WO2015082011A1

WO2015082011A1 - A method for controlling a laundry drying machine of the type comprising a heat pump system and a corresponding laundry drying machine

Info

Publication number: WO2015082011A1
Application number: PCT/EP2013/075722
Authority: WO
Inventors: Francesco Cavarretta; Maura Pasquotti; Giuseppe Rossi; Alessandro DALLA ROSA
Original assignee: Electrolux Appliances AB
Current assignee: Electrolux Appliances AB
Priority date: 2013-12-05
Filing date: 2013-12-05
Publication date: 2015-06-11
Anticipated expiration: 2016-06-05
Also published as: EP3077588A1; EP3077588B1; PL3077588T3

Abstract

The present invention relates to a method for controlling a drying cycle in a laundry drying machine (1) of the type comprising a heat pump system (20) having a refrigerant circuit (30) for a refrigerant and comprising a drying air circuit (10) for conveying a volume flow of drying air (A) in a laundry drum (9) suitable for receiving laundry to be dried. The refrigerant circuit (30) comprises: a compressor (24) with a variable rotation speed (Cs); a first heat exchanger (21) for a thermal coupling between the drying air circuit (10) and the refrigerant circuit (30) and a second heat exchanger (23) for a further thermal coupling between the drying air circuit (10) and the refrigerant circuit (30). The method comprises an optimization procedure comprising the step of controlling the rotation speed (Cs) or the power (Cp) of the compressor (24) during a plurality of time intervals (Δt_n-2; Δt_n-1-; Δt_n) of the drying cycle. The optimization procedure comprises the steps of: determining an efficiency parameter (F(t_n-1)) related to the amount of water removed from the laundry during a first time interval (Δt_n-2); determining an efficiency parameter (F(t_n)) related to the amount of water removed from the laundry during a second time interval (Δt_n-1); comparing the efficiency parameter (F(t_n)) determined during the second time interval (Δt_n-t) with the efficiency parameter (F(t_n-1)) determined during the first time interval (Δt_n-2); adjusting the rotation speed (Cs) or the power (C_P) of the compressor (24) in a third time interval (Δt_n) according to the result of said comparison.

Description

A METHOD FOR CONTROLLING A LAUNDRY DRYING MACHINE

OF THE TYPE COMPRISING A HEAT PUMP SYSTEM AND A CORRESPONDING LAUNDRY DRYING MACHINE The present invention concerns the field of laundry drying techniques.

In particular, the present invention refers to a laundry drying machine equipped with a heat pump system and, more in particular, a method for controlling such a heat pump system. BACKGROUND ART

Laundry treating machines capable of carrying out a drying process on laundry, hereinafter simply indicated as laundry dryers, generally comprise a casing that houses a laundry container, like a rotating drum, where laundry to be treated is received. A closed air stream circuit carries out drying operation by circulating hot air through the laundry container containing the wet laundry.

In laundry dryers, the heat pump technology is the most efficient way to save energy during drying operation. In conventional heat pump laundry dryers a drying air stream flows in a close loop. The air passes through the laundry drum and removes water from wet clothes. Then the drying air stream is cooled down and dehumidified and then heated up in a heat pump system and finally reinserted again into the laundry drum.

The drying air stream is typically moved by air conveyance means, usually constituted of a fan arranged along the closed-loop air stream circuit. The volume flow of the drying air conveyed into the drum is set according to the fan speed. The heat pump system comprises a refrigerant flowing in a closed-loop refrigerant circuit constituted by a compressor, a condenser, an expansion device and an evaporator. The condenser heats up the drying air while the evaporator cools and dehumidifies the drying air leaving the drum. The refrigerant flows in the refrigerant circuit where it is compressed by the compressor, condensed in the condenser, expanded in the expansion device and then vaporized in the evaporator. The temperatures of the drying air stream and the refrigerant are strongly correlated to each other.

Performance of the heat pump system strongly depends on the compressor features. In a first type of laundry dryers of known type the heat pump system comprises a fixed speed compressor. This type of compressor works in an on/off-mode, so that the operating parameters of said compressor and of the heat pump system cannot be properly controlled during the drying cycle.

In a second type of laundry dryers of known type the heat pump system comprises a controlled speed motor. The compressor motor speed may be determined on the base of the drying cycle selected by the user through a user control interface.

For example, the user may select an "economic cycle" or a "fast cycle". In case an "economic cycle" is selected, the compressor motor speed may be maintained substantially constant during the time. In case a "fast cycle" is selected, on the contrary, the compressor motor speed may comprise peaks of high intensity. This reduces the cycle duration, while the power consumption increases.

In other preferred embodiments, the compressor motor speed may be determined on the base of the type of laundry textile to dry (cotton, wool, delicate, etc.), typically selected by the user through the user control interface.

Irrespective of the type of compressor motor, the heat pump system strongly affects the efficiency of the drying process.

The drying process in the laundry dryers of the known type above mentioned requires a large amount of energy. Also, it is know that a drying process in laundry dryers of the known type requires a large amount of time. This determines high cost for the user.

It is desirable, therefore, to optimize the drying process, in particular to optimize the energy consumption during the process.

It is therefore an object of the present invention to provide a method for drying laundry in a laundry drying machine and a laundry drying machine with higher energy efficiency compared to the known technique.

A main object of the present invention is therefore to provide a method for controlling a heat pump system for a laundry dryer which allows an additional saving of energy during the drying cycle.

Another object of the present invention is to provide a method for drying laundry in a laundry drying machine which allows the reduction of the drying cycle time compared to method of known type.

Advantages, objects, and features of the invention will be set forth in part in the description and drawings which follow and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention.

DISCLOSURE OF INVENTION

The applicant has found that by providing a method for controlling a laundry drying machine with at least one heat pump system comprising an air stream circuit including at least one drum for receiving laundry to be dried, at least one refrigerant circuit including at least one compressor with a variable rotation speed, a first heat exchanger for a thermal coupling between the air stream circuit and the refrigerant circuit and a second heat exchanger for a further thermal coupling between the air stream circuit and the refrigerant circuit, wherein the rotation speed, or the power, of the compressor is repeatedly adjusted on the base of the amount of water removed from the laundry, it is possible to save energy during the drying cycle.

