Capacity Controls; Figure 3 - Detail A - Compressor Pre-Rotation Vanes - York YST Operation Manual

FORM 160.67-O1
ISSUE DATE: 4/1/2015
figure 3 - DETAIL A – COMPRESSOR PRE-
ROTATION 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 temperature
and discharges it into the condenser. Water flowing
through the condenser tubes absorbs heat from the re-
frigerant vapor, causing it to condense. The condenser
water is supplied to the chiller from an external source,
usually a cooling tower. The condensed refrigerant
drains from the condenser into the liquid return line,
where the subcooler level control valve meters the flow
of liquid refrigerant to the cooler to complete the refriger-
ant circuit.
The Subcooler liquid level is continuously monitored to
provide optimum subcooler, refrigerant condenser and
evaporator performance. The level control valve au-
tomatically adjusts to all Real-World operating condi-
tions, providing the most efficient and reliable operation
of refrigerant flow control.
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.
JOHNSON CONTROLS
SECTION 1- DESCRIPTION OF SYSTEM AND FUNDAMENTALS OF OPERATION

CapaCiTY ConTrolS

During part load operation at off design conditions,
the chiller capacity is reduced to maintain a constant
leaving chilled liquid temperature by first decreasing
the speed, closing the compressor pre-rotation vanes
(PRV) (See Figure 3 on page 11), then opening the
Hot Gas Bypass valve.
The position of these vanes is automatically controlled
through a lever arm attached to an electric motor lo-
cated outside the compressor housing. The automatic
adjustment of the vane position in effect provides the
performance of many different compressors to match
various load conditions from full load with vanes wide
open to minimum load with vanes completely closed.
This provides capacity reduction from 100% to 10%
7619A(D)
of design for normal air conditioning applications. The
speed is controlled by a pneumatically actuated gov-
ernor valve which throttles the inlet steam flow to the
turbine to maintain the speed dictated by the capacity
control logic. If the tower water temperatures must be
held above 75ºF for other chillers, the capacity control
logic automatically limits the amount of speed reduc-
tion and PRV closure to maintain stable operation. The
hot gas by-pass valve is then modulated to admit con-
denser gas into the evaporator and reduce the cooling
effect as required. This will maintain a constant leav-
ing chilled liquid temperature with loads down to 10%
of design.
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