Expansion Valve; Evaporator; Oil System - Trane R Series Installation, Operation And Maintenance Manual

Operating Principles

Expansion Valve

Pressure drop occurs in an electronic expansion valve.The
unit controller (CH530) uses the valve to regulate the flow
through the liquid line to match the flow produced by the
compressor. The valve has a variable orifice that is
modulated by a stepper motor.
High pressure, subcooled liquid refrigerant enters the
expansion valve from the liquid line. As refrigerant passes
through the valve the pressure is dropped substantially,
which results in vaporization of some of the refrigerant.
The heat of vaporization is supplied by the two phase
mixture resulting in low temperature low pressure
refrigerant which is supplied to the evaporator (state 4) to
provide cooling.

Evaporator

The evaporator is composed of a liquid-vapor distributor
and falling film evaporator.
A liquid-vapor refrigerant mixture enters the distributor
(state 4).The mixture is distributed over the length of the
evaporator tubes (state 4b). Liquid is evenly distributed
over the length of the evaporator tubes by the two-phase
distribution system. A portion of the liquid boils as it falls
by gravity from tube to tube, wetting all the tubes of the
evaporator.To ensure that the tubes at the bottom of the
evaporator do not experience "dry out, " a liquid pool is
maintained in the bottom few inches of the bundle.Tubes
located in the bottom of the evaporator will evaporate the
liquid refrigerant by boiling (pool boiling).
Heat is transferred from the water or glycol inside the
tubes to the liquid refrigerant as the film of refrigerant
evaporates on the surface of the tube.Thin film heat
transfer requires a smaller temperature difference for a
given amount of heat transfer than nucleate boiling, which
is the heat transfer process used in flooded evaporators.
Hence, efficiency is enhanced by the use of falling film
evaporation. Additionally, the evaporator requires less
refrigerant than a comparable flooded evaporator and the
evaporator boils the entire refrigerant supply at constant
pressure. Refrigerant vapor exits the evaporator through
the suction line (state 1).

Oil System

Screw compressors require large quantities of oil for
lubricating and sealing the rotors and lubricating the
bearings.This oil is mixed with refrigerant at the discharge
of the compressor.To enhance the performance of the
heat exchanger surfaces an oil separation system is placed
into the discharge line.The oil separator is located
between the compressor and the condenser. It separates
oil using highly efficient centrifugal force. Approximately
99.5% of the oil is removed from the refrigerant in the
separator.
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Oil that is removed from the refrigerant falls by gravity into
the oil sump.This oil is directed back to the compressor
through the oil lines. Internal to the compressor is a high
efficiency filter to clean the oil before it is delivered to the
rotors and bearings. Once oil is injected into the
compressor rotors it mixes with the refrigerant again and
is delivered back to the discharge line.
Oil that gets past the oil separators flows through the
condenser, subcooler and expansion valve into the
evaporator.This oil is collected in the pool of refrigerant
that is maintained in the bottom of the evaporator. A small
amount of oil and refrigerant from this pool (state 4b) is
returned through a line that is connected to the
compressor down stream of the motor.This oil and
refrigerant mixes with the refrigerant vapor that was
drawn out of the evaporator, prior to injection into the
compressor rotors.
Figure 33. RTAC oil system
Evaporator
Refrigerant
Pressure
Transducer
Evaporator
Oil Return Line Filter
Compressor
Compressor
Heater
Bearing and Rotor
Restrictors and
Oil Injection
Intermediate
Oil Pressure
Transducer
Refrigerant with
small amount of Oil
Refrigerant, Oil Mixture
(refrigerant vapor and oil)
Oil Recover System
(liquid refrigerant and oil)
Primary Oil system
Condenser
EXV
Evaporator
Condenser
Refrigerant
Pressur
Transducer
Oil
Separator
Oil
Internal
Separator
Manual
Compressor
Sump
Service
Oil Filter
Heater
Valve
Compressor Oil
Temperature Sensor
Optional Oil
Cooler
RTAC-SVX01J-EN