Expansion Tank Air charge Pressure

[BradM]

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?

 

[fredb]

Hi

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

 

[Goorah]

Fred: I'd like a copy of that info as well. Could you please fax it to me at: (608) 786 - 3745?

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."

 

[TBP]

The static fill pressure and the tank pressure should be the same. If you've got a 20 PSI static head, make sure that there's 20 PSI on the air side of the bladder. Check and adjust the tank pressure when it's NOT connected to the system.

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.

 

[Goorah]

Thanks for the info TBP. Is there any concern about pump cycling? Or is this addressed in the rule of thumb? 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."

 

[TBP]

I've never heard of pump cycling being a factor. The arrangement for a heating system should be: the boiler or heat exchanger, air separator, then the pump, pumping away from the tank and heat source. This gives the air sep the best running start, as it's seeing water closeset to the boiling point (hottest water at lowest pressure). The expansion tank is connected to the air sep. The make-up water line is connected to the short section of pipe between the sep and the expansion tank.

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.

 

[Goorah]

TBP:

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."

 

[TBP]

I don't know much about geothermal systems, but I do know if you have a plain steel bladderless expansion tank in a closed loop hot water heating system, and don't have an "Airtrol" fitting screwed into the bottom of it where it connects to the system, it'll gradually waterlog. Many people think they can just use an ordinary pipe nipple. It will work for a while, but the water will gradually absorb the air in the tank. The air gets into the piping system, is kicked out by the air sep, and the make-up valve adds water to maintain the system pressure. Over a heating season or 2, the tank gradually floods, the system loses it's expansion cushion, and the relief valve pops every time the boiler comes on. This steady diet of raw water that is added when the system cools and the pressure falls, is full of dissolved oxygen that corrodes the system to death in a few years.

You might consider posting your problem at

http://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.

 

[Goorah]

TBP:

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."

 

[TBP]

The equation just tells you how big the tank must be to handle the expansion of the water as it heats up. The system will heat just fine without it, except that the relief will pop every time it cycles on.

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.

 

[Goorah]

TBP: Well it's "out of my field" as well! I was talking COLD ground water (48F)in an OPEN LOOP (once thru);whereas your talking a HOT water CLOSED LOOP.. but your explanation makes sense: What's the difference (except time) if the tank takes 5 mos to waterlog (cold) at "reasonable" pump cycle rates... or if the pump cycles with cold water as many times (in a short time due to small volume and high drawdown) as in an entire season at lower flows?? Answer: No difference!! And the bangin' on and off agitates the water, which is favorable for air absorbtion. Right? (No?)

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;-)) for a normal residence. Do you see my point?

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!!:-() by using a DEDICATED pump WITHOUT a tank!! Damn!!! ... but that costs more $$$$!!

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"!! But is there a "high motion" bladder or diaphragm P tank available?? Maybe I'll check with manufaturers... NAH... no time... too "boring"!:_) 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."

 

[BradM]

Sorry, I don't have a fax machine, fredb. Is there any way that you can scan this page and email it to me at tomchrist2000@yahoo.com? I would appreciate that.

 

[YorkMech1]

Regularly when selecting an expansion tank for hot water (bladder type), what kind of air fill pressure should be?

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/

 

[walkes]

Goorah, Back to your March 17 posting, it seems to me that you need a pressure tank for the system. What you want is a tank that fills up to maintain system pressure (open system) and to prevent pump cycling. System tanks are generally for domestic water applications where you may have a well source.

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

 

[Goorah]

YorkMechI and Walkes:

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."