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ENG
6.9.2
Floating suction set point
For the suction line, the fl oating set point is managed by the supervisor.
The suction pressure set point set by the user is changed by the supervisor
in range between a settable minimum and maximum. The operation is
illustrated in the following fi gure:
Floating setpoint
Max
Real variation
Max variation
Min
The set point is calculated by the supervisor and acquired by the pRack
PR300T controller at set intervals, the maximum variation allowed for the
set point in each sampling period can also be set; if the value acquired
diff ers from the previous value by more than the maximum variation
allowed, the variation is limited to the maximum value. If the supervisor is
disconnected, after 10 minutes (fi xed) the pRack PR300T controller starts
decreasing the set point with variations equal to the maximum variation
allowed each sampling period, until reaching the minimum set point
allowed with fl oating suction pressure.
Note: if set point compensation from scheduler, digital input or
supervisor is also active, the off set is added to the minimum and
maximum limits for the fl oating set point.
6.9.3
Floating condensing set point
For the condenser line, the fl oating set point is based on the outside
temperature. The fl oating condensing pressure set point is achieved by
adding a constant programmable value to the outside temperature and
limiting the resulting value between a settable minimum and maximum,
as shown in the fi gure:
Floating setpoint
Max
Min
external temperature
Note: if set point compensation from scheduler, digital input or
supervisor is also active, the off set is added to the minimum and
maximum limits for the fl oating set point.
Copyright HPL March 2021
pRack PR300T +0300018EN rel. 1.5 - 02.05.2017
time
Sampling time
Fig. 6.ai
offset
time
Fig. 6.aj
6.10 Accessory functions
pRack PR300T can manage several accessory functions. Of these, the
economizer and liquid injection have already been described in paragraph
6.3 on compressor operation, while the others are described below.
6.11 Oil management
pRack pR300T allows some additional functionalities for oil management,
per individual compressor or per line:
• Individual compressor: oil cooling, oil injection.
• Line: common oil receiver
The functionalities can be enabled and the relative parameters can be set
from main menu branch E.a.a/E.a.b.
6.11.1 Individual compressor oil management
Oil cooler
An oil cooler can be managed for the fi rst 6 compressors in line 1, in order
to keep the oil temperature under constant control. For each compressor,
based on the value read by the oil temperature probe, an oil cooler digital
output can be activated with a settable threshold and diff erential, as
shown in the fi gure.
Oil cooler
Differential
For each compressor, two alarms can also be managed for high or low oil
temperature, setting the threshold, diff erential and delay.
Oil injection
An oil injection valve can be managed for each of the fi rst 6 compressors
in each line as shown schematically for three compressors in Fig. 6.ah.
Valve activation is performed when the corresponding oil level digital
input is active. The valve is opened in intermittent mode with settable
opening and closing times, for a total time that is also settable. Once
exceeded, if the digital input is still active a low oil alarm is generated.
When the oil level digital input is not active, the valve is activated with
opening and closing times which can be set at a diff erent value, in order
to allow the passage of a certain quantity of oil.
6.11.2 Oil management per line
A solenoid valve can be managed which connects the oil separator to
the receiver based on the digital input reading of the oil level, which
can be only minimum level or minimum and maximum level. Separator,
receiver and valve are illustrated schematically in Fig. 5.a. If no oil level
input is present, the solenoid valve can still be activated by connecting
its operation to the status of the compressors. If only the minimum level
is present, activation of the solenoid valve occurs intermittently for the
entire time in which the minimum level is not active. The opening and
closing times of the valve during activation can be set by a parameter. If
the minimum level signal deactivates again, the valve remains deactivated
for at least a minimum set closure time, as shown in the fi gure:
44
Page 93 of 172
Oil temperature
Setpoint
Fig. 6.ak
P4114S Rev 03
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