!
USER MANUAL – Release 1.2 of 01/01/2010
Page 117 of 191
Version: 02/03/2010
WARNING:
In the case of pump-down from dedicated pressure switch or analogue input, it is possible that the pressure
at which compressor/step switch-off is allowed is lower than the activation threshold of the LP pressure
switch. In this case, during the pump-down phase, the input relative to the LP pressure switch will be surely
activated and will remain so at least until the re-opening of the solenoid valve, with one compressor/step still
active.
Similarly, it is possible that the pressure at which the compressor/step is enabled is lower than the
deactivation threshold of the LP pressure switch. In this case, there will be a switch-on request of one
compressor/step with the digital input relative to the LP pressure switch active.
These are limit conditions that can be managed via parameter AL02, but which should be avoided in order to
prevent possible alarm conditions that are difficult to overcome
44.6
WORKING EXAMPLES:
44.6.1 Pump-down active only in switch-off based on LP pressure switch
On the switch-off request of the last step active in the circuit, the solenoid valve is closed. The
evaporation pressure lowers following closure of the valve until dropping below the LP pressure switch
intervention threshold (let's suppose that the time PD04 is long enough to allow this to happen). The
compressor is switched off at this point.
On the successive activation request of a compressor in the circuit in question, with all probability the
evaporation pressure will still be below the LP pressure switch deactivation threshold. 2 cases can occur:
1. AL02=2: The compressor is made to start even with the LP pressure switch active, on expiry of the
by-pass time AL01, if the pressure switch is still active an LP alarm will be signalled with “normal”
management (automatic or manual reset depending on the number of times the value of parameter
AL05 has been repeated in the last hour and however possible ONLY if the input relative to the
pressure switch is activated).
2. AL02=1: the solenoid valve is opened to favour the re-equilibrium of the pressure inside the circuit
but compressor working is not enabled while the LP pressure switch remains active. If the pressure
switch deactivates within time AL01 from the opening of the solenoid valve, the compressor/step is
activated, otherwise the LP alarm is signalled with normal methods.
44.6.2 Pump-down active in switch-off and re-start, based on dedicated pressure switch. Pump-
down pressure switch intervention threshold higher than the intervention threshold of the LP
pressure switch.
On the switch-off request of the last step active in the circuit, the solenoid valve is closed. The
evaporation pressure lowers following closure of the valve until dropping below the pump-down pressure
switch intervention threshold (let's suppose that the time PD04 is long enough to allow this to happen).
The compressor is switched off at this point.
On the successive activation request of a compressor in the circuit in question, the circuit solenoid valve
is activated to favour the re-equilibrium of the pressure inside the circuit. The compressor/step is not
enabled on working while the pump-down pressure switch remains active. If the pressure rises enough to
deactivate the pump-down pressure switch within time PD04, the compressor/step is activated. If this is
not the case, there will be a pump down alarm on start-up.
44.6.3 Pump-down active in switch-off and re-start, based on dedicated pressure switch. Pump-
down pressure switch intervention threshold lower than the intervention threshold of the LP
pressure switch.
On the switch-off request of the last step active in the circuit, the solenoid valve is closed. The
evaporation pressure lowers following closure of the valve until dropping below the LP pressure switch
intervention threshold. At this point, depending on the setting of parameter AL02, there may be 2 cases:
1. AL02=0: the compressor/step is switched-off and a LP alarm is signalled, which remains active until
the pressure switch is deactivated.
This configuration inevitably leads to unit block due to the
LP alarm, therefore it must be avoided.
If the intervention pressure of the pump-down pressure
switch is lower than the intervention pressure of the LP pressure switch, the parameter AL02 should
not be set at 0.
2. AL02=1: the compressor/step remains active and the pressure continues to decrease until dropping
below the activation threshold of the pump-down pressure switch (supposing that this occurs within
time PD04). The compressor/step is switched-off only at this point. On the successive activation
request of a compressor in the circuit in question, the circuit solenoid valve is activated to favour the
re-equilibrium of the pressure inside the circuit. The compressor/step is not enabled on working while
the pump-down pressure switch remains active. Three situations can occur:
