Chiller Operation; Chilled Liquid Control Settings; Table 1 - Input Current Limit Threshold - York YZ MODEL A Operation And Maintenance Manual

SECTION 2 - SYSTEM OPERATING PROCEDURES
The chiller will start if the following conditions are
met:
• Leaving Chilled Liquid Temperature is above the
setpoint
• Chilled liquid flow is established
• Condenser liquid flow is established
• No uncleared faults or start inhibits exist

CHILLER OPERATION

Upon a start request, the following occurs in sequence:
1. The chiller's system pump run contacts close
2. The VSD pre-charges (~12 seconds)
3. The VSD pre-regulates (~3 seconds)
4. The Motor runs
The chiller will vary capacity to maintain the leaving
chilled liquid temperature setpoint. Throughout capac-
ity control, the compressor speed is maintained above
the minimum required to avoid surge. Otherwise, the
device maintaining capacity is controlled based on the
leaving chiller liquid temperature. Pressure and motor
current overrides also apply as necessary to maintain
operating limits.
The Input Current limit threshold value is determined
from several settings, depending on the chiller control
source selected according to the Table 1.

TABLE 1 - INPUT CURRENT LIMIT THRESHOLD

CONTROL ACTIVE INPUT CURRENT
SOURCE LIMIT THRESHOLD
Lowest of:
• Local Input Current Limit Setpoint (% Input
Local
Job FLA)
• Pulldown Input Current Limit (when active)
BAS Remote Input Current Limit Setpoint (comms)
Lowest of:
• Local Input Current Limit Setpoint (% Input
Job FLA)
Hardwire
• Analog Remote Input Current Limit
Setpoint
• Pulldown Input Current Limit (when active)
22

CHILLED LIQUID CONTROL SETTINGS

Temperature Control Setpoint
The temperature to which the chiller will control the
chilled fluid leaving the evaporator must be set by the
operator. The way it is selected depends on whether the
operator wants this value set locally or modulated by a
remote input.
Regardless of which method is used to select the de-
sired Leaving Chilled Liquid Temperature (LCHLT),
the chiller will control to its own Active LCHLT
Setpoint. The Active Setpoint varies but is targeted to
the programed setpoint when the chiller is running.
When the chiller is not running, the active setpoint is
set to Entering Chilled Liquid Temperature minus a
programmable offset (default 5 °F (-15 °C)), but not
adjusted to less than the programmed LCHLT setpoint.
When the compressor motor starts, the Active LCHLT
Setpoint is ramped from this value to the programmed
LCHLT Setpoint at the programmable LCHLT Setpoint
Ramp Rate (default 0.1 °F or °C/second). This keeps
the chiller from undershooting setpoint excessively
during pulldown. Any time the programmed setpoint is
changed during operation, the active setpoint is ramped
to the new value at this rate.
Automatic Temperature Shutdown
This setting defines the temperature offset below the
LCHLT setpoint where shutdown is expected. It is pro-
grammable over a range of 1 °F to 70 °F (-17.2 °C
to 21.1 °C). However the actual shutdown temperature
will never be calculated to lower than 34 °F (1.1 °C)
(water) or 6°F (-14.4 °C) (brine). Anytime the LCHLT
setpoint is decreased, the shutdown threshold decreas-
es to the new LCHLT active setpoint minus offset at a
rate equal to the programmed LCHLT Setpoint Ramp
Rate. Anytime the Leaving Chilled Liquid Tempera-
ture setpoint is increased, the shutdown threshold in-
creases to the new LCHLT active setpoint minus offset
at a rate equal to 1/2 the programmed LCHLT Setpoint
Ramp Rate. This allows time for the chiller to change
temperature without shutting down first.
FORM 161.01-OM1
ISSUE DATE: 6/8/2018
JOHNSON CONTROLS