In a first aspect the present invention relates, therefore, to a method for controlling a drying cycle in a laundry drying machine of the type comprising a heat pump system having a refrigerant circuit for a refrigerant and comprising a drying air circuit for conveying a volume flow of drying air in a laundry drum suitable for receiving laundry to be dried, said refrigerant circuit comprising:

- a compressor with a variable rotation speed;

- a first heat exchanger for a thermal coupling between said drying air circuit and said refrigerant circuit wherein the temperature of said drying air increases and the temperature of said refrigerant decreases; and

- a second heat exchanger for a further thermal coupling between said drying air circuit and said refrigerant circuit wherein the temperature of said drying air decreases and the temperature of said refrigerant increases; wherein said method comprises an optimization procedure comprising the step of controlling the rotation speed or the power of said compressor during a plurality of time intervals of said drying cycle, wherein the optimization procedure comprises the steps of:

al) determining an efficiency parameter related to the amount of water removed from said laundry during a first time interval;

a2) determining an efficiency parameter related to the amount of water removed from said laundry during a second time interval; b) comparing the efficiency parameter determined during said second time interval with the efficiency parameter determined during said first time interval; c) adjusting the rotation speed or the power of said compressor in a third time interval according to the result of said comparison performed in said step b); wherein said second time interval is subsequent to said first time interval and said third time interval is subsequent to said second time interval.

Preferably, the step of controlling the rotation speed or the power of the compressor comprises increasing or decreasing the rotation speed or the power of the compressor during the plurality of time intervals.

Preferably, the step c) of adjusting the rotation speed or the power comprises increasing or decreasing the rotation speed or the power of the compressor in the third time interval according to the result of the comparison in the step b).

Preferably, the step c) of adjusting the rotation speed or the power comprises increasing or decreasing the rotation speed or the power of the compressor in the third time interval according to the result of the comparison in the step b) and according to an increasing or a decreasing of the rotation speed or the power of the compressor in the second time interval with respect to the first time interval. Preferably, the step c) of adjusting the rotation speed or the power comprises increasing or decreasing the rotation speed or the power of the compressor in the third time interval with respect to the second time interval according to the result of the comparison in the step b) and according to an increasing or a decreasing of the rotation speed or the power of the compressor in the second time interval with respect to the first time interval.

In a preferred embodiment of the invention, the step c) of adjusting the rotation speed or the power comprises increasing the rotation speed or the power of the compressor in the third time interval if the comparison performed in the step b) indicates that the efficiency parameter determined during the second time interval increases with respect to the efficiency parameter determined during the first time interval and the rotation speed or the power of the compressor was increased in the second time interval with respect to the first time interval.

More preferably, the step c) of adjusting the rotation speed or the power comprises increasing the rotation speed or the power of the compressor in the third time interval with respect to the second time interval if the comparison performed in the step b) indicates that the efficiency parameter determined during the second time interval increases with respect to the efficiency parameter determined during the first time interval and the rotation speed or the power of the compressor was increased in the second time interval with respect to the first time interval.

Alternatively, the step c) of adjusting the rotation speed or the power comprises decreasing the rotation speed or the power of the compressor in the third time interval if the comparison performed in the step b) indicates that the efficiency parameter determined during the second time interval increases with respect to the efficiency parameter determined during the first time interval and the rotation speed or the power of the compressor was decreased in the second time interval with respect to the first time interval.

More preferably, the step c) of adjusting the rotation speed or the power comprises decreasing the rotation speed or the power of the compressor in the third time interval with respect to the second time interval if the comparison performed in the step b) indicates that the efficiency parameter determined during the second time interval increases with respect to the efficiency parameter determined during the first time interval and the rotation speed or the power of the compressor was decreased in the second time interval with respect to the first time interval.

Alternatively, the step c) of adjusting the rotation speed or the power comprises increasing the rotation speed or the power of the compressor in the third time interval if the comparison performed in the step b) indicates that the efficiency parameter determined during the second time interval decreases with respect to the efficiency parameter determined during the first time interval and the rotation speed or the power of the compressor was decreased in the second time interval with respect to the first time interval.

More preferably, the step c) of adjusting the rotation speed or the power comprises increasing the rotation speed or the power of the compressor in the third time interval with respect to the second time interval if the comparison performed in the step b) indicates that the efficiency parameter determined during the second time interval decreases with respect to the efficiency parameter determined during the first time interval and the rotation speed or the power of the compressor was decreased in the second time interval with respect to the first time interval.

Alternatively, the step c) of adjusting the rotation speed or the power comprises decreasing the rotation speed or the power of the compressor in the third time interval if the comparison performed in the step b) indicates that the efficiency parameter determined during the second time interval decreases with respect to the efficiency parameter determined during the first time interval and the rotation speed or the power of the compressor was increased in the second time interval with respect to the first time interval.

More preferably, the step c) of adjusting the rotation speed or the power comprises decreasing the rotation speed or the power of the compressor in the third time interval with respect to the second time interval if the comparison performed in the step b) indicates that the efficiency parameter determined during the second time interval decreases with respect to the efficiency parameter determined during the first time interval and the rotation speed or the power of the compressor was increased in the second time interval with respect to the first time interval.

Preferably, the step al) takes place at the end of the first time interval.

Preferably, the step a2) takes place at the end of the second time interval.

Preferably, the step c) takes place at the beginning of the third time interval. Preferably, the efficiency parameter is calculated by:

bl) determining a first parameter related to the amount of water removed from the laundry during said time interval;

b2) determining the power consumption of the compressor during said time interval;

b3) determining a relation between the first parameter and the power consumption of the compressor.

More preferably, the step b3) comprises the step of calculating the ratio between the first parameter and the power consumption of the compressor.

In a further preferred embodiment of the invention, the efficiency parameter is calculated by:

b4) determining the sum of the power consumption of the compressor and the power consumption of at least one other electric component of the laundry drying machine during said time interval;

b5) determining a relation between the first parameter and the sum of power consumptions.

