| Overview
The DAB Modulating AHU with DCWB is a profile that is used to moderate the waterside of the AHU, this is a part of the DAB Fully Modulating AHU profile.
Dynamic airflow balancing optimizes the air side distribution and also the fan power required for conditioning the space. However, it does not control the cooling output signal to match optimal parameters of 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.
Since the solution would only be used with a fully modulating AHU(and primarily for scenarios where we DAB is used), it's been extended to fully modulating AHU profile for DAB to integrate the water control as an option.
| How it works
Once you enable the DCWB from the DAB fully modulating AHU profile, 4 Analog outputs and two relays are available for the user to configure based on site requirements.
Unlike the fully modulating AHU profile, in DCWB enabled the Analog out 1 is defaulted to Chilled Water Valve (CHW Valve) which is modulated based on the loop output, and the Analog out 4 is defaulted to the Cooling loop, optionally it can be mapped to a CO2 Loop.
Also as a part of the Dynamic Chilled Water Balance Control, the profile needs the user to connect a BTU meter to the CM board of the CCU which reports CHW In/Out Temperatures and is used to calculate ΔT across the coil which is also used in the algorithm. In addition, it also reports CHW flow across the line and calculated energy usage across the coils.
| Wiring
| Configuration
Below is the configuration screen for the Below is the configuration screen for the DAB Fully Modulating AHU with DCWB, the relays are enabled or disabled based on the preferences.
Note: Only one of the above options can be enabled at a time.
- Set up the required configuration.
- Click Set to confirm configuration.
The Modbus - connects to the BTU meter to read following inputs .
- Modbus BTU Meter Input parameters
- Flow rate
- Inlet water temperature
- Outlet water temperature
- Current Power
- Cumulative Energy
- Cumulative Volume
| Operation
The DCWB profile uses 2 algorithms in its operation to deliver efficiency in chilled water usage for the 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
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:
If | Then |
inletWaterTemperature + chilledWaterTargetDeltaT < adaptiveComfortThreshold (70F) |
The system uses PI to calculate chilledWaterDeltaTValveLoop to maintain delta T, which is (T1-T2 ). Example: When, inletWaterTemperature = 44 F
59F which is the (inletWaterTemperature + chilledWaterTargetDeltaT) < 70 which is the (adaptiveComfortThreshold)
Then the Valve modulates as per PI (chilledWaterTargetDeltaT , actualDeltaT) as load changes in the spaces. Where, actualDeltaT = OutletWaterTemp - InletWaterTemp. |
inletWaterTemperature + chilledWaterTargetDeltaT > adaptiveComfortThreshold (70F) |
The chilledWaterDeltaTValveLoop linearly modulates the valve from current position to 100% as outletWaterTemperature rises to averageDesiredCoolingTemp. Example: Now let's say, chilledWaterDeltaTValveLoop= 60% inlet water temp= 56F Then,
71 > 70 (adaptiveComfortThreshold)
For an inlet water temp= 57F the same would be
|
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
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:
If | Then |
Chilled Water Exit Temperature < Chilled Water Target Exit Temperature |
Then the valve will start closing using PI and settle at minimum of 1% if the chilledWaterExitTemperature remains below the chilledWaterTargetExitTemperature for a period long enough to cause the integral component to decrease to zero. Example: When, averageDesiredCoolingTemp = 74F
Lets say the chilledWaterExitTemperature = 69 69<70 ( chilledWaterExitTemperature < chilledWaterTargetExitTemperature)
|
Chilled Water Exit Temperature > Chilled Water Target Exit Temperature |
The chilled water return valve opens linearly to 100% so that the temperature from 4 degrees below the desired temperature reaches the desired cooling temperature. Example: When, averageDesiredCoolingTemp = 74F
Lets say the chilledWaterExitTemperature = 71 71>70 ( chilledWaterExitTemperature > chilledWaterTargetExitTemperature) |
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:
| Override 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.
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