Trane R Series, Installation Operation & Maintenance

Page: 87 / 188

RLC-SVX07A-EN 87
Operating Principles
Compressor Rotors
The compressor is a semi-hermetic, direct-drive helical rotary type
compressor. Each compressor has two rotors - “male” and “female” - which
provide compression. See
Figure 37 . The male rotor is attached to, and driven
by, the motor, and the female rotor is, in turn, driven by the male rotor.
Separately housed bearing sets are provided at each end of both rotors.
The helical rotary compressor is a positive displacement device. The refrig-
erant from the evaporator is drawn into the suction opening at the end of the
motor barrel, through a suction strainer screen, across the motor, and into the
intake of the compressor rotor section. The gas is then compressed and
discharged directly into the discharge line.
There is no physical contact between the rotors and compressor housing.
The rotors contact each other at the point where the driving action between
the male and female rotors occurs. Oil is injected along the top of the
compressor rotor section, coating both rotors and the compressor housing
interior. Although this oil does provide rotor lubrication, its primary purpose is
to seal the clearance spaces between the rotors and compressor housing.
A positive seal between these internal parts enhances compressor efficiency
by limiting leakage between the high pressure and low pressure cavities.
Capacity control is accomplished by means of two unloader valve assemblies
in the rotor section of the compressor. The female rotor valve is a two-
position valve and the male valve is an infinitely variable position valve. See
Figure 37 .
Compressor load capacity is dictated 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 a refrig-
erant 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 (RTWA and RTUA only)
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 (RTWA and RTUA Only)
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 37 .