York YZ MODEL A, Руководство по эксплуатации и обслуживанию

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JOHNSON CONTROLS
15
SECTION 1 - SYSTEM FUNDAMENTALS
FORM 161.01-OM1
ISSUE DATE: 6/8/2018
1
• Y – Commercial Chiller 03630 Microboard
• OPT - OptiView
• 01 – YZ Mod A Chiller
• xx - Controls Revision level (00, 01, etc)
• y – Language Package (0=English only, 1=NEMA,
2=CE, 3=NEMA/CE )
• zz – Language Package Revision level (00, 01,
etc)
Each time the controls portion or language section is
revised, the respective revision level increments.Soft-
ware upgrades should only be performed by a service
technician.
SYSTEM OPERATION
In operation, a liquid to be chilled (water or brine)
flows through the evaporator tubes, where its heat is
transferred to low pressure liquid refrigerant sprayed
over and pooled outside the tubes, boiling the refriger-
ant. The chilled liquid is then piped to air conditioning
or process terminal units, absorbing heat. The warmed
liquid is then returned to the chiller to complete the
chilled liquid circuit cycle.
The refrigerant vapor, which is produced by the boiling
action in the evaporator, is drawn into the suction of
the compressor where the rotating impeller increases
its pressure and temperature and discharges it into the
condenser. The cooling fluid flowing through the con-
denser tubes absorbs heat from the refrigerant vapor,
causing it to condense. The cooling fluid is supplied to
the chiller from an external source, usually a cooling
tower. The condensed refrigerant drains from the con-
denser into the subcooler section. There, it is cooled by
the entering condenser water and exits to into the liq-
uid return line. The level control valve meters the flow
of liquid refrigerant to the evaporator to complete the
refrigerant circuit. The level control valve continually
adjusts position as load changes to meet the changed
mass flow rate of refrigerant required to keep the sys-
tem balanced. It does this by maintaining a constant
level in the condenser, enough to maintain a liquid seal
to the outlet.
Capacity Control
The major components of a chiller are selected to
handle the required refrigerant flow at full load design
conditions. However, most systems will be called upon
to deliver full load capacity for only a relatively small
part of the time the unit is in operation.
The speed at which the compressor rotates establishes
the pressure differential that the chiller can operate
against. This is referred to as ‘lift’. Speed must always
be maintained above the minimum necessary to create
the lift required for the pressure difference between the
condenser and evaporator, regardless of load. Below
that speed, gas surge occurs. That pressure difference
is a function of the LCHLT and the leaving condenser
liquid temperature and the heat transfer between those
liquids and the refrigerant.
Reduced speed also reduces the available capacity of
the chiller. If speed is reduced, chiller power use is
reduced. Therefore, at reduced capacity requirements
where condenser pressure is also reduced, the motor
speed is reduced as much as possible while maintain-
ing chilled liquid temperature and sufficient lift. When
the speed cannot be further reduced due to lift required
for the specified leaving chilled water temperature set-
ting and available cooling to the condenser and capac-
ity must be further reduced, a mechanism called the
Variable Geometry Diffuser (VGD) at the exit of the
impeller is used to reduce refrigerant gas flow. The
VGD not only controls capacity, but serves to mitigate
“stall.” Stall is an effect caused by slow refrigerant gas
passing through the compressor at reduced flow rates
needed for low capacity operation.
A final optional means to reduce capacity called Hot
Gas Bypass (HGBP) is available regardless of com-
pressor model. When selected for an application,
HGBP is used to re-circulate some refrigerant through
the compressor without using it for cooling the chilled
liquid. Although this does not reduce power consump-
tion, it greatly reduces the capacity of the chiller for
maximum turndown. The YZ uses these mechanisms
in a controlled order to maintain the best possible ef-
ficiency.
The YZ Chiller controls capacity by adjusting the com-
pressor VGD position, the compressor motor Variable
Speed Drive (VSD), and optional Hot Gas Bypass
valve (HGBP) position (if equipped) in a specific se-
quence depending on whether loading or unloading is
required to keep LCHLT at setpoint. Motor speed is
additionally and simultaneously adjusted as necessary
to maintain the minimum compressor lift required to
prevent surge. The sequence for operation of the con-
trol devices is as follows: