What is the primary basis for sizing a closed expansion tank in chilled water systems?

What is the primary basis for sizing a closed expansion tank in chilled water systems?

• A. System pressure
• B. Thermal expansion of water volume
• C. Ambient temperature
• D. Maximum velocity

Answer: (B)

The amount of water expansion due to temperature change is what sets the size of a closed expansion tank in a chilled water system. In a closed loop there’s no free space for the liquid to expand, so as the water warms it increases in volume and would raise system pressure unless absorbed. The expansion tank provides an air chamber that compresses to accommodate that extra liquid volume, keeping pressure within limits. Therefore the tank is sized to accommodate the maximum expected change in liquid volume from the lowest to the highest operating temperatures, using the system’s total water volume and the coefficient of thermal expansion of water. Ambient temperature or maximum flow velocity don’t determine the expansion volume; while system pressure and flow controls manage operating conditions, the primary sizing factor is how much the water expands with temperature.

The amount of water expansion due to temperature change is what sets the size of a closed expansion tank in a chilled water system. In a closed loop there’s no free space for the liquid to expand, so as the water warms it increases in volume and would raise system pressure unless absorbed. The expansion tank provides an air chamber that compresses to accommodate that extra liquid volume, keeping pressure within limits. Therefore the tank is sized to accommodate the maximum expected change in liquid volume from the lowest to the highest operating temperatures, using the system’s total water volume and the coefficient of thermal expansion of water. Ambient temperature or maximum flow velocity don’t determine the expansion volume; while system pressure and flow controls manage operating conditions, the primary sizing factor is how much the water expands with temperature.