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Hot Gas Bypass
• Minimum Load (0ºF to 4ºF; default 0ºF) – This sets
the Minimum Load override thresh old. It is the off-
set below the Leaving Chilled Liq uid Tem per a ture
Setpoint at which the Hot Gas By pass valve will be
opened to the po si tion allowed per the MAX I MUM
OPEN setpoint (25% to 100%). If "0" is entered for
this val ue, this fea ture is disabled.
• Maximum Open (25% to 100%; default 100%)
– This is the maximum allowed po si tion for the
Hot Gas valve during a Minimum Load over ride
con di tion. Allows the user to adjust the quantity of
Hot Gas for the local re quire ments.
OPERATION
While the chiller is shutdown, the Hot Gas valve is
driv en to the fully closed position. While the chiller is
running, the valve is modulated in response to low load,
high load or surge conditions. However, manual control
can over ride this operation.
If the Leaving Chilled Liquid Temperature de creas es
to less than the Minimum Load setpoint, the valve
is opened to the maximum allowed by the Maximum
Open setpoint and "Override" is dis played as the Hot
Gas Bypass Control mode on the Hot Gas Bypass
Screen. For example, if the Minimum Load is set for
4ºF and the Maximum Open is set for 80%, the valve will
be po si tioned to 80% open when the Leaving Chilled
Liq uid Temperature de creas es to more than 4ºF below
the Leaving Chilled Liq uid Temperature setpoint. After
this Minimum Load Override is initiated, as the Leav ing
Chilled Liquid Temperature rises to the Leav ing Chilled
Liquid Temperature setpoint, the valve is closed by an
amount proportional to the dif fer ence be tween the tem-
per a ture delta and the min i mum Load setpoint. In this
example, when the Leaving Chilled Liquid Tempera-
ture in creas es to 2ºF be low the Leaving Chilled Liquid
Tem per a ture setpoint, the valve will be positioned to
40% open. The valve is closed accordingly until the
tem per a ture delta is 0ºF.
If the Pre-Rotation Vanes are more than 95% open and
the Leaving Chilled Liquid Temperature is at least 5ºF
above the Leaving Chilled Liquid Temperature setpoint,
the valve is set to one-half of its present position for 10
minutes. After the 10 minutes have elapsed, the valve
is driven fully closed.
If the chiller is equipped with a Variable Speed Drive
(VSD), whenever the VSD is running at < full speed
(50/60 Hz), the Hot gas Bypass valve is driven to the
126
fully closed position and "Override" is displayed as the
Hot gas Bypass Control Mode.
If none of the above conditions are in effect, the Hot Gas
Bypass valve is driven to the fully closed position, until
a surge condition is detected. When a surge is de tect ed,
the Hot Gas valve is opened a certain per cent age every
few minutes until the surging stops or the valve is fully
opened as follows:
• If Hot Gas valve is fully closed, it is driven to the
50% position. There will be no valve response to
surge events for the next 5 minutes.
• If Hot Gas valve position is < 35%, it is driven to
the 50% position. There will be no valve response
to surge events for the next 3.5 minutes.
• If Hot Gas valve position is > 35% but < 50%, it is
driven to the 50% position. There will be no valve
response to surge events in the next 2 minutes.
• If Hot Gas valve position is >50%, it is driven open
another 10%. There will be no valve response to
surge events in the next 2 minutes.
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
in ter vals by incremental amounts de ter mined by the
CLOSE PERCENTAGE setpoint. Af ter it is fully
closed, it re mains there until an oth er 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
(-2.5ºF to +25ºF) to (-5.0ºF to +25ºF). 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
Microboard and the Analog I/O Board are lost con tin u-
ous ly for 20 seconds, "Warning – External I/O – Serial
Com mu ni ca tions" is displayed on the System Details
line of the Display and the Hot Gas valve will remain
at the position when communications were lost.
MANUAL CONTROL
The Hot Gas Bypass valve can be manually controlled
from the Hot Gas Bypass Screen in Service access
lev el. Manual control has priority over Minimum
Load Over ride, Variable Speed Drive Override and
Au to mat ic con trol.
FORM 160.54-M1(503)
YORK INTERNATIONAL
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