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supply and equalize oil pressure to and around the pis-
ton. This allows the slide valve to unload and load.
• seals the gap between the male and female rotors. The
oil hydrodynamically seals this space to allow the refrig-
erant vapor to be compressed. A specific flow rate of oil
is injected into the compressor rotor housing at the point
where the compression process is initiated.
• cools the compressed refrigerant vapor. The oil that is
injected into the compressor for sealing also acts as a
heat sink by absorbing a portion of the heat from com-
pression. Thus, constant and cool compressor discharge
gas temperature, relative to an oil-less screw compres-
sor, is maintained.
Oil is charged into the system through a hand valve located
on the bottom of the oil sump (TC frame 1 and 2 chillers) or
separator (TD frame 4 chillers). Sight glasses on the oil sump
(TC frame 1 and 2 chillers) and/or oil separator (TD frame 4
chillers) permit oil level observation. When the compressor is
shut down, an oil level should be visible in the oil sump (TC
frame 1 and 2 chillers) or the lower oil separator sight glass
(TD frame 4 chillers). During operation, the oil level should
rise and be visible in the oil separator sight glass (TC frame 1
and 2 chillers) or the upper oil separator sight glass (TD frame
4 chillers). Approximately 4.2 gal. (15.9 L) of an oil and refrig-
erant mixture accumulates in the sump of TC frame 1 and 2
chillers. Approximately 10 gal (38 L) of oil accumulates in the
separator and 2 gal. (7.6 L) accumulates on the cooler of TD
frame 4 chillers.
Oil is driven from the oil separator through an oil filter to
remove foreign particles. The oil filter has a replaceable car-
tridge. The filter housing is capable of being valved off to per-
mit removal of the filter (see Maintenance sections, pages 72-
76, for details). The oil then travels through a shutdown sole-
noid and past a pressure transducer to three separate inlets on
the compressor. The oil pressure measured by the transducer is
used to determine the oil pressure differential and pressure drop
across the oil filter. The oil pressure differential is equal to the
difference between the oil pressure transducer reading and the
evaporator pressure transducer reading. It is read directly from
the Chiller Visual Controller (CVC) default screen.
Part of the oil flow to the compressor is directed to the slide
valve and is used for capacity control positioning. The remain-
ing oil flow is divided between the rotors and bearings. A spe-
cific quantity is sent to the rotors and injected at the start of
compression to seal the clearances between the rotors. Another
portion is sent to the bearings and used for lubrication.
Oil leaves the compressor mixed with the compressed dis-
charge refrigerant vapor. The mixture then enters the oil sepa-
rator, where oil is removed from the refrigerant and collected at
the bottom to complete the cycle.
TC FRAME 1 AND 2 CHILLERS — The oil and refrigerant
vapor mixture enters the oil separator through a nearly tangen-
tial nozzle, giving a rotational flow pattern. Oil is thrown to the
sides of the oil separator and runs down the walls to a chamber
in the bottom where it drains to the sump. A baffle separates
this chamber from the vortex flow to prevent re-entrainment.
Gas flows up through a vortex funnel to a removable coalesc-
ing element where the rest of the oil collects. This oil runs
down the element surface to a scavenge line which is piped to
the first closed lobe port.
TD FRAME 4 CHILLERS — The oil and refrigerant vapor
mixture is directed against the rear wall of the oil separator as it
enters the side of the oil separator. This action causes the bulk
of the oil to drop from the refrigerant and collect at the bottom
of the oil separator. A mesh screen is provided near the oil sep-
arator outlet to remove any additional oil which may still be en-
trained in the refrigerant vapor.
The oil sump (TC frame 1 and 2 chillers) contains a com-
bined level switch and temperature sensor, 500-watt oil heater
(TC frame 1 and 2 chillers), and oil filter. Oil temperature is
measured and displayed on the CVC default screen. During
shutdown, oil temperature is maintained by the Product Inte-
grated Control II (PIC) II). See Oil Sump Temperature Control
section on page 39.
NOTE: TD frame 4 chillers do not have an oil heater.
Operating oil pressure must be at least 20 psi (138 kPa) for
HCFC-22 [7 psi (48.3 kPa) for HFC-134a] and is dependent
upon system pressure differential (lift). The oil pressure trans-
ducer is located downstream of the filter, so the value displayed
on the CVC will be slightly less than the lift value. Under nor-
mal full load conditions, oil pressure is approximately 120 psi
(827 kPa) [76 psi (517 kPa)]. If sufficient system differential
pressure is not established or maintained, oil pressure will not
be established (or will be lost) and chiller shutdown will result.
The compressor provides a pressure differential, but the sys-
tem pressure differential is constrained by the temperatures of
the chilled and tower water circuits. Cold tower water, rapid
tower water temperature swings, and high return chilled water
temperature are among the factors which could contribute to
frequent low oil pressure alarms. To help ensure that suitable
oil pressure is established at start-up, sufficient tower water
control should exist. Increasing the chiller ramp loading rate
will allow faster compressor load up. This will quickly estab-
lish the refrigerant and, therefore, oil pressure differential.
Conversely, rapid loading of the compressor could cause
any refrigerant in the oil to flash due to the sudden drop in suc-
tion pressure. During initial start-up, the 23XL PIC II control
follows a ramped oil pressure requirement algorithm for the
first 160 seconds. Therefore, the PIC II control follows and in-
ternal oil pressure ramp loading schedule during initial start-up.
See The Troubleshooting Guide section on page 76 for further
information.
If the start-up oil pressure falls below the values specified in
Table 1, the PIC II control will shut down the chiller.
Table 1 — Oil Pressure Ramp-Up Rate
MINIMUM START-UP OIL PRESSURE
TIME
(SEC)
HCFC-22
psi
40
1.4
80
4
120
7
Oil Reclaim System —
to return oil from the cooler back to the compressor.
TC FRAME 1 AND 2 CHILLERS — The oil reclaim sys-
tem returns oil to the compressor using discharge gas pressure
to power an ejector. The oil and refrigerant mixture is vacu-
umed from the top of the cooler liquid refrigerant level and dis-
charged into the compressor suction port.
TD FRAME 4 CHILLERS — TD frame 4 chillers do not re-
quire an external oil reclaim system.
Oil Loss Prevention —
top of the cooler, where oil collects during low-load operation.
The cooler is designed so that when oil drains into the cooler
from the compressor during low loads, it will be re-entrained
with the suction gas flow.
In addition, the PIC II Controls minimize oil loss to the
cooler once the rotor inlet temperature sensor detects hot oil
draining down the suction pipe.
If the rotor inlet temperature increases 4 F (2.2 C) in TC
(frame 1 and 2) chillers or TD (frame 4) chillers above the
leaving chilled water temperature, the slide valve is proportion-
ately moved in the load direction to increase suction gas veloc-
ity. The chiller will continue to load until the rotor inlet temper-
ature is equal to LCWT+1° F or the chiller recycles.
9
REQUIREMENT
HFC-134a
kPa
psi
kPa
9.7
1.4
9.7
27.6
4
27.6
48.3
7
48.3
The oil reclaim system operates
The suction pan is located on
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