[SplyChw21] Supply chain chilled water, data exchange with rooms (TRA integration), via grouping mechanism

SplyChw21 collects and analyzes chilled water demand data from room automation area consumers through a group mechanism for use by the cooling plant, and distributes the cooling plant data back to the area consumers.

SplyChw21 can be connected to the cooling plant either directly or via an intermediary coordinator (SplyChw23).

The most important functions in the program block are:

• Cooling demand collection and evaluation
• Chilled water temperature setpoint calculation and its correction based on valve position evaluation of the consumers
• Cooling lockout (limit) based on outside air temperature
• Central valve override and valve kick
• H/C changeover (2-pipe)
• Condensation prevention
• Free cooling logic (requires CenOpMod21)

Cooling demand collection and evaluation

The chilled water demand signal collected from area consumers is multi-state with the following enumeration:

• Off: Area is not using or requesting chilled water.
• CoolDmd: Area is using or requesting chilled water to meet cooling demand.
• FreeCool: Area is using chilled water but does not need it to meet current demand. The area may pre-cool to efficiently meet anticipated demand.

SplyChw21 evaluates the data received and calculates the two binary output pin signals [CReq] and [FreeCReq] for communication with the plant.

Chilled water temperature setpoint calculation and its correction based on valve position evaluation of the consumers

SplyChw21 calculates the chilled water setpoint for the cooling request [SpCReq] taking into account the following factors:

• Base setpoint, SpTFl
• Valve positions of the members [ChwVlvPosEvl]
• Number of active condensation monitors in the room automation areas
• Dew points in the room automation areas

SplyChw21 collects valve position values from room automation area consumers via group data exchange, and calculates a representative valve position percent value [ChwVlvPosEvl]. [ChwVlvPosEvl] is the average of the 10 highest valve positions that:

• Are ≥ 10 %
• Have no disturbances
• Are from area consumers with an active chilled water demand or free cooling signal

A linear function (configurable) uses [ChwVlvPosEvl] to calculate a chilled water supply temperature setpoint correction value between SpCorrTFlHi and SpCorrTFlLo:

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The calculated setpoint correction value is added to the base setpoint SpTFl, which is further adjusted by the condensation prevention function that prevents condensation on cooling elements in the room automation areas. See Condensation prevention below. The result is the chilled water cooling request setpoint value [SpCReq] (chart output pin). To prevent abnormally low setpoints, a 3°C lower limit is hard-coded in the chart.

The suggested setpoint value [SpCReq] can be delivered to the cooling plant or other coordinator(s) (SplyChw23). The cooling plant program, which may receive additional input from other coordinators, determines the actual setpoint value.

If a Central valve override is active, SplyChw21 stops using area valve positions to calculate the setpoint. In this case, SplyChw21 delivers the previous setpoint value (the setpoint can still change in response to the condensation alarm sequence). When the override is no longer active, the normal process for setpoint calculation resumes.

Chilled water supply temperature

SplyChw21 receives the measured chilled water supply temperature from the cooling plant, and distributes it to the areas by group data exchange with priority 15.

Cooling lockout (limit) based on outside air temperature

Chilled water use in consumers is blocked if the outside air temperature is below a configured setting, CLm. By default, device modes of chilled water consumers (chilled ceiling or cooling coil) are set to 'Off' by a multistate Group Write with priority 9, if the outside air temperature falls below 14°C. However, in 2- or 4-pipe systems, the destination is the 'chilled water available' BACnet object (xxChwAvl) commanded by a binary Group Write with priority 9. This disables cooling and allows heating to remain enabled.

Logic comparing [TOa] to the lockout parameters is local to the function in SplyChw21. A hysteresis (1K by default) prevents frequent switching of the lockout function, and a filter with a configurable time constant is also used. Both the filter and the hysteresis are editable in the chart.

