## September 09, 2008

### Expansion Tank Sizing for Thermic Fluids

The design of the expansion tank in a liquid phase heat transfer system using Thermic fluids or other organic fluids is an important parameter in the total system’s successful operation.

It is important to design an expansion tank correctly as it can result in lower maintenance, lesser failure, low downtime, higher onstream factor etc.

Purpose
The main function of the expansion tank in a heat transfer system is to provide
for fluid expansion, which can be greater than 25% of its original volume depending on the fluid used and the operating temperature.

Since the tank is usually installed at the highest point in the system, it also can serve as the main venting point of the system for excess levels of low boilers and moisture which may accumulate in the heat transfer fluid. The highest point installation also creates positive head pressure to the pump’s inlet, providing flooded pump suction with uninterrupted flow of fluid to the user station. A simplified drawing showing a suggested positioning of the expansion tank in a heat transfer system is given below.

Sizing
The expansion tank should be sized so that it is 25% full at ambient temperature and 75% full at normal operating temperature. This sizing should cause positive fluid pressure to the pump’s suction side during system startup and should minimize the
vapor space in the tank during normal operation.

Fluid expansion between two temperatures can be calculated by dividing the fluid’s density at the lower temperature by the density of the fluid at the higher temperature — i.e., the density of Therminol® 66 at 40°F is 8.47 lb./gal. and changes to 6.72 lb./gal. at 600°F. Thus the expansion of Therminol 66 is 8.47/6.72 = 126% of the original volume at 40°F when heated to 600°F.

Therefore, an expansion tank for a 1,000-gallon Therminol 66 system operating between 40°F and 600°F should be sized for 260 gallons of expansion. Since this expansion represents 50% of the tank volume (the volume between 25% and 75% full), the expansion tank should be 520 gallons in size.

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