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With the optional Hot Gas Bypass feature, the Con-
trol Center mod u lates a valve located in the Hot Gas
By pass connection between the condenser and the
evap o ra tor to control the fl ow of gas to the evap o -
ra tor. The valve is modulated in response to load and
surging con di tions.
A Hot Gas Bypass Screen, accessed from the COM-
PRES SOR Screen dis plays all the applicable pa ram e ters
and allows a Service Tech ni cian to program the appli-
cable setpoints and manually control the Hot Gas valve.
If the chiller is equipped with the optional Hot Gas By-
pass control, it must be Enabled from the OP ER A TIONS
Screen using a procedure in the "System Cal i bra tion
Service Setpoints and Reset Procedures" section of this
book. If Disabled, the valve is driven to the fully closed
position.
The Microboard controls the Hot Gas valve by send-
ing a positioning com mand over the COM 3 RS-485
serial commumications link to the optional Analog
I/O Board that is mounted inside the con trol Center.
The Analog I/O Board converts this command into a
2-10VDC signal and applies it to the Hot Gas Valve
Actuator. A 2VDC signal drives the valve fully closed
(0% position); a 10VDC sig nal drives the valve fully
open (100% po si tion). Positions be tween these ex tremes
are linearly scaled. For ex am ple, 50% position would be
achieved with a 6VDC signal. The actual valve po si tion
is dis played on the HOT GAS BYPASS Screen as 0%
to 100%.
A 2.5K Ohm PRV Potentiometer mounted on the Pre-
rotation Vanes (PRV) assembly provides the PRV po-
si tion (0 to 100%) to the Analog I/O Board. A +12VDC
source is applied to the potentiometer. This position
val ue is sent over the RS-485 serial com mu ni ca tions link
to the Microboard, where it is displayed on the COM-
PRES SOR Screen. When the PRV are fully closed, the
po si tion is display as 0%; fully open displayed as 100%.
Positions be tween these extremes are linearly scaled.
To as sure accuracy, a PRV Calibration procedure must
be performed as detailed in the "System Cal i bra tion,
Service Setpoints and Reset Procedures" section of this
book. The PRV position is dis played as XX on the COM-
PRES SOR Screen and "Warn ing – Vanes Uncalibrated"
is displayed on the Sys tem Details line of the display
until the calibration is per formed.
YORK INTERNATIONAL
SECTION 20
HOT GAS BYPASS
(REFER TO FIG. 64)
The Evaporator and Condenser pressure trans duc ers
provide these pressure values to the Microboard. The
Microboard uses these values to calculate the DEL TA
P/P parameter as follows: [(con dens er pressure – evap-
o ra tor pressure) / evaporator pressure]. Al though this
parameter is not used in the Hot Gas Control, it rep re -
sents compressor "Head" and is displayed on the Hot
Gas Bypass Screen for ref er ence only. These pres sures
are also used to detect when a surge oc curs.
The Leaving Chilled Liquid Temperature ther mistor pro-
vides the temperature to the Microboard. This value is
subtracted from the Leaving Chilled Liquid Tem per a ture
Setpoint to produce the TEMPERATURE DIF FER -
EN TIAL parameter. This parameter is in dic a tive of
chiller load.
When a surge is detected, the SURGE DE TECT ED
LED illuminates for 5 seconds and the TOTAL SURGE
COUNT is incremented. This count is the ac cu mu lat ed
surge events that have been de tect ed over the lifetime
of the chiller. This value can be reset to zero using the
procedure detailed in the "System Calibration, Service
Setpoints and Reset Procedures" section of this book.
This should not be arbitrarily performed. The SURGE
SEN SI TIV I TY setpoint can be used to make the de-
tec tion more or less sensitive.
SETPOINTS
• Surge Sensitivity (0.3 to 1.3; default 0.3) – De ter -
mines the surge detection sensitivity. The small er the
number, the greater the sen si tiv i ty. Pro gram ma ble
in 0.1 increments. This setpoint is pro grammed on
the Surge Pro tec tion Screen and is common to the
Surge Pro tec tion feature.
• Hold Period (30 to 120 minutes; default 30) – This
is the period of time after no more surg es are de-
tect ed that the Hot Gas valve closing will begin. It
will be driven toward the closed po si tion in in cre -
ments equal to the Close Per cent age setpoint at 10
minute intervals until fully closed. Programmable
in 1 minute in cre ments.
• Close Percentage (5 to 15%; default 5%) – This is
the incremental amount that the Hot Gas valve will
be closed at 10 minute intervals after the HOLD
PERIOD has elapsed. Refer to HOLD PERIOD
above.
FORM 160.54-M1(503)
20
125
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