HPS Rotary Screw Compressor Units
070.700-IOM (MAR 21)
Page 12
Installation
Figure 16: Multiple compressor economizer system
INTERMEDIATE PRESSURE
GAS TO COMPRESSOR
SUCTION
STR
VCK
BPR
CONTROLLED PRESSURE
SATURATED LIQUID TO EVAPORATOR
ECONOMIZER
VESSEL
ECON4
HV-2
Economizer load balancing
The most energy efficient manner to operate an econo
-
mizer system, when using multiple compressors on a com
-
mon economizer vessel, is to take as much of the flash gas
as possible to the compressors that are fully loaded. This
can be done in at least two ways.
• Use the economizer output from the control panel to
turn off a solenoid, or to actuate the electric shutoff
option on a
back-pressure regulator, based on percent
compressor speed This directs all the
flash vapor to the
other higher speed compressors.
•
A dual-setpoint, back-pressure regulator valve can be
used in each of the individual economizer vapor lines.
When a compressor is running near full speed, the BPR
valve operates on the desired setpoint, or basically
wide open, to minimize pressure drop in the line. When
one compressor reduces operating speed to the point
that the economizer output on the control panel turns
on, the dual-setpoint feature of the regulator can be
actuated by this output to control the pressure, on the
vessel side of the regulator, to be a few psi higher.
Consequently, the
flash gas is sent to the higher speed
compressors first, until the capacity of the economizer
ports is exceeded, and the pressure in the vessel starts
to rise. Then, some of the vapor goes to the lower speed
compressor to help maintain the vessel at the desired
pressure. An example of a back-pressure regulator with
electric shutoff and the dual-setpoint feature is an R/S
A4ADS.
Economizing with VSDs
Economizing on a variable speed machine can pres
-
ent challenges. At low load conditions, the mass flow
through the economizer line is reduced. This may cause
the economizer check valve to chatter. Chattering leads to
premature failure of the check valve.
This can be avoided through the use of a smaller check
valve or by limiting the minimum speed of the compres
-
sor. The disadvantage of using a smaller check valve is
increased pressure drop during full load conditions. If ex
-
tended periods of low compressor loading are anticipated,
use an automated (non-modulating) full port ball valve
in lieu of a check valve. This valve must be equivalent in
size to the economizer line size. The ball valve must be
opened when the compressor starts, and closed when the
compressor stops. The ball valve must close automatically
upon power failure.
Electrical
Notice
Before proceeding with electrical installation, read
the instructions in the section
tronic equipment in an industrial environment.
HPS units are supplied with a
Quantum™ HD
control sys-
tem. Care must be taken that the controls are not exposed
to physical damage during handling, storage, and installa-
tion. The single-box control door must be kept tightly
closed to prevent moisture and foreign mat ter from entry.
Notice
All customer connections are made in the single-
box control mounted on the oil separator. This is
the ONLY electrical enclosure. Keep it tightly closed
whenever work is not being done in it.
Motor VSD package
Caution
When starting at full voltage or across-the-line, a
shunting device must be installed or the Analog I/O
board in the Quantum™ HD panel may be severely
damaged at start-up. See
.
Motor drive and interlock wiring require ments are shown
in the
. All of the equipment shown is supplied by
the installer, unless a variable speed drive (VSD) pack-
age is pur chased separately from Johnson Controls-Frick.
Drive packages consist of:
•
The variable speed drive of the specified horsepower
and voltage.
•
If specified, the starter package can be supplied as
a combination VSD with circuit breaker disconnect.
Howev er, other can apply the motor overcurrent pro-
tection/disconnection device, usually as a part of an
electrical power distribution board.
• A 3.0 KVA control power transformer (CPT) to supply
120 V control power to the Quantum HD control system
and separator oil heaters is included. If environ mental
condi tions need more than the usual two
500 W oil heaters, this requires an appropriately over-
sized
control transformer. If frequent power fluc tuations
are anticipat ed or extremely noisy power lines are
encoun tered, consider using a regulating control trans
-
former. Contact Johnson Controls-Frick for assistance.
