Controls, Start-Up, Operation, Service and ... - Climayoreo

P126 (Circuit A High Head Pressure, Comp Shutdown)
T126 (Circuit A High Head Pressure Alert)
A126 (Circuit A High Head Pressure Alarm)
P127 (Circuit B High Head Pressure, Comp Shutdown)
T127 (Circuit B High Head Pressure Alert)
A127 (Circuit B High Head Pressure Alarm) — This alert/
alarm is used to keep the saturated condensing temperature below
the compressor operating envelope outlined in Fig. 18.
This alert/alarm also attempts to prevent the saturated condensing
temperature from reaching the high pressure switch trip
point by reducing the upper horizontal portion of the compressor
operating envelope to a level slightly below the saturated
condensing temperature recorded upon a high pressure switch
trip (T057, T058).
When TemperaturesREF.TSCTA or Temperatures
REF.TSCTB rise above the compressor operating envelope
for the corresponding TemperaturesREF.TSSTA or
TemperaturesREF.TSSTB, a compressor of the affected
circuit will be immediately shut down with pre-alert (P126,
P127) and a 10-minute timeguard will be added to the
compressor. If the saturated condensing temperature remains
above the envelope for 5 more seconds, another compressor of
the affected circuit, if it exists, will be shut down with pre-alert
(P126, P127) and a 10-minute timeguard will be added to the
compressor. This sequence will continue until the last compressor
on the circuit is shut down, at which time the circuit will be
shut down with alert (T126, T127).
This failure follows a 3 strike methodology. When the circuit
is shutdown entirely, an alert (T126, T127) is generated
and a strike is logged on the circuit. On the third strike, alarm
(A126, A127) will be generated which will necessitate a manual
reset to get the circuit back running. It is important to note
that a strike is called out only if all compressors in the circuit
are off at the time of the alert.
To prevent nuisance alerts, P126 and P127 show up in the
alarm history and locally at the display, but are never broadcast
to the network. To recover from these alerts, both a 10-minute
hold off timer and saturated condensing temperature returning
under the compressor envelope must occur. If recovery occurs,
staging will be allowed on the circuit once again. Again, a
a48-8508
SDT (F)
112
strike is tied to the circuit going off entirely, not reducing capacity
and recovering. Therefore it is possible that multiple
P126 and P127 alerts may be stored in alarm history but not
broadcast.
T128 (Digital Scroll High Discharge Temperature Alert)
A128 (Digital Scroll High Discharge Temperature Alarm)
— This alert/alarm is for units with a digital scroll compressor
only. The digital scroll compressor is equipped with a temperature
thermistor that is attached to the discharge line of the compressor.
The alert occurs when the discharge temperature thermistor
has measured a temperature above 268 F or the thermistor is
short circuited. The digital scroll compressor will be shutdown
and alert T128 will be generated. The compressor will be allowed
to restart after a 30-minute delay and after the thermistor
temperature is below 250 F.
If five high discharge temperature alerts have occurred
within four hours, alarm A128 will be generated which will necessitate
a manual reset to start the compressor.
A140 (Reverse Rotation Detected) — This alarm performs a
check for correct compressor rotation upon power up of the
unit. The method for detecting correct rotation is based on the
assumption that there will be a drop in suction pressure upon a
compressor start if the compressor is rotating in the correct
direction.
A test is made once, on power up, for suction pressure
change on the first compressor of the first circuit to start.
Reverse rotation is determined by measuring suction pressure
at 3 points in time:
• 5 seconds prior to compressor start.
• At the instant the compressor starts.
• 5 seconds after the compressor starts.
The rate of suction pressure change from 5 seconds prior to
compressor start to compressor start (rate prior) is compared to
the rate of suction pressure change from compressor start to 5
seconds after compressor start (rate after).
If (rate after) is less than (rate prior minus 1.25), alarm
A140 is generated.
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SST (F)
Fig. 18 — High Pressure/SCT Alarm Upper Envelope