More preferably, the step b5) comprises the step of calculating the ratio between the first parameter and the sum of power consumptions.

In this case, preferably, the method further comprises a step d) of adjusting the rotation speed or the power of said at least one other electric component in the third time interval according to the result of the comparison performed in the step b), wherein the at least one other electric component is an electric motor.

Preferably, the step d) of adjusting the rotation speed or the power of the at least one other electric component in the third time interval according to the result of the comparison performed in the step b), comprises increasing or decreasing the rotation speed or the power of the of the at least one other electric component in the third time interval according to the result of the comparison in the step b) and according to an increasing or a decreasing of the rotation speed or the power of the at least one other electric component in the second time interval with respect to the first time interval.

Preferably, the at least one other electric component is a fan motor or a drum motor.

In another preferred embodiment of the invention, the method further comprises a step d) of increasing or decreasing the rotation speed or the power of a fan motor in the third time interval according to the result of the comparison in the step b) and according to an increasing or a decreasing of the rotation speed or the power of the fan motor in the second time interval with respect to the first time interval.

In a preferred embodiment of the invention, the step b4) of determining the sum of the power consumption of the compressor and the power consumption of at least one other component is carried out using a weighted sum.

Preferably, the first parameter is obtained according to one of the following criteria: estimation by a direct measure of the condensed water generated at the second heat exchanger; estimation by considering the trend of the power consumption of a drum motor; estimation by considering the trend of the electrical current and/or of the torque of the drum motor; estimation by measuring the weight variations of the laundry in the laundry drum; estimation by considering the number of switch on/off of a draining pump of a draining pump associated to the condensed water generated at the second heat exchanger; estimation by detecting the signal of a water level or flow sensor associated to a water collecting container; estimation by considering the trend of the temperature and/or the humidity level of the moist air leaving the laundry drum; estimation by considering the trend of the temperature and/or the pressure level of the refrigerant in the refrigerant circuit; estimation by the difference of the drying air temperature at the second heat exchanger outlet and the drying air temperature at the second heat exchanger inlet; estimation by the difference between two refrigerant temperature levels at the second heat exchanger; estimation by the difference between the drying air humidity at the second heat exchanger outlet and the drying air humidity at the second heat exchanger inlet.

Alternatively, the first parameter is obtained according to one of the following criteria: estimation by considering the difference between the relative humidity of the drying air at the drum inlet and outlet; estimation by considering the difference of the absolute humidity of the drying air at the drum inlet and outlet; estimation by considering the difference of the temperature of the drying air at the drum inlet and outlet.

In a preferred embodiment of the invention, the optimization procedure is performed after an initial transitional phase of the drying cycle.

Preferably, the optimization procedure is performed after a prefixed period of time from the beginning of the drying cycle.

Opportunely, the optimization procedure is performed when the drying cycle reaches a steady-state condition.

The steady-state condition occurs, preferably, after an initial transitional phase and when the condensation temperature and the evaporation temperature of the refrigerant in the first heat exchanger and in the second heat exchanger remain within a respective prefixed temperature range.

Preferably, the optimization procedure is carried out in the laundry drying machine by means of a central processing unit.

In a further aspect, the present invention relates to a laundry drying machine suited to implement the method above described.

BRIEF DESCRIPTION OF THE DRAWINGS

Further characteristics and advantages of the present invention will be highlighted in greater detail in the following detailed description of preferred embodiments of the invention, provided with reference to the enclosed drawings. In said drawings:

- Figure 1 shows a perspective view of a laundry drying machine with a heat pump system according to a preferred embodiment of the invention;

- Figure 2 illustrates a schematic diagram of the laundry drying machine of Figure 1;

- Figure 3 is a simplified flow chart of the basic operations of a method for drying laundry in the laundry drying machine of Figures 1 and 2 according to a preferred embodiment of the invention;

- Figure 4 illustrates a schematic diagram of the compressor speed and of the efficiency function as a function of the time;

- Figure 5 illustrates a schematic diagram of the evaporation and condensation temperatures of the refrigerant as a function of the time;

- Figure 6 is a simplified flow chart of the basic operations of a method for drying laundry in the laundry drying machine of Figures 1 and 2 according to a further preferred embodiment of the invention. DETAILED DESCRIPTION OF THE INVENTION

The present invention has proved to be particularly successful when applied to a front-loading laundry drying machine with a rotatable laundry container; however it is clear that the present invention can be applied as well to a top- loading laundry drying machine and also to laundry drying machines of cabinet type, i.e. laundry drying machines where the laundry container does not rotate.

Furthermore, the present invention can be usefully applied to all the machines requiring a drying phase for wetted clothes, as for example a combined laundry washing and drying machine.

In the present description the term "laundry drying machine" will refer to both simple laundry drying machines and laundry washing- drying machines.

Figures 1 and 2 illustrate a laundry drying machine 1, or laundry dryer, with a heat pump system 20 according to a preferred embodiment of the present invention.

The laundry dryer 1 preferably comprises, though not necessarily, a substantially parallelepiped-shaped outer boxlike casing 2 which is preferably structured for resting on the floor. A laundry container comprising a rotatably drum 9 is provided within the casing 2. A front door 8, pivotally coupled to the front upright side wall 2a, is provided for allowing access to the drum interior region to place laundry to be dried therein. The drum 9 is advantageously rotated by a drum motor 27, preferably an electric motor, which preferably transmits the rotating motion to the shaft of the drum 9, advantageously by means of a belt/pulley system. In a different embodiment of the invention, the drum motor can be directly associated with the shaft of the drum 9.

A drum motor control unit 28 is also provided for controlling the drum motor 27. In particular, the drum motor control unit 28 controls the speed Ds of the drum motor 27. The drum motor control unit 28 for controlling the rotation speed Ds of the drum motor 27 can be part of a central processing unit, not illustrated.

The laundry dryer 1 is provided with a drying air circuit 10, as illustrated in Figure 2, which is structured to circulate inside the drum 9 a volume flow of drying air A.

The drying air A circulates over and through the laundry located inside the drum

9 to dry the laundry. The drum 9 itself is therefore part of the drying air circuit 10.

