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FORM 160.54-M1
ISSUE DATE: 10/25/2019
After the chiller has not surged for the period of time
programmed as the Hold Period setpoint, the valve is
driven toward the closed position at 10 minute inter-
vals by incremental amounts determined by the Close
Percentage setpoint. After it is fully closed, it remains
there until another surge is detected.
Whenever the Hot gas valve is partially or fully open,
the existing safety check that subtracts the evaporator
saturation temperature from the leaving chilled liquid
temperature changes the range from standard range of
(-2.5°F to +25°F) to (-5.0°F to +25°F). Whenever the
Hot gas bypass Valve is closed or not used, this safety
check uses the standard values.
If RS-485 serial communications between the mi-
croboard and the Analog I/O Board are lost continu-
ously for 20 seconds, WARNING – EXTERNAL I/O
– SERIAL COMMUNICATIONS is displayed on the
System Details line of the Display and the Hot gas
valve will remain at the position when communica-
tions were lost.
HOT GAS OPERATION WITH VARIABLE
GEOMETRY DIFFUSER (VGD)
Since the VGD operates by pulsing the diffuser ring
open until stall is detected, there are some conditions
where this probing can actually cause a surge. There-
fore, to eliminate unnecessary Hot gas valve move-
ment, the Hot gas valve is not opened on the first surge
or when the VGD is closing. When a surge is detected,
the Hot gas valve will not be opened until a second
surge is detected. This gives the VGD a chance to close
and stabilize the chiller without opening the Hot gas
valve. While the VGD operation is inhibiting Hot gas
operation, VGD OVERRIDE (OVERRIDE in software
version C.OPT.01.18.307 (and earlier)) is displayed as
the Hot gas Control Mode.
Whenever the Hot gas valve is not closed, the VGD is
maintained at the last position. This keeps the VGD at
a position of similar surge stability when the system
head is later lowered and the Hot gas valve closes.
MANUAL CONTROL
The Hot gas bypass Valve can be manually controlled
from the Hot gas bypass Screen in SERVICE access
level. Manual control has priority over Minimum Load
Override, Variable Speed Drive Override and Auto-
matic Control.
JOHNSON CONTROLS
When the OPEN or CLOSE key is pressed, the valve
position will be increased or decreased by 5% to a
maximum of 100% or minimum of 0%. Each time ei-
ther key is pressed, the LED in the respective key will
illuminate for 2 seconds. The Hot gas bypass Control
Mode will display MANUAL.
I/O BOARD
The I/O Board supplied for Hot gas bypass applica-
tions is mounted on the right hand wall inside the Con-
trol Center. This board receives a Hot gas bypass Valve
position command via RS-485 serial communications
from the microboard COM3 port. It converts this com-
mand into a 0 to 10 VDC signal that is applied to the
valve. This signal positions the valve over the range
of 0% (fully closed) to 100% (fully open). Positions
between these extremes are linearly scaled (a 50% po-
sition would be achieved with a 5 VDC signal).
There are two different I/O Boards that could be present
for Hot gas bypass Control. Early vintage chillers use
Analog I/O Board 371-02514-000. After June 2009, I/O
Board 031-02895-000 is used (See SECTION 24 - SMART
FREEZE PROTECTION; Microboard 031-02430-001
equipped with software version C.OPT.01.21.307 or
later is required). Since these boards use different pro-
tocols, the microboard must know which board is pres-
ent in order to be able to communicate with it. The
microboard determines which board is present by the
features that are enabled:
• If Heat Recovery and/or or Head Pressure con-
trol is enabled, it knows the 031-02895-000 board
is present and only the 031-02895-000 board is
polled.
• If only Hot gas bypass is enabled, the microboard
automatically determines which board is present. At
boot-up, the microboard alternates between polling
the 031-02895-000 board and the 371-02514-000
Board. Once a valid response is received from one
of these two boards, the communications will con-
tinue to only poll that board type.
Below is a description of the interface for each board.
Analog I/O Board 371-02514-000
This board communicates with the microboard COM3
serial port using Opto-22 protocol. It is compatible
with Microboard 031-01730-000 and 031-02430-000/-
001, equipped with any software version. See Figure
79 on page 215.
JCI COMPANY CONFIDENTIAL
SECTION 23 - HOT GAS BYPASS
213
23
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