Compressor Loading Sequence; Oil System Operation; Oil Separator - Trane RTAA-70 Installation & Maintenance Manual

Operating Principles
Compressor load capacity is determined by the positions of the unloader
valves. They divert refrigerant gas from the rotors to the compressor suction,
thus unloading the compressor. This varies the compressor capacity to match
the load and reduces the KW draw of the compressor motor.
The two-position female unloader will fully open or fully close a port on the
rotor housing, at the discharge end of the female rotor. This relieves the
refrigerant gas to suction and unloads the compressor. The female unloader
valve is the first stage of loading after the compressor starts and the last
stage of unloading before the compressor shuts down.
The modulating male unloader valve opens or closes ports in the rotor
housing along the side of the male rotor. It can move to a more loaded
(closed) position after the female unloader valve is in the loaded position or
can relieve refrigerant gas to suction to unload the compressor.

Compressor Loading Sequence

When there is a call for chilled water, the UCM will start the compressor
which has the least number of starts. If the first compressor cannot satisfy
the demand, the UCM will start the other compressor and then balance the
load on both compressors by pulsing the load/unload solenoids.
The load on the compressors will be kept in balance, as load fluctuates, until
the demand for chilled water is reduced to a level that can be handled by one
compressor. At this time, the UCM will drop off the compressor that has the
greatest number of operating hours and will adjust the load on the other
compressor, as required.

Oil System Operation

Overview
Oil that collects in the bottom of the oil separator is at condensing pressure
during compressor operation; therefore, oil is constantly moving to lower
pressure areas. Refer to Figure 33.
As the oil leaves the separator, it passes through the oil cooler at the top of
the condensing coils. It then goes through the service valve and filter. At this
point, some of the oil is used to control the slide valve movement in the
compressor, via the male load/unload solenoids. The remaining oil passes
through the master oil valve and performs the functions of compressor
bearing lubrication and compressor oil injection.
If the compressor stops for any reason, the master oil valve closes, isolating
the oil charge in the separator and oil cooler during "off" periods. The master
oil valve is a pressure activated valve. Discharge pressure off the rotors, that
is developed when the compressor is on, causes the valve to open.
To ensure proper lubrication and minimize refrigerant condensation in the
compressor, a heater is mounted on the bottom of the compressor housing.
A signal from the UCM energizes this heater during the compressor "Off"
cycle to keep refrigerant from condensing in the compressor. The heater
element is continuously energized when the compressor is not running.

Oil Separator

The oil separator consists of a vertical tube, joined at the top by the refrig-
erant discharge line from the compressor. The discharge line is essentially
tangential to the tube. This causes the refrigerant to swirl in the tube and
throws the oil to the outside, where it collects on the walls and flows to the
bottom. The compressed refrigerant vapor, stripped of oil droplets, exits out
the top of the oil separator and is discharged into the condensing coils.
The oil separators on chillers with the remote evaporator option incorporate
heaters to prevent refrigerant migration when the machine is off.
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