The drying air circuit 10 is also structured for drawing moist air from the drum 9 and cooling down the moist air leaving the drum 9 so to extract and retain the surplus moisture. The dehumidified air is then heated up to a predetermined temperature preferably higher than that of the moist air arriving from the drum 9. Finally the heated, dehumidified air is conveyed again into the drum 9, where it flows over and through the laundry stored inside the rotatable drum 9 to rapidly dry the laundry, as said above.

The drying air circuit 10 forms, therefore, a closed-loop for the drying air A, as schematically illustrated with dashed line in Figure 2.

An air conveyance device 12 is preferably arranged along the drying air circuit

10 for generating the volume flow of drying air A.

The air conveyance device 12 preferably comprises a fan. A fan control unit 46 is also provided for controlling the fan 12. In particular, the fan control unit 46 is provided for controlling the fan speed Fs of the fan 12. The fan control unit 46 for controlling the rotation speed Fs of the fan 12 can be part of a central processing unit, not illustrated.

In a preferred embodiment of the invention, the fan 12 comprises an electric motor 45 and the control unit 46 comprises an inverter.

In further preferred embodiments, the fan 12 and the drum 9 may be preferably driven by the same electric motor. This may advantageously reduce the cost and/or the size of the laundry dryer. On the contrary, two electric motors 27, 45, as described above, may be advantageously driven and controlled independently so that the fan 12 and the drum 9 may be controlled independently.

The air conveyance device 12 is preferably arranged upstream of the drum 9. In different embodiments, nevertheless, the air conveyance device 12 may be arranged in any place along the drying air circuit 10.

Preferably, and more particularly, the drying air circuit 10 then comprises a dehumidifying unit 23 arranged downstream of the drum 9 and a heater unit 21 arranged downstream of the dehumidifying unit 23 and upstream of the drum 9. It is underlined that in the present application the terms "upstream" and "downstream" are referred to the flowing direction of the air, heated air and/or moist air, during the standard functioning of the laundry dryer; for example saying that the fan is arranged upstream of the drum means that in the standard functioning of the laundry dryer the air firstly passes through the fan and then flows into the drum; saying that the dehumidifying unit is arranged downstream of the drum means that in the standard functioning of the laundry dryer the air firstly circulates inside the drum and then passes through the dehumidifying unit. In the dehumidifying unit 23 the moist air leaving the drum 9 condenses and cools down and the water generated therein is preferably collected in a removable water container 14, visible in Figure 1, arranged below the dehumidifying unit 23.

In the preferred embodiment here described, the dehumidifying unit 23 is the evaporator of the heat pump system 20 and the heating unit 21 is the condenser of said heat pump system 20.

Therefore, the evaporator 23 cools down and dehumidifies the moist air coming from the drum 9 and then the condenser 21 heats up the dehumidified air coming from the evaporator 23. The heated air is then conveyed again into the drum 9. In further embodiments, the drying air circuit may not form a closed-loop. In this case, for example, the drying air may be conveyed to a condenser from outside, then conveyed into the drum, from the drum conveyed to the evaporator and finally expelled to the outside.

The heat pump system 20 with its evaporator 23 and condenser 21, therefore, interacts with the drying air circuit 10. In fact, the drying air circuit 10 and the heat pump system 20 are thermally coupled by the condenser 21 and the evaporator 23. In particular, the heat pump system 20 advantageously comprises a refrigerant circuit 30 forming a closed-loop circuit where a refrigerant flows.

The refrigerant circuit 30 comprises a compressor 24, a first heat exchanger 21, i.e. the condenser 21 in the preferred embodiment here described, an expansion device 22 and a second heat exchanger 23, i.e. the evaporator 23 in the preferred embodiment here described. The compressor 24, the condenser 21, the expansion device 22 and the evaporator 23 are connected in series to form said closed-loop circuit.

The refrigerant flows in the refrigerant circuit 30 wherein is compressed by the compressor 24, condensed in the condenser 21, expanded in the expansion device 22 and then vaporized in the evaporator 23.

In different embodiments, the first heat exchanger may comprise a gas cooler (instead of the condenser) and the second heat exchanger may comprise a gas heater (instead of the evaporator). In this case the refrigerant is advantageously a gas, such as C0₂, which maintains its gaseous state along all the closed-loop circuit, and in particular in the gas cooler and in the gas heater. In this type of heat pump system the gas temperature changes while passing through the gas cooler and the gas heater.

Generally, the first heat exchanger 21 defines a thermal coupling between the drying air circuit 10 and the refrigerant circuit 30 wherein the temperature of the drying air A increases and the temperature of the refrigerant decreases.

Analogously, the second heat exchanger 23 defines a further thermal coupling between the drying air circuit 10 and the refrigerant circuit 30 wherein the temperature of the drying air A decreases and the temperature of the refrigerant increases.

A compressor control unit 26 is also provided for controlling the compressor 24. In particular, the compressor control unit 26 is provided for controlling the rotation speed Cs of the compressor 24. The compressor control unit 26 for controlling the rotation speed Cs of the compressor 24 can be part of a central processing unit, not illustrated.

It should to be noted that with rotation speed Cs of the compressor 24 it is meant the rotation speed of a driving motor which is part of the compressor 24.

In a preferred embodiment of the invention, the compressor 24 comprises an electric motor and the compressor control unit 26 comprises an inverter.

Preferably, the compressor control unit 26, the fan control unit 46 and the drum motor control unit 28 communicate one to the other. More preferably, the compressor control unit 26, the fan control unit 46 and the drum motor control unit 28 are part of said central processing unit. The central processing unit advantageously manages and controls data from/for said units.

An interface unit 15 is preferably arranged on the top of the casing 2. The interface unit 15 is preferably accessible to the user for the selection of the drying cycle and insertion of other parameters, for example the type of fabric of the load, the degree of dryness, etc.. The interface unit 15 preferably displays machine working conditions, such as the remaining cycle time, alarm signals, etc. For this purpose the interface unit 15 preferably comprises a display 13.

