CET chilled water cooling coil, thermal power control, supply air temperature control (CetCclChw12)

This AF has its own PID controller(s) for cascaded supply air temperature control and quicker response. A supply air temperature sensor is required.

The figure below shows BACnet objects associated with this application function. Primary signal flow is summarized as follows:

Basic function: Accept request signal from associated room controller and map it to appropriate control setpoint; Compare setpoint to present value of supply air temperature sensor; Pass result through a device mode logic switch prior to outputting as a command to the object that controls the device.

Interlocks

Room automation interlocks are internal signals that coordinate interaction between HVAC devices. They are not visible in ABT Site or web interface, but parameters associated with them are. See comment column for hints on parameters that affect interlock functionality.

Chilled water valve modulation: In response to a cooling request (or a dehumidification request, if configured) the cooling valve is modulated to control the room temperature at setpoint or to dehumidify the air. In response to external triggers for special modes such as warm-up, cool down, or safety conditions, the cooling valve may be set to off or fully open.

Request signal(s) from a room temperature PID controller and/or a dehumidification PID controller are received from the associated "room" AF. If both a request for cooling and a request for dehumidification are received at the same time, then the larger of the two signals will be used.

Cascaded supply air temperature control: The cooling demand signal (CclCReq) from room temperature PID is mapped to a setpoint value for supply air temperature (CclSpTSuC), and used as input to the cascaded supply air temperature PID located in this AF. The supply air PID compares the setpoint value to the present value of the supply air temperature sensor, and any difference is used to increase or decrease the analog output signal to the command object.

If the supply air temperature sensor is not valid, temperature control is performed by the room temperature PID.

Dehumidification control: If dehumidification is used, the cooling coil dehumidification request signal (CclDhuReq) is converted into the analog output signal (CclVlvPos) without passing through the cascaded supply air temperature PID in this AF (this means that dehumidification is non-cascade control).

Device mode: The input signal for device mode is a multistate value.

• Off
• Control mode (modulation)
• Fully open

Cooling available status: When the cooling coil is available for cooling, the binary output signal for "Cooling coil available for cooling" (CclAvlC) will be "Yes" (available). CclAvlC is used by the system to regulate cooling resources. For example, if there is a call for cooling but the cooling coil is not available because it is already at maximum capacity (because device mode is fully open), then CclAvlC will signal "No" and the next available cooling resource in the temperature control sequence will be activated.

For cooling available status to be "Yes", device mode must equal "Control mode" (modulation).

Dehumidification available status: When the cooling coil is available for dehumidification, the binary output signal for "Cooling coil available for dehumidification" (CclAvlDhu) will be "Yes" (available). For the status to be "Yes", device mode must equal "Control mode" (modulation).

Supply chain interface, ChwDmd: The output signal for chilled water demand is a multistate value.

• Off (no demand signal)
• Cooling demand (demand signal is active)
• Free cooling (cooling device is in free cooling mode)

Free cooling can be initiated by the central plant when surplus cooling capacity is available. Under certain conditions, the room can take advantage of surplus cooling capacity in the absence of an explicit cooling demand; the room temperature must be below the setpoint associated with the current room climate mode (Comfort, Pre-Comfort etc.). This type of free cooling is different from "economizer" free cooling, which uses an outside air damper.

The following figure(s) illustrates control modulation and related functions.

Interface

Engineering and commissioning

Parameter EnMonAirFlSta (Enable monitoring for air volume flow state) must be set to 1: Yes. This is the default setting for CclChw12 (unlike CclChw11 where this setting is optional). It is important to understand that this setting is NOT optional for CclChw12.