JOHNSON CONTROLS
11
SECTION 1 – DESCRIPTION OF SYSTEM AND FUNDAMENTALS OF OPERATION
FORM 160.79-O3
ISSUE DATE: 11/09/2018
1
intervals. If the rate of change of the Liquid Tempera-
ture (
Delta T Rate
) is less than the programmed
Mini-
mum Rate Setpoint
(0.5 ºF/minute to 2.0 ºF/minute;
default 1.0 ºF/minute) for a period equal to the
Mini-
mum Rate Time Setpoint
(1 to 20 minutes; default 5
minutes) and the Pulldown Demand Limit Setpoint is
above 50%, the Lag compressor is brought on line. If
a Lead Compressor Pulldown Demand Limit Setpoint
has been entered, the Lag compressor will not be
brought on line until this time has elapsed.
When the first 3 minutes of compressor operation has
elapsed, if the refrigerant level is greater than or equal
to the
Level Control Setpoint
, the level is controlled
to the Level Setpoint. However, if the level is less than
the Level Control setpoint, a linearly increasing ramp
called the
Refrigerant Level Target
is applied to the
Level Control Setpoint. This ramp allows the level to
go from the present level to the Level Control Setpoint
over a period programmed as the
Ramp Up Time
Setpoint
. After the Ramp Up time has elapsed, the
level is controlled to the Level Control Setpoint.
After the Chiller Pulldown Period ends, if the Lag
compressor has not been brought on line during the
Chiller Pulldown Period, the
Chiller Steady State
Period
begins. During this period, the Pre-rotation
Vanes are modulated to maintain the Leaving Chilled
Liquid Temperature Setpoint. In order to determine if
the Lead compressor is meeting the load demand dur-
ing this period, the Leaving Chilled Liquid Tempera-
ture minus the Leaving Chilled Liquid Temperature
Setpoint (
Temperature Differential
) is monitored. If
the
Temperature Differential
exceeds the
Maximum
Delta T Setpoint
(1.0 ºF to 5.0 ºF; default 1.0) for a pe-
riod equal to the
Maximum Delta T Time Setpoint
(1
to 20 minutes; default 5) and the Chiller Current Limit
Setpoint is above 50%, the Lag compressor is brought
on line.
With only one compressor running, the compressor
Variable Geometry Diffuser is modulated according to
the stall and surge activity.
When bringing the Lag chiller on line, it enters the
Prelube period. If a high head condition exists, the
Lead compressor’s Pre-rotation vanes are driven
closed as described in “Lag Start with High Head”
below. The Refrigerant Level Control Lag Start mode
is initiated as described below under “Condenser Re-
frigerant Level Control Lag Start”. During the Lag
Prelube, all normal Prelube functions are performed
except the Oil Pressure Transducer Offset calibration
is not performed. The offset calculation from the Lead
compressor start is used. The Prelube time is fixed at
50 seconds. At the completion of the Prelube, the Lag
compressor motor is started and after a 50 second de-
lay, the Lag compressor Discharge Valve is opened (If
it doesn’t fully open within 40 seconds as indicated by
the Discharge Valve Limit Switch, a Safety shutdown
is performed on both compressors and “Discharge #X
– Valve Not Opened” is displayed). After the Discharge
Valve has fully opened, the Lag compressor Pre- rota-
tion Vanes are modulated to follow Lead motor cur-
rent. Meanwhile, the Lead compressor Pre-rotation
Vanes have been modulated to maintain the Leaving
Chilled Liquid Temperature setpoint (unless “Lag Start
with High Head” was performed). When the Lag mo-
tor current is 5% of the Lead motor current,
the run time of the compressors are evaluated to deter-
mine which should be the Lead compressor (The one
with the least amount of run time is identified as the
Lead compressor) and if a “Lag Start with High Head”
was performed, the Lead Pre-rotation vanes are now
allowed to open and control per the Leaving Chilled
Liquid Temperature Setpoint. Two minutes after the
Lag compressor Discharge Valve has opened, the Con-
denser Refrigerant Control ramp-up period begins as
Described below under “Condenser Refrigerant Level
Control – Lag Start”. While both compressors are run-
ning, the Variable Geometry Diffusers are modulated
according to the stall activity and Pre-rotation Vanes
position. There is no VGD response to surge activity.
When both compressors have been brought online,
they will be run for at least 30 minutes (unless the anti-
recycle feature has been disabled, a fault is detected or
a Local or Remote stop is applied). After the 30 minute
waiting period, a Soft Shutdown (as described below)
will be performed on the Lag compressor if the Lead
compressor motor current goes below the
Low Load
Setpoint
(20% to 60%; default 45%) for a period equal
to the
Low Load Time Setpoint
(1 to 20 minutes; de-
fault 5).
Lag Compressor Start with High Head
When operating at high head conditions, surges can
occur while going from one to two compressor opera-
tion. A high head condition is detected by measuring
the Delta P/P [(Condenser pressure – Evaporator pres-
sure) / Evaporator pressure] and comparing it to the
HIGH HEAD DP/P LIMIT Setpoint. By closing the
Lead compressor’s pre-rotation vanes during a Lag
compressor start when a high head condition is pres-
ent, surge conditions can be minimized.
