Trane RTWD Installation Operation & Maintenance page 74

Operating Principles
Evaporation of refrigerant occurs in the evaporator.
A metered amount of refrigerant liquid enters a
distribution system in the evaporator shell and is then
distributed to the tubes in the evaporator tube bundle.
The refrigerant vaporizes as it cools the water fl owing
through the evaporator tubes. Refrigerant vapor leaves
the evaporator as saturated vapor (State Pt. 1).
The refrigerant vapor generated in the evaporator fl ows
to the suction end of the compressor where it enters the
motor compartment of the suction-gas-cooled motor.
The refrigerant fl ows across the motor, providing
the necessary cooling, then enters the compression
chamber. Refrigerant is compressed in the compressor
to discharge pressure conditions. Simultaneously,
lubricant is injected into the compressor for two
purposes: (1) to lubricate the rolling element bearings,
and (2) to seal the very small clearances between the
compressor's twin rotors. Immediately following the
compression process the lubricant and refrigerant are
effectively divided using an oil separator. The oil-free
refrigerant vapor enters the condenser at State Pt. 2. The
lubrication and oil management issues are discussed
in more detail in the compressor description and oil
management sections that follow.
For RTWD units, a discharge baffl e within the condenser
shell distributes the compressed refrigerant vapor
evenly across the condenser tube bundle. Cooling tower
water, circulating through the condenser tubes, absorbs
heat from this refrigerant and condenses it.
For RTUD units, airfl ows across the condenser coils,
absorbing heat from the refrigerant and condensing it.
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As the refrigerant leaves the bottom of the condenser
(State Pt. 3), it enters an integral subcooler where it is
subcooled before traveling to the electronic expansion
valve (State Pt. 4). The pressure drop created by the
expansion process vaporizes a portion of the liquid
refrigerant. The resulting mixture of liquid and gaseous
refrigerant then enters the Evaporator Distribution
system (State Pt. 5). The fl ash gas from the expansion
process is internally routed to compressor suction, and
while the liquid refrigerant is distributed over the tube
bundle in the evaporator.
The RTWD/RTUD chiller maximizes the evaporator
heat transfer performance while minimizing refrigerant
charge requirements. This is accomplished by
metering the liquid refrigerant fl ow to the evaporator's
distribution system using the electronic expansion
valve. A relatively low liquid level is maintained in
the evaporator shell, which contains a bit of surplus
refrigerant liquid and accumulated lubricant. A liquid
level measurement device monitors this level and
provides feedback information to the CH530 unit
controller, which commands the electronic expansion
valve to reposition when necessary. If the level rises,
the expansion valve is closed slightly, and if the level is
dropping, the valve is opened slightly such that a steady
level is maintained.
RLC-SVX14H-GB