Refrigeration Cycle; Motor And Lubricating Oil; Cooling Cycle - Carrier AquaEdge 19XRV series Operation, Maintenance And Installation Manual

REFRIGERATION CYCLE

The compressor continuously draws refrigerant vapor from
the cooler at a rate set by the amount of guide vane opening. As
the compressor suction reduces the pressure in the cooler, the
remaining refrigerant boils at a fairly low temperature (typical-
ly 38 to 42 F [3 to 6 C]). The energy required for boiling is ob-
tained from the water flowing through the cooler tubes. With
heat energy removed, the water becomes cold enough to use in
an air conditioning circuit or for process liquid cooling.
After taking heat from the water, the refrigerant vapor is
compressed. Compression adds still more heat energy, and the
refrigerant is quite warm (typically 98 to 102 F [37 to 40 C])
when it is discharged from the compressor into the condenser.
Relatively cool (typically 65 to 90 F [18 to 32 C]) water
flowing into the condenser tubes removes heat from the refrig-
erant and the vapor condenses to liquid.
The liquid refrigerant passes through orifices into the
FLASC (Flash Subcooler) chamber (Fig. 4 and 5). Since the
FLASC chamber is at a lower pressure, part of the liquid refrig-
erant flashes to vapor, thereby cooling the remaining liquid.
The FLASC vapor is re-condensed on the tubes which are
cooled by entering condenser water. The liquid drains into a
float valve chamber between the FLASC chamber and cooler.
Here the AccuMeter™ float valve forms a liquid seal to keep
FLASC chamber vapor from entering the cooler. When liquid
refrigerant passes through the valve, some of it flashes to vapor
in the reduced pressure on the cooler side. In flashing, it re-
moves heat from the remaining liquid. The refrigerant is now at
a temperature and pressure at which the cycle began. Refriger-
ant from the condenser also cools the oil and optional variable
speed drive.
The refrigeration cycle for a 19XRV chiller with two-stage
compressor is similar to the one described above, with the fol-
lowing exception: Liquid refrigerant from the condenser
FLASC chamber linear float valve flows into an economizer at
intermediate pressure (see Fig. 5). As liquid enters the cham-
ber, due to the lower pressure in the economizer, some liquid
flashes into a vapor and cools the remaining liquid. The sepa-
rated vapor flows to the second stage of the compressor for
greater cycle efficiency. A damper valve located on the econo-
mizer line to the compressor acts as a pressure regulating de-
vice to stabilize low load, low condensing pressure operating
conditions. The damper will back up gas flow and thereby raise
the economizer pressure to permit proper refrigerant flow
through the economizer valve during those conditions. The
damper also is closed during start-up conditions to allow the
second stage impeller to start unloaded.
The subcooled liquid remaining in the economizer flows
through a float valve and then into the cooler.

MOTOR AND LUBRICATING OIL

COOLING CYCLE

The motor and the lubricating oil are cooled by liquid re-
frigerant taken from the bottom of the condenser vessel
(Fig. 4 and 5). Refrigerant flow is maintained by the pressure
differential that exists due to compressor operation. After the
refrigerant flows past an isolation valve, an in-line filter, and a
sight glass/moisture indicator, the flow is split between the mo-
tor cooling and oil cooling systems.
IMPORTANT: To avoid adverse effects on chiller opera-
tion, considerations must be made to condenser water tem-
perature control. For steady state operation, the minimum
operating refrigerant pressure differential between cooler
and condenser is approximately 20 psi (138 kPa) with a
maximum evaporator refrigerant temperature of 65 F
(18 C). Consult Chiller Builder for required steady state
operational limits. Inverted start conditions are acceptable
for short durations of time, but for periods exceeding
5 minutes, a special control solution strategy should be
used to allow the chiller to establish a minimum refrigerant
pressure differential (and thereby adequate equipment
cooling).
Flow to the motor cooling system passes through an orifice
and into the motor. Once past the orifice, the refrigerant is
directed over the motor by a spray nozzle. The refrigerant
collects in the bottom of the motor casing and is then drained
back into the cooler through the motor refrigerant drain line.
An orifice (in the motor shell) maintains a higher pressure in
the motor shell than in the cooler. The motor is protected by a
temperature sensor embedded in the stator windings. An
increase in motor winding temperature past the motor override
set point overrides the temperature capacity control to hold,
and if the motor temperature rises 10 F (5.5 C) above this set
point, closes the inlet guide vanes. If the temperature rises
above the safety limit, the compressor shuts down.
Refrigerant that flows to the oil cooling system is regulated
by thermostatic expansion valves (TXVs). The TXVs regulate
flow into the oil/refrigerant plate and frame-type heat exchang-
er (the oil cooler in Fig. 4 and 5). The expansion valve bulbs
control oil temperature to the bearings. The refrigerant leaving
the oil cooler heat exchanger returns to the chiller cooler.
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