4
ECONOMIZER OPERATION — Economizers are factory
installed on 30GX105-350 units and 30HXA,C161-271 units.
All other sizes use standard EXVs. The economizer improves
both the chiller capacity and efficiency as well as providing
compressor motor cooling. Inside the economizer are both a
linear stepper motor (same as standard EXV motor) and a float
valve. The stepper motor is controlled by the processor to
maintain the desired liquid level in the cooler (as is done for
chillers without economizers). The float valve maintains a liq-
uid level in the bottom of the economizer.
Liquid refrigerant is supplied from the condenser through
the end to the bottom of the economizer. A bubbler tube sup-
plies a small amount of discharge gas to ensure that the float
will be able to work properly. As the refrigerant passes through
the EXD, its pressure is reduced to an intermediate level of
about 75 psig (517 kPag). This pressure is maintained inside
the economizer shell. Next, the refrigerant flows through the
float valve where its pressure is further reduced to slightly
above the pressure in the cooler.
The increase in performance is achieved when some of the
refrigerant passing through the EXD flashes to vapor, further
subcooling the liquid that is maintained at the bottom of the
economizer. This increase in subcooling provides additional
capacity. Also, since the additional power required to accom-
plish this is minimal, the efficiency of the machine improves.
The vapor that flashes rises to the top of the economizer where
it passes to the compressor and is used to provide motor cool-
ing. After passing over the motor windings, the refrigerant
reenters the cycle at an intermediate port in the compression
cycle.
Oil Pumps —
The 30GX,HX screw chillers use one exter-
nally mounted prelubricating oil pump per circuit. This pump
is operated as part of the start-up sequence. On 30GX units, the
pumps are mounted to the base rails on the oil separator side of
the unit. The pumps are mounted to a bracket on the condens-
ers of 30HXC units and to the oil separator on 30HXA units.
When a circuit is required to start, the controls energize the
oil pump first and read the oil pressure transducer reading. The
pump is operated for a period of 20 seconds, after which the oil
solenoid is energized to open the oil inlet valve at the compres-
sor. The control again reads the pressure from the oil pressure
transducer. If the pump has built up sufficient oil pressure, the
compressor is allowed to start.
Once the compressor has started, the oil pump is turned off
within 10 seconds. If the pump is not able to build up enough
oil pressure, the pump is turned off. Within 3 seconds, the
pump is re-energized and makes one additional attempt to build
oil pressure. The control generates an alarm if the second at-
tempt fails.
The oil pump is also used to supplement system pressure
under certain operating conditions. The oil flow requirements
of the compressor vary based on pressure differential across the
compressor. The oil pump is designed to provide differential oil
pressure during low pressure differential conditions. It is not
designed to overcome high pressure drop across filters during
high pressure differential conditions.
If the differential oil pressure (oil pressure – economizer
pressure) for a compressor is less than 13 psi then the oil pump
will be started. Just before the oil pump is started the control
measures the pressure differential between the discharge pres-
sure and oil pressure (oil system pressure drop). The oil system
pressure drop is saved and used to determine when the oil
pump should be shut off.
When the oil pump is operating, it is capable of increasing
oil pressure from 0 psi to 50 psi depending on the oil flow
requirements of the compressor. For example, if the compres-
sor needs 2 gpm (high pressure differential condition) and the
oil pump is capable of 1.2 gpm, there is no pressure rise and the
oil flow will bypass the check valve and supply the 2 gpm to
the compressor. If the compressor requires .75 gpm, the oil
pump will increase pressure to satisfy the oil pressure require-
ment.
The pump will continue to operate until the discharge pres-
sure minus economizer pressure is greater then 17 psi plus the
oil system pressure drop.
Example:
Discharge pressure
80 psi
Oil pressure
65 psi
Oil system pressure drop 80 –65 = 15 psi
Economizer pressure
55 psi
Suction
pressure 42
psi
Based on the above conditions the oil pump will be started
because differential oil pressure equals 10 psi.
The oil pump will continue to operate until the discharge
pressure minus economizer pressure (which equals 25) is
greater than 17 plus 15 (oil system loss before pump was start-
ed). The only way this can be satisfied is if the discharge pres-
sure increases or the compressor unloads at which point the oil
pump will be shut off.
Motor Cooling —
Compressor motor winding tempera-
tures are controlled to a set point of 200 F (93.3 C). The control
accomplishes this by cycling the motor cooling solenoid valve
to allow liquid refrigerant to flow across the motor windings as
needed. On units equipped with economizers, flash gas leaves
the top of the economizer and continually flows to the motor
windings. All refrigerant used for motor cooling re-enters the
rotors through a port located midway along the compression
cycle and is compressed to discharge pressure.
Back Pressure Valve (30GX and 30HXA
only) —
This valve is located on the oil separator outlet on
30GX units and mounted on the oil separator shell of 30HXA
units. The valve’s function is to ensure that there is sufficient
system differential pressure to allow for oil to be driven back to
the compressor. A small copper line (economizer pressure) is
connected to the top of the valve, which contains an internal
spring that closes a piston if the pressure in the oil separator is
not at least 15 psig greater than the economizer pressure.
Sensors —
The 30GX,HX control system (based on the
Flotronic™ II chiller control system) gathers information from
sensors to control the operation of the chiller. The units use up
to 10 standard pressure transducers, up to 8 standard ther-
mistors (including 4 motor temperature thermistors), and 2 liq-
uid level thermistors to monitor and control system operation.
The sensors are listed in Table 2.
Compressor Protection Module (CPM) —
One
CPM controls up to 2 compressors. The CPM provides the fol-
lowing functions:
• compressor main contactor control|
• Wye-Delta contactor transition
• compressor ground current protection
• motor temperature reading
• high-pressure protection
• reverse rotation protection
• current imbalance protection
• compressor oil solenoid control
• motor cooling solenoid control
• sensor bus communications
• starting and running overcurrent protection