Laundry dryer of the type comprising a heat pump system having a refrigerant circuit for a refrigerant and comprising a process air circuit in communication with a laundry container suited for receiving laundry to be dried using process air, said refrigerant circuit comprising: a central processing unit, a refrigerant, a first heat exchanger for heating said process air and cooling said refrigerant, said first heat exchanger comprising a heat exchanger outlet for expelling the heated process air for said laundry container; a second heat exchanger for cooling said process air and heating said refrigerant; a refrigerant expansion device arranged in said refrigerant circuit between said first heat exchanger and said second heat exchanger; and a rotary or a scroll compressor arranged in said refrigerant circuit between said second heat exchanger and said first heat exchanger, said compressor comprising a compressor chamber and a compressor inlet where said refrigerant is sucked for said compressor chamber, wherein the refrigerant is compressed in the compressor chamber by movement of the compressor's mechanism; characterized in that said compressor is driven at a variable rotational speed, wherein a compressor control unit for controlling the rotational speed (Cs) of the compressor is part of the central processing unit; and sensor elements are provided for detecting a refrigerant temperature at the evaporator inlet and/or a refrigerant pressure at the evaporator inlet and/or at the evaporator outlet, wherein the sensor elements are connected to the central processing unit; wherein the product of the volume (Vd) of said compressor chamber and the latent heat of evaporation of said refrigerant at the evaporation temperature in steady state condition of said heat pump system is higher than 2500 cm 3∗ kJ/kg; wherein the latent heat of evaporation of the refrigerant is calculated using the refrigerant temperature detected at the evaporator inlet and/or the refrigerant pressure detected at the evaporator inlet and/or at the evaporator outlet; wherein the correlation between the latent heat of evaporation of the refrigerant at the evaporation temperature and the detected values are stored as a function or as a table in a memory of the central processing unit; and wherein said steady state condition is a state condition reached by said heat pump system after a period of time (t1-t0) from its activation when thermodynamic conditions are substantially stable over time so that the latent heat of evaporation of the refrigerant reaches a substantially stable value, and said steady state condition is obtained and/or maintained by controlling/varying the rotational speed of said compressor. a central processing unit, a refrigerant, a first heat exchanger for heating said process air and cooling said refrigerant, said first heat exchanger comprising a heat exchanger outlet for expelling the heated process air for said laundry container; a second heat exchanger for cooling said process air and heating said refrigerant; a refrigerant expansion device arranged in said refrigerant circuit between said first heat exchanger and said second heat exchanger; and a rotary or a scroll compressor arranged in said refrigerant circuit between said second heat exchanger and said first heat exchanger, said compressor comprising a compressor chamber and a compressor inlet where said refrigerant is sucked for said compressor chamber, wherein the refrigerant is compressed in the compressor chamber by movement of the compressor's mechanism; characterized in that said compressor is driven at a variable rotational speed, wherein a compressor control unit for controlling the rotational speed (Cs) of the compressor is part of the central processing unit; and sensor elements are provided for detecting a refrigerant temperature at the evaporator inlet and/or a refrigerant pressure at the evaporator inlet and/or at the evaporator outlet, wherein the sensor elements are connected to the central processing unit; wherein the product of the volume (Vd) of said compressor chamber and the latent heat of evaporation of said refrigerant at the evaporation temperature in steady state condition of said heat pump system is higher than 2500 cm 3∗ kJ/kg; wherein the latent heat of evaporation of the refrigerant is calculated using the refrigerant temperature detected at the evaporator inlet and/or the refrigerant pressure detected at the evaporator inlet and/or at the evaporator outlet; wherein the correlation between the latent heat of evaporation of the refrigerant at the evaporation temperature and the detected values are stored as a function or as a table in a memory of the central processing unit; and wherein said steady state condition is a state condition reached by said heat pump system after a period of time (t1-t0) from its activation when thermodynamic conditions are substantially stable over time so that the latent heat of evaporation of the refrigerant reaches a substantially stable value, and said steady state condition is obtained and/or maintained by controlling/varying the rotational speed of said compressor.