
YORK INTERNATIONAL
34
GENERAL
GENERAL DESCRIPTION
The Air Cooled Screw Chiller utilizes many components
which are the same or nearly the same as a standard
reciprocating chiller of a similar size. This includes
modular frame rails, condenser, fans and evaporator.
The chiller package consists of two modules, each con-
trolled by its own microprocessor with one module be-
ing designated as the fiMasterfl and one module as
the
fi
Subordinate
,fl four screw compressors in four
separate re-frigerant circuits, two shell and tube DX
evaporators,
economizers, air cooled condensers,
and expansion
valves. Each refrigerant system has
four fans which
operate in pairs.
COMPRESSOR
The Frick semi-hermetic rotary twin-screw compressor
utilizes a twin screw design with a single slide valve for
capacity control. The compressor is a positive displace-
ment type characterized by two helically grooved rotors.
The 60 Hz motor operating at 3570 RPM drives a geared
speed increaser to drive the male rotor between 4125 -
7600 RPM. The female rotor is driven by the male rotor
on a light film of oil. Compressors with gear sets running
at higher speeds will have greater capacity.
Refrigerant gas is injected into the void created by the
un-meshing of the 5 lobed male and 7 lobed female
rotor. Further meshing of the rotors closes the rotor
threads to the suction port and progressively com-
presses the gas in an axial direction toward the dis-
charge port. The gas is compressed in volume and in-
creased in pressure before exiting at a designed vol-
ume at the discharge end of the rotor casing. Since the
intake and discharge cycles overlap, a resulting smooth
continuous flow of gas is maintained.
Contact between the male and female rotor is primarily
rolling on a contact band on each of the rotors 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 check valve is installed
in the compressor discharge housing to prevent com-
pressor rotor backspin due to system refrigerant pres-
sure gradients during shutdown.
Motor cooling is accomplished by injecting intermedi-
ate pressure wet vapor into the motor which allows for
better efficiency than the traditional use of lower pres-
sure suction gas which requires more energy to raise
to discharge pressure. On demand high pressure liq-
uid injection provides additional motor cooling when
internal oil temperatures rise. Additional motor cooling
required during low load operation is supplied by a 5
ton TXV. A drain line drains excess liquid from the mo-
tor housing and feeds it into the gear cavity.
The compressor is lubricated by removing oil from the
refrigerant using an external oil separator. The pres-
surized oil is then piped back to the compressor for lu-
brication. The compressor design working pressure is
144 PSIG on the discharge side. Each chiller receives
a 300 PSIG low side and 450 PSIG high side factory
test. A 500 watt (115-1-60) immersion heater is located
in the compressor. The heater is temperature activated
to prevent refrigerant condensation.
EVAPORATOR
The system uses a Shell and Tube type Direct Expan-
sion Evaporator. Each of the two refrigerant circuits in
each of the 2 evaporators consists of 4 passes with the
chilled liquid circulating back and forth across the tubes
from one end to the other. The design working pres-
sure of the cooler shell on the liquid side is 150 PSIG,
and 235 PSIG for the tube (refrigerant side). The cooler
is equipped with a heater to provide freeze protection
to -20°F (-28.8°C).
Water connections are grooved to accept victaulic cou-
plings.
FIG. 17 – COMPRESSOR ROTORS
ACTUAL
ROTOR
ASSEMBLY
27936A(D)
LD01095
Содержание Millennium YDAS 410
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