Trane RTRA Series Installation Operation & Maintenance page 20

Cycle description
Vaporized refrigerant leaves the evaporator and is drawn
into the compressor. Here it is compressed and leaves
the compressor as a mixture of hot gas and oil (which
was injected during the compression cycle).
The mixture enters the oil separator at the two in/out
caps. The separed oil flows to the bottom of the separa-
tor, while the refrigerant gas flows out the top and pass-
es on the tubes in the condensing coils. Here circulating
air removes heat from the refrigerant and condenses it.
The condensed refrigerant passes through the electronic
expansion valve and into the tubes of the evaporator. As
the refrigerant vaporizes, it cools the system water that
surrounds the tubes in the evaporator.
Compressor description
The compressors used by the model RTRA air-cooled
chiller consists of two distinct components : the motor
and the rotors. Refer to figure 15.
Compressor motor
A two-pole, hermetic, squirrel-cage induction motor
directly drives the compressor rotors. The motor is
cooled by suction refrigerant gas from the evaporator,
entering the end of the motor housing through the suc-
tion line.
Compressor rotors
The compressor is a semi-hermetic direct-drive helical
rotary type compressor. Each compressor has only 3
moving parts : 2 rotors - «male» and «female» - provide
compression, and a slide valve controls capacity. See
figure 15. 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 slide valve is located
above, and moves along, the top of the rotors.
The helical rotary compressor is a positive displacement
device. The refrigerant 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 dis-
charge line.
There is no physical contact between the rotors and
compressor housing. The rotor 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 pro-
vide 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 a slide
valve assembly located in the rotor section of the com-
pressor. Positioned along the top of the rotors, the slide
valve is driven by a piston/cylinder along an axis that
parallels those of the rotors.
Compressor load condition is dictated by the position of
the slide valve over the rotors. When the slide valve is
fully extended over the rotors and away from the dis-
charge end, the compressor is fully loaded. Unloading
occurs as the slide valve is drawn towards the discharge
end. Slide valve unloading lowers refrigeration capacity
by reducing the compression surface of the rotors.
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