ELECTRIC CONTROLS
RETURN LIQUID TEMPERATURE CONTROL (TC)
The YORK model TC-6A return liquid temperature control is a
4 stage solid state thermostat that maintains the desired chilled
liquid temperature by sensing and regulating the liquid tem-
perature returning to the cooler. The control is mounted in the
compressor compartment, to the left of the main control box in
front of the system 2 compressor. The return water temperature
is sensed by a thermistor (SENSOR) located in the return liquid
line. Its resistance varies with changes in return liquid tempera-
ture. Changes in the thermistor resistance are interpreted by
the controller and are amplified to energize output relay con-
tacts. The circuitry is such that the controller shuts down the
compressor in the event the thermistor is disconnected or
accidentally cut. The return water temperature is adjustable
between 4
°
F and 70
°
F. This adjustable range is much greater
than applicable to this chiller. See Table 1 for limitations in
leaving liquid temperatures. The Between Stage Differential is
adjustable between 1
°
F and 3.75
°
F.
On both the W1LC420 and 530, the control is factory adjusted
to 54
°
F Return Liquid Temperature with the Between Stage
Differential set at 2.5
°
F which results in a 44
°
F (10
°
Range)
leaving liquid temperature at nominal conditions. Check that
these settings are still correct or change the setting as required
but do not exceed the application limitations in Table 1.
If the factory temperature control settings which provide 44
°
F
leaving liquid temperature with a 10
°
F liquid temperature range
are not desired for the W1LC420 or 530 installation, the re-
quired leaving liquid temperature and liquid temperature range
must be specified before the new temperature control settings
can be determined. To determine the Between Stage Differen-
tial setpoint on the temperature control for the W1LC420/530,
divide the specified range by 4. The Return Liquid Temperature
setpoint on the control is calculated by adding the specified
liquid temperature range to the specified leaving liquid tem-
perature.
EXAMPLE:
(W1LC420/530) Specified 45
°
F leaving liquid temperature with
an 8
°
F liquid temperature range.
Calculate Between Stage Differential Setpoint.
8
°
F - 4
°
F = 2
°
F
Calculate Return Liquid Temperature Setpoint.
45
°
F + 8
°
F = 53
°
F
As an example of how the TC-6A control works in application,
consider the W1LC420 chiller with the control set to maintain
a 54
°
F return water temperature with a 2.5
°
F Between Stage
Differential. The flow rate is set to 2.4 GPM/measured TON,
(10
°
F range) and the working fluid is water. The W1LC420 is a
4 stage machine with stages at 25%, 50%, 75% and 100%
capacity. All temperatures considered are for a TC-6A control
and thermistor working at design conditions. Allowable toler-
ances could change the actuation temperatures by
±
1.25
°
F.
With the RETURN water temperature to the cooler rising due
to increasing cooling load for the application, the #1 compres-
sor will start at part load when the RETURN water temperature
reaches 47.75
°
F. The LEAVING water temperature will quickly
drop to 45.25
°
F. If the cooling load continues to increase, the
#1 compressor will be fully loaded by the controller when the
RETURN water temperature reaches 50.25
°
F. With increasing
load, the controller will start the #2 compressor at 52.75
°
F
RETURN water temperature and fully loaded it at 55.25
°
F.
Under increasing load, the minimum LEAVING water tempera-
tue will be 45.25
°
F and the maximum 47.75
°
F. As the demand
for cooling load decreases, the controller will unload compres-
sor stages in reverse order to the way they were loaded at
52.75, 50.25, 47.75 and 47.75
°
F RETURN water temperature.
During decreasing load, the minimum LEAVING water tem-
perature will be 42.75
°
F and the maximum 45.25
°
F.
FREEZE CONTROL PREVENTION THERMOSTAT (FC)
The leaving liquid freeze prevention thermostat is designed to
prevent the liquid temperature in the cooler from dropping
below the freezing point of the fluid while the unit is running.
CAUTION: Freezing fluid in the cooler can cause damage
which can lead to total replacement of the cooler.
If the temperature of the liquid leaving the cooler drops below
its set point the control energizes 2R. 2R-1 and 2R-2 open and
shut both compressors off immediately. The control must be
manually reset when the bulb temperature rises to 6
°
F above
the setpoint cutout temperature. The body of the control is in
the compressor compartment attached to the left hand side of
the control box. The sensing bulb is the capillary type located
in a well in the leaving water connection at the far end of the
cooler. This is a cross ambient sensing device that is not
effected by changes in ambient temperature.
The freeze control is factory set to shut down the unit at 36
°
F
(
±
2) and is manually reset when the bulb temperature reaches
42
°
F. This setting is for a working liquid of pure water and many
light brine applications. In heavy brine applications, the control
may be field adjusted using the adjustment screw at the top of
the control.
CAUTION: It is recommended that the adjusted cutout tem-
perature be at least 4
°
F above the freezing point of
the fluid. In no brine application should the setpoint
be adjusted below 30
°
F.
If during delivery to the job site or during non-op-
erational periods when the ambient temperature of
the freeze control bulb drops below 36
°
F, the freeze
control may have tripped and needs to be manually
reset before the unit will start.
570.05-N4Y
Unitary Products Group
11
