I just spent five work days double checking, analyzing, re-calculating, and troubleshooting an undersized closed compression expansion tank selection. To make life more fun, the tank was already shipped and onsite. So we were stuck with what little we had. Luckily, it all worked out with some changed parameters. However, through this ordeal I discovered there just was not a clear description on what parameters are needed and why they matter when sizing a tank.
So here's what you need to know about the tank selection:
Total System Water Content (gallons):
What:This is pretty straightforward. Determine the total volume of water content in your system. You can use the tables to the left as an approximation for the pipe and components in your system. Or you can always be more accurate and ask the equipment manufacturers what volume of water is in their equipment.
Why:This is the baseline. You state how much water the system is dealing with.
Minimum Operating Temperature (degrees F)
What: Minimum operating temperature can be defined in two different ways.
Some engineers define this as the ambient temperature of the water if the system is shutdown. This is generally ~70F.
Other engineers define this as the minimum temperature during system operation. For a heating system, it maybe something like 180F. For a chilled water system it could be something like ~45F.
Design your tank for the worst case scenario which is the lowest possible temperature whether in operation or shutdown.
Maximum Operating Temperature (degrees F)
What: Like the Min temp, Max operating temperature is defined can be defined in two different ways.
Maximum operating temperature can be the weekend, ambient, or shutdown temperature.
Maximum operating temperature can also be the maximum temperature during operation. For heat it maybe 200F, for chilled water it can be 55F. Use the highest of the two for worst case scenario.
Why: The delta T between the min and max operating temperature will determine the expansion factor and therefore the expanded volume of the water. The larger the delta T, the more the water expands which requires more tank volume.
Calculation #1
Now use the minimum and maximum temperatures to determine the Expansion Factor from Table E.
Calculation #2
Multiply the Expansion Factor by the Total System Volume to calculate the expanded water volume.
Expanded Water Volume =
Expansion Factor x Total System Volume
Minimum System Operating Pressure (psi):
What:a.k.a. Cold Fill Pressure. Defined as the static pressure plus adequate positive pressure at the top.
This ensures that there is enough pressure to completely fill up the hydronic system to the top and have pressure at the top of the system (typically 5 PSI) so that air can be properly vented.
Why:The lower the minimum pressure, the less pressure is required to pre-charge the tank and the tank can accept more of the waters expanded volume. Simply there is more tank to utilize.
Maximum System Operating Pressure (psi):
What:Defined as the pressure setting of safety relief valve less 10%.
The pressure rating of each component should be checked and the valve adjusted to relief pressure at perhaps 10 % lower than the lowest pressure rated component. The relief valve should be set at a pressure value that will still protect the entire system.
Why:The higher the maximum operating pressure the more water the tank can accept to maintain that pressure.A small tank can hold the same volume of water at high pressure as a larger one at a lower pressure. Make sure to properly set the relief valve. For example, the safety valve will be set for 50 psi if the maximum operating pressure is 45 psi. If you accidentally set the safety valve at 45 psi, the max operating pressure would be 40.5 psi. This will increase the required tank volume.
Calculation #3
With the minimum and maximum operating pressures determine the acceptance factor from Table A to the left.
Calculation #4
Calculate the total tank volume by dividing the expanded water volume from calculation #2 by the acceptance factor from calculation #3.
Total Tank Volume = Expanded Water Volume/Acceptance Factor
And viola! Pick a tank that can hold the total tank volume with ease. If you pick one right on the border of acceptance, the tank could overflow if the system parameters change.
When in doubt, ask your manufacturer to confirm your selection is suitable for your conditions.
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