ST expansion tank pressure

Elliot

I have always pressurized the ex tank 3 to 5 psig below the system operating pressure because that is the way I was taught and it has always worked well. Recently I was talking with someone and they said it should be 5 above.

What is best and where can I find the science behind it.

Elliot

Is Steamback any different?

I forgot to ask, what should the expansion tank pressure be and what should the system operating pressure be for a steamback ?

hot_rod

pressure

I think the concept is, if the tank is pre-charges a couple psi lower than fill pressure some fluid will fill into the tank. As the collector temperature drops in cold weather you have some fluid capacity to handle the pressure drop.



A rule of thumb is pre charge to the static head and add 5 psi to assure any air vent on top of the collectors can function properly.



I noticed the Euro systems i visited had a high fill pressure 45 psi or more to help supress the fluid from flashing under stagnation conditions. This is why most of their solar pump stations ship with a 6 bar @ 90 psi relief valve. But the expansion tanks size grows also at higher fill pressures, or the relief setting has to be elevated to handle that pressure increase.



Solar tanks size a bit larger than hydronic tanks due to the much larger delta T they work across. Use a solar expansion tank sizer to select a solar tank properly.



If the fluid does "steam" the tank needs to accomodate the liquid volume expansion, plus the fluid displaced by the vapor. I believe that is accounted for in a true solar expansion tank sizer formula.



I'm not sure why you would over-pressurize if the tank capacity and relief valve are properly sized to the system?



The Zilmet website has some good info on solar expansion tank sizing, or page 34 of idronics 3 at www.caleffi.us Idronics archive.



www.zilmetusa.com



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