Fig. 2 - Screw Compressor - York MILLENNIUM YCAS Installation Operation & Maintenance

formance. Capacity control is achieved through a single
slide valve. The compressor is a positive displacement
type characterized by two helically grooved rotors which
are manufactured from forged steel. The 50 Hz motor
operates at 2975 RPM to direct drive the male rotor which
in turn drives the female rotor on a light film of oil.
Refrigerant gas is injected into the void created by the
unmeshing of the five-lobed male and seven-lobed fe-
male rotors. Further meshing of the rotors closes the
rotor threads to the suction port and progressively com-
presses the gas in an axial direction to the discharge
port. The gas is compressed in volume and increased in
pressure before exiting at a designed volume at the dis-
charge end of the rotor casing. Since the intake and
discharge cycles overlap, a resulting smooth flow of gas
is maintained.
The rotors are housed in a cast iron compressor hous-
ing precision machined to provide optimal clearances
for the rotors. Contact between the male and female
rotors is primarily rolling on a contact band on each of
the rotor's pitch circle. This results in virtually no rotor
wear and increased reliability, a trademark of the screw
compressor.
The compressor incorporates a complete anti-friction
bearing design for reduced power input and increased
reliability. Four separated, cylindrical, roller bearings
handle radial loads. Angular-contact ball bearings handle
axial loads. Together they maintain accurate rotor posi-
tioning at all pressure ratios, thereby minimizing leak-
age and maintaining efficiency. A springless check valve
is installed in the compressor discharge housing to pre-
vent compressor rotor backspin due to system refriger-
ant pressure gradients during shutdown.
FIG. 2 – SCREW COMPRESSOR
YORK INTERNATIONAL
Motor cooling is provided by suction gas from the evapo-
rator flowing across the motor. Redundant overload pro-
tection is provided using both thermistor and current over-
load protection.
The compressor is lubricated by removing oil from the
refrigerant using an external oil separator. The pressur-
ized oil is then cooled in the condenser coils and piped
back to the compressor for lubrication. The compressor
design working pressure is 31 bar (450 PSIG). Each
chiller receives a 21 bar (300 PSIG) low-side and a 31
bar (450 PSIG) high-side factory test. A 350 watt
(115-1-50) cartridge heater is located in the compres-
sor. The heater is temperature-activated to prevent re-
frigerant condensation.
The following items are also included:
• An acoustically tuned, internal discharge muffler to
minimize noise, while operating flow for maximum
performance.
• Discharge and suction shutoff valves.
• A rain-tight terminal box.
• A suction gas screen and serviceable, 0.5 micron
full-flow oil filter within the compressor housing.
Cooler
The system uses a Shell-and-Tube, Direct Expansion
Evaporator. Each of the refrigerant circuits consists of
4 passes with the chilled liquid circulating back and forth
across the tubes from one end to the other.
The design working pressure of the cooler on the shell
side is 10 bar (150 PSIG), and 21 bar (300 PSIG) for
the tube (refrigerant side). The water baffles are fabri-
cated from galvanized steel to resist corrosion. Remov-
able heads are provided for access to internally en-
hanced, seamless, copper tubes. Water vent and drain
connections are included.
The cooler is equipped with a thermostatically controlled
heater for protection to –29ºC (–20ºF) ambient and in-
sulated with flexible closed-cell foam.
The water nozzles are provided with grooves for me-
chanical couplings and should be insulated by the con-
tractor after pipe installation.
LD03674
FORM 201.18-NM2
2
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