JOHNSON CONTROLS
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SECTION 1 - DESCRIPTION OF SYSTEM AND FUNDAMENTALS OF OPERATION
FORM 160.78-O2
ISSUE DATE: 5/22/2017
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7619A(D)
FIGURE 3 -
COMPRESSOR PREROTATION VANES
The refrigerant vapor, which is produced by the boiling
action in the cooler, flows to the compressor where the
rotating impeller increases its pressure and tempera-
ture and discharges it into the condenser. Water flow-
ing through the condenser tubes absorbs heat from the
refrigerant vapor, causing it to condense.
The condenser water is supplied to the chiller from
an external source, usually a cooling tower. The con-
densed refrigerant drains from the condenser into the
liquid return line, where the variable orifice meters the
flow of liquid refrigerant to the cooler to complete the
refrigerant circuit.
The major components of a chiller are selected to han-
dle the refrigerant, which would be evaporated 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.
CAPACITY CONTROL
The speed at which the compressor rotates establishes
the pressure differential that the chiller can operate
against. As speed is reduced, the chiller power use is
reduced. At reduced capacity requirements where con-
denser pressure is also reduced, the motor speed is re-
duced as much as possible while maintaining chilled
water temperature and sufficient pressure differen-
tial. When the speed cannot be further reduced due to
pressure difference required for the specified leaving
chilled water temperature setting and available cool-
ing to the condenser, other means to reduce refriger-
ant gas flow are used to manage capacity. Compressor
models M1B-197FAA and M1B-205FAA use a device
called prerotation vanes (PRV) at the entrance to the
impeller to reduce capacity (See Figure 3). Regard-
less of chiller compressor model, the chiller also has a
mechanism called Variable Geometry Diffuser (VGD)
at the exit of the impeller that was designed 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. Compressor models
with gas path revision level “B” do not have operating
prerotation vanes, they are fixed full open, but use the
VGD also as a capacity control device instead.
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
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uses these mechanisms in a controlled order
to maintain best efficiency.
MAGNETIC BEARINGS
The chiller driveline operates on magnetic bearings
which are controlled by the Magnetic Bearing Con-
troller (MBC). This controller receives commands to
levitate the rotor and to enable a rotation interlock as
required.
