Air Management and Pressurization Part 4: Expansion Tank Sizing

By Chad Edmonson

Properly sized expansion tanks (standard or bladder/diaphragm) are critical to a successful air management in a hydronic system.

Every expansion tank manufacturer has a written form that provides the steps and calculations for sizing an expansion tank.  Most, if not all, have developed software that does the work for you.  Bell & Gossett has a great one, ESP-Plus which can be downloaded here.  But regardless of whether you use software or charts and longhand calculations to size an expansion tank, there are a few pieces of information you’re going to have to have either way.  These key values are:

1. Total system water volume

2. Minimum and maximum system operating temperatures

3. Maximum system operating pressure

4. Minimum operating pressure at the tank to maintain 4 psig at the high point in the system.

5. Maximum operating pressure at the tank

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We like to add one more to the list: The system component with the lowest pressure rating.  Obviously, you don’t want the system pressure to ever exceed the pressure rating on the boiler or other component, lest you blow the ASME relief valve!

After you know these values, choosing an expansion tank for the system you are designing is a matter of completing a relatively simple manufacturer’s sizing worksheet or plugging in a few numbers into a sizing program. But before you get to that point, it’s important that you understand the logic (and physics) that dictates the sizing of your expansion tank. 

First, it is important to remember the two primary purposes of an expansion tank in a hydronic system:

The tank (whether it is a bladder tank or a standard tank) must be sized such that it can store the required volume of expanded water without exceeding the maximum pressure allowable.  It must also be able to maintain the required pressure when the system is cold and water volume is at its lowest.  That’s where an old chemistry lesson comes into play.

Remember Boyle’s Law?  Boyles Law, and its associated equation, p1V1=p2V2, states that the absolute pressure exerted by a gas (such as air) is inversely proportional to the volume it occupies within a closed system, assuming the temperature and volume of the gas remain the same.  In a closed hydronic system, the volume of water changes constantly as a result of even the slightest temperature change.  As this water volume increases, the volume air within a bladder tank decreases, as the free-floating molecules of air get squeezed together.  As the volume of the air and water changes, so does the pressure. As you decrease (“squeeze”) the air volume in the expansion tank, you actually increase the pressure within the system proportionately.  If you decrease the air volume by half, you double the pressure within the system.

That’s Boyle’s Law, and it serves as the basis for how we size expansion tanks. 

Still Confused?

Here’s another way to think about it. In sizing an expansion tank for a hydronic system, we are actually predicting the expansion volume of the system and determining how much of an air cushion is needed to keep the system pressurized under low operating temperatures and provide enough space for expansion under higher operating temperatures to avoid over pressurization and blowing relief valves. 

Another potential problem when dealing with compression tanks is water logging the tank.  Thru gravity recirculation air can be reabsorbed from the compression tank.  Water-logging occurs when the compression tank reaches the point where all of the air has been removed from the tank and there is no room left for expansion. The addition of an Airtrol Tank Fitting helps prevent this recirculation. 

REMEMBER - Size and pipe your expansion tank correctly, and you will never have to worry about a water-logged tank or a blown relief valve!

To view a webinar how to size expansion tanks, click here!