Low flow pumps - Is Air in Solar system ok?

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Re: Low flow pumps - Is Air in Solar system ok?

I still think your best option is really to remove the solar pump and run the main pump at a higher RPM. I know you don't want to do this but you are fighting physics here. There is a critical flow rate required to purge water out of a vertical pipe (check valve or no check valve). This flow rate is approximately 40 GPM for a two story install. So if the solar pump does not have enough head to raise the water to the panels at 40 GPM, then it is unlikely this setup will ever work properly.

What is the make/model of your solar pump?

What is the height of the panels over the pump?

Plus the slower you run water through panels, the less efficient they are. If you are interested in getting the maximum heat out of the panels, you need to run at higher flow rates. Generally 0.1 GPM/sq-ft is optimal. The efficiency drops off pretty quickly when operating at lower flow rates so you will be forced to run the pump longer for the same heat gain. If you are truly interested in efficiency, it may be better to run the pump at higher RPM for a shorter period of time to get the same heat gain than try to get the system to work they way it is now.
 
Re: Low flow pumps - Is Air in Solar system ok?

needsajet said:
It's an important point that you would not want the solar booster pump to create enough suction to zero out the pressure at the filter. If that's going to occur, and I think there's a good chance it won't, it will happen after the solar system is full. If it does, you may need to tweak your low speed up from the default setting just a bit, or throttle a bit more.

Be sure that your main pump timer always overlaps solar system run times. Let us know how it goes.
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I just checked the pressure difference when the solar pump kicks in.

At low flow alone the filter pressure is ~12kpa (mid flow 70, high flow 100).

When I turn on the solar booster the gauge drops to ~5kPa briefly then creeps back to ~10kPa

Be interesting to see the pressures when I install the throttle valve. I hope it doesn’t zero it due to siphoning.
 
Re: Low flow pumps - Is Air in Solar system ok?

Mas985,

Solar pump is Davey wisper 500

You mention 40gpm to purge air from 2 storey pipe. Do you have a link to some tables/graphs? I’m curious as I want to check my vertical pipe.
 
Re: Low flow pumps - Is Air in Solar system ok?

tomfrh said:
You mention 40gpm to purge air from 2 storey pipe. Do you have a link to some tables/graphs? I’m curious as I want to check my vertical pipe.
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It is based upon my own experimentation and confirmed from formulas used to calculate the time it takes air to rise in a water column. The water has to be faster than the air rising or it will never purge the pipe. Although as you found, you can continue to run that way it just may mean adding air to the returns. But as I mentioned earlier, the efficiency of the panels drops quite a bit with lower flow rates.

https://pdfs.semanticscholar.org/b076/cea61f959bc4d228c1548e81dbd48696e5c0.pdf


If your panels are at 8 meters at the highest point and the main pump is adding about 1 meter of head, then the solar pump needs to add 7 meters of head at 150 lpm which it can't. At 7 meters of head, the pump can only produce about 100 lpm which is less than what is needed to push out the air. This also means that it probably won't work if you have the solar pump by itself.

However, if you were to run the 3 speed on speed 2 by itself, the head curve says that the pump will deliver 150 lpm which may be just enough for priming the return pipe.


https://davey.com.au/media/custom/upload/Whisper_Datasheet_DWP0187-1.pdf
http://www.davey.com.au/media/custom/upload/DWP0016-5_PowerMaster_ECO.pdf

Note too that according to the spec sheet the 500 uses 450 watts.

I just wanted to add that what probably matters more than this is that the pressure at the VRV must be above atmospheric or air will continually get sucked into the panels and return pipe. The throttling valve can help increase this pressure at the expense of flow rate. So while it may prevent more air from entering the panels via the VRV, the panel efficiency will drop and the flow rate may not be sufficient to push out the remaining air in the return pipe.
 
Re: Low flow pumps - Is Air in Solar system ok?

I actually measured the 40 GPM on my own system. Below 40 GPM, and the air never completely exited the plumbing. Above 40 GPM and the panels primed fairly quickly. Now this could be a combination of two things. The flow rate for purging and the pressure for the VRV, they are both tied to the flow rate through the plumbing. If the flow rate is sufficient to push out the bubbles but not sufficient to generate high enough pressure at the VRV, the bubbles continue forever out the return. If the opposite is true, you may never know because the air will just sit in the panels and/or return pipe but water still may come out of the returns flowing past the trapped air.

