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Chilled and Condensing Water Systems (CHWS and CDWS)

Type Definition

Hydronic chilled and condensing water systems are two distinct systems. A typical chilled water system (CHWS) consists of chillers (that use refrigerant to cool a closed loop of circulating water known as the "chilled water") and pumps to distribute the chilled water to other equipment in the building and/or to circulate water through the chillers. Typically, air-side equipment (e.g., AHU, FCU) contains chilled water coils connected to the main CHWS; as air passes over the chilled water coils it is cooled. As chilled water systems cool the chilled water, they must reject the heat absorbed from the chilled water loop. In a CHWS with "air-cooled" chillers, the heat is rejected directly into the atmospehre. In a CHWS with "water-cooled" chillers, the heat is rejected to another water distribution system (typically a condensing water system).

Condensing water systems (CDWS) serve those mechanical devices which rely on water-source heat rejection. Typical condensing water systems consist of open-air cooling towers (which reject the heat absorbed from the condensing water into the atmosphere) and pumps to circulate the condensing water between the cooling towers and the condenser side of a water-cooled chiller.

There are many different types of chilled and condenser water system configurations; some common examples include the following:

  • Constant volume: the system pumps run at a constant speed to provide fixed flowrates at pre-defined stages (predominantly on condensing water systems)
  • Variable-primary: there is a single (primary) loop between the chillers and the end-devices and the system pumps operate at variable speed (typically controlling to a differential pressure setpoint)
  • Primary/Secondary: there are two separate loops, with the primary loop serving as the "production" loop (the chillers) and the secondary loop serving as the distribution loop (to downstream end-devices).

Type Requirements

  • Chillers (CH) require the mechanical cooling of water.
  • Condensers require that, regardless of their own cooling method (sensible or evaporative), they serve downstream devices which require water as a condensing source.
  • Heat exchangers (HX) provide heat transfer between two fluid streams, without intermixing the fluids. They are modeled separately only if they are stand-alone devices.

Connection Requirements

  • The equipment which is part of each system should be assigned a HAS_PART connection to it.
  • The specific interconnections between equipment in the system can also be defined through FEEDS connections. In these cases, it is best to feed from the pumps.
  • To simplify distribution specifics, the system (rather than the system pumps) should be used to feed downstream equipment.

Example: Variable Primary CHWS (Air-Cooled)

This version of CHWS has a single set of variable speed pumps which serve air-cooled chillers and downstream equipment.

BMS Example

CHWS Variable Primary Notes:

  • The fields associated with individual devices (e.g., individual chiller commands) get mapped to those devices; system level telemetry (e.g., the system return water temperature) gets assigned to the system directly.

System Diagram and Connections

CHWS Variable Primary

Sample Building Configuration

# System definition. Everything that it serves will be fed by it, everything that comprises it will be contained by it.
CHWSYS-BLDG-1:
  cloud_device_id: 1234
  type: HVAC/CHWS_...
  translation:
    supply_water_temperature_sensor:
      present_value: points.sys_swt.present_value
      units:
        key: pointset.points.sys_swt.units
        values:
          degrees_fahrenheit: 'deg-F'
    differential_pressure_sensor:
      present_value: points.sys_dp.present_value
      units:
        key: pointset.points.sys_dp.units
        values:
          pascals: 'Pa'
    ...

# Plant equipment, all contained by the system.
# Various interconnections between the pumps and the chillers can also be defined as feeds connections.
CH-1:
  cloud_device_id: 2345
  connections:
    CHWSYS-BLDG-1: HAS_PART
    PMP-1: FEEDS
    PMP-2: FEEDS
  type: HVAC/CH_...
  translation:
    supply_water_temperature_sensor:
      present_value: points.swt.present_value
      units:
        key: pointset.points.swt.units
        values:
          degrees_fahrenheit: 'deg-F'
    ...

PMP-1:
  cloud_device_id: 3456
  connections:
    CHWSYS-BLDG-1: HAS_PART
  type: HVAC/PMP_...
  translation:
    run_command:
      present_value: points.cmd.present_value
      states:
        OFF: 'false'
        ON: 'true'
    ...

PMP-2:
  cloud_device_id: 4567
  connections:
    CHWSYS-BLDG-1: HAS_PART
  type: HVAC/PMP_...
  translation:
    run_command:
      present_value: points.cmd.present_value
      states:
        OFF: 'false'
        ON: 'true'
    ...

# Equipment served by the chilled wwater system.
FCU-1:
  cloud_device_id: 5678
  connections:
    CHWSYS-BLDG-1: FEEDS
  type: HVAC/FCU_DFSS_DFVSC_...
  translation:
    chilled_water_valve_percentage_command:
      present_value: points.clg_valve_percentage_command.present_value
      units:
        key: pointset.points.clg_valve_percentage_command.units
        values:
          percent: '%'
    ...

AHU-1:
  cloud_device_id: 6789
  connections:
    CHWSYS-BLDG-1: FEEDS
  type: HVAC/AHU_DFSS_...
  translation:
    chilled_water_valve_percentage_command:
      present_value: points.chwv.present_value
      units:
        key: pointset.points.chwv.units
        values:
          percent: '%'
    ...

Example: Variable Primary CHWS (Water-Cooled)

This version of CHWS has a single set of variable speed chilled water pumps serving water-cooled chillers and downstream equipment. The condensing side serves the chillers via two pumps.

BMS Example

CHWS Variable Primary WC Notes:

  • This example includes a parallel air-cooled chiller. It is simple enough to add; it doesn't change the way the rest of the system is modeled.

