Trane SCXG-SVX01B-EN manual UnitAirside Components

general

Operation information

UnitAirside Components

The unit’s air delivery system consists of dampers, enthalpy switch option, airside economizer option, filters, low ambient sensors, and factory mounted single or double wall plenums.

Supply Air Fan

The unit has a single supply fan that runs at a constant speed. However, the fan may have the IGV or VFD option that modulates airflow based on supply air temperature control. Pressing the stop key on the HI will turn the supply fan off. The fan is on continuously when a CV unit is in occupied mode and except when a unit is in the night heat/morning warmup mode. During the night heat and setback mode the fan cycles on and off in response to a call for heat. SeeTable O- GI-9 for available fan horsepower.

Low Entering Air Temperature Sensor This is standard on all units with a hydronic coil or waterside economizer. It can also be ordered as an option.

A thermostat limit switch is factory mounted on the unit’s entering air side with a capillary tube serpentine across the coil face. If the temperature falls below 35°F (2°C), the fan shuts down and the waterside economizer and/or hydronic heat valve options open to allow full water flow.The heat output also energizes. A manual reset is required. The low entering air temperature setpoint is adjustable at the HI.

High Duct Temperature Thermostat A factory-supplied temperature limit switch with reset element detects the supply air duct temperature. This sensor should be field-installed downstream from the unit’s discharge in the supply air duct. If the supply air duct temperature exceeds 240°F (115.6°C), the unit shuts down and displays a diagnostic. A manual reset is required at the unit. The high duct temperature can be adjusted at the thermostat.

Dirty Filter Sensor Option

A factory installed pressure switch senses the pressure differential across the filters. When the differential pressure exceeds 0.9-inches (23 mm) WG, contact closure occurs and the HI will display a diagnostic. The unit will continue to run until you replace the air filters.

A field installed indicator device may be wired to relay terminals to indicate when filter service is required. Contacts are rated at 115 VAC and are powered by a field supplied transformer.

Low Ambient Sensor (Air-Cooled Units) The low ambient sensor is field-installed on air-cooled units. Position it in a location subject to ambient temperatures only and not exposed to direct sunlight or exhaust fans.

The low pressure cutout initiates based on the ambient temperature. A time delay on the low pressure cutout initiates for ambient temperatures between 50 (zero minutes) and 0°F (10 minutes). This helps to prevent nuisance low pressure cutout trips.

Inlet GuideVane Option

Inlet guide vanes (IGV’s) are driven by a modulating 0-10 vdc signal from the RTM module. A pressure transducer measures duct static pressure, and the IGVs modulate to maintain the supply air static pressure within an adjustable user- defined range. The range is determined by the supply air pressure setpoint and supply air pressure deadband, which are set through the HI panel.

IGV assemblies installed on the supply fan inlet regulate fan capacity and limit horsepower at lower system air require- ments. When in any position other than full open, the vanes pre-spin the air in the same direction as the supply fan rotation. As the vanes approach the full-closed position, the amount of “spin” induced by the vanes increases at the same time that intake airflow and fan horsepower diminish. The IGVs will close when the supply fan is off.

Supply Air Static Pressure Limit

The opening of the IGVs andVAV boxes coordinate during unit startup and transition to/from occupied/unoccupied

modes to prevent supply air duct overpressurization. However, if for any reason the supply air pressure exceeds the user-defined supply air static pressure limit set at the HI panel, the supply fan/VFD shuts down and the IGVs close. The unit will attempt to restart, up to three times. If the overpressurization condition still occurs on the third restart, the unit shuts down and a manual reset diagnostic sets and displays at the HI.

Variable Frequency Drive Option The variable frequency drive (VFD) is driven by a modulating 0-10 vdc signal from the RTM module. A pressure transducer measures duct static pressure, and the VFD adjusts the fan speed to maintain the supply air static pressure within an adjustable user- defined range. The range is determined by the supply air pressure setpoint and supply air pressure deadband, which are set at the HI panel.

VFDs provide supply fan motor speed modulation. The drives will accelerate or decelerate as required to maintain the supply air static pressure setpoint.

VFD with Bypass

Bypass control is an option that provides full nominal airflow in the event of drive failure. The user must initiate the bypass mode at the HI panel. When in bypass mode,VAV boxes need to be fully open. The self-contained unit will control heating and cooling functions to maintain setpoint from a user-defined zone sensor. Supply air static pressure limit is active in this mode.

For more detailed information on VFD operation, reference the VFD technical manual that ships with the unit.

Airside Economizer Option

Units with the airside economizer option are equipped with the necessary control sequences to use outside air for the first

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