In closed hydronic systems expansion vessels must be placed in order to be able to cope with varying pressure and volume levels. Liquids are quasi-incompressible fluids and volume changes of the water due to temperature transients in the system must be made possible by the expansion vessel. This measure is needed to not strongly affect the pressure levels in the heating or cooling installation.
It is important to adequately design the expansion vessels in order to avoid air infiltration or to avoid too high-pressure levels in the system.
Practically, in the Hysopt software, the expansion vessel blocks are also used to adapt the type of fluid used in the system. (e.g. glycol or ethanol mixtures)
Changing the Fluid type
The fluid type can be changed by pressing the Brine scroll-down button. The mixture composition can be changed by the mixture percentage setting. (e.g.: 30% Mixture correlates to a mixture of 30% ethanol and 70% water). It is important to change your fluid type because brine mixtures have other heat transfer properties and will behave differently compared to pure water. If Brine is set to None, water will be used in the Hysopt model.
System volume
The calculation of the expansion vessel design volume is based on the DFTK17 of Flamco. More information can be found here:18501627 VL pdf Broch 0 (aalberts-hfc.com).
The system volume is calculated based on the installed capacity and a rule of thumb of 10 l/ propagated capacity. This rule of thumb can be changed by modifying parameter:
Estimated volume per propagated kW
When optimizing system components is executed, the design calculations are made for the calculation of the expansion vessel design volume. If it is not wanted to use this rule of thumb. The Hysopt user can visualize the different volumes in the system by hovering over the expansion vessel base circuit.
To adequately design the expansion vessel design volume, make sure to fill in:
Verify that the system design temperatures are correct as these will determine the expansion coefficient of the fluid.
Maximal static height difference: Difference between expansion vessel (usually in lowest point of installation) and highest point of the heating installation in meter water column.
Safety valve height difference: Static height difference between the expansion vessel and the pressure safety valve of the installation.
