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6.6.6.2 Temperature-driven defrost
Temperature-driven defrost follows the same operating principle as pressure-driven defrost, the only difference
being that the defrost threshold is expressed as a temperature value - not a pressure value - and it is compared
with the evaporation temperature.
6.6.6.3 Variable defrost
In conditions of low external air temperature the evaporation pressure of the unit reaches, on average, quite
low values that are close to the “start defrost” threshold. In other words, when defrosting is managed by either
pressure or temperature, the unit runs frequent defrost cycles, which results in reduced energy efficiency.
Variable defrost helps improve the overall efficiency of the unit. The defrost cycle is started exclusively when
the heat exchange of the external coil is actually reduced due to the build-up of frost on the coil.
A circuit defrost cycle is started as soon as the following conditions are met simultaneously:
1) the evaporation temperature is either inferior or equal to the defrost threshold (SPdfr), which is
calculated dynamically according to the temperature of the external air for longer than the “start
defrost” delay;
2) the external air temperature is either inferior or equal to the defrost deactivation threshold (SPnodfr);
3) the time between two consecutive defrost cycles has elapsed in the concerned circuit.
On the other hand, if the evaporation temperature is inferior to the forced defrost threshold (SPdfrf) for longer
than the “start forced defrost” delay, the defrost cycle is started without waiting for the time between two
consecutive defrost cycles to have elapsed.
The defrost threshold (SPdfr) is calculated as follows:
= (1 − )
−
where:
is the temperature of the external air, as measured by the dedicated probe;
is the angular coefficient of the straight line;
is the intercept, i.e. the “start defrost” temperature setpoint that corresponds to an external air
temperature value of 0°C.
Parameters “m” and “q” are set on the controller.
As is the case with pressure- or temperature-driven defrost, units featuring two refrigerant circuits can run the
defrost cycle either simultaneously in the two circuits or separately.
In this case too, the defrost cycle requires that the refrigerant cycle be reversed: the unit switches from heat
pump operation to cooling mode and this happens again with just one compressor ON in the circuit. The fans
in the external coil are disabled during the defrost cycle, which causes the condensing pressure to increase
and the temperature in the external coil to rise, thus melting the frost that had built on it.
As soon as the “end defrost” pressure threshold is reached, the controller switches off one compressor in the
defrosting circuit and then waits for a settable time - the “dripping” time - until water is dripped from the external
coil. Finally, it resumes operation in heat pump mode and restarts the fans in the external coil. Where the “end
defrost” pressure threshold fails to be reached, the defrost cycle ends after a max. time in any case.
Each circuit can start the defrost cycle after the min. time between two consecutive defrost cycles in the circuit
has elapsed.