In a known dryer or washing-drying machine, as far as the drying cycle is concerned, it is known to use an electric heater to heat air for use in a laundry treatment chamber (e.g., a laundry drum), called process air hereinafter. The process air is circulated in the laundry treatment chamber by means of a process air blower or fan. The laundry treatment chamber and the process air blower are rotated by a motor. Moisture may be optionally removed from process air using an air-air heat exchanger, or the process air may be exhausted outside the machine.
In order to save energy consumption, it is also known to use a heat pump system in place of or besides the electric heater, for heating the process air during the drying cycle.
More specifically, the heat pump system comprises a first heat exchanger (evaporator) for heating a refrigerator fluid, a second heat exchanger (condenser) for cooling the refrigerant fluid, an expansion device and a refrigerant loop, in which the refrigerant fluid is circulated through the first and second heat exchangers and the expansion device. A compressor is provided which is adapted to operate for circulating the refrigerant fluid through the refrigerant loop.
Process air flow rate influences the efficiency and reliability of the electric heater or the heat pump system of the dryer or washing-drying machine.
With specific reference to a dryer or washing-drying machine using a heat pump system for heating the process air during the drying cycle, in particular working situations, temperatures of the heat pump system may rise too much, thus reaching the critical working range of the compressor. In such conditions, it is necessary to switch off the compressor for a determined time period in order to let its temperature to decrease down to the normal working range while the process air fan is still running. Then the compressor can be switched on again keeping the drying cycle going on properly.
Such a solution has the drawback that the drying cycle lengthens considerably due to compressor stop(s). A particularly critical working situation occurs at low process air flow rate and high temperature of the ambient or environment where the laundry treatment apparatus is installed. In such a condition, heat pump system temperatures are quite high from the very beginning of the drying cycle and therefore the compressor can reach its critical working range quite quickly and frequently.