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AC-SVX001A-EN
Operating Principles
This section contains an overview of the operation and
maintenance of units equipped with UC800 control
systems. It describes the overall operating principles of
the Ascend™ ACRA design.
Refrigeration Circuits
Each unit has two refrigerant circuits, with one rotary
screw compressor per circuit. Each refrigerant circuit
includes a compressor suction and discharge service
valve, liquid line shutoff valve, removable core filter,
liquid line sight glass with moisture indicator, charging
port and an electronic expansion valve. Fully
modulating compressors and electronic expansion
valves provide variable capacity modulation over the
entire operating range. Lower condensing
temperatures and higher suction temperatures along
with more efficient compressors and fans result in the
premium efficiency level.
Refrigeration Cycle
The refrigeration cycle of the chiller is represented in
the pressure enthalpy diagram shown in figure below.
Key state points are indicated on the figure. The cycle
for the full load AHRI design point is represented in the
plot.
Figure 24. Pressure-enthalpy (P-h) diagram
The chiller uses a shell and tube evaporator design
with refrigerant evaporating on the shell side and water
flowing inside tubes having enhanced surfaces (states
4 to 1). The suction lines are designed to minimize
pressure drop.(states 1 to 1b). The compressor is a
twin-rotor helical rotary compressor designed similarly
to the compressors offered in other Trane Screw
Compressor Based Chillers (states 1b to 2). The
discharge lines include a highly efficient oil separation
system that removes 99.8% of the oil from the
refrigerant stream going to the heat exchangers (states
2 to 2b). De-superheating, condensing and sub-cooling
is accomplished in a fin and tube air cooled heat
exchanger where refrigerant is condensed in the tube
(states 2b to 3b). Refrigerant flow through the system is
balanced by an electronic expansion valve (states 3b to
4).
Refrigerant R-134a
The Ascend™ ACRA chiller uses environmentally
friendly R-134a. Trane believes responsible refrigerant
practices are important to the environment, our
customers, and the air conditioning industry. All
technicians who handle refrigerants must be certified.
The Federal Clean Air Act (Section 608) sets forth the
requirements for handling, reclaiming, recovering and
recycling of certain refrigerants and the equipment that
is used in these service procedures. In addition, some
states or municipalities may have additional
requirements that must also be adhered to for
responsible management of refrigerants. Know the
applicable laws and follow them.
Refrigerant R-134a is a medium pressure refrigerant. It
may not be used in any condition that would cause the
chiller to operate in a vacuum without a purge system.
Ascend™ ACRA is not equipped with a purge system.
Therefore, the chiller may not be operated in a
condition that would result in a saturated condition in
the chiller of –15°F (-26°C) or lower.
Refrigerant R-134a requires the use of specific POE oils
as designated on the unit nameplate.
IImmppoorrttaanntt:: Use only R-134a and Trane Oil 00311 (bulk)/
00315 (1gal)/00317 (5gal) .
Compressor and Oil System
The rotary screw compressor is variable Vi (variable
pressure ratio) semi-hermetic, direct drive with
capacity control via a variable speed drive, rolling
element bearings, differential refrigerant pressure oil
pump and oil heater. To maximize efficiency, the
variable Vi compressor is controlled to one of two
possible states depending on the chiller system
operating point and to provide ease of starting. The
motor is a suction gas cooled, hermetically sealed,
permanent magnet motor. An oil separator is provided
separately from the compressor. Oil filtration is
provided internal to the compressor. Check valves in
the compressor discharge and lube oil system are also
provided.
Condenser and Fans
Air-cooled condenser coils have aluminum fins
mechanically bonded to internally finned seamless
aluminum tubing. The tubing is a long life alloy
designed to deliver corrosion performance that meets
or exceeds microchannel coils. The condenser coil has
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