Header configuration
The ‘Header configuration’ templates consist of various configurations of headers with flow rate and/or temperature controls.
Header configuration - Low loss header
The low loss header is a frequently used hydronic component and is applied to ensure a minimum boiler flow by providing a constant primary flow or to diminish the pressure drop on the primary side of the low loss header in order to reduce the hydraulic interactivity between the downstream control valves.
However, in this template, the return temperature will be significantly higher on the primary side of the low loss header compared to the secondary side because the primary flow rate is constant and the secondary flow rate is variable. The high return temperature can result in decreased production efficiency.
To avoid high return temperatures because of a low loss header, advanced controls can be used to lower the return temperature. Further information on these advanced controls can be found in https://hysopt.atlassian.net/wiki/spaces/HRM/pages/3089370633.
Notice that the pump on the secondary side of the low loss header can perfectly regulate the flow rate to its absolute minimum. The design flow rate is 42 m³/h and the flow rate is mostly regulated between 2 and 5 m³/h. Not all pumps can regulate so deep. The user should check the specifics of a pump because not all pumps can regulate so deep. To ensure a minimum flow rate for the pump, the templates below can give inspiration.
Header configuration - Collector bypass
The collector bypass ensures a constant flow rate through the boiler. When the flow rate of every branch decreases, the remaining flow will go through the collector bypass. Increased flow through the collector bypass results in an increased return temperature. The high return temperature can result in decreased production efficiency.
To avoid high return temperatures, other configurations are possible and a few are explained in the templates below.
Header configuration - Full load overflow
The full load overflow is dimensioned with a certain flow rate, which guarantees a minimum flow rate through the boiler or pump at full load. In partial load, however (when valves in branches are modulating), the flow rate through the bypass will increase, resulting in a higher flow rate than the design flow rate of the valve. A higher bypass flow rate results in a higher return temperature, which can also result in decreased production efficiency.
In the example below the design flow rate through the bypass valve is 5 m³/h, although it mostly varies between 6.3 and 5.8 m³/h.
Header configuration - Pressure independent full load overflow
The operation is the same as with the full load overflow, but now the pressure independent control valve will ensure a constant flow rate through the bypass even in partial load. This results in a lower return temperature than previous templates, but this can still be improved.
Header configuration - Zero load overflow
The zero load overflow doesn’t allow any flow to go through if the minimal flow is already met. This means in full load and even in partial load, the valve will be closed. From the moment the flow through the boiler and pump is lower than its minimum required flow rate, the valve will open to ensure the minimum required flow rate. This overflow configuration has the lowest return temperature.
Header configuration - Maximum pressure overflow - Pump constant speed
The maximum pressure overflow should be used with constant speed pumps and not with constant head pumps. More information on constant speed and constant head pumps can be found in https://hysopt.atlassian.net/wiki/spaces/HRM/pages/3089205023and https://hysopt.atlassian.net/wiki/spaces/HRM/pages/3089370615.
The maximum pressure overflow will open when the preset pressure is met on the supply pipe of the valve. If the pressure is lower than the preset value, the valve will be closed. When using a constant speed, the pressure will build up in partial load when the flow is reduced by all the valves. This is visualised in the pump curve below.
If for instance, the minimal volume flow rate is 20 m³/h, the preset pressure drop of the maximum pressure overflow should be equal to 7.5 meter pump head (or almost 75 kPa). This is only valid if there is almost no pressure drop between the pump and the maximum pressure overflow valve. If not, the other components should be included in the 75 kPa, which makes the preset value lower.
The moment the volume flow rate through the pump is lower than 20 m³/h, the valve will sense a pressure drop higher than its preset value, which will open the valve. Because of this, the minimal flow rate is guaranteed. An example is given for a flow rate of 5 m³/h.
The return temperature is in this case pretty low, but the pump energy cost is mostly higher because of the higher pressure drop. Another issue is the difficulty to set the correct preset value. If the pump curve is analysed, the pressure drop required to get 5 m³/h is hard to set as a preset value, because it’s almost the same as the maximal pressure the pump can give at 0 m³/h. So if a small mistake is made, it’s possible to have a valve which will never open, or is set to 20 m³/h instead of 5 m³/h which will increase the return temperature.
Header configuration - Maximum pressure overflow - Pump constant head
In this template, a maximum pressure overflow is combined with a constant head pump, which is a common mistake. More information on how a maximum pressure overflow should work is explained in the previous template “Header configuration - Maximum pressure overflow - Pump constant speed“.
Because of the constant head, the preset value of the valve is either too high or too low, meaning the valve will always be completely open or completely closed. A fully open valve gives the same results as previously mentioned template “Header configuration - Collector bypass“, and a fully closed valve doesn’t guarantee a minimum flow rate.