Sizing thermal fluid expansion and the expansion tank
Heat transfer fluid swells noticeably when it heats up. Expansion volume is total system volume × cubic expansion coefficient × temperature rise, and this tool uses 0.000417 /°F for mineral oil based fluids, 0.000472 /°F for synthetic aromatics, 0.000250 /°F for a 50 % ethylene glycol solution and 0.000272 /°F for water. System volume has to include the heater, exchangers and user coils, not just the piping.
Take a 500 gallon system taken from 70 °F to 480 °F. Expansion comes to about 85.5 gallons, or roughly 17.1 % of the charge. Systems running near 570 °F pass 20 %, which matches the long standing rule of thumb that hot oil grows about a quarter of its cold volume by the time it reaches full operating temperature.
Standard practice is to size the expansion tank so it sits about one quarter full cold and about three quarters full at maximum temperature. That is why the recommended tank here is twice the calculated expansion. It leaves liquid in the tank for the pump when the loop is cold and still leaves freeboard when it is hot.
Mount the tank at the highest point in the loop and keep it cool, since hot oil in contact with air oxidizes quickly. Water carried into a new charge flashes off violently during first heat-up, so bring the system up in stages to vent it. Confirm final tank volume, vent and relief sizing against the fluid supplier data and with a mechanical engineer.
Frequently Asked Questions
Fluid overflows during heat-up, which is a burn and fire hazard, and after cooling the loop is short of liquid so the pump loses suction. Err on the generous side.
No. It rises with temperature, so the values here are averages across the range. For a precise design, use the density versus temperature table for the specific fluid and work from actual density change.