pressure at the panels pre or post panels?

Can you confirm the exact inner diameter of the solar pipe? I am not sure if it is the same as US 1.5" pipe (1.61" ID). That might explain the difference.

 

[mas985]

The inner diameter of the pipe is dependent on the type of pipe that was used and inner diameter can make a big difference in head loss:

 

[Bill1974]

@dschembri not sure how easy this would be but out a curiosity check and a sanity check. Can you measure the flow to the solar panels by just filling a bucket (trying to get a feel of the flow in liter/min or min/liter)? This will give an idea of how well the system can prime. After that do the same test with the outlet from the solar panels (still at the elevation where the solar panels are). Interested to see if the panel make much of a difference.

If the flow is really low it will never prime well or take a really long time. Looking to see what if the flow is liter/min or min/liter magnitude differences.

@mas985, when you are doing your pipe loss are you taking into account the the entire length of the pipe run (the supply and the return? It was state that the pipe was 43 mm which is close to 1-1/4" sch 40 I believe. With that size pipe/length/fitting and the estimated flow, is getting to the point where friction losses start to become noticeable/significant. Based on what @dschembri has tried, with as little restriction on the suction side the flow is enough that either the solar piping is the restriction and/or the filter is or both.

@dschembri the distance of pipe between fittings does not have much of an effect on the total loss.

 

[mas985]

@dschembri not sure how easy this would be but out a curiosity check and a sanity check. Can you measure the flow to the solar panels by just filling a bucket (trying to get a feel of the flow in liter/min or min/liter)? This will give an idea of how well the system can prime. After that do the same test with the outlet from the solar panels (still at the elevation where the solar panels are). Interested to see if the panel make much of a difference.

If the flow is really low it will never prime well or take a really long time. Looking to see what if the flow is liter/min or min/liter magnitude differences.

@mas985, when you are doing your pipe loss are you taking into account the the entire length of the pipe run (the supply and the return?

It depends on if the pipe is in priming phase or fully primed. I looked at it both ways. During priming, the supply pipe has head loss and there is static loss due to the lift but there is no return head loss/gain due to the water falling because there is air in the pipe so it cannot create backpressure and/or a siphon. This is why it has to be analyzed in two different ways.

Assuming PN10 pipe and 40mm OD, the ID is then 38.4mm

Pre-Primed: 29.5 GPM @ 15.5' dynamic head & 28' static head, 17 PSI Filter pressure
Post-Primed: 39.5 GPM @ 39' of dynamic head & 0' of static head, 14 PSI filter pressure

Just a reminder that the OP measure 21 PSI so there is clearly a difference. Something is not being accounted for.

After the return pipe is primed, a siphon is setup so the panels are under slight suction of around -7 PSI.

During the priming phase, pressure in the filter tends to bounce around a lot as the return fills with water.

It was state that the pipe was 43 mm which is close to 1-1/4" sch 40 I believe. With that size pipe/length/fitting and the estimated flow, is getting to the point where friction losses start to become noticeable/significant. Based on what @dschembri has tried, with as little restriction on the suction side the flow is enough that either the solar piping is the restriction and/or the filter is or both.

Actually, early on the OP stated 42mm and then later 40 mm so there is some ambiguity. But as the table showed, there are still multiple choices for each pipe OD depending on the pressure rating so that needs to be clarified.

@dschembri the distance of pipe between fittings does not have much of an effect on the total loss.

Actually, that is not quite true. I have been doing some CFD simulations and as fittings become closer, they can interact. For example back to back 45s have less head loss than 2 45s separated. In some cases, depending on the orientation of the fitting, the head loss can be more. This is because head loss is dependent not only on the fitting geometry but also what also comes after the fitting because the velocity profile exiting a fitting is skewed to one side due to the fitting.

I looking into modeling the OPs setup for the fitting cluster below but these simulations take quite a bit of time to setup and run.

