Specifications

36
the EXV flashes to vapor, further subcooling the liquid that is
maintained at the bottom of the economizer. This increase in
subcooling provides additional capacity. Since no additional power
is required to accomplish this, the efficiency of the machine also
improves. The vapor that flashes will rise to the economizer where
it passes to the compressor and is used as needed to provide motor
cooling. After passing over the motor windings, the refrigerant
reenters the cycle at an intermediate port in the compression cycle.
Oil pumps
The 30GX/HXC screw chillers use one externally mounted pre-
lubricating oil pump per circuit. This pump is operated as part ot
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 condensers of 30HXC units. When a
circuit is required to start, the controls will energize the oil pump
first and read the current oil pressure transducer reading. The
pump will be operated for a period of 20 seconds, after which the
oil solenoid will be energized to open the oil inlet valve at the
compressor. The control will again read the pressure from the oil
pressure transducer. If the pump has built up sufficient oil pressure,
the compressor will be allowed to start. Once the compressor has
started, the oil pump will be turned off within 10 seconds and
will not be used again until the next start-up. If the pump was not
able to build up enough oil pressure, the pump will be turned off.
Within 3 seconds, the pump will be re-energized and will make
one additional attempt to build oil pressure. The control will
generate an alarm if the second attempt fails.
Motor cooling valves
Compressor motor winding temperatures are controlled to a
setpoint of 85°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 (GX only)
On GX units, this valve is located on the oil separator outlet. 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 and an internal spring is
connected to a valve which will close if the pressure in the oil
separator is not at least 100 kPa greater than the economizer
pressure.
Sensors
The units use eight standard thermistors (including two motor temperature
thermistors) and two level thermistors and eight standard pressure
transducers to monitor and control system operation.
Thermistors
Cooler leaving fluid
This temperature is used to measure the leaving cooler fluid
temperature (water or brine). The temperature is used for leaving
fluid temperature control and to protect against cooler freeze-up. It
is located in the cooler fluid nozzle.
Cooler entering fluid
This sensor is used to measure the cooler entering fluid temperature.
It is located in the entrering cooler nozzle. It is used to provide
automatic temperature compensation for the leaving fluid
temperature control with entering fluid compensation.
Discharge gas temperature (circuits A & B)
This sensor used to measure the discharge gas temperature and
control the discharge temperature superheat. It is located on the
discharge pipe of each circuit (30HXC) or on the top of the oil
separator (30GX).
Motor temperature
The Compressor Protection Module (CPM) monitors the motor
temperature. Thermistor terminals are located in the compressor
junction box.
Cooler liquid level (circuits A & B)
Cooler liquid level thermistor is used to provide optimized flow
control in the cooler. It is installed in the top of the cooler.
Condenser entering fluid (optional on 30HXC)
This sensor is used to measure the temperature of the fluid entering
the water cooled condensers. It is located in the common fluid line
entering the condensers (field installed). On Heat Machines it is
used by the capacity control routine. On water cooled condensers it
is only used for monitoring of the condenser fluid temperature.
Condenser leaving fluid (optional on 30HXC)
This sensor is used to measure the temperature of the fluid leaving
the water cooled condensers. It is located in the common fluid line
leaving the condensers (field installed). On Heat Machines it is used
by the capacity control routine. On water cooled condensers it is
only used for monitoring of the condenser fluid temperature.
Space temperature, outdoor air temperature (optional)
These temperatures are used to measure the temperature of the space
or the outside air temperature respectively for reset control based
on Outside Air or Space Temperature reset options.
Pressure transducers
Discharge pressure (circuits A & B)
This input is used to measure the high side pressure of each circuit
of the unit. It is located on the discharge pipe of each circuit (30HXC)
or on the top of the oil separator (30GX). It is used to provide the
pressure to replace the discharge pressure gauge and to control the
head pressure.
Suction pressure (circuits A & B)
This input is used to measure the pressure of the low side of the
unit. It is located on the top of the cooler of each circuit.
Oil pressure (each compressor)
This input is used to measure the oil pressure of each compressor of
the unit. The ecomizer pressure is substracted from this reading to
determine the oil pressure differential. It is located on the oil pressure
port of each compressor.
Economizer pressure (circuits A & B)
This input is used to monitor the oil pressure differential supplied to
the compressor. It is located on the economizer suction pipe of each
circuit.