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Refrigerant gas is injected into the void created by the
unmeshing of the five lobed male and seven lobed fe-
male rotor. Further meshing of the rotors closes the rotor
threads to the suction port and progressively compresses
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 discharge 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 housing
precision machined to provide optimal clearances for
the rotors. Contact between the male and female rotor
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 han-
dle axial loads. Together they maintain accurate rotor
positioning at all pressure ratios, thereby minimizing
leakage and maintaining efficiency. A springless check
valve is installed in the compressor discharge housing
to prevent compressor rotor backspin due to system
refrigerant pressure gradients during shutdown.
Motor cooling is provided by suction gas from the evap-
orator flowing across the motor. Redundant overload
protection is provided using both thermistor and current
overload protection.
FIG. 2 – SCREW COMPRESSOR
16
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 450 PSIG (31 bar). Each
chiller receives a 300 PSIG (21 bar) low side and a 450
PSIG (31 bar) high side factory test. A 350 watt (115-
1-60) cartridge heater is located in the compressor. The
heater is temperature activated to prevent refrigerant
condensation.
The following items are also included:
• Internal discharge check valve to prevent rotor
backspin or shutdown.
• 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.
Evaporator
The system uses a high efficiency Shell and Tube type
Direct Expansion Evaporator. Each of the 2 refrigerant
circuits consists of 4 passes with the chilled liquid cir-
culating back and forth across the tubes from one end
to the other.
The design working pressure of the cooler on the shell
side is 150 PSIG (10 bar), and 350 PSIG (24 bar) for the
tube (refrigerant side). The water baffles are fabricated
from galvanized steel to resist corrosion. Removable
heads are provided for access to internally enhanced,
seamless, copper tubes. Water vent and drain connec-
tions are included.
The cooler is equipped with a thermostatically controlled
heater for protection to -20°F (-29°C) ambient and
insulated with 3/4" (19 mm) flexible closed-cell foam.
The water nozzles are provided with grooves for
mechanical couplings and should be insulated by the
contractor after pipe installation.
Condenser
The fin and tube condenser coils are manufactured from
00485VIP
seamless, internally enhanced, high condensing coeffi-
cient, corrosion resistant copper tubes arranged in stag-
JOHNSON CONTROLS
2
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