RWB II ROTARY SCREW COMPRESSOR UNITS
INSTALLATION
070.200-IOM (DEC 11)
Page 14
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 (Figure 13) in order to maintain some preset pres‑
sure 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 antici‑
pated 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.
Figure 12 - Direct Expansion Economizer System
Figure 11 - Shell and Coil Economizer System
Figure 14 - Multiple Compressor Economizer System
Figure 13 - Flash Economizer System
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 economizer vessel and use one on each
compressor, as shown in Figure 14. It is also recommended
that the back‑pressure regulating valves, used on economizer
lines, should be specified 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
the PRR unit and the liquid feed solenoid on the DX vessel
off of a common signal to avoid liquid overfeed on the DX
economizer system.
If multiple compressors are operated with a common econ‑
omizer vessel, it is necessary to install a back‑pressure
regulator valve with an electric shutoff option in the vapor
line piped to the compressor's economizer port. If an electric
shut‑off is not installed in the economizer vapor line, valve
HV‑2 must remain closed to avoid a gas bypass from the
economizer line through the suction check valve bypass, back
to the suction line on a compressor that is shut down.
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