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Check valves for passive solar hot water system - Page 3

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Posted by Anthony Matonak on October 28, 2007, 6:11 am
 
Mike Scaife wrote:
...

I'm no expert in these things but if it only happens with very hot
water then the pump impeller might be experiencing cavitation. This
is when the impeller generates a low enough pressure 'behind' it
that some of the water flashes to steam.

http://www.mcnallyinstitute.com/01-html/1-3.html

Anthony

Posted by Mike Scaife on October 28, 2007, 10:50 am
 

It only occurs with the ELSID pump, when another brand of pump is used, the
automotive type, the problem doesn't show itself. Could be the design of the
impeller, both are totally different, the SID's use a simple flat blade,
whilst the other uses a compound curved design.
If I slowly close off the ball valve on the return from the solar panel, ie
increases the flow resistance the pump is working against, there is a point
when the noise greatly reduces, just before the flow stops.
So you could well be correct. Trouble is I cannot increase the pressure on
the system which could possibly stop it from occurring.
Thanks for the link, it was very informative.

Cheers
Mike



Posted by daestrom on October 28, 2007, 10:23 pm
 

If reducing the flow causes the noise to stop, that points towards
cavitation.  If the temperature was causing the impeller and casing to
expand and start interfering, flow wouldn't make much difference.

To help reduce cavitation...

Put the pump as low in the system as possible (even below the tank or at
least level with the bottom of the tank)
Pump the 'cooler' water from the tank to the collector instead of the
'hotter' water from the collector to the tank.
If the system has any sort of expansion tank with an air bladder, consider
raising the pressure in the air bladder.

daestrom


Posted by nicksanspam on October 29, 2007, 8:39 am
 

You can't just turn off the pump? ("Here is your new car. Whatever you do,
don't turn off the engine" :-)

Nick


Posted by Mike Scaife on October 29, 2007, 7:30 pm
 

I could turn the pump off, this will cause the water in the panels to boil
and be vented by the OT relief valves located on each bank of the array. I
would prefer not to have this occur.
The dissipater was an easy solution to make, its a serpentine pipe
arrangement clamped between 2 sheets of mini corrugate aluminium, painted
black mounted vertically under the floor below the hot water cylinder. A 3
port temperature limiting valve was modified so when the heated water
approaches the set level the valve begins to open diverting the overheated
water through the dissipater, the hotter the water the valve opens further.
Works very well, so I can have a large array to get reasonable heating in
winter.

No here when one purchases a new car, it is never put in the garage, it is
left parked outside the house so the whole street knows you've go it.

Cheers
Mike



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