4 - 2 RTHC-IOM-1C
Refrigeration (Cooling) Cycle
The refrigeration cycle of the Series R chiller is
conceptually similar to that of other Trane chiller
products. It makes use of a shell-and-tube
evaporator design with refrigerant evaporating on the
shell side and water flowing inside tubes having
enhanced surfaces.
The compressor is a twin-rotor helical rotary type. It
uses a suction gas-cooled motor that operates at
lower motor temperatures under continuous full and
part load operating conditions. An oil management
system provides oil-free refrigerant to the shells to
maximize heat transfer performance, while providing
lubrication and rotor sealing to the compressor. The
lubrication system ensures long compressor life and
contributes to quiet operation.
Condensing is accomplished in a shell-and-tube heat
exchanger where refrigerant is condensed on the
shell side and water flows internally in the tubes.
Refrigerant is metered through the flow system using
an electronic expansion valve, that maximizes chiller
efficiency at part load.
A unit-mounted starter and control panel is provided
on every chiller. Microprocessor-based unit control
modules (UCP2) provide for accurate chilled water
control as well as monitoring, protection and adaptive
limit functions. The “adaptive” nature of the controls
intelligently prevents the chiller from operating
outside of its limits, or compensates for unusual
operating conditions, while keeping the chiller
running rather than simply tripping due to a safety
concern. When problems do occur, diagnostic
messages assist the operator in troubleshooting.
Cycle Description
The refrigeration cycle for the RTHC chiller can be
described using the pressure-enthalpy diagram
shown in
Figure 25
. Key State Points are indicated
on the figure and are referenced in the discussion
following. A schematic of the system showing the
refrigerant flow loop as well as the lubricant flow loop
is shown in
Figure 26
.
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 shell and is then distributed to the tubes
in the evaporator tube bundle. The refrigerant
vaporizes as it cools the water flowing through the
evaporator tubes. Refrigerant vapor leaves the
evaporator as saturated vapor (State Pt. 1).
The refrigerant vapor generated in the evaporator
flows to the suction end of the compressor where it
enters the motor compartment of the suction-gas-
cooled motor. The refrigerant flows 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.
Baffles within the condenser shell distribute 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.
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
liquid-vapor separator chamber (State Pt. 5). At this
point the available refrigerant vapor is routed directly
to the compressor suction (State Pt. 1). All remaining
liquid refrigerant enters the evaporator (State Pt. 6).
The RTHC chiller maximizes the evaporator heat
transfer performance while minimizing refrigerant
charge requirements. This is accomplished by
metering the liquid refrigerant flow 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 UCP2 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.
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