Direct expansion evaporator cooling coil 12, two binary outputs (CclDx12)

This application function operates a 2-stage direct expansion cooling coil device.

The device stages are calculated based on request signals (from the room controllers) received for cooling and dehumidification.

There is an optional binary input for an evaporator fault.

This application function is NOT available for:

DXR2x09

Required elements and how to configure them:

Element	I/O	Signal type
CclDx1StPos "Cooling coil direct expansion evaporator position first stage" CclDx2StPos "Cooling coil direct expansion evaporator position second stage"	On-board output, Cooling coil valve position	DX normally open

Function

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

The demand signal(s) are received (CclDhuReq / CclCReq) and processed into binary output signals for DX coil stage position (CclDx1StPos / CclDx1StPos).

Basic function: Accept request signal from associated room controller and pass it through a device mode logic switch; Output the result as a command to the BACnet object that controls the device.

	Command or request (or related)
	Notification of condition or status, or availability
	Device mode
	Interlock (internal signal, not a BACnet object; see Interlocks section for additional information)

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.

Signal	Type	Direction	Description	Comment
AirFlSta	Boolean	In	Air flow status ▶ Interlock signal (from fan) that keeps the coil locked at equipment protection priority (PrPrio = 5) unless the fan is running.	No related parameters in DX coils. AirFlSta is hard-coded.
AirFlCReq	Boolean	Out	Air flow cooling request ▶ Initiates air flow (fan turns on) to satisfy air flow support request for cooling.	See Configuration section for parameters named "...AirFlCReq".
AirFlDhuReq	Boolean	Out	Air flow dehumidification request ▶ Initiates air flow (fan turns on) to satisfy air flow support request for dehumidification.	See Configuration section for parameters named "...AirFlDhuReq".
AirFlHldC	Boolean	Out	Air flow hold for cooling ▶ Interlock signal to fan requesting equipment protection air flow support for a period of time, such as when the DX coil has just turned off.	See Configuration section for parameters named "...AirFlHldC".
AirManOff	Boolean	In	Air manual off ▶ Interlock signal to turn off DX cooling if air source (fan or damper) is manually turned off. If AirManOff is True, CclDevMod will be set to 1:Off at priority 4 (protection mode 1).

Sequence

The 0-100% demand signal (cooling or dehumidification) is converted to binary output via two hysteresis switches (stage 1: 33% on, 17% off; stage 2: 83% on, 67% off). A single stage of operation has a minimum on / minimum off time of 3 minutes (3 minutes on, 3 minutes off; see TiOnMin and TiOffMin in the Configuration section).

Alarm input: Several DX safety switches can be connected to shut off the equipment, including:

Low suction pressure
High compressor pressure
High refrigerant pressure
Low refrigerant pressure

An optional binary input object picks up any DX equipment alarm switch. When the switch indicates a fault, the component sets device mode off at equipment safety priority. The "Available" signals cooling, dehumidification) also turn off. When the switch indicates no fault, the command to the device mode is disabled.

Equipment output limits: For units that use outside air, some specifications call for logic that turns off the DX below a threshold temperature. This allows the use of free cooling (cold outside air). This AF will set device mode to off at equipment safety priority when the OA temperature is below a configured cut out temperature. This also drives the "Available" signal; if device mode is off, the signal is "No".

This feature (low outside air temperature cut out) is enabled by default. The Boolean parameter that enables it is EnLockDxTOaLo (Enable lockout DX evap.at low outs.temp). See Configuration for additional information.

Note
This lock out feature requires a TOa (temperature outside air) value to be sent from a field panel. If EnLockDxTOaLo is set to Yes and a TOa sensor is not present (or signal is not valid), device mode (CclDevMod) is set to Off at equipment protection priority, shutting off the DX coil.

Minimum on/off time: When the min on or off timers are active, the staged DX cooling coil will be placed at equipment protection priority (PrPrio=5). See TiOnMin and TiOffMin in the Configuration section.

Dehumidification control: If dehumidification is used, the cooling coil dehumidification request signal (CclDhuReq) is converted into binary output signal for DX coil position.

Note
This AF does not perform loop control; its main purpose is to run commands. Loop control processes (for this AF) are managed in the "room" AF(s) that collaborate with this AF to command the end device.

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

CclDevMod supports the following states:

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).

DX Cooling compressor power: The nominal electric power usage of the compressor is provided for energy monitoring and coordination with other devices. When the DX coil is on, the compressor electric power (CclElPwr) is equal to the cooling power of the cooling stages that are active. CclElPwr equals zero when the DX coil is off.

