Free Cooling is Available - Johnson Controls - Metasys - LIT-12011147 - Software Application - Controller Configuration Tool - 13.1

Controller Tool Help

Product name
Controller Configuration Tool
Document type
User Guide
Document number
LIT-12011147
Version
13.1
Revision date
2019-12-04

The Free Cooling is Available logic block provides economizer decision logic to determine when outdoor air, in excess of the minimum amount required for ventilation, can be used to reduce the cooling coil load.

The output is evaluated when the Supply Fan Status input changes and then subsequently only on its Interval (configurable property in the Advanced Details dialog box). This block does not constantly calculate its outputs as the inputs change.

Different methods include:

  • Dry Bulb Temperature Economizer where if the supply fan is running and the Outside Air Temp is less than the Change Over Temp, then Free Cooling Available is True.

  • Temperature and Enthalpy Economizer where measurements of Outside Air Temp, Outside Air Humidity, Return Air Temp, and Return Air Humidity are used to calculate the enthalpy of the outside air and return air streams. Free Cooling Available is True when:

    • the supply fan is running

    • the calculated outside air enthalpy is less than the calculated return air enthalpy

    • the outside air dry bulb temperature is less than the return air dry bulb temperature

  • Model Based Economizer where measurements of Outside Air Temp, Outside Air Humidity, Return Air Temp, and Return Air Humidity as well as an estimate of the temperature leaving the cooling coil are used for calculations.

The following table describes the inputs and outputs of the Free Cooling is Available logic block.

Table 1. Free Cooling is Available Block Inputs and Outputs

Name

Data Type

Exposed for Connection by Default

Description

Default Value

Inputs

Bypass Percent

Float

No

Indicates the percent of air passing through the coil that is unaffected by the coil (%). This value is used for the Model-Based Economizer Type.

20%

Range: 0-50%

Change Over Temperature

Float

No

Indicates the change over dry bulb temperature used to determine if free cooling is available for the Dry Bulb Temperature economizer option.

20 °C,

68 °F

Cooling Discharge Temperature

Float

Yes

Indicates the effective discharge air temperature for the cooling coil. This value is used for the Model-Based Economizer Type.

12 °C,

55 °F

Economizer Type

Enum

Yes

Use to select the type of economizer logic.

Temperature & Enthalpy

Elevation

Float

No

Indicates the distance above or below sea level in m or ft. This value is used in the psychrometric calculations performed for Economizer Type = Temperature & Enthalpy or Model Based.

0

Min OA Percent

Float

No

Indicates the percent (%) of maximum supply fan flow rate required for the minimum amount of ventilation air.

20%

Range: 0-100%

Outside Air Humidity

Float

Yes

Indicates the Relative Humidity of the outside air.

50%

Range: 0-100%

Outside Air Temp

Float

Yes

Indicates the dry bulb temperature of the outside air stream.

25 °C,

77 °F

Return Air Humidity

Float

Yes

Indicates the Relative Humidity of the return air.

50%

Range: 0-100%

Return Air Temp

Float

Yes

Indicates the dry bulb temperature of the return air stream.

20 °C,

68 °F

Supply Fan Status

Boolean

Yes

Indicates the operating status of the AHU supply fan (Running = True, Off = False).

False

Unit Set

Enum

No

Indicates the units of measurement that appear in the tool.

SI, IP

Outputs

Free Cooling Available 1

Boolean

Yes

Indicates the following:
  • This output is True when increasing the volume of outside air can reduce the cooling coil load.
  • This output is False when the volume of outside air should be at the minimum required for ventilation.

False

Properties

Interval

Float

N/A

Indicates the time between scheduled executions of the block.

900 Seconds

1 This input or output is the Default Element of the block.