Trane CITY RTSF Series Manual page 28

Operating Principles Mechanical
Cycle Description
The refrigeration cycle for the chiller can be described using the pressure-enthalpy diagram shown in Figure 5.
Key State Points are indicated on the fi gure and are referenced in the discussion following. Typical schematics of
the system showing the refrigerant fl ow loop as well as the lubricant fl ow loop is shown in Figure.
Evaporation of refrigerant occurs in the evaporator that maximizes the heat transfer performance of the heat
exchanger while minimizing the amount of refrigerant charge required. A metered amount of refrigerant liquid enters
a distribution system in the evaporator and is then distributed to plates in the evaporator.
The refrigerant vaporizes as it cools the water fl owing through the evaporator plates. Refrigerant vapor leaves
the evaporator as superheated vapor (State Point 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 Point 2. The lubrication and oil management
issues are discussed in more detail in the compressor description and oil management sections that follow.
Cooling device water, circulating through the condenser plates, absorbs heat from this refrigerant and condenses it.
Refrigerant leaves the condenser as subcooled liquid and travels to the electronic expansion valve (State Point 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 Point 5).
The 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 compressor suction superheat measurement provides feedback information to the Tracer UC800 unit controller,
which commands the electronic expansion valve to reposition when necessary.
Figure 5 – Pressure /Enthalpy Curve
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