Trane SCXG-SVX01B-EN manual general Operation information, Airside Economizer Interface

general

Operation information

stage of cooling, in occupied or unoccupied mode and when ambient conditions are favorable for economizing. Inherent in the unit controller is the ability to suppress the setpoint below the normal unit setpoint. This allows the building to improve comfort levels when possible, and at the same time, optimize building mechanical cooling operation for peak operating efficiency. An outside air temperature and relative humidity sensor are provided to allow monitoring of reference enthalpy and are field installed.

If the unit has the ECEM board, econo- mizer operation enables when the outside air enthalpy is less than 25 BTU’s/ lb. default (adjustable 19-28 BTU’s/lb). During occupied mode, the outside air damper opens to 15% (adjustable 0- 100% at the HI) for ventilation purposes. Also, the ability to alter the outside air damper position to compensate forVAV supply air modulation is inherent in the unit controls, and can be enabled by the operator.

If the unit does not have an ECEM board, it will economize when the O/A tempera- ture falls below the O/A economizer setpoint.

The mixing box fabrication is galvanized steel. Opposed low leak damper blades are fabricated from galvanized steel and rotate on rustproof nylon bushings. A factory installed 24V modulating spring return actuator controls both damper positions.

When outdoor conditions are not suitable for economizer cooling, the enthalpy control disables the economizer function and permits the outdoor air damper to open only to the minimum position.

On water-cooled units, compressor operation lockout will not occur at low ambient air temperatures. However, lockout will still occur via low condenser water temperature.

The outdoor air dampers drive fully closed whenever the supply air fan is off, provided there is power to the unit.

Comparative Enthalpy Control Comparative enthalpy controls the economizer operation and measures temperature and humidity of both return air and outside air to determine which

source has lower enthalpy.This allows true comparison of outdoor air and return air enthalpy by measurement of outdoor air and return air temperatures and humidities. A factory-installed control board, with field-installed outside and return air temperature and relative humidity sensors, allows monitoring of outside and return air.

Note: If comparative enthalpy is not ordered, the standard method is to compare outdoor air enthalpy with the fixed reference enthalpy. The reference enthalpy is set through the human interface panel.

Units with comparative enthalpy control are equipped with the necessary control sequences to allow using outside air for the first stage of cooling, in occupied or unoccupied mode and when ambient conditions are favorable for economizing. Inherent in the unit controller is the ability to suppress the setpoint below the normal unit setpoint. This allows the building to improve comfort levels when possible, and at the same time, optimize building mechanical cooling operation for peak operating efficiency.

Economizer operation enables when the outside air enthalpy is 3 BTu/lb less than the return air enthalpy. During occupied mode, the outside air damper opens to 15% (adjustable 0-100%) for ventilation purposes. Also, the ability to alter the outside air damper position to compen- sate forVAV supply air modulation is inherent in the unit controls, and can be enabled by the operator.

The mixing box fabrication is galvanized steel. Opposed low leak damper blades are fabricated from galvanized steel and rotate on rustproof nylon bushings. A factory installed 24V modulating spring return actuator controls both damper positions.

Airside Economizers withTraqTM Damper Outside air enters the unit through the TraqTM damper assembly and is measured by velocity pressure flow rings. The velocity pressure flow rings are connected to a pressure transducer/ solenoid assembly, which compensates for temperature swings that could affect the transducer.The ventilation control

module (VCM) utilizes the velocity pressure input, the RTM outdoor air temperature input, and the minimum outside air cfm setpoint to modify the volume (cfm) of fresh air entering the unit as the measured airflow deviates from setpoint.

When the optional preheat temperature sensor is installed at the auxiliary temperature on the VCM and the preheat function is enabled, the sensor will monitor the combined (averaged) fresh air and return air temperatures. As this mixed air temperature falls below the preheat actuate temperature setpoint, the VCM activates the preheat binary output to control a field-installed heater. The output deactivates when the tem- perature rises 5°F above the preheat actuate temperature setpoint.

Using a field-installed CO2 sensor with CO2 reset enabled, as the CO2 concentra- tion increases above the CO2 reset start value, the VCM modifies the minimum outside air cfm setpoint to increase the amount of fresh air entering the unit. The setpoint adjusts upward until reaching the CO2 maximum reset value. The maxi- mum effective (reset) setpoint value for fresh air is limited to the system’s operating cfm. As the CO2 concentration decreases, the effective (reset) setpoint value adjusts downward toward the minimum outside air cfm setpoint. See Figure O-GI-6 for an airflow cfm vs. CO2 concentration curve.

Standard Two-Position Damper Interface Units with the two-position damper interface are provided with a 0-10 VDC control output suitable for controlling a field-provided modulating actuator. In occupied mode, the output drives to the maximum position.

Airside Economizer Interface

Units with airside economizer interface are equipped with the necessary control sequences to allow using outside air for the first stage of cooling, in occupied or unoccupied mode and when ambient conditions are favorable for economizing. Inherent in the unit controller is the ability to suppress the setpoint below the normal unit setpoint. This allows the building to improve comfort levels when possible, and at the same time, optimize building mechanical cooling operation for

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