Johnson Controls Frick RWF 100, Руководство по установке

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RWF ROTARY SCREW COMPRESSOR UNITS
INSTALLATION
S70-600 IOM (OCT04)
Page 9
ECONOMIZER - HIGH STAGE (OPTIONAL)
The economizer option provides an increase in system ca-
pacity and efficiency by subcooling liquid from the condenser
through a heat exchanger or flash tank before it goes to the
evapora tor. The subcooling is provided by flashing liquid in
the economizer cooler to an intermediate pressure level.The
intermediate pressure is provided by a port located part way
down the compres sion process on the screw compressor.
As the screw compressor unloads, the economizer port will
drop in pressure level, eventually being fully open to suc-
tion. Because of this, an output from the microproces sor
is generally used to turn off the supply of flashing liquid on
a shell and coil or DX economizer when the capacity falls
below approximately 45%-60% capacity (85%-90% slide
valve position). This is done because the compressor will
be more efficient operating at a higher slide valve position
with the economizer turned off, than it will at a low slide valve
position with the economizer turned on. Please note however
that shell and coil and DX economizers can be used at low
compressor capaciti es in cases where efficien cy is not as
important as assuring that the liquid supply is subcooled. In
such cases, the economi zer liquid solenoid can be left open
whenever the com pressor is running.
Due to the tendency of the port pressure to fall with de creasing
compressor capacity, a back-pressure regulator valve (BPR)
is generally required on a flash economizer system (FIG. 8) in
order to maintain some preset pressure dif ference between
the subcooled liquid in the flash vessel and the evaporato rs.
If the back-pressure regulator valve is not used on a flash
economizer, it is possible that no pressure difference will exist
to drive liquid from the flash vessel to the evaporators, since
the flash vessel pressure will approach suction pressure at a
decreased slide valve position. In cases where wide swings
in pressure are anticipated in the flash econo mizer vessel, it
may be necessary to add an outlet pressure regulator to the
flash vessel outlet to avoid overpressurizing the economizer
port, which could result in motor overload. Example: A system
feeding liquid to the flash vessel in batches.
The recommended economizer systems are shown below.
Notice that in all systems there should be a strainer (STR)
and a check valve (VCK) between the economizer vessel
and the economizer port on the compressor. The strainer
prevents dirt from passing into the compressor and the check
valve prevents oil from flowing from the compressor unit to
the econo mizer vessel during shutdown.
Other than the isolation valve
needed for strainer cleaning, it is
essential that the strainer be the
last device in the economizer line before the compres-
sor. Also, piston-type check valves are recom mended
for installation in the economizer line, as opposed to
disc-type check valves. The latter are more prone to gas-
pulsation-induced failure. The isolation and check val ves
and strainer should be located as closely as possible to
the compressor, preferably within a few feet.
For refrigeration plants employing multiple compressors on a
common economizing vessel, regardless of economizer type,
each compressor must have a back-pressure regulat ing valve
in order to balance the economizer load, or gas flow, between
compressors. The problem of balancing load becomes most
important when one or more compressors run at partial load,
exposing the economizer port to suction pressure. In the case
of a flash vessel, there is no need for the redundancy of a
back-pressure regulating valve on the vessel and each of the
multiple compressors. Omit the BPR valve on the flash econo-
mizer vessel and use one on each compressor, as shown
in Figure 9. It is also recommended that the back-pressure
regulating valves, used on economizer lines, should be speci-
fied with electric shutoff option. The electric shutoff feature
is necessary to prevent flow from the common economizer
vessel to the suction side of a stopped compressor, through
the suction check valve bypass line, if the other compressors
and the common economizer vessel are still operating and
the HV2 valve on the suction bypass is open.
For refrigeration plants using a Packaged Refrigerant Recir-
culation (PRR) unit and a direct expansion (DX) economizer
system it is necessary to operate the liquid feed solenoid on
Figure 7 - Direct Expansion Economizer System
Figure 6 - Shell and Coil Economizer System
Figure 9 -Multiple Compressor Economizer System
Figure 8- Flash Economizer System