In different embodiments, for example in a combined laundry washing and drying machine, the user may selects and inserts other types of parameters, for example the washing temperature, the spinning speed, etc..

In further embodiments, the interface unit may be differently realized, for example remotely arranged in case of a remote-control system.

Further, the laundry dryer 1 may comprise several kinds of sensor elements, which are not shown in the figures. For example, the sensor elements may be provided for detecting the temperature, the relative humidity of the drying air A and/or the electrical impedance at suitable positions of the laundry dryer 1, the pressure and/or the temperature of the refrigerant, etc..

The central processing unit above mentioned is advantageously connected to the various parts of the laundry dryer 1, or peripheral units or sensor elements, in order to ensure its operation.

Typically, the laundry to be dried is first placed inside the drum 9. By operating on the interface unit 15 the user selects the desired drying cycle depending, for example, on the type of laundry textile to dry or on the dryness degree of the laundry which is expected at the end of the drying cycle, for example totally dry or with residual moisture for a best ironing.

Once the user has selected the desired drying cycle, the central processing unit sets the laundry drying machine 1 so that the drying cycle may start.

In a further embodiment, the selection of the desired drying cycle may be performed before placing the laundry into the drum 9.

The drying cycle is preferably defined by controlling many parameters which allows the laundry to be dried according to the user selection.

Parameters which typically affect a drying cycle are: the drum rotation speed Ds and its rotational direction of rotation; the performance of the heat pump system 20, in particular the rotation speed Cs or the power Cp of the compressor 24; the volume flow of the drying air A in the drying air circuit 10.

In particular, the heat pump system 20 strongly affects the efficiency of the drying process.

At this purpose, a comprehensive and simple way to express the efficiency of a drying process in a laundry dryer can be expressed by the following efficiency function:

Pcompressor(t)

where:

F(t) is a time-dependent function;

m(t) is a parameter related to the mass flow rate of the water removed from the laundry at the generic time t;

Pcompressor t) is the electrical power consumption of the compressor 24 at the generic time t.

It is clear, also as expressed from said function F(t), that the efficiency of the drying process is affected by the electrical power consumption of the compressor 24.

With the term "mass flow rate" we mean the rate of water extracted by evaporation from laundry placed in the drum 9. The mass flow rate is advantageously expressed as weight over the time (typically gr/min).

In a preferred embodiment of the invention, the parameter rh(t) eventually used in said formula is m_Condense(t), where m_Condense(t) is the water mass flow rate of the water which condenses on the dehumidifying unit 23 (evaporator).

A value for the mass flow rate m_Condense(t) can be estimated in different ways. ^mcondense 0D ^may be preferably estimated by a direct measure of the condensed water. The measure of the condensed water may be preferably carried out, for example, using a flow meter or a scale arranged in correspondence of the removable container 14 which collects the condensed water generated in the evaporator 23.

In further embodiments, m_Condense(t) may be preferably estimated in different ways: by considering the trend of the power consumption of the drum motor 27; by considering the trend of the electrical current and/or of the torque of the drum motor 27; by measuring the weight of the clothes in the drum 9 during the cycle; by considering the number of switch on/off of a draining pump in case a draining pump for the condensed water is provided; by detecting the signal of a water level or flow sensor associated to the water collecting container 14; by considering the trend of the temperature and/or the humidity level (relative humidity or absolute humidity) of the moist air leaving the drum 9; by considering the trend of the temperature and/or the pressure level of the refrigerant in the refrigerant circuit 30; by considering the difference of the drying air temperature at the second heat exchanger outlet and the drying air temperature at the second heat exchanger inlet; by considering the difference between two refrigerant temperature levels at the second heat exchanger; by considering the difference between the drying air humidity at the second heat exchanger outlet and the drying air humidity at the second heat exchanger inlet. In particular, the weight of the clothes in the drum 9 may be estimated by measuring the electric resistance and/or conductivity of the same wet clothes, preferably by means of electric resistance sensor and/or conductivity sensor arranged inside the drum 9.

It should be noted that the laundry dryer may be equipped with a draining pump for the condensed water, as cited above. In a preferred realization of the laundry dryer, the condensed water drained by the draining pump is conveyed to a water tank preferably arranged on the top of the laundry dryer, which may be easily and periodically emptied by the user. In this preferred embodiment, m_Condense(t) may be preferably estimated by detecting the signal of a water level sensor associated to the water tank.

In a further preferred embodiment, the condensed water drained by the draining pump may be conveyed directly to the outside, preferably by means of a dedicated discharge pipe connected to the draining pump. In this preferred embodiment, m_Condense(t) may be preferably estimated by detecting the signal of a water flow sensor associated to the discharge pipe.

In another preferred embodiment of the invention, the parameter rh(t) eventually used in said formula is m_evap0ratecj (t) , where m_evap0ratecj (t) is the mass flow rate of the water extracted by evaporation from laundry in the drum 9.

A value for the mass flow rate m_evaporated (t) can be assessed in different ways. ^mevaporated 00 may be preferably estimated by: considering the difference between the relative humidity of the drying air A at the drum inlet and outlet, considering the difference of the absolute humidity of the drying air A at the drum inlet and outlet, considering the difference of the temperature of the drying air A at the drum inlet and outlet.

The method of the present invention maximizes said function F(t) by properly controlling the compressor 24 during the time.

In fact, once the user has selected the desired drying cycle, the central processing unit sets the course of the compressor speed Cs over the time until the drying cycle ends. Alternatively, the central processing unit sets the course of the compressor power Cp over the time until the drying cycle ends, instead of the compressor speed Cs.

For the purpose of the present invention, by "course" it is meant a trend over the time.

In preferred embodiments of the drying cycle, the compressor speed Cs, or the compressor power Cp, is adjusted during the time according to the evolution of the cycle. Preferred criteria for determining the courses of the compressor speed Cs or of the compressor power Cp in a drying cycle will be better described later. According to the method of the invention, the compressor speed Cs or the compressor power Cp is controlled on the base of the performance of the heat pump system 20 in terms of quantity of water removed from the laundry during the time. In particular, the compressor speed Cs or the compressor power Cp is adjusted during the time according to the variation of the parameter related to the mass flow rate rh(t) of the water removed from the laundry, more particularly by checking and maximizing the efficiency function F(t) as defined above.

