µRack
Cod. CAREL +03P220431 rel. 0.0 del 08/09/05 - preliminary version
9
4. Compressor management
Inputs used:
• Suction pressure probe/probes
• Digital inputs dedicated to the compressor safety devices
• Multifunction input for generic alarm (general suction pressure switch 1 and 2)
Outputs used:
• Compressor outputs
4.1. General settings
Parameters used for ON OFF control:
• number of compressors
• compressor times
• type of control
The compressors are managed by the controller based on a pressure set point (parameter r01) and differential (parameter r02), measured by the suction probe.
In the case of two circuits, the set point and the differential also need to be set for the second circuit (parameters r03 and r04).
4.2. Compressor rotation
Rotation (parameter r05) of the compressor calls ensures that the number of operating hours and the number of starts of the different compressors balance out.
Rotation automatically excludes any compressors with alarm or that are disabled.
If a compressor is stopped due to an alarm or has been disabled, another will be immediately called to satisfy the load.
In the default configuration, FIFO rotation has been selected.
Three different types of rotation can be set:
LIFO rotation
The first compressor to start will be the last to stop.
• Start: C1,C2,C3,C4.
• Stop:C4,C3,C2,C1.
FIFO rotation
The first compressor to start will be the first to stop.
• Start: C1,C2,C3,C4
• Stop: C1,C2,C3,C4.
This selection enables the rotation of the compressors so as to even out as much as possible the number of compressor operating hours.
Rotation by time
The compressor that starts will be the one with the lowest number of operating hours. When stopping the exact opposite is true, that is, the compressor with the
highest number of operating hours will stop.
4.3. Compressor control
In the default configuration, “dead band” control is activated (parameter r06).
Proportional band
Proportional band control calculates, based on various parameters (SP, DF and the number of devices set) the points where the devices must switch on and off,
inside the differential band. Parameters r01 (set point) r02 (differential).
Figure 4.1 shows the activation points for a system with 4 steps.
Setting the parameters listed above, each individual step will have a differential as follows:
SP + 1 *DF/ (No. of steps) for the first;
SP + 2 *DF/ (No. of steps) for the second;
…
SP + DF for the last.
RP
DF
ON
OFF
Fig 4.1
Dead band
This type of control features the definition of a dead band to the side of the set point, within which no device is started or stopped.
Key
:
SP Compressor set point
DF Compressor differential
RP Pressure read
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