With 2- or 4-pipe systems, the heating limit (HtgLm) for SplyHw21 must be equal or less than the cooling limit (CLm) for SplyChw21. Otherwise, there may be times between heating and cooling seasons when consumer device modes are set to 'Off' unintentionally.

Central valve override

Distribute a signal to manually command all room automation area chilled water consumer valves with priority 8. BACnet objects are provided for enabling the command (EnVlvPosOvrr) and setting the valve position to be commanded (VlvPosOvrr).

Valve kick

To prevent actuators from getting stuck due to prolonged periods of inactivity, SplyChw21 commands the valves for chilled water consumers briefly and periodically with priority 10.

H/C changeover (2-pipe)

Applies only to systems with 2-pipe changeover. Disable or delete otherwise.

SplyChw21 collects heating/cooling requirements from area consumers (via HCDmd SplyChw grouping tag) and decides whether the water system should be used for heating or cooling. The largest number of heating or cooling segments wins. Example: 50 segments require cooling, 30 segments require heating. The evaluation = Cooling.

The calculated h/c state is distributed to the area consumers (via ChovrCnd2Pipe SplyChw grouping tag) with priority 15 to activate the heating or cooling sequence. During heating, the cooling controllers are deactivated, during cooling the reverse is true.

In addition, the h/c state can be manually overridden by commanding the Cho2PipeModCmd object (Command for changeover mode 2-pipe) with a higher priority. Cho2PipeModCmd is in the Central AF for 2-pipe changeover, SplyCho2Pipe11. The h/c state distributed to the area consumers will still be sent with priority 15.

Condensation prevention

To avoid condensation at chilled radiant ceiling devices, the condensation prevention function raises (shifts up) the chilled water setpoint by:

• Collecting and evaluating dew points
• Collecting and evaluating active condensation detectors

SplyChw21 collects active condensation detectors from the devices via group mechanism and evaluates the results. If necessary, the chilled water setpoint is shifted up based on the percentage of active condensation alarms.

SplyChw21 collects and evaluates dew points at chilled water devices from room automation areas to determine the average of the two highest values. The result is written to ChwTDwp (multistate process value in SplyCdnPrev11) with priority 16 via chart connection. If no disturbance-free dew points exist, the ChwTDwp default is used, 14 [°C] / 57 [°F].

The maximum value – whether it is the shifted chilled water setpoint or the calculated dew point value – is used in the chilled water temperature setpoint evaluation functions.

ChwTDwp can also be overridden with a higher priority, for example the source could be the extract dew point of the air handling unit.

Free cooling logic

SplyChw21 collects free cooling requirements from consumers and calculates if excess cooling capacity exists. See Cooling demand collection and evaluation above. Typical applications are chiller plants that use re-coolers to chill the water when outside conditions are favorable.

The following conditions apply:

• Average room temperature collected from the areas is above 23 [°C] / 73.4 [°F] and there is no disturbance in the collection. A configurable hysteresis (default 1K) applies to the switch-on temperature.
• Central plant distributes the free cooling available signal [FreeCAvl] indicating that free cooling is available with no disturbance in the signal.
• Chilled water temperature for free cooling signal [FreeCTChw] is below 20 [°C] / 68 [°F] with no disturbance in the signal. A configurable hysteresis (default 1K) applies to the switch-on temperature.

The central plant can distribute the free cooling request signal via SplyChw grouping tag to the room automation areas. The areas can take this offer and go to Comfort conditions by using the free energy to cool with Comfort setpoints.

Free cooling demand is collected from the room automation areas via SplyChw group mechanism. However, for the activation of the function and distribution of the free cooling request, the CenOpMod21 function and group are needed. The [FreeCReq] output pin must be sent to the CenOpMod21 function from where the request signal is sent to the Desigo room automation HvacPltModxx function.

The free cooling function in SplyChw21 requires CenOpMod21. The following illustration shows the signal flow between SplyChw21, CenOpMod21 and Desigo room automation.

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