185
JOHNSON CONTROLS
SECTION 8 – UNIT OPERATION
FORM 150.67-NM1
ISSUE DATE: 4/28/2017
8
RETURN CHILLED LIQUID CONTROL
(Can be used on Dual System 4, 5 and 6 compres-
sor 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 17). At return water
temperatures between the Cooling Setpoint and Cool-
ing Setpoint plus Range, compressor loading and un-
loading will be determined by the formulas in Table 18.
Return Chilled Liquid Control MUST
only be used when constant chilled
liquid flow is ensured.
The range MUST always be pro
-
grammed to equal the temperature
drop across the evaporator when the
chiller is “fully loaded”. Otherwise,
chilled liquid temperature 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).
As an example of compressor staging (refer to Table 17),
a chiller with six compressors using a Cooling Setpoint
programmed for 45 °F (7.20 °C) and a Range Setpoint
of 10 °F (5.56 °C). Using the formulas in Table 18, the
control range will be split up into six (seven including
hot gas) segments, with the Control Range determin-
ing the separation between segments. Note also that the
Cooling Setpoint is the point at which all compressors
are OFF, and Cooling Setpoint plus Range is the point
all compressors are ON. Specifically, if the return wa-
ter temperature is 55 °F (12.8 °C), then all compressors
will be ON, providing full capacity. At nominal gpm,
this would provide approximately 45 °F (7.2 °C) leaving
water temperature out of the evaporator.
If the return water temperature drops to 53.4 °F (11.9
°C), one compressor would cycle OFF leaving five
compressors running. The compressors would contin-
ue to cycle OFF approximately every 1.7 °F (.94 °C),
with the exception of hot gas bypass. Notice that the
hot gas bypass would cycle ON when the return water
temperature dropped to 46.25 °F (7.9 °C). At this point
one compressor would be running with hot gas.
Should the return water temperature rise from this
point to 46.7 °F (8.2 °C), the hot gas bypass would
shut OFF, still leaving one compressor running. As the
load increased, the compressors would stage on every
1.7 °F (.94 °C).
Also note that Table 18 not only provides the formu-
las for the loading (ON POINT) and unloading (OFF
POINT) of the system, the “STEP” is also shown in the
tables. The “STEP” is the increment in the sequence of
the capacity control scheme that can be viewed under
the OPER DATA key. Please refer to the section on the
DISPLAY/PRINT keys for specific information on the
OPER DATA key.
RETURN 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 of the 2 systems based on
average run hours of the compressors in each system.
Manual Lead/Lag selects specifically the sequence
which the microprocessor starts the systems.
*Unloading only
TABLE 17 -
SAMPLE COMPRESSOR STAGING FOR RETURN WATER CONTROL
COMPRESSOR STAGING FOR RETURN WATER CONTROL
4 COMPRESSOR
COOLING SETPOINT = 45 °F (7.2 °C) RANGE = 10 °F(5.6 °C)
# OF COMP ON
0
* 1+HG
1
2
3
4
RWT
45 °F
(7.2 °C)
46.25 °F
(7.9 °C)
47.5 °F
(8.6 °C)
50.0 °F
(10.0 °C)
52.5 °F
(11.4 °C)
55.0 °F
(12.8 °C)
Summary of Contents for YCAL0019
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