expantion tank

alex9453

hello i am geting ready to fill the heating loop in  a 25 floors  bldg do i need  air pressure in the top of the expantion tank all the way down the basement since they are pipe out with a line at the top off the tank at 200 psi

Jamie Hall

Eek

200 psi?  Why I prefer steam...  But yes, somewhere in the system there has to be room for the water to expand, which means somewhere in the system there has to be air, or access to air.  I must admit that one something like that system, I would put the expansion space or tank at the top, however, not at the bottom.  In fact, I would be very much inclined to go for a big tank at the top, vented to the atmosphere.  But that's just one man's opinion...

Mark Eatherton

Why 200 PSI???

Technically, if each floor were 10' tall, at 1/2 PSI per foot of elevation, you would only need 125 PSI just to raise the water to the top of the system, and then add 5 PSI to that to aid in burping the air out of the top of the system and avoid potential steam flash due to vacuum.



The diaphragm should be aired up to the fill pressure of the system, in your case, 130 PSI. This must be done with ZERO pressure on the water side of the tank. Make sure ALL components in the mechanical room are ASME rated for the pressures and temperatures it will be seeing.



A question for Jamie, what in your minds eye, is the pump going to do, if it is at the bottom of the system, and the PONPC is at the top of the system? WIll the pumps produce positive pressure, negative pressure or what?



If the tank is located on the roof, but piped into the basement, does this change?



Just wanted to get your M.E. take on it.



Thanks!



ME

brstr1

1psi = 2.31' of head

10' a floor for 25 floors would need an expantion tank with 113.25lbs of pressure to keep high temp water from going to steam at the top of the system.

Jamie Hall

Pump will do

what pumps always do -- create a pressure differential between the inlet and the outlet.  You'd need the same size pump for a given system whether your static (pump off) pressure was 12 psi or 200 -- the big difference is that the pump body and seals would have to be rated for the higher pressure.  Keep in mind that it is that pressure difference that makes the water go round, not the total pressure!

Mark Eatherton

My thoughts on this...

You are correct. Pumps are simply pressure differential machines. They don't care if its positive,,or negative, or both. They just make a difference. If the pump is half way between the expansion tank (parallel direct return piping, with pump in basement and expansion tank in attic) and it is capable of producing say 7 feet of head at that given piping condition, then it will produce 3.5 feet of positive pressure, and 3.5 feet of negative pressure, for a total pressure differential of 7 feet. If the expansion tank is on the roof, and a pipe run separately from there back down to the inlet of the pump, the pump has no choice but to present its full pressure differential as POSITIVE pressure, which keeps microbubbles small..(remember Dan's carbonated water demonstration)The PONPC, is where the atmosphere (expansion tank) interfaces with the water. On older open loop gravity systems, that would be the open tank (atmospheric interface) in the attic....But that's just what I see in MY minds eye. It blinked. Didja see it ? :-)ME



EDIT: Here is a link to an article I penned a long time ago that may answer a lot of questions as it pertains to expansion tanks. This was the last of a series of articles, that basically summed up what was presented in previous articles.



http://contractormag.com/hydronics/cm_column_75/



Enjoy!



ME