The method of the invention performs, therefore, an optimization procedure for the efficiency function F(t) by adjusting the compressor speed Cs or the compressor power Cp.

Adjustment of the compressor speed Cs, or of the compressor power Cp, means increasing or decreasing the value of the compressor speed Cs, or of the compressor power Cp, of a preferred value Δω, or Δρ.

According to the method of the invention, as briefly illustrated in Figure 3, the compressor speed Cs (or the compressor power Cp) is adjusted in the same direction (step 102) of a previous adjustment action if the efficiency function F(t) in the meantime has increased (output "Yes" of block 101) or the compressor speed Cs (or the compressor power Cp) is adjusted in the opposite direction (step 103) of a previous adjustment action if the efficiency function F(t) in the meantime has decreased (output "No" of block 101).

A course of the compressor speed Cs on the base of the efficiency function F(t) in a drying cycle according to a first preferred embodiment of the invention is illustrated in Figure 4.

In an analogue and alternative representation, the course of the compressor power Cp on the base of the efficiency function F(t), instead of the compressor speed Cs, may be illustrated.

As illustrated in Figure 4, the optimization procedure takes place in subsequent time intervals, for example At₀=t₀÷ti, Ati=ti÷t₂, At₂=t₂÷t₃,

In the preferred embodiment here illustrated, the time intervals preferably have the same duration time At, i.e. At=At₀=Ati=

(for example At=4min). In different embodiments, nevertheless, the time intervals may be different one to the other. At the end of each time interval At_n_i , the efficiency function F(t_n) is calculated, advantageously by means of the central processing unit. The calculated value of the efficiency function F(t_n) is compared to the efficiency function F(t_n_i) calculated at the end of the previous time interval At_n_₂. According to this comparison, the compressor speed Cs is adjusted by increasing or decreasing its speed of a respective value Δω_η.

In the preferred embodiment here illustrated, the adjusting values at the end of the time intervals preferably have the same value Δω, i.e. Δω=Δωι=Δω₂= Δω₃= ....=Δω_η. In different embodiments, nevertheless, the adjusting values may be different one to the other.

In particular, if the efficiency function F(t_n) has increased compared to the efficiency function F(t_n_i) calculated at the end of the preceding time interval Δί_η_₂, the compressor speed Cs is increased of a respective value Δω_η if the compressor speed Cs was increased of a respective value Δω_η_ι at the end of the preceding time interval Δί_η_₂.

Alternatively, if the efficiency function F(t_n) has increased compared to the efficiency function F(t_n_i) calculated at the end of the preceding time interval Δί_η_₂, the compressor speed Cs is decreased of a respective value Δω_η if the compressor speed Cs was decreased of a respective value Δω_η_ι at the end of the preceding time interval Δί_η_₂ (which is the case shown in Figure 4 at time t_n ). Alternatively, if the efficiency function F(t_n) has decreased compared to the efficiency function F(t_n_i) calculated at the end of the preceding time interval Δί_η_₂, the compressor speed Cs is increased of a respective value Δω_η if the compressor speed Cs was decreased of a respective value Δω_η_ι at the end of the preceding time interval Δί_η_₂.

Alternatively, if the efficiency function F(t_n) has decreased compared to the efficiency function F(t_n-1) calculated at the end of the preceding time interval Δί_η_₂, the compressor speed Cs is decreased of a respective value Δω_η if the compressor speed Cs was increased of a respective value Δω_η_ι at the end of the preceding time interval Δί_η_₂.

The optimization procedure according to the invention starts at a particular time, or starting time, which corresponds to the above mentioned time t₀. The starting time t₀ may be defined according to different criteria.

In a preferred embodiment of the invention, the starting time t₀ is preferably set as a prefixed time after the beginning of the drying cycle, i.e. after an initial transitional phase. Preferably the starting time t₀ is a time comprised between 5 and 40 minutes. The choice of the starting time t₀ in this range may depends on the type of heat pump system 20 and/or the type of laundry dryer 1 used, for example depends on size of the heat pump system, the type or of the heat exchangers, the type of refrigerant used, etc..

Advantageously, it is assured that the optimization procedure will start after the heat pump system 20 has been switched on and has reached reasonable working conditions.

In further preferred embodiments of the invention, the starting time t₀ is preferably comprised between 30 and 60 minutes. In this case, advantageously, the optimization procedure will start after an initial transitional phase, which may typically last from 30 to 60 minutes, during which the heat pump system 20 from its switch-off condition reaches a substantially stable working condition.

In particular, after the initial transitional phase the compressor 24 and the heat pump system 20 reaches a steady-state condition.

The steady- state condition may be considered the period of time following an initial transitional phase during which both the condensation temperature CC and the evaporation temperature EC of the refrigerant in the condenser 21 and in the evaporator 23 remain within a respective prefixed temperature range DC and DE, as illustrated in figure 5 wherein CC represents the condensation temperature of the refrigerant at the condenser 21 as a function of the time and EC represents the evaporation temperature of the refrigerant at the evaporator 23 as a function of the time. In preferred embodiments, said temperatures are kept in the respective prefixed temperature range DC and DE by properly controlling specific parameters of the heat pump system 20. In more preferred embodiments, for example, the laundry dryer also comprises a cooling fan for the compressor 24. During the steady- state, the cooling fan is opportunely activated and de-activated to cool down the compressor 24 thus avoiding superheating of the refrigerant and aiming to maintain its temperatures in the respective prefixed temperature range DC and DE .

In further preferred embodiments of the invention, then, the starting time t₀ may be preferably set according to other preferred criteria, for example when the condensation temperature CC or the evaporation temperature EC reaches a prefixed threshold temperature, or when the condensed water reaches a prefixed threshold value or other particular conditions of the laundry dryer and/or of the heat pump system.

It should be noted that before the optimization procedure starts, i.e. during the transitional phase before the starting time t₀, the compressor speed Cs is preferably set to a reference value Cs₀ (2400 rpm in the graph of Figure 4).

