Trane RTAA-100, Installation & Maintenance Manual

Page: 70 / 178

70 RTAA-SVX01A-EN
Operating Principles
Compressor Bearing Oil Supply
Oil is injected into the bearing housings located at each end of both the male
and female rotors. Each bearing housing is vented to compressor suction, so
that oil leaving the bearings returns through the compressor rotors to the oil
separator.
Compressor Rotor Oil Supply
Oil flows through this circuit directly from the master oil valve, through the oil
filter to the top of the compressor rotor housing. There it is injected along the
top of the rotors to seal clearance spaces between the rotors and the
compressor housing and to lubricate the rotors.
Female Unloader Valve
The position of the female unloader valve determines compressor capacity.
Its position is dependent on whether the backside of the female unloader
valve is exposed to the compressor discharge or suction pressure. See
Figure 33 .
The female unloader valve solenoid receives a constant signal from the UCM,
based on system cooling requirements. To load the compressor, the female
unloader valve solenoid is energized and discharge pressure is passed
through the normally-closed port and into the cylinder. This pushes the female
unloader valve closed.
To unload the compressor, the female unloader valve solenoid is de-energized
and the discharge pressure is relieved to the suction of the compressor. The
female unloader valve retracts into the cylinder and the compressor is
unloaded.
Just prior to a normal compressor shut-down, the male unload valve solenoid
is energized and the slide valve moves to the fully-unloaded position, so the
unit always starts fully unloaded.
Oil Filter
Each compressor is equipped with a replaceable-element oil filter. The filter
removes any impurities that could foul the solenoid valve orifices and
compressor internal oil supply galleries. This also prevents excessive wear of
compressor rotor and bearing surfaces. Refer to the maintenance portion of
this manual for recommended filter element replacement intervals.
Condenser Fan Staging
The fans on the RTAA 70 -125 Ton units are staged by logic in the UCM. The
UCM takes several different pressures and temperatures into account, to
determine when fans should be added or subtracted. Input from the outside
air temperature sensor, the saturated condensing refrigerant temperature
sensor, and the saturated evaporator refrigerant temperature sensor are
monitored to determine fan staging.
The number of fans activated at startup is dependant upon the outdoor air
temperature.
Figure 34 shows fan activation at different temperatures.
During normal operation, the micro uses PID control to maintain a 70±5 psid
between the condensing pressure and the evaporator pressure. Through the
use of algorithm logic, a fan will be added if the pressure differential is greater
than 75 psid and the fan inverter is at maximum speed.
A “Low Differential Pressure” diagnostic will take the circuit off-line if the
pressure differential falls below 40 psid for more than two minutes.
A “High Differential Pressure” diagnostic will take the circuit off-line if the
pressure differential increases to 350 psid or greater. This diagnostic can also
be produced if the pressure differential increases to the range between 320