CCU DAB Fully Modulating AHU With Dynamic Chilled Water Balancing- Sequence of Operation

| Description of Equipment

The Dynamic Airflow Balancing (DAB) Modulating AHU with Dynamic Chilled Water Balancing (DCWB) is a system-level profile controlled by the CCU. It is used to moderate air and the waterside of the AHU.

Dynamic airflow balancing optimizes the air side distribution and the fan power required for conditioning the space. However, it does not control the cooling output signal to match the optimal parameters of the cooling coil, leading to suboptimal chilled water usage.

DCWB extends the fully modulating DAB profile for an AHU. DCWB along with DAB provides more energy saving than DAB alone. It controls the chilled water usage in the AHU based on multiple approaches to reduce the overall energy consumption (Electrical + Cooling).

DAB Fully Modulating AHU with DCWB optimizes energy consumption of fan and chilled water by lowering static pressure of fan through DAB algorithm and by having optimal exit water temperature through DCWB algorithm.

When considering the entire plant, having optimal CHWS leads to chiller efficiency and having optimal ΔT leads to lower hydronic pump energy use.

| Schematics

| Control System Object List

When the system profile is enabled only for a Dynamic Airflow Balancing  (DAB) based control, the following control system objects are applicable. 

When the system profile is enabled for a Dynamic Chilled Water Balancing  (DCWB) based control, additional control system objects are included to contribute to the additional algorithm applicable. Based on the algorithm opted for in the configuration the control system objects vary as shown below.

Additional configuration parameters to define the Analog Input & Output ranges for different control triggers are also available for configuration as below.

| Sequence of Operation

The sequence of operation is completely driven based on the Occupancy Mode, Zone load (Weighted Average), set Cooling and Heating Desired Temperatures, Return Water temperature, and Space CO2 levels.

For a sequence of operations related to the DAB part of the profile, based on the occupancy modes refer to CCU DAB System Profile - Sequence of Operation

| Sequence of Operation for DCWB Parts

When the system profile is enabled for a Dynamic Chilled Water Balancing  (DCWB) based control. During all the occupancy modes when the zones demand cooling, the 

The DCWB profile uses 2 algorithms in its operation to deliver efficiency in chilled water usage for HVAC needs.

With the DCWB feature enabled, the profile screen provides two more options to select from, to make the system work in two different approaches one based on the Inlet Water Temperature (Adaptive Delta T), and the other based on the Chilled water exit temperature (Maximized Exit Water Temperature).

As shown in the wiring diagram above, a BTU meter which is connected to the Chilled water Inlet Temperature and Chilled water Exit Temperature sensors constantly records the chilled water inlet and exit temperatures and communicates it to the system as input. 

The algorithm uses these temperatures and the predefined Adaptive Delta T, configurable parameter in the configuration screen to calculate a CHW Delta T Valve loop which is a percentage value, and the valve position is scaled to the valve loop, to increase or decrease based on the scaling percentage. 

| Adaptive Delta T Algorithm & Operation

The adaptive delta T algorithm uses adaptiveComfortThreshold to determine if the valve needs to open linearly to 100% or use a PI loop to calculate chilledWaterDeltaTValveLoop to maintain delta T.

The Delta T Valve loop generated operates the system in two different ways based on whether the inletWaterTemperature + chilledWaterTargetDeltaT </> adaptiveComfortThreshold (70F). The table below talks about the same:

Note: When actualDeltaT is 0, we would expect the valve to be completely closed,  but a PI Loop with setPoint 15 and control point 0,  would eventually generate 100% output.

Hence a PI Loop inversion is in place to generate the valveLoopOutput which will be 0 once
the loop is stabilized.

Illustration for InletWaterTemperature  + TargetChilledWaterDeltaT)< AdaptiveComfortThreshold:

Illustration for InletWaterTemperature  + TargetChilledWaterDeltaT) > AdaptiveComfortThreshold:

| Maximized Exit Water Temperature Algorithm & Operation

Maximized Exit Water Temperature is a method in which the chilled water exit temperature is used to calculate the CHW Delta T Valve loop.

Based on the chilledWaterTargetExitTemperature, the system controls the chilledWaterDeltaTValveLoop in two ways. The table below talks about the same:

Note:  When the chilledWaterExitTemperature falls below chilledWaterTargetExitTemperature the valve will start closing using PI and settle at a minimum of 1%, If the chilledWaterExitTemperature remains below the chilledWaterTargetExitTemperature for a period long enough to cause the integral component to decrease to zero.

Illustration Chilled Water Exit Temperature < Chilled Water Target Exit Temperature:

Illustration Chilled Water Exit Temperature > Chilled Water Target Exit Temperature:

| Overrode Control for Max GPM

Irrespective of the approach used, if at any point of time the flow reaches chilledWaterMaxFlowRate then the chilledWaterDeltaTValveLoop will stop increasing, and an alert "Chilled water flow limited by max limit" is generated.

| Control Mode Summary