24
To reduce the possibility of false oil level alarms, Carlyle
recommends that the oil level safety switch should be
open continuously for five seconds prior to initiating a
compressor shutdown.
4.5 Oil Filter
Provisions should be made to isolate the oil filter using
some combination of shutoff valves and/or check valves.
See Fig. 19.
This will allow for the filter element to be replaced without
remo
ving or isolating the charge in the unit. The oil filter
must be installed after the oil cooler and as close to the
compressor as feasible.
Because of the long bearing life requirements, filtration
for this compressor is very stringent. The Beta Ratio for
this filter is greater than or equal to 200 for a five micron
particle size evaluated using ISO 16889.
Filter areas must also be sufficient to avo
id premature
clogging of the filter during normal operation. The Carlyle
supplied filters have a filtration area of > 5000 cm
2
. Car-
lyle recommends monitoring the pressure differential
across the oil filter. An alarm should be activated if the
differential pressure exceeds 2 bar (29 psig) and the oil
filter should be replaced.
4.6 Oil Cooling Systems
Carlyle's 06TSR/TTR refrigeration-duty and 06TSM/TTM
medium-temperature-duty compressor models require oil
cooling for all operating points in the application enve-
lope. The oil temperature entering the compressor is to
be maintained at 115°F (46.1°C) at all times over the
range of operation.
Be aware that air-cooled oil coolers may not be suitable
for applications that have very high design saturated dis-
charge temperatures. The maximum oil temperature
entering the compressor is 115°F (46.1°C). If the design
outdoor ambient air temperature for the app
lication is
greater than 115°F (46.1°C), an alternate oil cooling
method should be selected.
Carlyle offers a complete line of air-cooled oil coolers to
assist systems designers. Please see Section 9.9.1. A
mixing valve is recommended for all oil coolers circuited
through a remote air-cooled condenser.
4.7 Oil Cooler Selection
An oil cooler is required for all Paragon low/medium tem-
perature screw compressor applications. As noted in
Section 4.6, the m
aximum oil temperature entering the
compressor may not exceed 115°F (46.1°C).
Carlyle's Solutions software calculates the oil cooler load
based on the operating parameters and compressor
models selected. The oil cooler must maintain the oil
temperature entering the compressor at 115°F (46.1°C)
maximum over the range of operation.
Several methods exist to control oil temperature, including:
• Oil cooler fan cycling based on oil outlet temperature
(measured at the outlet of the oil cooler, controlled to
10 F T)
• Oil cooler bypass, via a solenoid valve, controlled by
the temperature of the oil entering the oil cooler.
• Use of a mixing valve to maintain a constant oil tem-
perature entering the compressor.
• Some combination of the three methods listed
above.
The oil may be cooled by means of air-cooled, refriger-
ant-cooled, or water-cooled oil
coolers. Following are
selection criteria for the various models, along with
dimensional information.
If using a refrigerant-cooled oil cooler, the oil cooling load
will need to be subtracted from either the compressor's
evaporator capacity or the subcooling capacity, depend-
ing on where the oil cooler's suction gas flow enters the
compressor. The superheated gas from refrigerant-
cooled oil coole
rs may be connected to the economizer
port or the suction line of the compressor.
Using a compressor suction port will lead to a reduction
in system capacity since some of the compressor suction
mass flow will now come from the oil cooler.
Using the compressor interstage port for oil cooling will
not reduce the compressor suction pumping capacity, but
will indirectly reduce system capacity by decrea
sing the
compressor's ability to perform liquid subcooling.
The additional mass flow from the oil cooler to the inter-
stage will increase the interstage pressure. This method
will marginally raise the economizer pressure, which may
increase the liquid temperature of the liquid refrigerant
subcooler.
Both methods will require holdback valves to prevent the
oil temperature from dropping below 80°F (27°C).
The heat rejected from the oil cooler may
be used for
heat reclaim processes, such as water heating. Since the
oil cooler rejects some discharge heat, the heat rejection
from the oil cooler must be considered when sizing the
system condenser (unless a refrigerant-cooled oil cooler
is used). This may allow the application of a smaller con-
denser.
Condenser circuiting may also be used for oil cooling;
however, pressure drops must be taken i
nto account for
minimum oil pressure differential to the compressors.
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