Boiler Facts

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By George Careygcarey@fainc.com

One of the most common components usedin every hydronic heating system is also oneof the least understood. I am referring to theexpansion tank…I mean compression tank…Imean steel expansion tank…I mean the dia-phragm compression tank…I mean the pre-charged expansion tank…see what I mean?It is called so many different names and eachone of them can be correct, depending uponthe tank. Years ago, when gravity hot waterwas king of the heating systems, there trulywas an expansion tank that was located atthe highest point in the system (usually inthe attic) and was open to the atmosphere(there was no top on it or if there was, ithad a vent pipe that was piped out throughthe roof). As the coal fired boiler heated thewater, the molecules would start to get allexcited and move around faster, causing thewater to expand. Since water is not compress-ible, the old-timers knew they needed a placeto put this "expanding" water so as to notcreate a pressure problem. So they would in-stall this tank at the highest point in the sys-tem and as the water heated/expanded, itwould move into the expansion tank. Thetank thus gave the water a place to expandinto and consequently would prevent the sys-tem from over-pressurizing. One of the nega-tives to having an open expansion tank wasevaporation and the potential increase for cor-rosion through iron oxidation. Because thesystem would have to replenish the missingwater with fresh-oxygenated street water,

once heated, it released oxygen intothe system. Also because the tankwas open to the atmosphere, youhad this constant interaction be-tween air and the system water,which could accelerate corrosionby having the oxygen attack thecast iron and black steel piping.

As early as 1911, some gravi-ty systems started to incorpo-rate a closed and thuspressurized expansion tank.One manufacturer in partic-ular, D & T Manufacturingfrom St Louis, MO devel-oped a system called The D& T Tank in Basement Sys-tem. Interestingly enough,the reason they went to apressurized tank in thebasement was to bettercontrol the firing of thecoal boiler. By using aset of pulleys andchains that were con-nected to the draftdamper and check

damper, and a diaphragmregulator piped into the bottom of

the basement expansion tank, they could in-crease or decrease the firing rate of the coal

An Expansion Tank Storyboiler and thus regulate the correspondingtemperature supplied to the system. As men-tioned earlier, as water is heated, the mole-cules expand causing an increase in thevolume of system water.

With a pressurized tank, there is a volumeof air that is maintained in the tank; it isthere to act as a cushion or a spring on thewater in the system. Since gases are com-pressible and water is NOT, when heated, thewater squeezes into the tank by compressingthe air in the tank. And like all gases, whenyou squeeze or compress the volume of air inthe tank, its pressure increases. Well the D& T Tank company used this changing ofpressures to influence the diaphragm regula-tor which in turn re-positioned the check anddraft dampers, thereby increasing or decreas-ing the fire in the boiler. Pretty ingenious!

Eventually the "Booster" pump was devel-oped and gravity hot water systems gave wayto forced hot water circulation systems. In factthe term "Booster" was a marketing termfrom Bell & Gossett in their efforts to showhow adding a pump to a gravity hot watersystem you could "boost" the heat fasterthroughout the system. Of course the expan-sion tank was needed even more than beforebecause now the tank performed two func-tions:• It was still the only place for the ex-panded volume of water to be accepted whilekeeping the pressure within the system be-low the boiler relief valve setting.• It became the only place where any airin the system was directed to…the develop-ment of what became known as an Air Con-trol System. And at the heart of this systemwas the Compression/Expansion Tank.

It was at this point that the industry start-ing referring to the tank as a compressiontank. Their reasoning was for the expandingwater to enter into the tank, it was now nec-essary to squeeze or "compress" the body ofair in the tank. And in doing so, the pres-sure in the system would start to increase.It quickly became apparent that the size ofthe tank was important if they did not wantthe system pressure to increase to the boil-er's relief valve setting. They did not want todump water all over the floor and they alsodid not want to introduce large amounts offresh water that would attack the boiler,pumps and piping.

So they had to come up with a method tosize the compression tank for any and all hotwater systems. They determined that to prop-erly size a compression tank, you need toknow:• The volume of water in the system.• The operating temperature range (Howhot will the water operate up to?)• What is the acceptable pressure range(Start with the system's fill pressure whencold and then as the water is heated, the

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pressure can rise to a point close to but be-low the boiler relief valve setting.)

