JOHNSON CONTROLS
15
SECTION 1 - SYSTEM FUNDAMENTALS
FORM 160.84-OM1
ISSUE DATE: 9/21/2017
1
The chiller operating program resides in the Opti-
View Control Center microboard. Software versions
(C.OPT.18.xx.yzz) are alpha-numeric codes that rep-
resent the application, language package and revision
levels per below. Each time the controls portion or lan-
guage section is revised, the respective revision level
increments.
• C – Commercial Chiller
• OPT - OptiView
• 18 – YMC
2
Mod B 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)
Software upgrades should only be performed by a Ser-
vice Technician.
SYSTEM OPERATION DESCRIPTION
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 con-
denser. Cooling water (or other fluid) flowing through
the condenser tubes absorbs heat from the refrigerant
vapor, causing it to condense. The cooling water is
supplied to the chiller from an external source, usu-
ally a cooling tower. The condensed refrigerant drains
from the condenser into the subcooler section. There it
is cooled by the entering condenser water and exits to
into the liquid return line. The level control valve me-
ters 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 system 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. A means exists
to modulate capacity for other loads.
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, when speed reduction is possible. If speed
is reduced, the 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 maintaining chilled liquid
temperature and sufficient lift. When the speed cannot
be further reduced due to lift required for the specified
leaving chilled water temperature setting and available
cooling to the condenser and capacity must be fur-
ther reduced, a mechanism called Variable Geometry
Diffuser (VGD) at the exit of the impeller is used to
reduce refrigerant gas flow. The VGD not only con-
trols 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 YMC
2
uses these mecha-
nisms in a controlled order to maintain best efficiency.
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