 

[Bill1974]

There are many assumption in FEA and CFD analysis. To make best use, typically a correlation needs to be made to a physical test to get potential accurate result. A more typical use of FEA and CFD analysis is a comparison between 2 or more simulations to determine if the results are as expected and how they differ or from each other (does the change give the expected improvement and is it of a reasonable magnitude). CFD analysis for water is pretty forgiving, but if flow rate/velocity, pressure, temperature, surface roughness and other factors are not accurate results can be easily skewed and possibly misleading. One thing to look for is how well the simulation can predict cavitation and how it deal with it. Does you model take into consideration the glue squeeze out or gaps present when a pipe is not full in a socket, the diameter differences between pipe and fittings? It is possible to get accurate results but it will take a significant amount of time and effort accurately modelling the system and figuring out pump curves (i don't know how you would model a sand filter other than vastly simplify it).

The proximity of the fitting to one another is not likely a likely a major contributor to the flow/pressure issues here. If it is, it's also because the flow is to high for the system and that is a bigger issue.

A booster pump for the solar or setting the valves to create a differential pressure between the inlet and outlet of the solar circuit are what can be done. A venturi could be be used to get the differential but I am not sure it would be easily sourced or fit within the plumbing. The booster pump is the more expensive fix but also probably the best for making best use sun, the other option should have values to prevent the solar panels from working as radiators when the sun isn't on them.

 

[dschembri]

The fitting with a wire on the solar return is a temperature sensor - correct.

The pipe fittings to/from the solar are 42/43mm in diameter, and I believe 40mm actually inside the pipe.

My exact solar panels are these -

Solar pool heater - 66 x 300cm incl. Accessories

Solar pool heater - 66 x 300cm incl. Accessories - 4260627428656 - - Pool Accessories

www.arebos.co.uk

I have a group of 5, then 2 sets of 2 each so a total of 9 in total. The link specifies 'This model offers an impressive flow rate of max. 10,000 liters per hour and a capacity of up to 15 liters. With an operating pressure of max. 3 bar'. Does this seem excessively high, and how does having 9 in series effect thess numbers?

@Bill1974 i can disconnect the panels and fill a bucket to try and get an estimate of gpm before, then after the panels but by doing this am I right in saying I would probably see very skewed numbers since there is no return downforce helping the pump achieve better flow? Is there a small device or something I can connect inline that will measure this maybe? I do wonder what the flow actually is pre/post panels as this may guide me further as to the resistence of the panels themselves, but also of how far off the optimal working efficiency I currently am.

In terms of valves to stop routing water through the panels when the sun isn't on them, my plan was to only use them to extend our summer really, and if I run my pump any time during the day you are pretty much guaranteed sun over here - lucky from that perspective really.

 

[mas985]

There are many assumption in FEA and CFD analysis. To make best use, typically a correlation needs to be made to a physical test to get potential accurate result. A more typical use of FEA and CFD analysis is a comparison between 2 or more simulations to determine if the results are as expected and how they differ or from each other (does the change give the expected improvement and is it of a reasonable magnitude).

I am using OpenFoam which many companies and universities use and there are numerous validation studies that I have duplicated to make sure I am taking the right factors into account. Plus for every simulation a mesh sensitivity study is done to make sure the mesh is not influencing the results.

Also, I started off with straight pipe matching results from Darcy-Weisbach to get a calibration point for at least the pipe. Then moved on to match single fitting head loss from minor loss tables such as this one.

PVC Pipes - Equivalent Length and Pressure Loss in Fittings

Minor loss in PVC and CPVC fittings expressed as equivalent length of straight pipe.

www.engineeringtoolbox.com

And both the 90 and 45 were pretty close to those numbers so this was not done in a vacuum. One thing that I have discovered is the minor losses differs between sources and sometimes, significantly so I don't a lot of people have bothered to confirm these losses.

I also used the CAD drawings from the manufactures so the geometry would be accurate. It turns out there are some differences between manufactures in terms of the geometry as well as head loss.

