Expansion tank is provided in hot oil system to accommodate for the increased thermic fluid volume due to thermal expansion. It also helps in removal of moisture and non-condensables during start-up. This article will show how to do sizing of an expansion tank.

Example

Size an horizontal expansion tank for hot oil system using Therminol VP-1 as thermic fluid. Fluid is heated from 30 °C to 350 °C. System hot oil volume is 7.87 m³. L/D to be used as 2.5. Thermic fluid density is 1056 Kg/m³ at 30 °C and 761 Kg/m³ at 350 °C.

A minimum volume (10% ~ 20%) is considered in expansion tank at start-up in cold conditions. As heating is started volume expands and tank sizing should be such that expanded volume fills (70% ~ 80%) of tank volume.

Consider vessel volume to be V. Initial volume of hot oil at cold conditions is as following.

 Vcold = 10% of V + VSys (in m³)
 Mcold = ( 0.1 V + 7.87 ) ρcold (in Kg)

Volume on expansion is determined by dividing by density at hot conditions.

 Vhot = ( 0.1 V + 7.87 ) ρcold/ ρhot (in m³)

Expansion in volume is obtained as

 Vexpansion = ( 0.1 V + 7.87 )( ρcold/ ρhot - 1 )

Liquid level in expansion tank increases and fills upto 70% of the volume.

 Vexpansion = 0.7 V - 0.1 V

Solving above equations for V provides capacity of expansion tank.

 V = 5.44 m³

Volume for 2:1 elliptical horizontal tank is provided by.

 V = πD²L/4 + πD³/12

With L/D = 2.5, solving above equation provides.

 D = 1.35 m
 L = 3.37 m

Resources

1. Spreadsheet for sizing expansion tank for hot oil system