The reference value Cs₀ may depend on different factors. For example, the reference value Cs₀ may be a fixed estimated value. For example, the reference value Cs₀ may be a substantially constant compressor speed value.

At the first control time t₀ of the optimization procedure (starting time t₀), the efficiency function F(ti) is calculated and the compressor speed Cs is increased of a respective value Δα^.

In a different embodiment, the compressor speed Cs at the control time t₀ may be decreased of a respective value Δα>ι (instead of being increased).

From the control time t₀ on, the optimization procedure is then carried out as described above.

It should be further noted that the evaluation of the efficiency function F(t_n) at the end of a time interval Δί_η_ι requires the evaluation of the parameter Pcompressor(t_n).

P 'compressor^ is the electrical power consumption of the compressor 24 during the respective time interval Δί_η_ι.

P 'compressor^ may be preferably measured directly or, in different embodiments, may be measured indirectly, for example evaluating the motor compressor torque or the rotational speed of the compressor motor. During the drying cycle, then, the compressor speed Cs is adequately monitored so that safe and suitable working conditions are ensured. Preferably, the compressor 24 is made working at compressor speed Cs comprised between a minimum value and a maximum value.

Typically, a maximum speed value for the compressor 24 is set in order to avoid over heating of the compressor 24 itself. A minimum speed value for the compressor 24 is set in order to avoid low performances of the heat pump system 20 which would lead to too long duration of the drying cycle.

A further control for performing and/or starting the optimization procedure above described is made on the parameter rh(t).

In particular, if rh(t) is lower than a prefixed minimum value the optimization procedure is not started. For example, if m(t)=m_Condense(t) is lower than 15 gr/min the optimization procedure is not started. This may happen, for example, in the initial transitional phase before the system reaches the steady-state condition. In the initial phase, in fact, the hat pump system 20 does not work at its maximum and the water removed from the laundry is low (lower than 15 gr/min). Eventually, the control of the parameter rh(t) is used in this case to start the optimization procedure.

In different embodiments, nevertheless, the optimization procedure may start from the beginning of the drying cycle.

Similarly, if the optimization procedure has started and the calculated rh(t)=m_Condense(t) falls below a prefixed minimum value of 15 gr/min, the optimization procedure is terminated. This may happen, in particular, when the drying cycle is about to terminate, the laundry is dry (or almost dry) and the water removed therefrom is low (lower than 15 gr/min). Eventually, the control of the parameter rh(t) is used in this case to terminate the optimization procedure.

Once the optimization procedure terminates, the compressor speed Cs is preferably keep working at a fixed value until the drying cycle ends. The fixed value is preferably set as the last value of the compressor speed Cs in the optimization procedure.

In different embodiments, nevertheless, the compressor speed may follow a predetermined speed curve until the drying cycle ends.

It is clear in the above described situations that the optimization procedure preferably terminates before the drying cycle ends. In different embodiments, nevertheless, termination of the drying cycle and of the optimization procedure may coincide.

Generally, the optimization procedure and/or the drying cycle terminates when the calculated m(t) falls below a prefixed minimum value or when the machine has detected that the drying load has achieved the required level of dryness and therefore stops its functioning.

According to the invention, advantageously, the proper control of the compressor parameters, namely the motor speed Cs or the motor power Cp, leads to a significant energy savings and reducing of the drying time with respect to laundry dryers of known type.

Figure 6 briefly illustrates a further embodiment of the method of the invention. This embodiment differs from the embodiment previously described with reference to figures 1 to 4 for the fact that the controlled parameters refer not only to the compressor 24, but also to other components of the laundry dryer 1. In the preferred embodiment here described, the other components comprise the fan motor 45 and the drum motor 27.

In this case, the optimization procedure uses the new efficiency function F'(t):

Pcompressor(t) + f an (t) + Pdrurn(t ) where:

F'(t) is a time-dependent function;

Pcompressor(i) is the electrical power consumption of the compressor 24 at the generic time t;

Pfan(t) is the electrical power consumption of the fan motor 45 at the generic time t;

Pdrum t) is the electrical power consumption of the drum motor 27 at the generic time t.

Analogously to what described above where the optimization of the efficiency function F(t) implied the control of the compressor parameters, now the method performs the optimization of the efficiency function F'(t) by adjusting not only the compressor speed Cs (or the compressor power Cp) but adjusting also the fan motor speed Fs (or the fan motor power Fp) and/or adjusting the drum motor speed Ds (or the drum motor power Dp).

In this case, it is useful to define a priority among the components in order to avoid conflicts among their controls.

For instance, the method first optimizes the efficiency function F'(t) acting on the compressor speed Cs (or the compressor power Cp) in a first period of time Tl, then optimizes the efficiency function F'(t) acting on the fan motor speed Fs (or the fan motor power Fp) in a second period of time T2 and finally optimizes the efficiency function F'(t) acting on the drum motor speed Ds (or the drum motor power Dp) in a third period of time T3.

Within any period of time Tl, T2 and T3, the optimization procedure for the efficiency function F'(t) will advantageously follow the same criteria described above in the first preferred embodiment of the method.

Therefore, during the first period of time Tl, the compressor speed Cs (or the compressor power Cp) is adjusted in the same direction (step 202 in Figure 6) of a previous adjustment action if the efficiency function F'(t) in the meantime has increased (output "Yes" of block 201) or the compressor speed Cs (or the compressor power Cp) is adjusted in the opposite direction (step 203) of a previous adjustment action if the efficiency function F'(t) in the meantime has decreased (output "No" of block 201).

Analogously, during the second period of time T2, the fan motor speed Fs (or the fan motor power Fp) is adjusted in the same direction (step 202 in Figure 5) of a previous adjustment action if the efficiency function F'(t) in the meantime has increased (output "Yes" of block 201) or the fan motor speed Fs (or the fan motor power Fp) is adjusted in the opposite direction (step 203) of a previous adjustment action if the efficiency function F'(t) in the meantime has decreased (output "No" of block 201).

