VVT-C Modulating AHU with DCWB

| Overview

The VVT-C Modulating AHU with DCWB is a profile that is used to moderate the waterside of the AHU, this is a part of the VVT-C 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 VVT-C profile for an AHU. DCWB along with VVT-C provides more energy saving than VVT-C alone. It controls the chilled water usage in the AHU based on multiple approaches to reduce the overall energy consumption (Electrical + Cooling).

The issues related to energy from AHU and downstream, which is affected by the chilled water supply and fan use are resolved.

For the Fan use: It reduces the static pressure with VVT-C which causes lesser fan energy.

On the water side: using a higher exit temperature causes least amount of latent heat rejection leading to lower energy use (indirectly cause ΔT).

When considering the entire plant, having higher CHWS leads to chiller efficiency and having higher 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 VVT-C is used), it's been extended to fully modulating AHU profile for VVT-C to integrate the water control as an option.

| How it works

Once you enable the DCWB from the VVT-C fully modulating AHU profile, from all the relays, analog inputs and output that are available as a part of the CM board of the CCU. Along with the three analog outputs, a relay, and an auxiliary relay that was made available as a part of the Fully modulating AHU and Additional Analog out 4 is also made available in the profile screen to enable the conditioning required based on the preferences.

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 let 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. 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 VVT-C 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 approaches 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 to work in two different approaches one based on the Inlet Water Temperature (Adaptive Delta T), 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 decreases based on the scaling percentage.  

Adaptive delta T:

Adaptive delta T is a method in which the chilled water inlet temperature is used for calculating the CHW Delta T Valve loop which is PI based loop.

The Delta T Valve loop generated behaves in two different ways:

Approach 1:

If the (inletWaterTemperature + chilledWaterTargetDeltaT) < adaptiveComfortThreshold (70F)

The system uses PI to calculate chilledWaterDeltaTValveLoop to maintain delta T, which is (T1-T2 ).

Below is an example calculation and Illustration for the same.

Where: 

• InletWaterTemperature = CHWS Temperature
• ChilledWaterTargetDeltaT (Configurable parameter) =15F usually can be increased or decreased based on the CHWS temperature)

AdaptiveComfortThreshold =

Cooling Desired temperature- adaptiveComfortThresholdMargin= 74F-4F=70F

Illustration:

Approach 2:

If InletWaterTemperature  + TargetChilledWaterDeltaT) > AdaptiveComfortThreshold:

The system responds by increasing chilledWaterDeltaTValveLoop linearly from current position to 100%, as outlet water temperature rises to average desired cooling temperature using the Proportional and integral parts of the error, taking the last valve position into consideration.

Illustration:

Maximized Exit Water Temperature:

Maximized Exit Water Temperature is a method in which the chilled water exit temperature is used for calculating the CHW Delta T Valve loop which is PI based loop.

Based on the value of Chilled water Exit temperature the system controls the CHW Delta T Valve loop in two different approaches.

Approach 1:

If the 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. 

Where: 

• Chilled Water Exit Temperature = CHWR Temperature

• AverageDesiredCoolingTemp = 74F 

• ChilledWaterExitTemperatureMargin  = 4 F 

ChilledWaterTargetExitTemperature  =

AverageDesiredCoolingTemp - ChilledWaterExitTemperatureMargin  = 74 - 4 = 70 

Illustration:

Approach 2:

If the Chilled Water Exit Temperature > Chilled Water Target Exit Temperature

The chilled water return valve opens linearly to 100% so that the temperature from 4 degree below the desired temperature reaches desired cooling temperature.