Configuration

Parameters

Description	Parameter	Default value
Enable fault input ▶ Enable fault input, when a cooling coil DX fault detector (binary input) is present. When cooling coil DX detector is closed, the cooling coil device mode will turn Off at priority (4:Protection 1). When cooling coil DX detector is open, the DX cooling coil device mode will be released to 15: Automatic mode 6). 0:No 1:Yes	EnFltIn	0:No
Nominal electric power ▶ The nominal electric power (kW) for the DX cooling coil. The total power for the DX cooling coil can be found on manufacturer’s data sheet.	NomElPwr	0.4 [kW]
Switch-on/-off times
Min.switch-on time ▶ Minimum-on time that the cooling coil DX contact must stay on to protect equipment against short cycling. When the minimum-on time delay is active, the Cooling coil DX position will be set to priority level (5: Protection mode 2).	TiOnMin	3 [min]
Min.switch-off time ▶ Minimum-off time that the cooling coil DX contact must stay off to protect equipment against short cycling. When the minimum-off time delay is active, the Cooling coil DX position will be set to priority level (5: Protection mode 2).	TiOffMin	3 [min]
Low outside air temperature lockout
Enable lockout DX evaporator at low outside air temperature ▶ Enables the lock out feature that keeps the DX from activating if outside temperature is low enough to cause freezing on the coil. 0:No 1:Yes	EnLockDxTOaLo	1:Yes
Lockout DX evaporator at low outside air temperature ▶ When the outside air temperature is below the Lock out DX evaporator low limit (i.e., several degrees above freezing), the DX cooling device mode turns Off at priority 4 (protection mode 1) and the DX cooling coil position turns Off at priority 5 (protection mode 2).	LockDxTOaLo	4 [°C] 39.2 [°F]
Outside air temperature hysteresis for lockout DX evaporator ▶ When the outside air temperature is above the DX Lock out evaporator low limit plus the hysteresis, the DX cooling coil will be available for control.	HysTOaLockDx	1 [K] 1.8 [°F]
Interlocks
Switch-on point for air volume flow hold cooling ▶ When the cooling coil position (in percent) is greater than the switch-on point for air volume flow hold cooling, the airflow volume hold cooling interlock to the fan goes to True.	SwiOnAirFlHldC	33 [%]
Hysteresis for air volume flow hold cooling ▶ When the cooling coil position (in percent) is less than the hysteresis value, the air volume flow hold cooling interlock goes False.	HysAirFlHldC	16 [%]
Switch-off delay for hold signal for air volume flow cooling ▶ The time after the switch-off point for air volume flow hold cooling is True, then the air volume flow hold cooling goes from True to False.	DlyOffAflHldC	0 [s]
Switch-on point for air volume flow cooling request ▶ When the cooling coil cooling request (in percent) is above this value, the air volume flow cooling request goes to True.	SwiOnAirFlCReq	33 [%]
Hysteresis for air volume flow cooling request ▶ When the cooling coil cooling request (in percent) is below this value, the air volume flow cooling request goes to False.	HysAirFlCReq	16 [%]
Switch-on delay for air volume flow cooling request ▶ The time after the switch-on point for air volume flow cooling request is True that air volume flow cooling request goes from False to True.	DlyOnAirFlCReq	0 [s]
Switch-on point for air volume flow dehumidification request ▶ When the cooling coil dehumidification request (in percent) is above this value, the air volume flow dehumidification request goes to True.	SwiOnAflDhuReq	33 [%]
Hysteresis for air volume flow dehumidification request ▶ When the cooling coil dehumidification request (in percent) is below this value, the air volume flow dehumidification request goes to False.	HysAirFlDhuReq	16 [%]
Switch-on delay for air volume flow dehumidification request ▶ The time after the switch-on point for air volume flow dehumidification request is True that air volume flow dehumidification request goes from False to True.	DlyOnAflDhuReq	0 [s]
Switch-off delay for air volume flow dehumidification request ▶ The time after the switch-off point for air volume flow dehumidification request is True that air volume flow dehumidification request goes from True to False.	DlyOfAflDhuReq	0 [s]
Internal settings – do not change
Temperature unit ▶ Defined by selected engineering unit – do not change.	TUnit	[°C] [°F]
Temperature difference unit ▶ Defined by selected engineering unit – do not change.	TDiffUnit	[K] [°F]

Interface

Interface	Description	Type	Ref.	Owned by
CclDx1StPos	Cooling coil direct expansion evaporator position first stage 0:Off 1:On	BO	◉	Room segment / Device
CclDx2StPos	Cooling coil direct expansion evaporator position second stage 0:Off 1:On	BO	◉	Room segment / Device
CclDxFlt	Cooling coil direct expansion evaporator fault 0:Normal 1:Fault	BI	◉	Room segment / Device
CclDevMod	Cooling coil device mode 1:Off 2:Control mode 3:Fully open	MPrcVal	●	-
CclCReq	Cooling coil cooling request	ACalcVal	●	-
CclDhuReq	Cooling coil dehumidification request	ACalcVal	●	-
CclAvlC	Cooling coil available for cooling 0:No 1:Yes	BCalcVal	●	-
CclAvlDhu	Cooling coil available for dehumidification 0:No 1:Yes	BCalcVal	●	-
CclElPwr	Cooling coil electric power	ACalcVal	●	-

Engineering and commissioning

Boolean parameter EnLockDxTOaLo (Enable lockout DX evap.at low outs.temp.) is set to Yes by default. This lock out feature requires a TOa (temperature outside air) value to be sent from a field panel. If EnLockDxTOaLo is set to Yes and a TOa sensor is not present (or signal is not valid), device mode (CclDevMod) is set to Off at equipment protection priority, shutting off the DX coil. To disable this feature, either for testing purposes during startup or for system design needs, set EnLockDxTOaLo to No.

The AF that operates the DX coil(s) works with the room cooling feedback controller, and is compatible with the feedback configured for PID or staged response. In either case, the timing characteristics needed to prevent frequent switching are implemented by tuning the feedback controller. Additionally you can adjust min on / min off times (TiOnMin and TiOffMin) and delay parameters in the Configuration section. Set parameters to prevent short cycles that might damage the equipment even if occurring only occasionally. But note that DX coil timing is closely related to the level of temperature error that the controller allows before turning the coil on and off.

Staged vs. PID control
Staged control operates with relatively consistent, selectable switch on and switch off points. PID operates with relatively consistent temperature averaged over time.

Troubleshooting tip: Why is the DX turned off at equipment safety priority... Probably because the fan is off. Check that first.