However, if you use the method in the paper I posted, assume a bubble half the diameter of a 2" pipe and you want the bubble to travel downwards to the bottom in 11 seconds, it comes fairly close to that number (~41 GPM). You might be able to push out the bubbles at slightly lower flow rates but it will take much longer to flush them out. If you match the bubble rise rate with the downward water flow rate, the bubble remains stationary in elevation so the water flow rate needs to be much higher than the rate at which the bubble rises to be able to push it out of the return.

When you go from low to medium speed while solar is on, does a lot more air come out of the returns?
 
Re: Low flow pumps - Is Air in Solar system ok?

The bubbling ceases when I go from low to medium speed with the solar pump on or off.

at low speed the water level is level with the T meaning air bubbles can be captured. At medium speed and above the water level rises up into the T (due to increased pressure from main pump), and this isolates the air from the returning water.



How did you confirm all the air was gone at 40gpm?
 
Re: Low flow pumps - Is Air in Solar system ok?

No more air came out of the returns at 40 gpm and when i raised it to 60 gpm, no more air came out either. If air is trapped, it will come out at higher flow rates.

So do you have clear pipe in certain sections? How do you see the air in the pipes?
 
Re: Low flow pumps - Is Air in Solar system ok?

I installed the Valve and the U trap. The solar return U-trap extends about 400mm below the horizontal main return, and connects to it from the underside.

with low flow pump and solar running all the pipes are filled with water. I installed more clear pipe so I can see exactly what’s happening. The solar return is full and purging all the air. For some reason I don’t need to close the valve at all. It’s working as is with no bubbling.


with low flow pump running and no solar pump running the water level in the u trap on the solar return side is about 200mm below the horizontal main return. No air entering the main return.
 

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Re: Low flow pumps - Is Air in Solar system ok?

Good to hear mate :) You definitely want it wide open if everything works fine that way, giving you maximum flow through the collectors. Your trap must have made it a bit easier for the air to get evacuated. Your valve wide open would be a very minor restriction but I'd say not very much. Well done!
 
Re: Low flow pumps - Is Air in Solar system ok?

So does it all work? I was following along... fascinating, and complicated! Traps and throttles and clear sections and extra pumps!?! Whew. (Guess I lucked out. My system is one pump, an actuator and one check valve, no issues.) Where I got lost was the extra pump. Was that necessary because of the location of the panels? Seems like running pumps in series is just asking for problems, but I wouldn't really know. As I was reading, I kept thinking the following, so I'm glad someone else pointed this out:

Plus the slower you run water through panels, the less efficient they are. If you are interested in getting the maximum heat out of the panels, you need to run at higher flow rates. Generally 0.1 GPM/sq-ft is optimal. The efficiency drops off pretty quickly when operating at lower flow rates so you will be forced to run the pump longer for the same heat gain. If you are truly interested in efficiency, it may be better to run the pump at higher RPM for a shorter period of time to get the same heat gain than try to get the system to work they way it is now.
Click to expand...

Was this addressed? Isn't that pretty important? Isn't the flow rate the most important factor, in terms of overall efficiency (and thus, overall cost)? Wouldn't trying to minimize WATTs and RPMs, to meet some arbitrary goal, be counter productive if the optimum flow rate for the panels gets compromised?

Wouldn't this be pretty easy to test, efficiency-wise? Take x number of days, with similar air temps, alternate between two separate tests, and average the results: run one pump higher (better flow rate) until pool reaches desired temp vs two pumps lower (compromised flow rate) until same temp is reached. Note the time each test takes. Calculate the watts used. Use the lower-costing solution.

My solution for my system: I installed a flow meter. No math, no guessing. I found one that fits into the same housing as my check valve, so I can switch the check valve guts for the flow meter guts and measure flow at two different points in the system, if I want to. With a variable speed pump, I just tweak the RPMs until my flow meter reads what the panel manufacturer suggests for flow. Done deal. Never seen or heard any bubbles (except during panel filling at beginning of the cycle). I figure heating my pool to the temp I want is going to require "X" amount of electricity, I don't think there's a way around that. So I don't try and cheat the flow to lower the WATTs, because I'm trusting the manufacturer's flow numbers as the most efficient use of the panels, and that the controller will shut down the panels sooner, thus saving me electricity in the long run. I realize you don't have a VS pump, but maybe you should. Maybe replacing two fixed speed pumps with one VS pump, that is powerful enough to run your solar system at the proper flow rate, by itself, would pay for itself in short order. Sometimes a little upfront cost can end up saving money. (Not to mention all the other great benefits a VS pump offers.) Speaking of which, you could also look into "part 2" of my solution, which was to install a PV solar system. A pile up front, but in six years I'll be running my pool, with all the solar heating I want, plus my whole house, for "free" (based on when the PV panels will pay for themselves).

