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B−Cooling Components
All units use independent cooling circuits consisting of sep-
arate compressors, condenser coils and evaporator coils.
See figures 8 through 11. Six draw−through type condenser
fans are used in all SCA240/SCB288 and three are used in
all SCA120 units. All units are equipped with belt-drive
blowers which draw air across the evaporator during unit
operation.
Cooling may be supplemented by a factory- or field-
installed economizer. The evaporators are slab type
and are stacked. Each evaporator uses a thermostatic ex-
pansion valve as the primary expansion device. Each evap-
orator is also equipped with enhanced fins and rifled tubing. In
all units each compressor is protected by a crankcase heater,
high pressure switch and low pressure switch. Additional
protection is provided by low ambient switches and freezes-
tats (on each evaporator).
1−Compressors B1 and B2 (all units)
B13 & B20 (240/288 units)
All SCA120 use two scroll compressors and
SCA240/SCB288 units use four. All units are equipped with
independent cooling circuits. Compressor capacity may
vary from stage to stage. In all cases, the capacity of each
compressor is added to reach the total capacity of the unit.
See SPECIFICATIONS" and ELECTRICAL DATA" (table
of contents) or compressor nameplate for compressor spec-
ifications.
WARNING
ELECTRICAL SHOCK HAZARD. COMPRESSOR
MUST BE GROUNDED. DO NOT OPERATE WITH-
OUT PROTECTIVE COVER OVER TERMINALS.
DISCONNECT POWER BEFORE REMOVING PRO-
TECTIVE COVER. DISCHARGE CAPACITORS BE-
FORE SERVICING UNIT. FAILURE TO FOLLOW
THESE PRECAUTIONS COULD CAUSE ELECTRI
-
Each compressor is energized by a corresponding com-
pressor contactor.
NOTE−Refer to the wiring diagram section for specific unit
operation.
2−High Pressure Switches
S4 and S7 (all units) S28 & S96
(SCA240/SCB288)
The high pressure switch is an auto-reset SPST N.C. switch
which opens on a pressure rise. All units are equipped with this
switch. The switch is located in the compressor discharge line
and is wired in series with the compressor contactor coil.
S4 (first circuit), S7 (second circuit), S28 (third circuit), and S96
(fourth circuit) are wired in series with the respective compres-
sor contactor coils.
When discharge pressure rises to 640 ± 10 psig (4413 ±
69 kPa) (indicating a problem in the system) the switch
opens and the respective compressor is de−energized
(the economizer can continue to operate). When dis-
charge pressure drops to 475 ± 20 psig (3275 ± 138 kPa)
the pressure switch will close.
Main control A55 has a three−strike counter before lock-
ing out. This means the control allows three high pres-
sure trips per one thermostat demand. The control can
be reset by breaking and remaking the thermostat de-
mand or manually resetting the control.
3−Low Ambient Switches S11 & S84(all units)
S85 & S94 (SCA240/SCB288)
The low ambient switch is an auto-reset SPST N.O. pres-
sure switch which allows for mechanical cooling opera-
tion at low outdoor temperatures. All units are equipped
with this switch. In all models a switch is located in each
liquid line prior to the indoor coil section.
In the SCA120, S11 and S84 wired in parallel are wired in se-
ries with outdoor fan relay K10 (compressor one). Compressor
two is under control of the IMC board. In the SCA 240/SCB288
units, S11 and S84 are in parallel wired to outdoor fan relay
K149 while S85 and S94 (compressor four) are in parallel
wired to third outdoor fan relay K150.
When liquid pressure rises to 450 ± 10 psig (3102 ± 69
kPa), the switch closes and the condenser fan is ener-
gized. When discharge pressure in one refrigerant circuit
drops to 240 ± 10 psig (1655 ± 69 kPa), the switch opens
and the condenser fan in that refrigerant circuit is de−ener-
gized. This intermittent fan operation results in higher
evaporating temperature allowing the system to operate
without icing the evaporator coil and losing capacity.
4−Low Pressure Switches S87 & S88(all units)
S98 & S97 (SCA240/SCB288)
The low pressure switch is an auto-reset SPST N.O. switch
(held N.C. by refrigerant pressure) which opens on a pres-
sure drop. All units are equipped with this switch. The
switch is located in the compressor suction line.
S87 (compressor one), S88 (compressor two), S98 (com-
pressor three), and S97 (compressor four) are wired in se-
ries with the main control module A55.
The main control module A55 governs the low pressure
switches by shunting the switches during start up until pres-
sure is stabilized. After the shunt period, the control has a
three-strike counter, during first thermostat demand, before the
compressor is locked out. The control is reset by breaking and
remaking the thermostat demand or manually resetting the
control.
When suction pressure drops to 40 + 5 psig (276 ± 34
kPa) (indicating low pressure), the switch opens and the
compressor is de−energized. The switch automatically
resets when pressure in the suction line rises to 90 + 5
psig (620 ± 34 kPa), due to many causes such as refrig-
erant being added.
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