Pumping water to a height in a closed loop circuit 5

[Goolakh]

I have a tank of water with a variable speed pump to pump water to solar collectors on the roof. The return pipe from the collectors comes back to the tank creating a closed flow loop. The roof is 15m above the tank. I am expecting the flow to run through the system if the pump just compensates the pressure drop due to friction in the pipe (because the fluid circuit is a closed loop). Whereas this doesn't happen. If the pump slows down and generates a head below a certain threshold the flow in the return pipe from the collectors separates. I am guessing that at the top of the circuit the pressure drops below the vapor pressure of water. Can anyone clarify on this and let me know how to run water at low flow rates in such cases?

 

[LittleInch]

Goolakh.

you've answered your own question. The system pressure ( usually maintained by some sort of expansion tank or bladder) needs to be at a pressure where the top of the system is at a positive pressure compared to atmosphere, so > 0.1 barg.

The easiest way for you is to have a tank located at the high point and keep it topped up to a level above the highest bit of piping.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[bimr]

You need more pressure at the bottom to pump up. The pressure at the bottom must be at least equal to the 15 m water head + friction loss + residual pressure at the top.

This was discussed recently:

http://www.eng-tips.com/viewthread.cfm?qid=393312

 

[Artisi]

If you need to reduce the flowrate you will require some method of restricting the flowrate by imposing more head on the system - not by reducing pump speed which reduces head as well as flow. Remeber you still need sufficient head to overcome the head being imposed by the system, - - - friction losses plus the additional imposed head.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[77JQX]

I second LittleInch. With the tank at the high point, you'll be able to use your VFD to its full extent. With the tank low, you need to add friction to the downpipe to keep it full, which defeats the purpose of the VFD.

 

[cvg]

you are heating a water tank. I imagine it is heavy. you certainly dont want to put that on the roof. you only need a small reservoir at the top to maintain positive pressure. A hydro pneumatic bladder tank would work.

http://westank.com/hydropneumatic-tanks/

 

[itdepends]

Nothing wrong with having a water tank on the roof- they're relatively common for solar systems (at least in Oz).

I've got a 300L tank mounted horizontally on my roof and there were no changes/structural reinforcing done when it was installed.

Question to the OP- can you pressurised the ground based tank? If it can handle pressure- close it up and run a small line up to a header tank to provide the required system pressure. Top up from the header tank. May require further thought if your area is subject to freezing temperatures.

As a chem eng/metallurgist the first part of any answer I give starts with "It Depends"

 

[moltenmetal]

LittleInch is right. You need the system to remain liquid full and under a small positive pressure at the top of the system. When you do so, you no longer need a pump capable of generating 15m of head, period, nor is there any risk of generating a vacuum unless you open a drain valve. Your pump will then need only to overcome frictional loss AND to overcome the density difference (to drive hot water on the roof back down to the basement, displacing denser cold water). If you could put the heating at the bottom, the density difference would be assisting your pump i.e. acting as a thermosiphon, but because the sun shines on the roof you are fighting the density difference with your pump.

If you live in a mainly heating climate, your main storage tank needs to be inside your house, not on the roof. The expansion tank on the roof is only needed to ensure the system remains liquid full without having thermal expansion problems so it should be small. If you're in a mainly cooling climate, the opposite is true.

 

[Goolakh]

Good responses everyone, particularly moltenmetal, This is giving a better understanding.