Heat exchangers (HX)
- Finn Hansenne
- Kristof Smits
- Dennis De Beuckeleer
Heat exchangers are common parts of hydronic systems. They transfer heat from one system to the other whilst hydraulically separating them.
Heat exchangers for heating
1) Primary temperature regime
In order to propagate the correct volume flow rates from the secondary side to the primary side, one should correctly choose the primary temperature regime. The software will calculate the secondary capacity, temperature regime, and flow using the design system components. Afterward, using this information and the filled-in supply and return temperature, the UA value can be calculated. If the UA-value is known, only the supply temperature should be filled in, as the return temperature will be calculated using this information.
For the supply temperature, you can choose between
absolute temperature
relative temperature → this is an offset to the secondary supply emperature
Common errors
Error message | Solution |
|---|---|
Override supply temperature and secondary supply temperature are too close together | The supply temperature before and after the heat exchanger can only become equal for an infinitely large HX. Make sure the primary supply temperature is higher than the secondary supply temperature. |
Override return temperature and secondary return temperature are too close together | The return temperature before and after the heat exchanger can only become equal for an infinitely large HX. Make sure the primary return temperature is higher than the secondary return temperature. |
Expected the override supply temperature to be higher than the secondary supply temperature | In order to transfer heat in the right direction. the primary supply temperature should always be higher than the secondary supply temperature. |
Expected the override return temperature to be higher than the secondary return temperature | In order to transfer heat in the right direction. the primary return temperature should always be higher than the secondary return temperature. |
2) Design of Heat exchanger (UA-value)
The UA value is a number which describes the heat exchanger effectiveness, dependent on the heat exchanger material type, size and used temperatures. It is the product of the heat exchanging area and the thermal transmittance. The thermal transmittance [W/m²K] or simply the U-value is the exchanged heat divided by the temperature difference over the heat exchanger per m².
There are some different possibilities in order to set this value correctly:
Note that if the UA-value is filled in, it should be locked, otherwise Hysopt will calculate an optimal UA-value.
UA- value | Â | Â |
|---|---|---|
Constant | Â | If a constant UA value is known. It can be filled in here. |
Look-up table | Â | If UA values are known for different flow rates. It can be filled in manually through a custom csv table |
Parameters | Â | Hysopt can calculate the desired UA value for different primary and secondary design temperatures and flow rates. |
Device | Â | Manufacturer specific heat exchanger that is included in Hysopt software |
Expression | Â | A mathematical expression for the calculation of the UA value can be entered here. |
3) Heat exchanger flow type
In order to calculate the amount of heat transfer realised in the heat exchangers, it is important to characterize the flow type. In Hysopt the following flow types can be chosen:
Heat exchanger flow type | Â | Â |
|---|---|---|
Counter flow |  Â | Most commonly used type |
Parallel flow | Not often used due to low heat exchanger effectiveness | |
Cross flow | Â | Often used for compactness. Heat exchanger effectiveness is somewhere in between counter and parallel flow. Not easy to visualize with a graph because the heat exchange is 2 dimensional in nature. |
Using the UA value and the heat flow type, the exchanged heat can be calculated for different boundary conditions.
KV value
In order to characterize the pressure resistance of the heat exchanger primary and secondary side, a KV value for both the primary and the secondary side should be filled in. This value can be found from HX technical documentation.
Heat exchanger for heating and cooling
The principle for this base circuit is comparable to the heat exchanger for heating. The only difference is the primary connection pipe type. For these kind of base circuits, Hysopt expects a cooling system connected at the primary side.