Expansion Tank Air charge Pressure
Expansion Tank Air charge Pressure
(OP)
Regularly when selecting an expansion tank for hot water (bladder type), what kind of air fill pressure should be? Is there any general rule of thumb of calculating it?





RE: Expansion Tank Air charge Pressure
I don't know of any quick rule. There are a number of factors to take into consideration which are variables
such as: Tank size, System volume, Max. operating pressure, etc. Email me your fax number and I'll give you a page that shows how it's done
RE: Expansion Tank Air charge Pressure
How does one trade off tank volume and pressure with an acceptable pump cycle rate based on the CI and CO settings? I've always just made the tank as big as possible and try to find a bladder that has a lot of movement from the high mark to the low mark. Are there any "bladder" types that maintain a relatively constant pressure over a wide tank level range? Who would know?
Thanks much.
Buddha's advice 2,500 yrs ago (I'm Christian!): "After deep, careful observation/analysis; and it agrees with reason, is conducive to the common good: accept it and live up to it."
RE: Expansion Tank Air charge Pressure
To size a tank:
Vt = (0.00041t-0.0466)Vs / (34/Pf-34/Po)
Vt = tank volume is US gal
Vs = system vol in US gal
t = max avg operating temp in F
Pf = initial fill pressure in ft of water (abs)
Po = Max operating pressure, including pump head, in ft of water (abs)
A general rule is to add 20 - 50% to the calculated volume to allow for modifications and additions to the system.
RE: Expansion Tank Air charge Pressure
Buddha's advice 2,500 yrs ago (I'm Christian!): "After deep, careful observation/analysis; and it agrees with reason, is conducive to the common good: accept it and live up to it."
RE: Expansion Tank Air charge Pressure
This type of system isn't really my long suit. I do mostly steam and compressed air. I had to become familiar with hot water systems, because I got tired of the steam-side of the heat exchanger being blamed for everything that went wrong. Someone who does more of these may have some better info for you.
RE: Expansion Tank Air charge Pressure
Thanks for the info. I'm new at designing hydronic & steam heating systems; so I really found your replys helpful and have put them in my design resource scrap book.
My experience with tackling the pump cycling issue has to do with a residential, open-loop water source heat pump (WSHP)in a residence with its own submersible supply pump.
The WSHP runs continuously about 75% of the time ( drawing 10 gpm)on subzero days, due to the building heat load. With the standard-sized residential "bladderless" pressure tank the 3/4 hp pump cycled like crazy and starter contacts were overheating and I'm sure the pump was stressed.The bladdrless tank was also continuously getting waterlogged which only exacerbated the problem!!
To solve the problem, I installed a triple-oversized bladder-type pressure tank in parallell with the much smaller air tank. Although the pump ran much longer to fill the tank, it had a good rest between cycles; and the fried starter problem was solved.
At the full draw-down CI point, the tank still had a substantial amount of water in it .... as do most bladder tanks.
If the bladder could continue to expand as the water is drawn down further, the pump rest period could be much longer. Hence my question about a bladder with more mobility. Are you aware of any?
Thanks
Buddha's advice 2,500 yrs ago (I'm Christian!): "After deep, careful observation/analysis; and it agrees with reason, is conducive to the common good: accept it and live up to it."
RE: Expansion Tank Air charge Pressure
You might consider posting your problem at www.heatinghelp.com on the sub-site called "The Wall". It's a bulletin board primarily for contractors who do residential and small commercial steam and hot water heating systems. Somebody there will very likely be able to help you better than I can.
RE: Expansion Tank Air charge Pressure
Thanks! I never really understood why the tanks got water logged. But what you say makes perfect sense! And the more AGITATED the water (many pump cycles), the quicker the air gets absorbed... because all those cycles is like a full season!!! EUREKA!!!
Thanks again, now I understand.
BUT even with a bladdr and an undersized tank, the pump will be short-cycled; so guess the answer is that tank volume has to be matched to the gpm to give a "resonable" cycle rate ... which also depends on the CI/CO settings???
This is probably addressed in your equation of March 16; which I'll have to study carefully.
Thanks, Much. I'll let you kknow if the eqn gives me any new "revelations"
Buddha's advice 2,500 yrs ago (I'm Christian!): "After deep, careful observation/analysis; and it agrees with reason, is conducive to the common good: accept it and live up to it."
