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6.3.10 Digital Scroll™ compressors
pRack pR300 can use a Digital Scroll
device for suction lines (one for each line). This type of compressor
features special operation, and is controlled by pRack pR300 as follows.
The related parameters can be set in main menu branch C.a.f/C.b.f.
The capacity is modulated by opening/closing a valve with PWM; when
the valve is ON the compressor delivers minimum capacity, while when
the valve is off the compressor delivers maximum capacity. In the
following description and fi gure, ON and OFF refer to the status of the
compressor, while operation of the valve is the exact opposite:
Compressor power request
Compressor PWM
modulation
ON
OFF
2s
13s
15s
Variable cycle time
ON
OFF
2s
18s
20s
Fig. 6.p
The following data are provided by the manufacturer of the compressor:
•
minimum ON time 2 s
•
maximum cycle time 20 s
•
optimum cycle time 12 s
There are three possible operating modes:
•
Fixed cycle time
•
Variable cycle time
•
Optimised cycle time
Based on the operating mode selected, pRack pR300 calculates the valve
activation percentage that satisfi es the required capacity.
Fixed cycle time
The compressor ON time is calculated as the percentage of the cycle time
corresponding to the required capacity:
T
= % Request * Cycle Time
ON
The cycle time can be set to the optimum value suggested by the
manufacturer to achieve maximum COP, or to a higher value to increase
resolution of the capacity delivered (a higher cycle time implies greater
continuity in the eff ective capacity that can be delivered).
Variable cycle time
The compressor ON time is set to 2 s and the cycle time is calculated
based on the required capacity:
T
= T
CYCLE
ON
Optimised cycle time
The compressor ON time is set to 2 s and the cycle time is calculated
based on the required capacity for capacities less than 17 %, after which
the cycle time is set to 12 s and the ON time varies. In essence, this mode
is a combination of the previous two. This guarantees the maximum
possible COP and control rate (obtained with the 12 s cycle time) and the
maximum control range (starting from 10 %).
Note:
•
the minimum capacity that can be delivered by Digital Scroll™
compressors is Minimum ON time/Maximum cycle time = 2/30 = 6.7
%, which also depends on the selected control mode (for example, in
the fi rst case shown in the fi gure the minimum capacity delivered is
Minimum ON time/Cycle time = 2/15 = 13%).
compressorä as the modulating
10% 67%
Fixed cycle time
10s
5s
15s
8s
5s
4s
12s
/% Request
•
if high pressure prevention is enabled with activation/deactivation
of the devices, the Digital Scroll™ compressor delivers the minimum
possible capacity.
Starting procedure
pRack pR300 can manage the specifi c starting procedure for Digital
Scroll™ compressors, as represented as in the following fi gure:
4s
Request
Valve
Compressor
There are three stages:
1.
balance: the PWM valve is activated for 4 s, so that the compressor
delivers minimum capacity;
2.
compressor activation with 50 % capacity for 3 minutes;
3.
forced operation at 100 % for 1 minute.
During the starting procedure, the request sent by the controller is
ignored and only at the end of the procedure does the capacity delivered
start refl ecting the request. If the request is cancelled during the starting
procedure, the compressor stops at the end, then the minimum ON time
for these types of compressors is set to 244 s. The starting procedure
is performed when the compressor is started, while it can be disabled
for a set time by parameter for subsequent starts, if the compressor has
not remained off for a minimum set time. After this time has elapsed the
procedure is performed again during the following start.
Note: the safety times for Digital Scroll™ compressors are
established by the manufacturer, and are as follows:
•
Minimum ON time: 244 s (starting procedure)
•
Minimum OFF time: 180 s
•
Minimum time between restarts: 360 s
Alarms
pRack pR300 can manage, in addition to the common alarms for all types
of compressors (see chapter 8 for details), some specifi c alarms for Digital
Scroll™ compressors:
•
high oil temperature
•
oil dilution
•
high discharge temperature
These alarms are managed as specifi ed by the manufacturer of the
compressor, and therefore pRack pR300 can only enable or disable them.
Activation of these alarms requires an oil temperature probe, which
can also be the common probe (see the paragraph relating to oil
management) and the compressor discharge temperature probe.
Note: pRack pR300 does not manages the envelope for Digital
Scroll™ compressors and consequently there is no corresponding
alarm when operating outside the envelope.
6.3.11 Screw compressors
pRack pR300 can manage up to two screw compressors, with control in
stages or continuous control (only the fi rst compressor with continuous
control, used as modulation device for the suction line), which can be a
generic device or pre-confi gured in accordance with the most common
devices supplied by the main manufacturers. Advanced functions are
also available, for example envelope control, described further on. The
parameters relating to screw compressors can be set in main menu branch
C.a.f/C.b.f. Screw compressors are fi tted with up to 4 capacity control valves
(hereinafter V1, V2, V3, V4), which can have 4 types of behaviour:
•
ON: the valve is open
•
OFF: the valve is closed
•
Intermittent: the valve is open/closed alternatively (around 10 to 15 s)
•
Pulsating: the valve is open/closed alternatively with very short
opening/closing times (around 1 to 2 s).
39
180s
60s
50s
100s
Startup
Fig. 6.q
pRack +0300025EN rel. 1.3 - 17.12.2015
ENG
time
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