...
Below is a screenshot of Hysopt’s Expansion Vessel with parameter list. The expansion vessel can be found in the heating and cooling library under “System peripherals”.
| Info |
|---|
Hysopt only provides an expansion vessel in the return pipe for reasons of good practice. Expansion tank membranes are sensitive to continuously high temperatures above 70°C (158°C), so it is recommended to place the component in the return pipe to avoid excessive temperatures. |
...
The design pre-charge pressure is chosen in such a way that evaporation, pump cavitation and underpressure are avoided at all times at any location in the system. Hysopt calculates the pre-charge pressure based on the maximal static height above vessel, plus an additional safety margin.
| Note |
|---|
The calculation of the pre-charge pressure assumes that the expansion vessel is located at the suction side of the circulation pump. If this is not the case, a manual correction by adding an additional pressure differential is necessary. The additional pressure differential equals the pump’s design differential pressure for the worst-case scenario. If well-understood, the additional pressure can also be limited to the pressure that ensures no evaporation nor cavitation anywhere in the system. It is advised to always place the expansion vessel at the suction side of the circulation pump whenever possible. |
Final pressure
The final pressure represents the pressure (gauge pressruepressure) at the expansion vessel when the complete system volume is heated to the maximal temperature.
During design, we choose the final pressure to be equal to the maximal allowable pressure at the expansion vessel location to minimise the required expansion vessel volume. However, the final pressure can also be chosen lower than the maximal allowable pressure, which will result in an oversized expansion vessel.
If the user overwrites the design volume, we will recalculate the final pressure following its definition, namely the pressure when the complete system is at the maximal temperature. This value will deviate from the maximal allowable pressure at the vessel location, which arises from the safety valve properties.
| Note |
|---|
The final pressure calculation assumes that the pump head, if a pump is located between the vessel and the safety relief valve, is negligible compared to the discharge pressure of the safety valve. If this is not the case, a manual correction of the final pressure is necessary by subtracting the maximal pump pressure from the final pressure. |
...
The acceptance factor represents the percentage of the expansion vessel that can effectively be used to take in any fluid. The acceptance factor is not 100% due to the minimal and maximal pressure limitations of the system, and is thus derived from the pre-charge and final pressure.
| Info |
|---|
Expansion automats | Constant pressure expansion vessels Expansion automats are devices that keep the pressure in the expansion vessel constant by taking away gas or water volume in a controlled manner in as a response to thermal expansion. As a result, expansion automats allow to use a greater part of the gross volume of an expansion vessel in a useful way. The acceptance factor can be set equal to the maximal acceptance factor that the membrane can withstand. If you want to calculate the required vessel volume using an expansion automat, manually enter & lock the maximal acceptance factor of your expansion vessel in the parameter “acceptance factor”. |
...
6. Example Sizing Scenarios
Example 1: Standard Sizing (No Special Conditions)
On default the Hysopt software will calculate the design volume and pressures of the expansion vessel based on ideal design conditions. These are:
...
Determination of the max/min system temperatures for expansion coefficient
On By default, the minimal system temperature is 4°C, if . If the system has a lower minimal temperature, it is advised to switch to use a glycol mixture. On
By default, the maximal system temperature is based on the supply temperature in the model + 5°C. In this example the supply temperature of the system is 70°C, therefore the software will use 75 °C as the maximum system temperature.
...
The expansion coefficient (e) is subsequently calculated with the formula below:
...
The density of water at the minimal and maximal system temperature can be determined by using the table density tables. A density table is shown as example below:
...
Density at minimal system temperature = 999.97 kg/m³
Density at maximal system temperature = 974.84 kg/m³
Expansion coefficient (e ) = 2.51%.
Determination of the maximal static height and safety valve height above the expansion vessel
The user needs to fill in these value depending on the configuration of the system. In this example, the values of these parameters are:
...
p0,gauge = Pre-charge pressure
pst = Static height pressure
pd = Vapour pressure, this factor is negligible at full liquid phase state.
Mpre = 30 kPa = Pre-charge pressure margin.
ρmin_temp = density of the water at the lowest system temperature
g = gravitational constant
Hmax,static = Maximal static height of the system above the expansion vessel
On By default, the software uses a pre-charge pressure margin of 30 kPa, if a different margin is desired, the user must . If you want to apply another pre-charge pressure margin, you can manually calculate the pre-charge pressure and lock itthe value.
Pre-charge pressure = 177.15 kPa
The final pressure in the design situation is calculated with the formula below:
...
pfinal = Final pressure
pmax,sv = Design discharge temperature
Msv = Safety valve setting margin
ρmax = Density of the water at the highest system temperature
Hsv,vessel = Static height of the safety valve above the expansion vessel
On By default, the Design discharge pressure of the safety valve is set on to 350 kPa , this value differs based on the installed safety valve, the user must overwrite this value(=3.5 bar). If this valus is different for your situation, you can adjust the value of the “Design discharge pressure” at the safety valve component. The default safety valve setting margin is 0.9. If the user you want to change this parameter, pressure margin, you need to manually calculate your desired final pressure, fill in that value and lock the final pressure needs to be manually calculated and filled in.
Determination of the Design volume of the expansion vessel
At last, the design volume of the expansion vessel is calculated with the formula below:
...
Example 2: Expansion vessel at discharge side of pump
If the expansion vessel is at the discharge side of the pump, additional pressure from the pump is must be considered in the “Pre-charge pressure” calculation. The additional pump pressure will certainly have an impact on the calculations of the “Design volume” of the expansion vessel.
Example model
...
In this example, the same system configuration is used , only except for the location of the expansion vessel is changed. This means that most of calculated parameters will not be affected. Only the pre-charge pressure, acceptance factor and design volume of the expansion vessel will be affected. Currently, the user have factor and design volume of the expansion vessel will be affected by having the vessel at a different location, more specifically at the discharge side of the pump. To take into account the pump influence correctly, you need to manually add the pressure from the pump(s) with to the calculated “Pre-charge pressure” , in order to calculate the correct design volume of the expansion vessel.
...
The pre-charge pressure can be calculated with the formula below, if the vessel stands at the discharge side oof the pump:
...
In this situation the acceptance factor ηg differs than with the standard sizing due to the change of in Pre-charge pressure, therefore increasing the required volume of the expansion vessel differs.
Example 3: Pump Between Vessel & Safety Valve
If the pump is located in between the expansion vessel and the safety relief valve, the final pressure is limited by the pump pressure. The pump pressure will certainly have an impact on the calculations of the “Design volume” of the expansion vessel.
...
In this example the same system configuration is used, only the location of the pump is changed. This means that most of calculated parameters will not be affected. Only the final pressure, acceptance factor and design volume of the expansion vessel will be affected. Currently, the user have has to manually subtract the pressure from the pump(s) with the calculated “Final pressure”, to calculate the correct design volume of the expansion vessel.
...
Example 4: Expansion automat (Constant pressure vessels)
When using an expansion automat, the required design volume of the expansion vessel can be lowered due to the higher efficiency of the expansion vessel. In this case the acceptance factor of the expansion vessel is significant significantly higher, depending on the maximal efficiency of the expansion automat. Currently, the user needs to manually fill in the acceptance factor in the Hysopt software.
...
In this situation the acceptance factor ηg depends on the maximal efficiency of the expansion automat. In this example an acceptance factor of 95% is taken into accountused.
...
Standard sizing | expansion vessel at discharge side of pump | pump in between expansion vessel and safety valve | Expansion automat | |
|---|---|---|---|---|
Design volume of expansion vessel | 0.197 m³ | 0.216 m³ | 0.208 m³ | 0.078 m³ |
...