Trane IntelliPak SAHL Series Installation, Operation And Maintenance Manual page 20

General Information
Head Pressure Control
This is accomplished using two saturated refrigerant temperature sensors. During a request for
compressor operation, when the condensing temperature rises above the "lower limit" of the
control band, the Compressor Module (SCM/MCM) starts sequencing condenser fans "On" . If the
operating fans can not bring the condensing temperature to within the control band, more fans are
turned on.
As the saturated condensing temperature approaches the lower limit of the control band, fans are
sequenced "Off" . The minimum "On/Off" time for condenser fan staging is 5.2 seconds. If the
system is operating at a given fan stage below 100% for 30 minutes and the saturated condensing
temperature is above the "efficiency check point" setting, a fan stage will be added. If the saturated
condensing temperature falls below the "efficiency check point" setting, the fan control will remain
at the present operating stage. If a fan stage cycles four times within a 10 minute period, the control
switches from controlling to the "lower limit" to a temperature equal to the "lower limit" minus
the "temporary low limit suppression" setting. It will utilize this new "low limit" temperature for
one hour to reduce condenser fan short cycling.
For Evaporative Condensing units, Head pressure is monitored with pressure transducers attached
to the Saturated Condensing line and converted to a temperature by the MPM. This temperature
is used to control the variable speed fan and control the sump pump. When the temperature rises
above the Upper Limit (120F) the Sump Pump is energized. If the Condensing Temperature drops
below the Lower Limit (70F) the Sump Pump is de-energized.
High Pressure Controls
High Pressure controls are located on the discharge lines near the scroll compressors. They are
designed to open when the discharge pressure approaches 650 ± 10 psig. The controls reset
automatically when the discharge pressure decreases to approximately 550 ± 10 psig. However, the
compressors on that circuit are locked out and a manual reset diagnostic is initiated after the fourth
occurrence of a high pressure condition.
Outdoor Air Humidity Sensor (3U63)
This is an analog input device used on applications with 100% economizer. It monitors the outdoor
humidity levels for economizer operation. It is mounted in the fresh air intake section and is
connected to the RTM (1U48).
Return Air Humidity Sensor (3U64)
This is an analog input device used on applications with the comparative enthalpy option. It
monitors the return air humidity level and compares it to the outdoor humidity level to establish
which conditions are best suited to maintain the cooling requirements. It is mounted in the return
air section and is connected to the ECEM (1U52).
Space/Duct Humidity Sensor (5U108)
Analog input device used on applications with modulating dehumidification option and/or
humidification field installed option. It is used to monitor the humidity level in the space and for
comparison with the dehumidification and humidification setpoints to maintain space humidity
requirements. It is field mounted in the space and connected to the RTM or ECEM (1TB16).
Low Ambient Control
The low ambient modulating output on the compressor module is functional on all units with or
without the low ambient option. When the compressor module has staged up to it's highest stage
(stage 2 or 3 depending on unit size), the modulating output will be at 100% (10 VDC). When the
control is at stage 1, the modulating output (0 to 10 VDC) will control the saturated condensing
temperature to within the programmable "condensing temperature low ambient control point" .
Status/Annunciator Output
This is an internal function within the RTM (1U48) module that provides;
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