| Controls 10, 15 Tons SEQUENCE OF UNIT OPERATION
Refer to the unit wiring schematic
pasted to the inside of the control panel cover when reviewing the control sequence
described below.
Refer to the legend on the previous
page for an explanation of the acronyms used in this sequence.
10-Ton Operation
With fused disconnect switch closed,
power is supplied to the crankcase heaters and the 24-volt control circuit.
Starting the chilled water pump
closes the CWPS auxiliary contacts and completes the flow switch.
When the water temperature rises
above the WTT’s setpoint, its first stage switch closes, allowing power to pass through CWPS auxiliary contacts, the flow
switch, the LTC, the ASCT1, the RR1 contacts, the
LPC01, and the HPC01 to energize the CC1 coil. This
starts compressor number one and the outdoor fan.
If compressor number one can’t
satisfy the cooling demand, WTT’s 2nd stage
switch closes, allowing power to pass through the CC1 auxiliary contacts, the DBC, the ASCT2, the RR2 contacts, the LPC02,
and the HPC02 to energize the CC2 coil which starts
compressor number two.
15-Ton Operation
With fused disconnect switch closed,
power is supplied to the crankcase heaters, and the 24-volt control circuit.
Starting the chilled water pump
closes the CWPS auxiliary contacts and completes the flow switch, allowing power to pass through the LTC to energize the
CWFIR.
When the water temperature rises
above the WTT’s setpoint, its first stage switch closes, allowing power to pass through the CWFIR contacts, the ASCT1, the RR1
contacts, the LPC01, and the HPC01 to energize the CC1
coil. This starts compressor number one and outdoor fan number one.
If compressor number one can’t
satisfy the cooling demand, WTT’s second stage switch closes, allowing power to pass through the CWFIR contacts, the DBC, the
ASCT2, the RR2 contacts, the LPC02, and the HPC02 to
energize the CC2 coil. This starts compressor number two and outdoor fan number two.
LOW AMBIENT OPERATION
Field Installed Head Pressure
Control Accessory
Standard units will operate in
outdoor ambient temperatures down to the values shown in the "General Data" section of this catalog. This accessory will enable units to operate down to much
lower temperature extremes (see "General Data" section of this catalog).
Head pressure control for CGA units
is regulated by means of a field-installed head pressure accessory which varies condenser fan speed in relation to discharge
pressure.
When discharge pressure is 270 psig
or higher, the condenser fan runs at full speed. At pressures between 270 psig and 180 psig, the fan speed is adjusted (increased
or decreased) in direct relation to the pressure, with
minimum fan speed (10 percent of rated motor rpm) occurring when the pressure reaches
180 psig. At pressures below 180 psig, the fan will not
run. When discharge pressure rises to 180 psig, the fan will start and run at the reduced speed. Fan speed will continue to increase, as
the pressure increases, until full speed is reached at
270 psig. |
