blockValveCondenserEvaporator

Information

This block implements the control logic for the chiller isolation valves, the HRC isolation and switchover valves, the CHW and HW minimum flow bypass valves, the HRC evaporator CW mixing valve, and the CW chiller bypass valve. It also computes the lead pump Enable signal for the CHW, HW, CWC and CWE pump groups.

Chiller evaporator isolation valve

When a chiller is enabled, the valve position is controlled as follows.

• If no HRC is concurrently operating and connected to the CHW loop, the valve is commanded to a fully open position,
• If any HRC is concurrently operating in cascading cooling mode, but no HRC is in direct heat recovery mode, the valve is commanded to a fixed position ensuring flow balancing proportionally to design flow.
• If any HRC is concurrently operating in direct heat recovery mode, the valve is modulated with a control loop tracking an evaporator flow setpoint which is reset as described hereunder. The loop output is mapped to a valve position of 10 % (resp. 100 %) at 0 % (resp. 100 %) output signal. The loop is biased to launch from 100 %.

Otherwise, the valve is commanded to a closed position.

Chiller evaporator flow setpoint

The setpoint is computed based on the logic implemented in Buildings.DHC.Plants.Combined.Controls.BaseClasses.DirectHeatRecovery.

Chiller condenser isolation valve

When a chiller is enabled, the condenser isolation valve is modulated with a control loop tracking a condenser flow setpoint which is reset as described hereunder. The loop output is mapped to a valve position of 10 % (resp. 100 %) at 0 % (resp. 100 %) output signal. The loop is biased to launch from 100 %.

Otherwise, the valve is commanded to a closed position.

Chiller condenser flow setpoint

The condenser flow setpoint varies based on the condenser loop mode and on the tank cycle index.

• When the condenser loop mode is Charge Assist, a control loop maintains the condenser loop return temperature at a target setpoint equal to the highest temperature setpoint of the active tank cycle. The loop output is mapped to a flow setpoint of 10 % (resp. 100 %) of design flow at 0 % (resp. 100 %) output signal. The loop is biased to launch from 20 %.
• When the condenser loop mode is Tank Charge/Discharge, a control loop maintains the chiller condenser leaving temperature at target setpoint equal to the highest temperature setpoint of the active tank cycle. The loop output is mapped to a flow setpoint of 5 % (resp. 100 %) of design flow at 0 % (resp. 100 %) output signal. The loop is biased to launch from 50 %.
• When the condenser loop mode is Heat Rejection, the condenser flow setpoint is set at design value.

HRC evaporator isolation valve

When a HRC is enabled, the valve position is controlled as follows.

• If the HRC is operating in cascading cooling mode, the valve is commanded to a fixed position ensuring flow balancing proportionally to design flow.
• If the HRC is operating either in cascading heating mode or in direct heat recovery mode, the valve is modulated with a control loop tracking an evaporator flow setpoint which is reset as described hereunder. The loop output is mapped to a valve position of 10 % (resp. 100 %) at 0 % (resp. 100 %) output signal. The loop is biased to launch from 100 %.

Otherwise, the valve is commanded to a closed position.

HRC evaporator flow setpoint

In direct heat recovery mode, the setpoint is reset based on the logic implemented in Buildings.DHC.Plants.Combined.Controls.BaseClasses.DirectHeatRecovery.

In cascading heating mode, the setpoint is reset with a control loop that maintains the evaporator leaving temperature at target setpoint equal to the lowest temperature setpoint of the active tank cycle. The loop output is mapped as follows. From 0 % to 50 %, the HRC evaporator CW mixing valve commanded position is reset from 0 % (full bypass flow) to 100 % (no bypass flow). From 50 % to 100 %, the evaporator flow setpoint is reset from minimum to design value. The loop is biased to launch from 75 %. When disabled, the loop output is set to 75 % to ensure that the HRC evaporator CW mixing valve is fully open (no bypass flow).

HRC condenser isolation valve

When a HRC is enabled, the valve position is controlled as follows.

• If the HRC condenser is indexed to the HW loop (cascading heating or direct heat recovery mode), the valve is commanded to a fully open position.
• If the HRC condenser is indexed to the CW loop (cascading cooling mode), the valve is modulated with a control loop tracking a condenser flow setpoint which is reset based on the same logic as for the chiller condenser flow setpoint (see above). The loop output is mapped to a valve position of 10 % (resp. 100 %) at 0 % (resp. 100 %) output signal. The loop is biased to launch from 100 %.

Otherwise, the valve is commanded to a closed position.

HRC condenser and evaporator switchover valve

Each valve is commanded to a fully open or fully closed position depending on the valve index and the current operating mode of the HRC (cascading cooling, cascading heating or direct heat recovery). In addition, the condenser switchover valve indexed to the HRC which is nearest to the interconnection with the condenser loop (highest index) and which is operating in direct heat recovery mode is modulated with a control loop tracking the condenser entering temperature. The condenser entering temperature setpoint is reset based on the logic implemented in Buildings.DHC.Plants.Combined.Controls.BaseClasses.DirectHeatRecovery. This allows false loading the HRC that is controlled to meet the HW supply temperature setpoint in direct heat recovery mode, and thus meeting the CHW supply temperature setpoint simultaneously.

HRC evaporator CW mixing valve

The valve is modulated based on two control loops: the HRC evaporator leaving temperature control loop (see the section HRC evaporator flow setpoint) and another control loop that maintains the HRC evaporator entering water temperature below the highest tank temperature setpoint. This latter control loop is enabled when any HRC is operating in cascading heating mode. When the loop is enabled, the loop output is mapped to a valve position of 100 % (resp. 0 %) at 0 % (resp. 100 %) output signal. When the loop is disabled, the loop output is set to 100 % (no bypass flow). The valve control signal is the minimum (maximum bypass flow) of the resulting signals of those two control loops.

CHW and HW minimum flow bypass valve

Each chiller and HRC has its own CHW (resp. HW) minimum flow control loop. The loop is enabled whenever the unit's evaporator (resp. condenser) is indexed to the CHW (resp. HW) loop and its evaporator (resp. condenser) isolation valve is commanded open (with a threshold of 10 %). When enabled, each loop tracks a flow setpoint equal to 1.1 times the minimum CHW (resp. HW) flow rate. When disabled, each loop output is set to 0 %. The valve control signal is the maximum (maximum bypass flow) of the resulting signals of all control loops.

CW chiller bypass valve

The valve control is enabled when the plant is enabled either in cooling or heating mode, the Charge Assist mode is active and all chiller condenser isolation valves are closed (based on their commanded position).

When the valve control is enabled the valve position is modulated by the same control loop used to maintain the condenser loop return temperature at a target setpoint equal to the highest temperature setpoint of the active tank cycle (see the section "Chiller condenser flow setpoint").

Otherwise, the valve is commanded to a closed position.

CHW, HW, CWC, CWE lead pump

The lead pump of each loop is enabled whenever any chiller or HRC is indexed to the loop and the corresponding evaporator or condenser isolation valve is commanded open (with a threshold of 10 %). In addition, the CWC lead pump may also be enabled if the CW chiller bypass valve is commanded open.

Parameters

Connectors

Components

Revisions

• February 24, 2023, by Antoine Gautier:
  First implementation.