Carel mRack - Compressors with Different Capacities

µRack
Cod. CAREL +03P220431 rel. 0.0 del 08/09/05 - preliminary version
11
4.4. Number of compressors started with probe 1 fault
In the event of a suction probe fault or not connected alarm, parameter /07 indicates the number of compressors forced on, so as to ensure minimum
cooling/operation of the installation.
For two circuits, the parameter relating to the second circuit /08 must also be set. This will be related to the probe in the 2nd circuit.
4.5. Compressors with different capacities
Parameter /02 is used to choose the option of compressors with different capacities.
This allows more load steps and therefore finer control.
Once the capacity of the individual compressors has been defined (parameters /03, /04, /05, /06), the software, based on the requirements of the installation and
the compressors available (without alarms or timers), will calculate the most suitable combination to satisfy the requirement. Whenever the requirement changes,
the software recalculates the most suitable combination. The combination will always be greater than or equal to the requirement.
If two compressors have the same capacity, the compressor with the lower index will always be the first to start.
4.5.1. Proportional band control with different capacity compressors
Based on the pressure, the set point and the differential, the software will proportionally calculate the capacity required to bring the pressure back near the set
point.
At the set point plus differential the requirement will be at the maximum value, while it will be null for pressure values around or less than the set point.
alDifferenti
pressSetpointCapacityMax
requiredCapacity
)(_
_
−×
=
4.5.2. Dead band control with different capacity compressors
The software will calculate the maximum number of combinations possible with the compressors available.
At certain intervals of time (see the paragraph on Compressor dead band with variable times), the software will call a sequence with a higher capacity.
In the deactivation phase, the opposite will occur, while in the dead band no compressors will be started or stopped.
An increase in the requirement will correspond to a different combination.
DOffZ
DOnZ
NZ
SP
DZN
RP
Fig 4.4
4.5.3. Example of compressors with different capacities
The following example looks at an installation featuring 3 compressors with different capacities, using proportional band control. As can be seen, there are 8
possible combinations available.
Set point
1.0
bar
“r01”
Differential 2.0 bar
“r02”
Comp1 5 kW
“/03”
Comp2 7 kW
“/04”
Comp3 15 kW
“/05”
Maximum capacity 27 kW
“/06”
Pressure Requirement kW Comp1 Comp2 Comp3 Total active capacity kW
1.1 1.35 X 5
1.6 8.1 X 7
1.8 10.8 X X 12
2 13.5 X 15
2.1 14.85 X 15
2.4 18.9 X X 20
2.5 20.25 X X 22
3 27 X X X 27
Table 4.1
Key:
DOffZ Device deactivation zone
DOnZ Device activation zone
NZ Dead band
DZN Dead band differential
RP Suction pressure read
SP Set point: compressors (S2); fans (S1)