JOHNSON CONTROLS
16
FORM 161.01-OM1
ISSUE DATE: 6/8/2018
SECTION 1 - SYSTEM FUNDAMENTALS
• Conditions require capacity increase:
1. HGBP (if present) is driven toward closed.
2. When the HGBP is full closed, the VGD is
driven toward open.
3. When the VGD is fully opened, VSD speed
is increased.
• Conditions require capacity decrease:
1. VSD speed is decreased.
2. When VSD speed is at the minimum limit
to avoid surge, the VGD is driven toward
closed.
3. When the VGD reaches closed, the HGBP (if
present) is driven toward open.
Also, high condenser pressure, low evaporator pres-
sure, high motor current, and high input current limits
and overrides limit or reduce the output to the appro-
priate devices (HGBP, VGD, or VSD) to mitigate the
condition to keep the chiller online. As any of these
physical thresholds are approached, the control will
proportionally limit the amount of capacity increase
permitted and if exceeded will issue unloading into the
capacity control command.
Anti-Surge Minimum Frequency
In order to maintain sufficient compressor lift to over-
come condenser pressure and prevent surge through-
out operation, the control maintains and continuously
updates a minimum limit for VSD speed. This limit is
the Active Anti-Surge Minimum Frequency. It is calcu-
lated and applied to the speed each cycle of the capac-
ity control routine.
Surge Protection
The surge protection feature detects surge events. It
provides a running count of the surges detected over
the lifetime of the chiller. It allows the user to define
how many surges are excessive and how the control
will react to an excess surge condition. When exces-
sive surging is detected, this feature can shutdown the
chiller.
Surge events are detected by monitoring the relation-
ship between the Condenser pressure and Evaporator
pressure while the chiller is running. An excess surge
condition is detected by comparing the number of
surge events that occur in a selectable time period to a
selectable threshold.
If the number of surge events (Surge Window Count)
detected in the time period programmed as the COUNT
WINDOW setpoint (1 to 5 minutes; default 3) exceeds
the threshold programmed as the COUNT LIMIT
setpoint (4 to 20; default 15) an excess surge condition
has been detected.
Unless the SHUTDOWN features have been enabled
as explained below, the chiller will continue running
under the same conditions displaying WARNING –
EXCESS SURGE DETECTED. This message will be
displayed until manually reset with the Warning Re-
set key in Operator access level. If the SHUTDOWN
setpoint is Enabled when an excess surge condition has
been detected, a safety shutdown will be performed
and SURGE PROTECTION - EXCESS SURGE is
displayed.
Head Pressure Control
The Head Pressure Control feature enables chiller
control of a field-mounted facility condenser water
temperature control means, if one is necessary for pro-
longed cold water startup as described in SECTION
3 - OPTIVIEW™ CONTROL CENTER FUNCTIONS
AND NAVIGATION. YZ chillers are capable of op-
eration within a wide range of condenser water tem-
peratures; however, a low minimum condenser water
temperature, as specified in the YZ Engineering Guide
(Form 161.01-EG1), is required to maintain a suffi-
cient pressure differential (head) between the condens-
er and evaporator for proper refrigerant management
in the chiller. The head pressure control function pro-
vides an analog output control signal from the OptiV-
iew Control Center that responds to the programmed
head pressure (condenser pressure minus evaporator
pressure) Setpoint. The 0-10VDC or 4-20mA output
is configurable from the Head Pressure Control screen
when the feature is enabled. Output wiring is described
in Unit Wiring and Field Connections for YZ Cen-
trifugal Chiller with Magnetic Bearing Control (Form
161.01-PW4).
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