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SECTION 23 - HOT GAS BYPASS
• Minimum Load (0°F to 4°F; default 0°F) – This
sets the Minimum Load override threshold. It is
the offset below the Leaving Chilled Liquid Tem-
perature Setpoint at which the Hot gas bypass
Valve will be opened to the position allowed per
the Maximum Open setpoint (25% to 100%). With
Heat Pump Duty enabled and operating in Heat-
ing Mode, it is the offset above the Leaving Con-
denser Liquid Temperature setpoint that the valve
will be opened. If "0" is entered for this value, this
feature is disabled.
• Maximum Open (25% to 100%; default 100%)
– This is the maximum allowed position for the
Hot gas valve during a Minimum Load override
condition. Allows the user to adjust the quantity
of Hot gas for the local requirements.
OPERATION
While the chiller is shutdown, the Hot gas valve is
driven 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 override this operation.
In standard operation or with Heat Pump Duty en-
abled and operating in Cooling Mode, if the Leaving
Chilled Liquid Temperature decreases to less than the
Minimum Load setpoint, the valve is opened to the
maximum allowed by the Maximum Open setpoint
and MINIMUM LOAD (OVERRIDE in software ver-
sion C.OPT.01.18.307 (and earlier)) is displayed as
the Hot gas bypass Control Mode on the Hot gas by-
pass Screen. For example, if the Minimum Load is set
for 4°F and the Maximum Open is set for 80%, the
valve will be positioned to 80% open when the Leav-
ing Chilled Liquid Temperature decreases to more than
4°F below the Leaving Chilled Liquid Temperature
setpoint. After this Minimum Load Override is initi-
ated, as the Leaving Chilled Liquid Temperature rises
to the Leaving Chilled Liquid Temperature setpoint,
the valve is closed by an amount proportional to the
difference between the temperature delta and the Mini-
mum Load setpoint. In this example, when the Leav-
ing Chilled Liquid Temperature increases to 2°F be-
low the Leaving Chilled Liquid Temperature setpoint,
the valve will be positioned to 40% open. The valve is
closed accordingly until the temperature delta is 0°F.
If Heat Pump Duty is enabled, the Hot gas operates
as it does in a standard YK chiller without Heat Pump
ability. However, in Heating Mode, Hot gas operation
212
is the same as Cooling Mode except for the operation
during low load conditions, which is as follows.
When the Leaving Condenser Liquid Temperature ex-
ceeds the Minimum Load setpoint (1 to 4°F above the
Leaving Condenser Liquid Temperature Setpoint) the
Hot gas valve is opened to the position allowed per the
Maximum Open setpoint (25 to 100%). For example,
if the Minimum Load setpoint equals 4°F, Maximum
Open setpoint equals 100%, Heating setpoint equals
105°F, and leaving Condenser Liquid Temperature is
greater than 109°F, the Hot gas valve would be opened
to the 100% position. When the leaving Condenser
Liquid Temperature falls to 107°F, the valve closes to
50% and finally to 0% when this temperature falls back
to 105°F. All other aspects of Hot gas control remain
unchanged in Heating Mode.
If the Pre-rotation Vanes are more than 95% open and
the Leaving Chilled Liquid Temperature is at least
5°F above the Leaving Chilled Liquid Temperature
setpoint, the valve is set to one-half of its present posi-
tion for 10 minutes. After the 10 minutes have elapsed,
the valve is driven fully closed.
With software version C.OPT.01.18.307 (and earlier),
if the chiller is equipped with a Variable Speed Drive
(VSD), whenever the VSD is running at less than full
speed (50/60Hz), the Hot gas bypass Valve is driven to
the 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-
tected, the Hot gas valve is opened a certain percentage
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 less than 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 greater than 35% but
less than 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 greater than 50%, it is
driven open another 10%. There will be no valve
response to surge events in the next 2 minutes.
JCI COMPANY CONFIDENTIAL
FORM 160.54-M1
ISSUE DATE: 10/25/2019
JOHNSON CONTROLS
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