Leaving Chilled Liquid System Lead/Lag And Compressor Sequencing; Return Chilled Liquid Control; Table 21 - Lead/Lag Return Chilled Liquid Control For 4 Compressors (6 Steps) - York YLAA Series Installation Operation & Maintenance

SECTION 8 – UNIT OPERATION
LEAVING CHILLED LIQUID SYSTEM LEAD/
LAG AND COMPRESSOR SEQUENCING
A Lead/Lag option may be selected to help equalize
average run hours between systems with 2 refriger-
ant systems. This may be programmed under the OP-
TIONS key. Auto Lead/Lag allows automatic Lead/
Lag of the two systems based on average run hours of
the compressors in each system. Manual Lead/Lag se-
lects specifically the sequence which the microproces-
sor starts systems.
On a hot water start, once a system starts, it will turn
ON all compressors before the next system starts a
compressor. The microprocessor will sequence com-
pressors within each circuit to maximize individual
compressor run time on individual compressors within
a system to prevent short cycling.
Each compressor in a system will be assigned an arbi-
trary priority number 1, 2, or 1, 2, 3. The non-running
compressor within a system with the lowest priority
number will always be the next compressor to start.
The running compressor with priority number 1 will al-
ways be the next to shut OFF. Whenever a compressor
is shut OFF, the priority numbers of all compressors
will be decreased by 1 with wrap-around. This control
scheme assures the same compressor does not repeat-
edly cycle ON and OFF.
Once the second system starts a compressor on a 2 sys-
tem chillers, the microprocessor will attempt to equally
load each system as long as the system is not limit-
ing or pumping down. Once this occurs, loading and
unloading will alternate between systems, loading the
lead system first or unloading the lag system first.

TABLE 21 - LEAD/LAG RETURN CHILLED LIQUID CONTROL FOR 4 COMPRESSORS (6 STEPS)

STEP COMP 1
OFF
0
1 ON + HG
2 ON
3 ON
4 ON
5 ON
6 ON
NOTES 1. Step is Hot Gas Bypass and is skipped when loading occurs. Hot Gas Bypass operation is inhibited during pumpdown. For Leaving
Chilled Liquid Control the Hot Gas Bypass solenoid is energized only when the lead compressor is running and the LWT < SP, the
Hot Gas Bypass solenoid is turned off when the LWT more than SP + CR/2.
2. Step 3 is skipped when loading occurs.
3. Step 4 is skipped when unloading occurs.
152
LEAD SYSTEM
COMP 2 -
OFF -
OFF -
OFF -
OFF -
ON -
ON -
ON -

RETURN CHILLED LIQUID CONTROL

(Can be used on Dual System 4, 5 and 6 Comp
Units Only)
Return chilled liquid control is based on staging the
compressors to match the cooling load. The chiller
will be fully loaded when the return water temperature
is equal to the Cooling Setpoint plus the Range. The
chiller will be totally unloaded (all compressors OFF)
when the return water temperature is equal to the Cool-
ing Setpoint (See sample in Table 19 on page 151).
At return water temperatures between the Cooling
Setpoint and Cooling Setpoint plus Range, compres-
sor loading and unloading will be determined by the
formulas in Table 20 on page 151.
Return Chilled Liquid Control MUST
only be used when constant chilled liquid
flow is ensured.
The range MUST always be programmed
to equal the temperature drop across
the evaporator when the chiller is "fully
loaded". Otherwise, chilled liquid tem-
perature will over or under shoot. Variable
flow must never be used in return chilled
liquid mode.
Normal loading will occur at intervals of 60 seconds
according to the temperatures determined by the for-
mulas. Unloading will occur at a rate of 30 seconds ac-
cording to the temperatures determined in the formulas
used to calculate the ON and OFF points for each step
of capacity.
The return chilled liquid setpoint is programmable
from 40 °F to 70 °F (4.4 °C to 21.1 °C) in water chill-
ing mode and from 10 °F to 70 °F (-12.2 °C to 21.1
°C) in glycol chilling mode. In both modes, the cooling
range can be from 4 °F to 20 °F (2.2° to 11.1 °C).
COMP 1
OFF
OFF
SEE NOTE 1
OFF
ON
SEE NOTE 2
OFF
SEE NOTE 3
ON
ON
FORM 150.72-ICOM6
ISSUE DATE: 12/29/2017
LAG SYSTEM
COMP 2 -
OFF -
OFF -
OFF -
OFF -
OFF -
OFF -
ON -
JOHNSON CONTROLS