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System Pressure and Expansion Vessels

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Posted by mckayt on January 18, 2007, 9:28 am

Can someone explain what the advantafes of a pressurised system are
over an un-pressurised system?

Also, am I right in thinking that all systems (apart from drain-back)
need an Expansion Vessel to accomodate expansion in the Glycol in the
closed and sealed loop (ciruit) within an indirect active system?

Sorry if the question seems simplistic or confusing,



Posted by sylvan butler on January 19, 2007, 11:59 pm

do you simply mean pressurized or do you mean direct water supply
connection so operating at system pressure and not atmospheric pressure?

If you really do mean pressurized, then the main advantage is the
ability to increase system max operating temperature.

drain-back is indirectly related...  Any system that heats a liquid
needs expansion room.  If the system is open to the atmosphere (and most
drain-back are but they are not the only ones) then the atmosphere is
the expansion.  A sealed system needs something, and that can be a
purpose built expansion vessel (e.g. well-x-trol) or simply a stub in
the system which contains air and which can be refilled with air when it
is eventually absorbed by the working fluid.


Wanted:  Omnibook 800 & accessories, cheap, working or not
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Posted by KDV on January 20, 2007, 12:56 pm

All systems need an expantion vessel!

When the system is overheating (pump is not running), basic difference are:
With a system  under pressure expantion, you prevent the water from boiling.
So overheating the system is not directly a problem with temperatures above
100C but the temperature can go higher alot then 100C!
With volume expantion the water can go boiling with temperature near the
100C, steam wil be vented at the top of the volume expantion vessel and you
have to refill the system afterwards but the temperature will not go higher
then 100C as long there is water in the system!

With a system under pressure you should have a better heat transfer in the
heat exchanger.
With a system under pressure you could have more leaks.

Sombody has other points?

Kris De Voecht
Homepage: www.devoecht.com

Posted by Jeff on January 20, 2007, 5:41 pm
 KDV wrote:

   That sounds like what could be used in a drainback system. Can you
elaborate on the volume expansion as this is not something I'm familiar
with. There has been talk here of drainback sytems that have long drops
(20' or 7m) and how the boiling point of the water is reduced by the
downward vacuum making them vulnerable to flashover. It would seem that
a volume expansion vessel on the return line would be helpfull.


Posted by Gary on January 21, 2007, 1:16 am
 Jeff wrote:

Hi Jeff,
Alan R. has drainback system with a 50 ft rise from the tank to the roof top
collectors.  He uses two Taco 009 pumps in series to push the water up this 50
ft.  Once flow is established, a time delay turns off one of the two pumps.
All this is working fine for him:

Pictures of the pumps and tank down the page a ways.


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