Most of what you were discussing was over my head, so I'll defer to those that know more about this than me. I was just looking at your problem from a few steps back, and it seemed overly complicated from my vantage point... Not even sure that's worth two cents! ;)
 
Re: Low flow pumps - Is Air in Solar system ok?

Dirk,

It always was a two pump system (as many are).

My main pump draws about 160W and the solar pump 370W when it’s running.

It would be hard to beat that with a single pump.
 
Re: Low flow pumps - Is Air in Solar system ok?

(as many are)
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Huh. When I was researching how to build my system (I installed it myself), I came across all kinds of drawings on the 'net. I never saw one with two pumps and no actuator. Always one pump and one actuator. Some with one check, some with two checks. So your system is greek to me. Maybe I was just lucky that I ended up with a solution that worked right off. I guess there's a lot that can go wrong.

I know very little about hydraulics. But it's counter intuitive to me that two pumps could be more efficient than one. Doesn't matter though, if you got it working the way you want, that's all that matters! ;)
 
Re: Low flow pumps - Is Air in Solar system ok?

Here’s a link needsajet posted previously:

http://www.ecoonline.com.au/content/manuals/oku_solar_pool_heating_install_manual.pdf

it shows various twin pump configurations if you’re interested.

I do have two pumps, but they are use ~200W (main pump) plus ~400W (solar pump when running), so it’s fairly efficient.

the two pumps used to use about 2500W in total. That chewed the power. Hence my change to smaller pumps, and this resultant bubbling issue...
 
Re: Low flow pumps - Is Air in Solar system ok?

Yep, right there. Two two-pump systems, one in series and the other a separate, independent solar pump. Interesting. Never ran into those before. I wonder if it's a USA/AUS thing: two pumps became more common down under than here? My panel's manual has no mention of a two-pump system. And my research was from sites of US-based solar install and manufacturing companies. Well, you all drive on the wrong side of the road, too, so there's that!! ;)
 
Re: Low flow pumps - Is Air in Solar system ok?

No, the toilets are a different matter. That's because your drain water twists counter-clockwise down there!! ;)
 
Re: Low flow pumps - Is Air in Solar system ok?

tomfrh said:
I installed the Valve and the U trap. The solar return U-trap extends about 400mm below the horizontal main return, and connects to it from the underside.

with low flow pump and solar running all the pipes are filled with water. I installed more clear pipe so I can see exactly what’s happening. The solar return is full and purging all the air. For some reason I don’t need to close the valve at all. It’s working as is with no bubbling.


with low flow pump running and no solar pump running the water level in the u trap on the solar return side is about 200mm below the horizontal main return. No air entering the main return.
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So what do you think the U-Trap is doing? I think what you are seeing is just extra head loss created from the valve and U-Trap fittings is increasing the pressure at the VRV and that alone is enough to keep it closed. Hydraulically the U-Trap really shouldn't do anything other than add head loss. When the panels drain, a siphon is created and that should empty the U-Trap. But even if it didn't I still don't see the point. Traps only work in gravity fed systems and not pressurized systems.
 
Re: Low flow pumps - Is Air in Solar system ok?

Putting the VRV at a lower elevation is an option. You'll get more gurgling when the system shuts down, but that may not be important to you.
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Can I hijack a little? (My question is air-related!) My VRV is just below my eves. Below my panels. I decided to place it there to make it easier to monitor and/or replace. And to keep it out of the sun. But it's pretty froggy at drain time, both outside and a bit inside my house. Croaks for about 15 minutes.

Had I put it at the very highest point in my system, would it have been quieter? I couldn't find a consensus about the optimum placement for a VRV when I was designing my system. Highest point of panels? Lowest point of panels? Just below the roof? A few feet above the equipment pad? Etc.

I have a pretty simple layout. One set of panels. Water goes in at lower left, comes out at upper right. Both connections come right off the roof with no tricky elevations to navigate. I did skew my panels and feed pipes a bit to enhance drainage. Other than the noisy VRV, my system seems to work as intended.

- - - Updated - - -

mas985 said:
So what do you think the U-Trap is doing? I think what you are seeing is just extra head loss created from the valve and U-Trap fittings is increasing the pressure at the VRV and that alone is enough to keep it closed. Hydraulically the U-Trap really shouldn't do anything other than add head loss. When the panels drain, a siphon is created and that should empty the U-Trap. But even if it didn't I still don't see the point. Traps only work in gravity fed systems and not pressurized systems.
Click to expand...

And if that's the case, isn't that making his system even less efficient, by compromising the flow even more?
 

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