FORM 160.87-OM1
ISSUE DATE: 8/24/2018
JOHNSON CONTROLS
12
SECTION 1 - DESCRIPTION OF SYSTEM AND FUNDAMENTALS OF OPERATION
7619A(D)
FIGURE 2 -
COMPRESSOR PRE-ROTATION VANES
When the compressor motor is driven by a YORK
Variable Speed Drive, there could be a Variable Speed
Drive (VSD) or a Medium Voltage Variable Speed
Drive (MVVSD) applied.
In operation, a liquid (water or brine to be chilled) flows
through the cooler, where boiling refrigerant absorbs
heat from the liquid. The chilled liquid is then piped to
fan coil units or other air conditioning terminal units,
where it flows through finned coils, absorbing heat
from the air. The warmed liquid is then returned to the
chiller to complete the chilled liquid circuit.
The refrigerant vapor, which is produced by the boil-
ing action in the cooler, flows to the compressor where
the rotating impeller increases its pressure and tem-
perature and discharges it into the condenser. Water
flowing through the condenser tubes absorbs heat from
the refrigerant vapor, causing it to condense. The con-
denser water is supplied to the chiller from an external
source, usually a cooling tower. The condensed refrig-
erant drains from the condenser into the liquid return
line, where the variable orifice meters the flow of liq-
uid 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 major components of a chiller are selected for full
load capacities, therefore capacity must be controlled
to maintain a constant chilled liquid temperature leav-
ing the cooler. Pre-Rotation Vanes (PRV), located at
the entrance to the compressor impeller, compensate
for variation in load (Refer to
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.