System Diagram and Connections

CHWS Variable Primary WC

Sample Building Configuration

# Chilled water system feeds the equipment, and contains the distribution/production equipment.
CHWSYS-BLDG-1:
  cloud_device_id: 1234
  connections:
    CDWSYS-BLDG-1: FEEDS
  type: HVAC/CHWS_...
  translation:
    supply_water_temperature_sensor:
      present_value: points.sys_swt.present_value
      units:
        key: pointset.points.sys_swt.units
        values:
          degrees_fahrenheit: 'deg-F'
    differential_pressure_sensor:
      present_value: points.sys_dp.present_value
      units:
        key: pointset.points.sys_dp.units
        values:
          pascals: 'Pa'
    ...

# Condenser wawter system feeds the chilled water system, 
CDWSYS-BLDG-1:
  cloud_device_id: 2345
  type: HVAC/CDWS_...
  translation:
    supply_water_temperature_sensor:
      present_value: points.sys_swt.present_value
      units:
        key: pointset.points.sys_swt.units
        values:
          degrees_fahrenheit: 'deg-F'
    ...

# Equipment contained by chilld water system.
# Various interconnections between the pumps and the chillers can also be defined as feeds connections.
CH-1:
  cloud_device_id: 3456
  connections:
    CHWSYS-BLDG-1: HAS_PART
    CWP-4: FEEDS
  type: HVAC/CH_...
  translation:
    supply_water_temperature_sensor:
      present_value: points.swt.present_value
      units:
        key: pointset.points.swt.units
        values:
          degrees_fahrenheit: 'deg-F'
    ...

CH-2:
  cloud_device_id: 4567
  connections:
    CHWSYS-BLDG-1: HAS_PART
    CWP-3: FEEDS
  type: HVAC/CH_...
  translation:
    supply_water_temperature_sensor:
      present_value: points.swt.present_value
      units:
        key: pointset.points.swt.units
        values:
          degrees_fahrenheit: 'deg-F'
    ...

ACC-1: # Note: this is a non-compliant equipment name, which may exist in brownfield sites. 
  cloud_device_id: 5678
  connections:
    CHWSYS-BLDG-1: HAS_PART
  type: HVAC/CH_...
  translation:
    supply_water_temperature_sensor:
      present_value: points.swt.present_value
      units:
        key: pointset.points.swt.units
        values:
          degrees_fahrenheit: 'deg-F'
    ...

CHWP-1:
  cloud_device_id: 6789
  connections:
    CHWSYS-BLDG-1: HAS_PART
    CH-1: FEEDS
    CH-2: FEEDS
    CHWP-7: FEEDS
  type: HVAC/PMP_...
  translation:
    run_command:
      present_value: points.cmd.present_value
      states:
        OFF: 'false'
        ON: 'true'
    ...

CHWP-2:
  cloud_device_id: 7890
  connections:
    CHWSYS-BLDG-1: HAS_PART
    CH-1: FEEDS
    CH-2: FEEDS
    CHWP-7: FEEDS
  type: HVAC/PMP_...
  translation:
    run_command:
      present_value: points.cmd.present_value
      states:
        OFF: 'false'
        ON: 'true'
    ...

CHWP-7:
  cloud_device_id: 8901
  connections:
    CHWSYS-BLDG-1: HAS_PART
    ACC-1: FEEDS
  type: HVAC/PMP_...
  translation:
    run_command:
      present_value: points.cmd.present_value
      states:
        OFF: 'false'
        ON: 'true'
    ...

# Condensing water system equipment.
CT-1:
  cloud_device_id: 9012
  connections:
    CDWSYS-BLDG-1: HAS_PART
    CWP-3: FEEDS
    CWP-4: FEEDS
  type: HVAC/CT_...
  translation:
    supply_water_temperature_sensor:
      present_value: points.swt.present_value
      units:
        key: pointset.points.swt.units
        values:
          degrees_fahrenheit: 'deg-F'
    ...

CT-2:
  cloud_device_id: 11234
  connections:
    CDWSYS-BLDG-1: HAS_PART
    CWP-3: FEEDS
    CWP-4: FEEDS    
  type: HVAC/CT_...
  translation:
    supply_water_temperature_sensor:
      present_value: points.swt.present_value
      units:
        key: pointset.points.swt.units
        values:
          degrees_fahrenheit: 'deg-F'
    ...

CWP-3:
  cloud_device_id: 12234
  connections:
    CDWSYS-BLDG-1: HAS_PART
  type: HVAC/PMP_...
  translation:
    run_command:
      present_value: points.cmd.present_value
      states:
        OFF: 'false'
        ON: 'true'
    ...

CWP-4:
  cloud_device_id: 12334
  connections:
    CDWSYS-BLDG-1: HAS_PART
  type: HVAC/PMP_...
  translation:
    run_command:
      present_value: points.cmd.present_value
      states:
        OFF: 'false'
        ON: 'true'
    ...

# Equipment served by the chilled wwater system.
FCU-1:
  cloud_device_id: 12344
  connections:
    CHWSYS-BLDG-1: FEEDS
  type: HVAC/FCU_...
  translation:
    chilled_water_valve_percentage_command:
      present_value: points.clg_valve_percentage_command.present_value
      units:
        key: pointset.points.clg_valve_percentage_command.units
        values:
          percent: '%'
    ...

AHU-1:
  cloud_device_id: 12345
  connections:
    CHWSYS-BLDG-1: FEEDS
  type: HVAC/AHU_...
  translation:
    chilled_water_valve_percentage_command:
      present_value: points.chwv.present_value
      units:
        key: pointset.points.chwv.units
        values:
          percent: '%'
    ...

Note: All BMS screenshots taken from Google's WebCTRL instances. WebCTRL is a building automation system owned by Automated Logic.