Sizing Expansion Tank for Hot Oil System

Mon, 14 Dec 2015

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

Spreadsheet for sizing expansion tank for hot oil system




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