FORM 150.66-NM1 (708)
135
JOHNSON CONTROLS
8
The micro will sequence compressors load and unload
systems according to Table 21. The microprocessor will
lead/lag compressors within each circuit to maximize
individual compressor run time for the purpose of
lubrication. It will also prevent the same compressor
from starting 2 times in a row. The micro will not attempt
to equalize run time on individual compressors within
a system.
Each compressor in a system will be assigned an
arbitrary number 1, or 2. The non-running compressor
within a system with the lowest priority number will
always be the next compressor to start. The running
compressor with priority number 1 will always be the
next compressor to shut off. Whenever a compressor is
shut off, the priority numbers of all compressors in each
system will be decreased by 1 with the wrap around.
This control scheme assures the same compressor does
not repeatedly cycle on and off.
ANTI-RECYCLE TIMER
The programmable anti-recycle timer assures that
systems do not cycle. This timer is programmable
under the PROGRAM key between 300 - 600 seconds.
Whenever possible, to reduce cycling and motor heating,
the anti-recycle timer should be adjusted to 600 seconds.
The programmable anti-recycle timer starts the timer
when the fi rst compressor in a system starts. The timer
begins to count down. If all of the compressors in a
circuit cycle off, a compressor within the circuit will
not be permitted to start until the anti-recycle timer
has timed out. If the lead system has run for less than 5
minutes, 3 times in a row, the anti-recycle timer will be
extended to 10 minutes.
ANTI-COINCIDENCE TIMER
Two timing controls are present in software to assure
compressors within a circuit or between systems, do not
start simultaneously. The anti-coincidence timer assures
there is at least a one minute delay between system starts
on 2-circuit systems. This timer is NOT programmable.
The load timers further assure that there is a minimum
time between compressor starts within a system.
EVAPORATOR PUMP CONTROL
The evaporator pump dry contacts (CTB2 – terminals 23
- 24) are energized when any of the following conditions
are true:
1. Low Leaving Chilled Liquid Fault
2. Any compressor is running
3. Daily Schedule is not programmed OFF and Unit
Switch is ON
The pump will not run if the micro panel has been
powered up for less than 30 seconds or if the pump
has run in the last 30 seconds to prevent pump motor
overheating.
EVAPORATOR HEATER CONTROL
The evaporator heater is controlled by ambient air
temperature. When the ambient temperature drops
below 40°F (4.4
°C)
the heater is turned on. When the
temperature rises above 45°F (7.2
°C)
the heater is turned
off. An under voltage condition will keep the heater off
until full voltage is restored to the system.
Notes:
1. Step 1 is Hot Gas Bypass and is skipped when loading occurs. Hot Gas Bypass operation is inhibited during Pumpdown.
2. Step 3 is skipped when loading occurs.
3. Step 4 is skipped when unloading occurs.
TABLE 20 – RETURN CHILLED LIQUID CONTROL FOR 4 COMPRESSORS (6 STEPS)
* STEP can be viewed using the OPER DATA key and scrolling to COOLING DEMAND.
*STEP
COMPRESSOR
COMPRESSOR ON POINT
COMPRESSOR OFF POINT
0
0 SETPOINT
SETPOINT
1
1 W/HGB
SP + CR/8 (Note 1)
SETPOINT
2
1 NO HGB
SP + CR/4
SP + CR/8
3
2
SP + 2*CR/4 (Note 2)
SP + CR/4
4
2
SP + 2*CR/4
SP + CR/4
(Note 3)
5
3
SP + 3*CR/4
SP + 2*CR/4
6
4
SP + CR
SP + 3*CR/4