RE: Expansion Tank Air charge Pressure
I think the problem you're experiencing sounds more like a well pump and cushion tank. (Now this is REALLY out of my field.) It's my understanding that if the cushion tank loses it's air, the pump just bangs on and off constantly as the system pressure is made, then lost in short order, over and over.
RE: Expansion Tank Air charge Pressure
Due to the relatively large flow-to-tank-volume (75% run time = 10 gpm @ 45min/hr) , the analogy in the EXTREME "LIMIT" would be to have a P tank the size of an 8oz glass ( I could choose a thimble
I was going to see if in your eqn (still no time to look at)there is as relationship between volume and flow. IE: with a flow of 10 gpm and a desired cycle rate of say every 10 minutes, you'd need a 100 gal storage volume: 100 gallons would have to exit the tank before the CI pressure is reached. Since most residences have a 40 to 50 gal capacity (TOTAL that is!) and the draw down until CI is much less less: (~25% of 50 gal =~ 12 gals); one gets a cycle rate (at 10 gpm) of almost 1x per minute!! (1.2500?000????.. to be "exact'... no one can be exact!)
If there were a bladder that could expand way down until almost all of that 50 gal capacity were drawn... say 10 gals left at CI, then the time from CO until CI would be longer. "Ballpark" would be: 40 gals / 10gpm = 4 mins ... BUT: as the pressure decayed to CI, the 10 gpm WOULD DECAY TOO! ... due to the fixed flow resistance. Perhaps to an unacceptable level (?) depending on the load's (WSHP) requirements. To "solve" that problem, one would have to increase the CI pressure... and .. and... and ... and...
OH "My Buddaha"!!! You'd be back to short cycles... water logging (air cushion case)...... and burned contactors!!:_(
BUT:...... (Oh no!!.. here I go again!!.... DAMN!!!!!!) CONSTANT RUNNING is BETTER than short cycling( RIGHT??.. EVERYONE "KNOWS that!!!?)); so it'd be good if the pump "never caught up"! (RIGHT? ... right!!
I guess I was kinda doing the right thing by putting in a larger tank!! Due to my concern about short cycles ......BUT!.. DANG... here I go AGAIn (Jim CAREY!); the problem COULD be solved (for more money!!
EUREKA!!! .......I got it: a pressure tank BYPASS.... so that the pump RUNS CONTINUOUSLY!!!... when the WSHP kicks in, solenoids bypass the pressure tank. That'll work.. Right???
THANKS!!!!, TBP, for making me THINK and BRAINSTORM with you...(my brain can be lazy... except when someone keeps it going for me!!)
But before I rush off to the patent office: what do you think of my ideas???
Hey if you think it'll work, just let me know and I'll let YOU "rush to the patent office" ... and go thru all the paperwork hassle!
Me?..... I'm moving on to the NEXT interesting problem... they're "fun"
I know: this is all "obvious" to all you pumping "experts". Please excuse me while I have fun "reinventing the wheel"!!
Buddha's advice 2,500 yrs ago (I'm Christian!!???!!): "After deep, careful observation/analysis; and it agrees with reason, is conducive to the common good: accept it and live up to it."
RE: Expansion Tank Air charge Pressure
RE: Expansion Tank Air charge Pressure
The "Standard" pressure, or the ones most used & sometimes "Fine-Tuned" in the field is "12PSIG" on an empty expansion tank.
Without the coveted "Airetrol" valve to insure the "Air Cushion" is maintained, the possibillity of Reliief Valve "Popping", can & will be a problem..
For more information go to;
http://www.bellgossett.com/
RE: Expansion Tank Air charge Pressure
AMTROL makes tanks for that purpose. the model is a Well-x-trol. They also make diaphragm, bladder and air type expansion tanks for closed heating systems.
Just my $0.02
RE: Expansion Tank Air charge Pressure
Thanks for the input. I'll visit the Bell & Gossett site. Two cents is a lot when you're otherwise "broke"!
Buddha's advice 2,500 yrs ago (I'm Christian!!???!!): "After deep, careful observation/analysis; and it agrees with reason, is conducive to the common good: accept it and live up to it."