Acceptance Volume of Expansion and Compression Tanks – Part 3

Norm Hall
October 17, 2011
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In part 2 of the R L Deppmann Monday Morning Minutes we determined the initial and final temperatures of the hydronic system in order to calculate the expansion tank selection. When we heat the water in a hydronic system, the water expands. This expansion is expressed as Ew in the formula:

Ew – Ep = The expansion of the water minus the expansion of the pipe.

This expansion is calculated for you in the Bell and Gossett ESP-PLUS selection program when you enter the initial and final temperatures and the fluid. The term Ew is not a mystery. It is expressed as:

Ew = [((specific volume at hot Temp.)/ (specific volume at cold Temp.))-1]

The calculation of the required tank acceptance volume is (Ef – Ep) X system volume. Let’s look at an example and assume the system has 50% Ethylene Glycol but you did not change the default from water.: Assume you have a closed hydronic heating system with a maximum temperature of 200°F and a 20°F ΔT. Assume the volume is 3000 gallons. What happens to the acceptance volume? The acceptance volume required in this example using 50%.

The in tool calculation Acceptance Volume of Expansion and Compression Tanks the in tool calculation of the acceptance volume for 50% E.G. is 125.4 gallons
The acceptance volume for water is 93.6 gallons The acceptance volume for 50% E.G. is 125.4 gallons

As you can see in the example, forgetting to change the fluid type default results in a tank which is 25% undersized. The result of this might be weeping pressure relief valves at the elevated temperatures.

Next week we look at pressures in a hydronic system.
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