York YCAV0569, Installation Operation & Maintenance

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15
JOHNSON CONTROLS
FORM 201.21-NM2 (616)
2
Flash Tank Feed Valve/Drain Valves
A Flash Tank is fitted to both refrigerant circuits. The
Flash Tank is a shell type refrigerant reservoir designed
to sustain 2 phase refrigerant. The purpose of the Flash
Tank is to increase the efficiency of the system. A
portion of the liquid fed into the Flash Tank gases off,
cooling the remaining liquid in the tank another 25-35°F.
Both liquid and gas exist in the flash tank. The refriger-
ant gas in the flash tank is fed to the economizer port on
the compressor at a point on the rotors approximately
1.7X suction when the economizer solenoid is activated.
The liquid in the tank is fed to the evaporator.
The vapor feed to the economizer port of the compres-
sor is at an intermediate pressure between discharge
and suction (1.7 x suction) and therefore little energy
is required to pump it back through the compressor to
condenser pressure. This results in a very small loss to
system efficiency.
The design working pressure of the flash tank is 450
PSIG (31 bar). The Drain and Feed Valves on the flash
tank are activated on start-up. The Feed Valve on the
Flash Tank acts like a liquid line solenoid, but also func-
tions to control the liquid level in the flash tank. The
Drain Valve functions similar to an electronic expan-
sion valve (EEV). The Drain Valve controls refriger-
ant flow to the evaporator based on suction superheat.
Both valves are stepper motor valves. An economizer
solenoid is placed between the flashtank and the econo-
mizer port of the compressor. The economizer solenoid
valve is generally activated at speeds above 90-120 Hz,
depending upon a number of other factors.
Both valves are controlled by 2 phase drive signals from
a stand-alone controller in the Control Cabinet. Signals
from sensors such as suction pressure and temperature
are sent to the Chiller Control Board, which in turn sends
control signals to the Drain and Feed Valve Controller.
The control algorithm in the Chiller Control Board will
attempt to control the liquid level in the flash tank to
35% on the level sensor and the system will fault if the
flash tank level exceeds 87.5%.
During operation, it will be noted the flash tank level
will typically remain between 30-40% level when the
economizer solenoid is ON. The economizer solenoid
valve will typically be on most of the time. When the
economizer solenoid is OFF, the liquid level will vary
greatly as the Drain and Feed Valves directly affect the
level as they open and close.
Oil Separator/Oil System
The external oil separators, with no moving parts and
designed for minimum oil carry-over, are mounted in
the discharge line of the compressor. The high pressure
discharge gas is forced around a 90 degree bend. Oil is
forced to the outside of the separator through centrifugal
action and captured on wire mesh where it drains to the
bottom of the oil separator and is then forced into the
condensor.
The oil (YORK “L” oil – a POE oil used for all refriger-
ant applications), flows from the oil separator, through
the condenser where it is cooled, and back into the com-
pressor through a replaceable 0.5 micron oil filter at high
pressure. This high pressure “oil injection” forces the oil
into the compressor, where it is fed to the bearings and
rotors for lubrication. After lubricating the bearings, it is
injected through orifices on a closed thread near the suc-
tion end of the rotors. The oil is automatically injected
because of the pressure difference between the discharge
pressure and the reduced pressure at the suction end of
the rotors. This lubricates the rotors as well as provides
an oil seal against leakage around the rotors to ensure
refrigerant compression efficiency.
The oil also provides cooling by transferring much of
the heat of compression from the gas to the oil, keeping
discharge temperatures down and reducing the chance
for oil breakdown. Oil injected into the rotor cage
flows into the rotors at a point about 1.2x suction. This
ensures that a required minimum differential of at least
30 PSID exists between discharge and 1.2x suction, to
force oil into the rotor case. A minimum of 10 PSID
(0.6 bar) is all that is required to ensure protection of
the compressor. The oil pressure safety is monitored as
the difference between suction pressure and the pressure
of the oil entering the rotor case.
Maximum working pressure of the oil separator is 450
PSIG (31 bar). Oil level should be above the midpoint
of the “lower” oil sight glass when the compressor is
running. Oil level should not be above the top of the
“upper” sight glass.
Relief Valves
Two relief valves are installed in each refrigerant circuit.
A 325 PSIG relief valve is located on each Flash Tank
and a 250 PSIG relief valve is located on the suction
line of the compressor near the evaporator.