What the manufacturers quickly realizedthat although it was easy to establish thetemperature range and the system's operat-ing pressure range, it was very difficult in es-tablishing the water content of a system.Unfortunately, that was also a very impor-tant piece of information to successfully sizea compression tank. Eventually, they came upwith sizing charts that included system wa-ter content for the various pieces of equip-ment including boilers, different forms ofradiation and piping systems. All the chartshad a column that listed the BTUs and thenthey calculated the water content according-ly. They put a lot of time, behind the scenes,to come up with these sizing charts. Unfor-tunately, the industry mistook the BTU/H ca-pacity of a boiler as the definitive piece ofinformation needed to size a tank, when infact it has nothing to do with it; the engi-neers just calculated water content basedupon standard conditions of such and suchBTU boiler and its corresponding radiationand piping.

The other important role the steel compres-sion tank played was in controlling any oxy-gen in the form of air bubbles that could befloating around in a hydronic system.

This whole system, which was developed andpromoted by Bell & Gossett around 1935 orso, became known as "Air Control" (drawing,right). The intent of this system is to "catch"any air found in the piping or radiation anddirect it up into the compression tank. Theair was the cushion that acted like a "spring"to maintain the appropriate pressures withinthe system. This type of system could neveremploy automatic air vents because if thevents vented air, the system pressure woulddrop, causing the automatic pressure-reduc-ing valve to open, bringing in additional wa-ter to maintain the proper system fillpressure. And where would this new watergo? The only place it could go…into the com-

pression tank. Now you have changed the re-lationship between the volume of air and wa-ter in the tank. Without the proper amount ofair in the tank, there wasn't enough "cushion"to accept the expanding water without caus-ing an exorbitant pressure increase, thus caus-ing the relief valve to open and dump gallonsupon gallons of water onto the basement floor!The compression tank had to be mounted upin the ceiling, high above the boiler and its airseparator. This was to allow "Mother Nature"to take over. Any air bubbles that were cap-tured at the air separator would float up intothe bottom of the compression tank. One lastimportant characteristic about compressiontanks is that the air in the tank was in con-stant contact with the system water—theywere "touching" each other. So as the systemwater temperature changed, some of the air inthe tank would go back into solution and then,as the system water was heated, this airwould come back out of solution. It was criti-cal to have a proper air separating device to"catch" any of these bubbles and put themback into the compression tank.

As early as 1954, a company in Warwick, RInamed Amtrol patented a pre-pressurized di-aphragm compression/expansion tank thatwould eventually change the hydronics indus-try when it came to expansion tanks. Thesediaphragm tanks are much smaller comparedto the equivalent-in-size steel compressiontank. And the main reason for that is the tankis pre-charged with the appropriate air pres-sure before it is connected to the system. Ap-propriate air pressure for what? For thatparticular system's required fill pressure (as-suming the tank is installed in the basementwith the boiler). See, the idea with these styletanks is there is no system water (theoreti-

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cally) in the tank before the water is heated. Once thewater starts expanding due to a temperature increase, itthen "squeezes" into the tank, compressing the air. It isimportant to know what the system's required fill pres-sure is, because that becomes the pre-charged air settingin the diaphragm tank. Remember, there is no water intothe tank until it is heated!

To calculate the required fill pressure, just measure fromthe boiler/fill valve to the highest piece of radiation in thehouse in feet. Divide that number by 2.31 to convert feetinto psi. Now add 4 psi to that number (for positive pres-sure at the high point) and the total becomes the sys-tem's required fill pressure. We take that number andpre-charge the diaphragm tank to the same psi (beforethe tank is attached to the system) and the tank is readyto go. Another big difference with these tanks is they havea butyl diaphragm SEPARATING the air charge in thetank from the system water that enters the tank. There-fore we do not have to control the air in the system; rath-er we have to ELIMINATE it by venting any air found inthe system out to the atmosphere. In this type of systemyou will typically see a large air separator with a largecapacity vent located somewhere near the outlet of theboiler. Any air that passes through the separator will get"caught" and vented out through the air vent. Because thetanks have a pre-charge of air, they can be located almostanywhere, but the pipe connecting the tank to the systemshould come from the air separator or very close to it. Theyare subject to the same influences when it comes to siz-ing:

• System volume• Temperature (how hot will the water become)• Pressure range (from the fill valve's setting to thepressure relief valve's setting)

Well, there you have a brief history of the evolution ofThe Expansion Tank.

If you have any questions or comments, please call meat 1-800-423-7187 or email me at gcarey@fiainc.com

Amtrol design

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An Expansion Tank Story