However, it is difficult to find anyone who has done this type of measurement but Pond Magazine had an article that did a measurement comparing 45s to 90s but the link is not broken for that so I don't know if they retracted that or what happened. However, I do plan on testing this and have a rig setup ready to go.

CFD analysis for water is pretty forgiving, but if flow rate/velocity, pressure, temperature, surface roughness and other factors are not accurate results can be easily skewed and possibly misleading.

PVC is very smooth and even Darcy-Weisbach equations do not show a significant variance with roughness or temperature, for typical pipe and pool.

One thing to look for is how well the simulation can predict cavitation and how it deal with it. Does you model take into consideration the glue squeeze out or gaps present when a pipe is not full in a socket, the diameter differences between pipe and fittings?

As for the glue joints, I am actually using a chamfer for the transitions as it is easier to converge the analysis.

Keep in mind that the intent here is not to directly match watch any one installer might have done only to see if certain configurations can make things better or worse. So more of a comparison study.

It is possible to get accurate results but it will take a significant amount of time and effort accurately modelling the system and figuring out pump curves (i don't know how you would model a sand filter other than vastly simplify it).

I have already created models for most plumbing items. The spreadsheets are in my signature. Pretty much any piece of plumbing can be modeled (first order) as a parabolic because of the V2 term in the head loss equations. In fact, the regulatory agencies have adopted this approach as well and all now using parabolic plumbing curves, in addition to head curves, to model pump performance on different plumbing. The models that I have create have been validated/calibrated many times over the last couple of decades with my own as well as other members plumbing measurements. So they can usually get pretty close to reality.

The proximity of the fitting to one another is not likely a likely a major contributor to the flow/pressure issues here. If it is, it's also because the flow is to high for the system and that is a bigger issue.

At this point I would not expect it to be either. Even in the case of the 2x45s, the difference was a reduction in head loss of only about 10% so not very much influence even if you double it.

A booster pump for the solar or setting the valves to create a differential pressure between the inlet and outlet of the solar circuit are what can be done. A venturi could be be used to get the differential but I am not sure it would be easily sourced or fit within the plumbing. The booster pump is the more expensive fix but also probably the best for making best use sun, the other option should have values to prevent the solar panels from working as radiators when the sun isn't on them.

I agree that a booster is the less desirable option.

 

[dschembri]

Out of curiosity, if I had to consider replacing the combination of 8 x 90's/45's (each way) around the beam with perhaps a flexible hose to have a more 'rounded' route to the roof rather than the angles, could this make a significant difference in order to avoid the additional pump?

 

[mas985]

Out of curiosity, if I had to consider replacing the combination of 8 x 90's/45's (each way) around the beam with perhaps a flexible hose to have a more 'rounded' route to the roof rather than the angles, could this make a significant difference in order to avoid the additional pump?

I am still not convinced that would change things much and it could actually make things worse if the wrong flex were used. Some flex pipe is corrugated which is worse than the fittings.

i can disconnect the panels and fill a bucket to try and get an estimate of gpm before, then after the panels but by doing this am I right in saying I would probably see very skewed numbers since there is no return downforce helping the pump achieve better flow?

That is correct because you would have the static lift as additional head loss.

Is there a small device or something I can connect inline that will measure this maybe?

I asked this previously but is there a tap up on the roof where you could measure pressure? That could give a calibration point for the model. Pressure and flow rate are directly related so a measurement of either is useful. Plus pressure/suction would give an indication if the panels are primed or not.

I do wonder what the flow actually is pre/post panels as this may guide me further as to the resistence of the panels themselves, but also of how far off the optimal working efficiency I currently am.

I thought you already ran this experiment by bypassing the panels and reporting that the filter pressure did not change much. This would indicate that there is not much head loss in the panels.

 

[mas985]

Just to illustrate how unimportant those fittings are, I increased the number of fittings by a factor of 4x and it only increased filter pressure by 1 PSI for the pre-prime case and 2 PSI for the post-prime case. So fairly insignificant.

