Frick HPS 36 Installation Manual Download Page 11

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HPS Rotary Screw Compressor Units

070.700-IOM (MAR 21)

Page 11

Installation

Economizer (optional)

The economizer option provides an increase in system

capacity 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.

See

Figure 13

Figure 16

for the example economizer

systems. Notice a strainer (STR) and a check valve (VCK)
between the economizer vessel and the economizer port
on the compressor in all systems. 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.

Warning

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 necessary for
installation in the economizer line, as opposed to
disc-type check valves. The latter are more prone to
gas-pulsation-induced failure. Place the isolation and
check val ves and strainer 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. 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

back pressure regulator (BPR) valve on the flash

economizer vessel and use one on each compressor, as
shown in

Figure 14

. It is also best practice to specify the

back-pressure regulating valves, used on economizer lines,

with electric shutoff option. The electric shutoff feature

is necessary to prevent flow from the common econo

mizer 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

Recirculation Unit and a direct expansion (DX) economizer
system it is necessary to operate the liquid feed solenoid
on the 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
economizer vessel, it is necessary to install a back-pres-

sure regulator valve with an

electric shutoff option in the

vapor line piped to the compressor's economizer port.

Figure 13: Shell and coil economizer system

HIGH

PRESSURE

LIQUID

INTERMEDIATE PRESSURE

GAS TO COMPRESSOR