Oil Pumps; Motor Cooling; Back Pressure Valve; And 30Hxa Only) - Carrier ComfortLink 6 Series Controls, Start-Up, Operation, Service, And Troubleshooting

Oil Pumps —
The 30GXN,GXR,HX screw chillers use
one externally mounted prelubricating oil pump per circuit.
This pump is operated as part of the start-up sequence. On
30GXN,R units, the pumps are mounted above the base rails
on the oil separator side of the unit (see Fig. 3). The pumps are
mounted to a bracket on the condensers 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 after 15 seconds.
Once the compressor has started, the oil pump will continue
to run for 120 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 two additional attempts, if necessary, to build oil
pressure. The control generates an alarm if the third attempt
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 too low the oil pump will be
started. Just before the oil pump is started the control measures
the pressure differential between the discharge pressure 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
requirement.
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
Oil pressure
Oil system pressure drop
Economizer pressure
Differential oil pressure
Suction pressure
Based on the above conditions the oil pump will be started
because differential oil pressure equals 10 psi. See Table 2.
80 psi
65 psi
80 – 65 = 15 psi
55 psi
(65 – 55) = 10 psi
40 psi
OIL FEED
TO COMPRESSOR
OIL SUPPLY
LINE TO PUMP
Fig. 3 — Oil Pump
Table 2 — Oil Pump Suction Pressure
Requirements
SUCTION PRESSURE
(SP)
≤ 35 psig
35 psig < SP < 51 psig
≥ 51 psig
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
started). The only way this can be satisfied is if the discharge
pressure increases or the compressor unloads at which point the
oil pump will be shut off.
Motor Cooling —
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 30GXN,R units equipped with economizers, flash
gas leaves the top of the economizer (when the circuit is fully
loaded for 30GXN,R models only) and continually flows to the
motor windings. All refrigerant used for motor cooling reenters
the rotors through a port located midway along the compres-
sion cycle and is compressed to discharge pressure.
Back Pressure Valve (30GXN,R and 30HXA
only) —
This valve is located on the oil separator outlet on
30GXN,R 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 (economiz-
er 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 30GXN,GXR,HX ComfortLink™ control
system gathers information from sensors to control the operation
of the chiller. The units use up to 10 standard pressure transduc-
ers and up to 10 standard thermistors (including 4 motor
temperature thermistors). The sensors are listed in Table 3.
5
CHECK
VALVE
OIL
SEPARATOR
FILTER
OIL PUMP TURNS ON
WHEN DIFFERENTIAL
PRESSURE IS LESS THAN:
12 psig
14.5 psig
17 psig
Compressor motor winding tempera-
OIL
PUMP