But one additional observation, you measured 21 PSI for when the suction valves were 100% open and the return was solar only. 21 PSI is the equivalent of ~15 meters of head and if you add 1 meter for the gauge above the pump, that is 16 meters of head but that is above the published head curve so the pump should actually have very little flow < 2.5 m3/h and almost no dynamic head loss. This doesn't make any sense.

Are you sure that pressure gauge is working properly? (this is the most likely source of the error)

Are you absolutely sure that the difference in elevation from the pump to the panels is 28'?

Are you sure the model # of the pump is SPK12607XY?

Here is the head curve of the pump you identified:

 

[dschembri]

I asked this previously but is there a tap up on the roof where you could measure pressure? That could give a calibration point for the model. Pressure and flow rate are directly related so a measurement of either is useful. Plus pressure/suction would give an indication if the panels are primed or not.

by tap, do you mean just a water supply? if so yes - I'm not sure how I would test this though?

Are you absolutely sure that the difference in elevation from the pump to the panels is 28'?

I've just double checked this again now, and the total elevation is correct yes. in reality, its from after the valve going to the panels, so if anything its a further 2 feet or so.

Are you sure the model # of the pump is SPK12607XY?

Yep, attaching a photo of the label

 

[mas985]

by tap, do you mean just a water supply? if so yes - I'm not sure how I would test this though?

A fitting where you can screw in a pressure gauge to measure the pressure up on the roof near the panels.

I've just double checked this again now, and the total elevation is correct yes. in reality, its from after the valve going to the panels, so if anything its a further 2 feet or so.

Yep, attaching a photo of the label

Then the filter pressure you posted (21 PSI) must be incorrect because that pump cannot deliver that much pressure.

Do you have a second pressure gauge you can install on the pressure side pump drain plug to get a direct measurement of pump pressure?

 

[mas985]

One other thing that I may have missed. Is the equipment below water level? What is the height of the pressure gauge relative to the water level in the pool?

 

[dschembri]

One other thing that I may have missed. Is the equipment below water level? What is the height of the pressure gauge relative to the water level in the pool?

I don't have a tap on the roof, or any other gauge unfortunately. The one I am currently using is the one on the sand filter. Is it easy to install an inline pressure gauge or flow meter? Maybe I should add one of these on the roof before the solar panels?

The equipment is adjacent to the pool, but the pool is not as deep as the room where the equipment is. I would say the bottom of the pool is around 2 feet higher than the equipment.

 

[mas985]

How deep is the pool?

The floor of the equipment room is 2 feet below the bottom of the pool?

So the top of the pool is near the top of the room? How high is the ceiling of that room?

I guess I am more interested in where the top of the pool water is relative to the pressure gauge? Can you estimate that distance?

 

[mas985]

BTW, if I assume that the pool water level is 8' above the filter pressure gauge and I assume that the return pipe is not fully primed, I get exactly 21 PSI as you measured with all suction lines open and all water going to the panels.

One way to tell if the return side plumbing is full primed, is to open the hose clamp on the return side pipe on the roof while the pump is running. If water comes out, the return pipe is not primed but if air gets sucked in, then the pipe is primed.

 

[dschembri]

The pool is around 1.5m deep, the height distance between the water level of the pool and the pressure gauge is around 1.7-1.8m I would estimate.

Interesting, just to ensure I understood correctly - pump running, 75% to solar, and i disconnect the pipe between the panels and the return down from the roof?

Which side should I be looking at, the end of the solar panels or the other end that goes back down from the roof?

 

[mas985]

The pool is around 1.5m deep, the height distance between the water level of the pool and the pressure gauge is around 1.7-1.8m I would estimate.

Interesting, just to ensure I understood correctly - pump running, 75% to solar, and i disconnect the pipe between the panels and the return down from the roof?

100% to solar would be better and more definitive. Open all suction valves as well.

Which side should I be looking at, the end of the solar panels or the other end that goes back down from the roof?

The end the goes back down the roof into the pool although it probably doesn't make that much of a difference.