Sizing An Expansion Tank For A Heat Transfer System
Expansion tanks are important to the operation of heat transfer systems. They are designed to account for the volumetric expansion of heat transfer fluids as a result of temperature increase. Expansion tanks also remove moisture, non-condensables, degradation by-products and entrained air during startup and operation.
In this article, we present the sizing of an expansion tank using Relatherm HT-1, a high flash heat transfer fluid as an example.
Size an horizontal expansion tank for hot oil system using Relatherm HT-1 as Heat Transfer Fluid. The fluid is heated from 30 °C to 330 °C. System hot oil volume is 7.87 m³. Assume L/D to be used as 2.5. Thermal fluid density is 861.91 Kg/m³ at 30 °C and 752.17 Kg/m³ at 330 °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.
If VSys = Volume of heat transfer system (piping and heat exchangers)
Vcold = Volume of the entire heat transfer (including piping, heat exchangers and cold fluid in expansion tank at start up)
Mcold = Total mass of thermal fluid in heat transfer system
ρcold = Density of thermal fluid in heat transfer system
Vhot = Volume of thermal fluid on expansion
Vexpansion = Volumetric expansion
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 up to 70% of the volume.
Vexpansion = 0.7 V – 0.1 V
Solving above equations for V provides capacity of expansion tank.
V = 1.96 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