JOHNSON CONTROLS
134
FORM 150.66-NM1 (708)
Unit Operation
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 Cooling Setpoint
(See sample in Table 19)
. At return water temperatures
between the Cooling Setpoint and Cooling Setpoint
plus Range, compressor loading and unloading will be
determined by the formulas in Table 20.
Return Chilled Liquid Control MUST
only be used when constant chilled
liquid fl ow 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 fl ow 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 formulas.
Unloading will occur at a rate of 30 seconds according
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 chilling 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 19),
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 20, the
control range will be split up into six (seven including
hot gas) segments, with the Control Range determining
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. Specifi cally, if the return water
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 fi ve compressors
running. The compressors would continue 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 20 not only provides the formulas
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. R
efer to the section on the
DISPLAY/PRINT keys for specifi c information on the
OPER DATA key.
RETURN CHILLLED LIQUID SYSTEM LEAD/LAG
AND COMPRESSOR SEQUENCING
A lead/Lag option may be selected to help equalize
average run hours between systems with 2 refrigerant
systems. This may be programmed under the OPTIONS
key. Auto Lead/Lag of the 2 systems based on average
run hours of the compressors in each system. Manual
Lead/Lag selects specifi cally the sequence which the
micro starts the systems.
TABLE 19 – COMPRESSOR STAGING FOR RETURN WATER CONTROL
*Unloading only
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)