JOHNSON CONTROLS
12
FORM 160.67-O2
ISSUE DATE: 10/9/2020
SECTION 1 - DESCRIPTION OF SYSTEM AND FUNDAMENTALS OF OPERATION
REFRIGERATION SYSTEM OPERATION
In operation, a liquid (water or other fluid to be chilled)
flows through the tubes in the evaporator , where boil-
ing refrigerant evaporates and absorbs heat from the
liquid. The refrigerant rises and passes through elimi-
nators that remove any entrained liquid droplets. The
dry refrigerant gas flows to the compressor suction.
The compressor raises the temperature and pressure
of the refrigerant through centrifugal force. The higher
pressure refrigerant gas exits the compressor and enters
the refrigerant condenser where the latent heat of the
refrigerant is removed and the refrigerant condenses to
a liquid by the water flowing through the tubes. The
liquid refrigerant passes through a subcooling section
in the bottom of the condenser where the liquid refrig-
erant is cooled to a lower temperature again by wa-
ter inside tubes. The higher pressure refrigerant liquid
then expands into the evaporator through a level con-
trol valve that is controlled by the control center based
on the signal from a refrigerant level sensor located
in the subcooler. The hot gas bypass is utilized during
slow roll start up and also during periods of very low
load operation or cold condenser water.
The open drive single impeller centrifugal compressor
is direct driven by a variable speed condensing steam
turbine. The reference to a condensing steam turbine
indicates the exhaust steam from the steam turbine is
condensed to water, which will be referred to in this
manual as condensate, which is usually returned to
the system boiler. The steam condenser package is
furnished as an integral part of the YST system. See
Steam and Condensate Flow, Figure 4 on page 16.
The process water or other fluid that is chilled in the
evaporator is pumped to the point of use which may
be air conditioning terminal units and/or central sta-
tion air handling units and/or other equipment requir-
ing cooling. The warmed liquid is then returned to the
chiller to complete the chilled liquid circuit. The con-
denser water is supplied from a cooling tower system
or sometimes other sources of water suitable for con-
densing service.
The chiller is controlled by a modern state of the art
Microcomputer Control Center that monitors its opera-
tion. The Control Center is programmed by the opera-
tor to suit job specifications. The chiller control panel
provides control of entire system, including turbine
and steam condenser operation and monitoring.
The control panel includes a color liquid crystal display
(LCD) surrounded by “soft” keys which are redefined
based on the screen displayed at that time, mounted
in the middle of a keypad interface and installed in
a locked enclosure. The screen details all operations
and parameters, using a graphical representation of the
chiller and its major components. Panel text is in Eng-
lish only. Data can be displayed in either English or
Metric units.
CAPACITY CONTROLS
During part load operation at off design conditions, the
chiller capacity is reduced to maintain a constant leav-
ing chilled liquid temperature by first decreasing the
turbine speed, secondly closing the compressor pre-ro-
tation vanes (PRV) (See Figure 2 on page 12), lastly
opening the Hot Gas Bypass valve.
7619A(D)
FIGURE 2 - COMPRESSOR PREROTATION VANES
Speed is controlled automatically by a pneumatically
actuated governor valve which throttles the inlet steam
flow to the turbine to maintain the speed dictated by
the capacity control logic.
The position of the compressor prerotation vanes is
automatically controlled through a lever arm attached
to an electric motor located outside the compressor
housing. The automatic adjustment of the vane posi-
tion in effect provides the performance of many differ-
ent compressors to match various load conditions from
full load with vanes wide open to minimum load with
To Next Page
Loading...
