JOHNSON CONTROLS
12
FORM 160.79-O3
ISSUE DATE: 11/09/2018
SECTION 1 – DESCRIPTION OF SYSTEM AND FUNDAMENTALS OF OPERATION
SECTION 1 – DESCRIPTION OF SYSTEM AND FUNDAMENTALS OF OPERATION
With the Lead compressor running and it is determined
the Lag compressor is required to run, the Delta P/P
is measured. If it is > the HIGH HEAD DP/P LIMIT
Setpoint, a close signal is applied to the Lead compres-
sor’s pre-rotation vanes coincident with the start of the
Lag Compressor’s Pre-lube. They will be allowed to
open when all the following conditions are met: the
lag compressor is running, the Lag Discharge Valve is
open, the difference between the Lead and Lag motor
currents is < 5%.
An LED on the Surge – Dual Compressor Setup Screen
and the Capacity Compressor Cycling Screen illumi-
nates whenever a high head Lag compressor start con-
dition exists. Also, “Warning – Head Pressure – High
Head Lag Start Limit” is displayed on the System Sta-
tus line of the Display during this condition.
Condenser Refrigerant Level Control – Lag
Start
When operating at low condenser liquid temperatures,
a Low Evaporator Pressure trip can occur when going
from one to two compressor operation.
During startup of the Lag compressor, as the Lag Dis-
charge Valve opens, the system will see a step change
in refrigerant flow. Although the Condenser Refrig-
erant Level Control keeps the refrigerant level stable
during normal operation, the variable orifice does not
open quickly enough when the Lag compressor is
started. This causes the refrigerant to stack in the in
the condenser, starving the evaporator, and causing a
low evaporator pressure shutdown.
The condenser Refrigerant Level Control Lag Start
mode is initiated when the Lag compressor enters
Prelube and is terminated 2 minutes after the Lag Dis-
charge Valve has opened. During this mode, if the re-
frigerant level is > 20%, the level control lower (valve
open) output is turned on for 5 seconds every 10 sec-
onds until the level is less than 20%. If the refrigerant
level is < 20%, the level control is held in place.
After the Lag Start mode has ended, a linearly increas-
ing ramp limit, called the Refrigerant Level Target,
is applied to the REFRIGERANT LEVEL setpoint.
This ramp allows the level to go from the present level
(20%) to the programmed Refrigerant Level Setpoint
over a period equal to one-half of the VALVE PRESET
TIME setpoint. During this ramp-up period, the Re-
frigerant Level target is used to control the refrigerant
level in the condenser. The Refrigerant Level Setpoint
is then used to control the level for the remainder of
the run period.
On the Refrigerant Level Control Screen, the Refriger-
ant Level Override Mode displays “Valve Preset” dur-
ing the pre-positioning of the Refrigerant Level Con-
trol Valve while bringing on the Lead compressor. It
displays “Lag Start” when performing the Refrigerant
Level Control Lag Start mode while starting the Lag
compressor. The Override Time Remaining displays
the time remaining in the “Valve Preset” mode or the
time remaining in the “Lag Start” mode. In the first
case, it is the time remaining in the Valve Preset Timer
plus the time remaining of the first 3 minutes of Lead
compressor run time. In the second case, it is the time
remaining in the Lag compressor Pre-lube cycle plus
the time remaining in the first 3.5 minutes of Lag com-
pressor run time.
The
Lead Compressor Pulldown Time
Setpoint
can be used to limit the chiller to single compressor
operation for a specified duration (0-255 minutes)
when starting the chiller. While this pulldown time is
in effect, “System In Lead Compressor Pulldown” is
displayed on the System Details line along with the
time remaining in the period. During this time, only
the lead compressor is permitted to run. The lag com-
pressor will not be brought on line. After this period
has elapsed, the lag compressor can be brought on
line per normal operation. While this period is in ef-
fect, the lead compressor motor current will be limited
to the value programmed for the
Lead Compressor
Pulldown Demand Limit
Setpoint (30-100% FLA).
This current limit applies to the full load amps of the
Lead compressor motor only, not the Chiller Full Load
Amps (total of both compressors). For example, if the
Chiller Full Load Amps is 2000A, then each compres-
sor’s FLA is 1000A. If set to 80%, then the lead com-
pressor motor current would be limited to 800A (80%
x 1000) for the duration of this period.
The CHILLER PULLDOWN DEMAND LIMIT
Setpoint (that was used during the CHILLER PULL-
DOWN PERIOD in previous software versions) has
been removed in this and later software versions. Al-
though this setpoint has been removed, the logic in
the CHILLER PULLDOWN PERIOD that is used to
determine if it is necessary to bring on the Lag com-
pressor (to meet load) still exists. This period is still
in effect from start until the Leaving Chilled Liquid
Temperature is within 2ºF of the Leaving Chilled Liq-
uid Temperature Setpoint. However, the “Chiller Pull-
down In Effect” LED (that indicated when the Chiller
Pulldown Period is in effect) has been eliminated from
the MOTOR Screen to avoid confusion with the Lead
Compressor Pulldown Demand Limit feature.