I was looking at a Caleffi solar brochure and seen two circs hooked back-to-back for high lift to roof mounted solar panels. Just wondering if anyone has done this and if they have, what size pumps they used.
Two circs tandem....?
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Gooserider
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Garnification said:
Haven't done it, but theory says that putting two circs in series will give the same flow, but twice the head pressure... Assuming both pumps are the same, the pump curve for two in series is to take the curve for one, and double the pressure at each volume point.
What pump sizes you need is something we'd need a lot more data to answer - starting with how high is the lift? What kind of volume will you need at that lift? Essentially the process is the same as you would use for selecting a single pump - find all the other answers to calculate your needed flow volume and head pressure, generate a set of pump curves for series pairs of likely pump candidates, and pick one where the volume and pressure falls in the middle third of the desired curve...
Note also that if you are talking about a "drainback" type system, you will only need to run both pumps for a short period of time as you refill the panels and return plumbing. Once the system is filled, you can ignore the lift height, and need only worry about the head resistance from the piping and fittings just like any other loop - usually this means you can turn off one of the two pumps to save power... If you loop is otherwise a low resistance loop, you can also mix pump types, with one (or two) high power pumps to fill the system, and a low power pump to carry the load once it's working. I've also seen discussion of setups that use one of the thermal control pumps for the "flow pump" so that the flow velocity can be changed in order to get the optimal heat flow from the system. This kind of setup is a little fancier on the control side, but the pump picking process doesn't change...
Gooserider
The only drainback solar system I've seen personally had a second pump like that but the second booster pump was a much larger rig than the full time circulator. You probably wouldn't need or want such a large power load to run all the time. The pump shot a lot of water up from the basement to the roof of a 2-story cape in a very short time.
But I'm not certain at this point how exactly it was plumbed in, just in tandem or parallel with some kind of valve.
But I'm not certain at this point how exactly it was plumbed in, just in tandem or parallel with some kind of valve.
Gooserider
Mod Emeritus
DaveBP said:
That is another valid combination - a smallish circulator pump and a big booster to get it started... If doing things that way, one could plumb the two pumps either in series or parallel - In series the water would flow through the booster pump even when it was off - according to the stuff I've seen this has very little flow resistance. Parallel would also be possible, but it might be tricky, as you would need some kind of flow check valve to keep the pumps from "short circuiting" through each other, but must always have some way for that line to flow backwards in order to allow the panels to drain when the pumps cut off. Not sure what kind of valve would be appropriate, maybe someone makes a "butterfly" type valve that would act as a flow-check on one of two lines while leaving the other one open? The other option might be something like a flow check on one side, and a normally open zone valve on the other?
I will say that more I look at drainback setups, the better I like the concept... It gets rid of the need for antifreeze in the loop, with all of it's expense, complications, and lowered performance, and it seems like it would handle stagnation a lot better - a stagnant system would be an empty system, so you wouldn't have to deal with having a system that vaporizes it's working fluid, so you wouldn't need as much expansion space. Unlike glycol, water doesn't break down and turn corrosive when over heated. The only downside I can see is the need for that big booster pump, but it probably would cost less to install that pump and run it as needed than it would to put in and keep replacing the required amounts of glycol...
Gooserider
Pumps in series double the head, or lift. Almost double as their is a little friction loss. So chose two pumps that "combined" equal the lift or head you need.
In a drainback solar you just need to run both until the fluid reaches the top. Then the fluid flowing down, raises the fluid going up. This drawing shows two pumps in series and the lift. The red lines are pump curves, the blue are system curves. Notice how the system curve shifts down when the siphon is established. Notice also how the flow rate changes.
Ever watch them load an empty Ferris wheel? they load the bottom car and move that car to the very top. Then load the car below it. This way as the folks come down, they lift the car opposite them.
Same happens in a closed loop solar or hydronic system. Really all the circ does is "splash" or keep the flow moving by creating a pressure difference. It just needs to overcome the resistence in the piping path.
In a drainback solar system that device starts out as a pump, lifting the fluid, then turns into a circulator when the fluid "rounds the bend"
Even better is to use a variable speed function on a larger circ sized to the lift. then the controller watches the delta T and adjusts the speed to always optimize the heat transfer.
With solar collectors you want a tight delta T, 3- 5 degrees across the S&r. it's not about the "linger" time in the collector you want to scrub those BTU out as fast as they are transfered. else you start losing them to the ambient air temperature as collectors have very little insulation or r- value in the glass.
Grundfos does build "Dolly Parton" pumps. They put two pumps and motors in parallel, side by side in one casting. This doubles flow, not head.
hr
In a drainback solar you just need to run both until the fluid reaches the top. Then the fluid flowing down, raises the fluid going up. This drawing shows two pumps in series and the lift. The red lines are pump curves, the blue are system curves. Notice how the system curve shifts down when the siphon is established. Notice also how the flow rate changes.
Ever watch them load an empty Ferris wheel? they load the bottom car and move that car to the very top. Then load the car below it. This way as the folks come down, they lift the car opposite them.
Same happens in a closed loop solar or hydronic system. Really all the circ does is "splash" or keep the flow moving by creating a pressure difference. It just needs to overcome the resistence in the piping path.
In a drainback solar system that device starts out as a pump, lifting the fluid, then turns into a circulator when the fluid "rounds the bend"
Even better is to use a variable speed function on a larger circ sized to the lift. then the controller watches the delta T and adjusts the speed to always optimize the heat transfer.
With solar collectors you want a tight delta T, 3- 5 degrees across the S&r. it's not about the "linger" time in the collector you want to scrub those BTU out as fast as they are transfered. else you start losing them to the ambient air temperature as collectors have very little insulation or r- value in the glass.
Grundfos does build "Dolly Parton" pumps. They put two pumps and motors in parallel, side by side in one casting. This doubles flow, not head.
hr
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