Cooling Sequence of Operation
Time delays are built into the controls to increase reliability and performance by protecting the compressors and maxi- mizing unit efficiency.
Sequence of Operation
Compressor Crankcase Heaters
Each compressor is equipped with a crankcase heater and is controlled by a 600 volt auxiliary switch on the compres- sor contactor. The proper operation of the crankcase heater is important to maintain an elevated compessor oil tempera- ture during the "Off" cycle to reduce oil foaming during com- pressor starts.
When the compressor starts, the sudden reduction in crank- case pressure causes the liquid refrigerant to boil rapidly causing the oil to foam. This condition could damage com- pressor bearings due to reduced lubrication and could cause compressor mechanical failures.
When power has been "Off" for an extended period, allow the crankcase heater to operate a minimum of 8 hours be- fore starting the unit.
Units without an Economizer
Upon entering an "occupied" mode of operation, the RTM receives input from the remote panel to start the supply fan. For constant volume applications, the RTM supply fan con- tacts K2 close which energizes the supply fan contactor 1K16. Units equipped with Inlet Guide Vanes (IGV), the fan is delayed until the inlet guide vanes are driven to the full closed position. When the supply fan starts, the fan proving switch (3S68) closes, signaling the RTM that airflow has been established. Inlet Guide Vanes will begin to drive open (if equipped), or the VFD will begin to ramp the fan, (if equipped).
When a cooling request is sent to the RTM from a zone temperature sensor, the RTM evaluates the operating con- dition of the system using the supply air temperature input and the outdoor temperature input before sending the re- quest to the SCM/MCM. Once the request is sent to the SCM/MCM, the compressor module checks the compressor protection circuit before closing "Stage 1" (K10 on SCM or K11 on MCM). After the first functional stage has started, the compressor module monitors the saturated refrigerant temperature and closes the condenser fan output contact "1A", when the saturated refrigerant temperature rises above the "lower limit" setpoint.
Units with an Economizer
Upon entering an "occupied" mode of operation, the RTM receives input from the remote panel to start the supply fan. For constant volume applications, the RTM supply fan con- tacts K2 close which energizes the supply fan contactor 1K16. Units equipped with Inlet Guide Vanes (IGV), the fan is delayed until the inlet guide vanes are driven to the full closed position. When the supply fan starts, the fan proving switch (3S68) closes, signaling the RTM that airflow has been established. The RTM opens the economizer dampers to the specified "minimum position".
When a cooling request is sent to the RTM from the zone temperature sensor, the RTM evaluates the operating con- dition of the system using the supply air temperature input and the outdoor temperature input before sending the re- quest to the SCM/MCM for mechanical cooling. If the out-
Unit Start-Up
door conditions are suitable for cooling (temperature and humidity are within specified setpoints), the RTM will at- tempt to maintain the zone temperature without using any compressors. If the zone temperature can not be main- tained within the setpoint deadband, the RTM sends a cool- ing request to the SCM/MCM. The compressor module checks the compressor protection circuit before closing "Stage 1" (K10 on SCM or K11 on MCM). After the first functional stage has started, the compressor module moni- tors the saturated refrigerant temperature and closes the condenser fan output contact "1A", when the saturated re- frigerant temperature rises above the "lower limit" setpoint.
Units with TraqTM Sensor
The fresh air enters the unit through the TraqTM Sensor as- sembly and is measured by velocity pressure flow rings. The velocity pressure flow rings are connected to a pres- sure transducer/solenoid assembly. The solenoid is used for calibration purposes to compensate 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 enter- ing the unit as the measured airflow deviates from setpoint.
When the optional temperature sensor is installed 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 Actu- ate Temperature Setpoint, the VCM will activate the preheat binary output used to control a field installed heater. The output will be deactivated when the temperature rises 5 above the Preheat Actuate Temperature Setpoint.
When the optional CO2 sensor is installed and the CO2 Re- set is enabled, as the CO2 concentration increases above the CO2 Reset Start Value, the VCM will modify the mini- mum outside air CFM setpoint to increase the amount of fresh air entering the unit. The setpoint will be adjusted up- ward until the CO2 Maximum Reset Value is reached. The maximum effective (reset) setpoint value for fresh air enter- ing the unit is limited to the systems operating CFM. As the CO2 concentration decreases, the effective (reset) setpoint value is adjusted downward toward the minimum outside air CFM setpoint.
FrostatTM Control
The compressor module utilizes an evaporator temperature sensor (3RT14 & 15), mounted on the suction line of each circuit, to protect the evaporator from freezing. If the evapo- rator temperature approaches the specified setpoint, adjust- able between 25 F and 35 F, the compressor(s) will be cycled "off". The compressors will not be allowed to restart until the evaporator temperature has risen 10 F above the specified cutout temperature and the compressor(s) have been off for a minimum of three minutes.
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