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13 CONTROL AND PROTECTION FUNCTIONS
13.1 Pressure control
13.1.1
Safety Reduction Load function (LP)
Also in S-MEXT units with Mr. Slim, the main purpose of the function is to keep the low pressure at a value set using the
parameter, should it fall below a "setpoint" value. This is to avoid the frost-free function from tripping at low temperatures, and
stopping the unit due to the probability of ice forming on the coil.
The function activates if the pressure falls below the setpoint value (P20.74), and operates on two levels:
•
"Alert" level (less less incisive).
•
"Protection" level.
INFORMATION:
For two-circuit units, when there are two pressure probes there is only one setpoint. When activating
the function, the lower of the two values read by the probe is taken into account.
INFORMATION:
The time between steps is set by parameters P20.76 for circuit 1 and P20.81 for circuit 2.
The number of steps is set by parameter P20.77 for both circuits and its minimum value is fixed at 1.
INFORMATION:
The function prevents the compressor from dropping below the minimum frequency, irrespective of
the number of steps and Hz value to be scaled for each step.
If the pressure does not increase again even after all the protection steps have been carried out, the frost-free function trips,
turning off the faulty circuit.
If the pressure rises above setpoint + differential (P20.74 + P20.75), the function increases the demand on the compressor step
by step, backwards, until it returns to normal operation in t-control.
INFORMATION:
The safety times of the compressors are observed.
The charts in the following pages show the operation described above:
Version 38
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