Analogously, during the third period of time T3, the drum motor speed Ds (or the drum motor power Dp) is adjusted in the same direction (step 202) of a previous adjustment action if the efficiency function F'(t) in the meantime has increased (output "Yes" of block 201) or the on the drum motor speed Ds (or the drum motor power Dp) is adjusted in the opposite direction (step 203) of a previous adjustment action if the efficiency function F'(t) in the meantime has decreased (output "No" of block 201).

In different embodiments, the priority among the components may be different and the method may provide for any other combination regarding the order during which the components parameters are controlled.

In a further preferred embodiment, than, the control may be directed only to the compressor 24 and the fan motor 45.

In this case, the optimization procedure uses the efficiency function F"(t):

m(t)

F"(t)

Pcompressor(t) + Pfan(t)

Advantageously, in another further preferred embodiment, the control may be directed only to the compressor 24 and the drum motor 27.

In this case, the optimization procedure uses the efficiency function F" '(t): rh(t)

F'"(t)

P compress or {t) + Pdrum(i)

In a further preferred embodiment, the efficiency function F*(t) uses "weights" applied to the power of each component, as expressed below:

m(t)

F * (t) =

A{t)Pcompressor{t) + B{t)Pfan{t) + C(t)Pdrum(t) where, A(t), B(t), C(t) are time-dependent "weights", whose sum is equal to 1. Alternatively, the weights can be time-independent, i.e. constant during the whole drying cycle.

For example, in a preferred embodiment of the invention, said weights may be: A=0,6, B=0,3 and C=0,1. In this case, the efficiency function F*(t) takes into account the fact that the compressor 24 is the component that has the highest impact on the efficiency function F*(t) during the whole drying cycle.

In another preferred embodiment of the invention, said weights may vary according to the drying cycle phase. For example, in the initial transitional phase, said weights may be: A=0,5, B=0,3 and C=0,2, while in the steady-state phase said weights may be: A=0,7, B=0,3 and C=0,1. In this case, the efficiency function F*(t) takes into account the fact that the compressor 24 has a lower impact on the efficiency function F*(t) during in the initial transitional phase than in the steady-state phase.

In this case, advantageously, the optimization procedure starts at the beginning of the drying cycle, i.e. also during the initial transitional phase.

Furthermore, it is still necessary to define a priority among the components in order to avoid conflicts among their controls, as said above.

It is clear that in different embodiments, weights A(t), B(t), C(t) may assume more complex trends over the time.

It has thus been shown that the present invention allows the set object to be achieved. In particular, it makes it possible to obtain a drying cycle which allow an additional saving of energy compared to machines of known type.

Although illustrative embodiments of the present invention have been described herein with reference to the accompanying drawings, it is to be understood that the present invention is not limited to those precise embodiments, and that various other changes and modifications may be affected therein by one skilled in the art without departing from the scope or spirit of the invention. All such changes and modifications are intended to be included within the scope of the invention as defined by the appended claims.

Claims

1. A method for controlling a drying cycle in a laundry drying machine (1) of the type comprising a heat pump system (20) having a refrigerant circuit (30) for a refrigerant and comprising a drying air circuit (10) for conveying a volume flow of drying air (A) in a laundry drum (9) suitable for receiving laundry to be dried, said refrigerant circuit (30) comprising:

- a compressor (24) with a variable rotation speed (Cs);

- a first heat exchanger (21) for a thermal coupling between said drying air circuit (10) and said refrigerant circuit (30) wherein the temperature of said drying air

(A) increases and the temperature of said refrigerant decreases; and

- a second heat exchanger (23) for a further thermal coupling between said drying air circuit (10) and said refrigerant circuit (30) wherein the temperature of said drying air (A) decreases and the temperature of said refrigerant increases; characterized in that said method comprises an optimization procedure comprising the step of controlling the rotation speed (Cs) or the power (Cp) of said compressor (24) during a plurality of time intervals (At_n_₂; At_n_i; At_n) of said drying cycle, wherein the optimization procedure comprises the steps of:

al) determining an efficiency parameter (F(t_n_i)) related to the amount of water removed from said laundry during a first time interval (At_n_₂);

a2) determining an efficiency parameter (F(t_n)) related to the amount of water removed from said laundry during a second time interval (At_n_i);

b) comparing the efficiency parameter (F(t_n)) determined during said second time interval (At_n_i) with the efficiency parameter (F(t_n_i)) determined during said first time interval (At_n_₂);

c) adjusting the rotation speed (Cs) or the power (Cp) of said compressor (24) in a third time interval (At_n) according to the result of said comparison performed in said step b);

wherein said second time interval (At_n_i) is subsequent to said first time interval (At_n_₂) and said third time interval (At_n) is subsequent to said second time interval (AW).

2. The method according to claim 1, characterized in that said step of controlling the rotation speed (Cs) or the power (Cp) of said compressor (24) comprises increasing or decreasing the rotation speed (Cs) or the power (Cp) of said compressor (24) during said plurality of time intervals (Δί_η_₂; Δί_η_ι; At_n).

3. The method according to claim 1 or 2, characterized in that said step c) of adjusting the rotation speed (Cs) or the power (Cp) comprises increasing or decreasing the rotation speed (Cs) or the power (Cp) of said compressor (24) in said third time interval (Δί_η) according to the result of said comparison in said step b).

4. The method according to any preceding claim, characterized in that said step c) of adjusting the rotation speed (Cs) or the power (Cp) comprises increasing or decreasing the rotation speed (Cs) or the power (Cp) of said compressor (24) in said third time interval (Δί_η) according to the result of said comparison in said step b) and according to an increasing or a decreasing of the rotation speed (Cs) or the power (Cp) of said compressor (24) in said second time interval (Δί_η_ι) with respect to said first time interval (Δί_η_₂).

5. The method according to any preceding claim, characterized in that said step c) of adjusting the rotation speed (Cs) or the power (Cp) comprises increasing the rotation speed (Cs) or the power (Cp) of said compressor (24) in said third time interval (Δί_η) if the comparison performed in said step b) indicates that the efficiency parameter (F(t_n)) determined during said second time interval (Δί_η_ι) increases with respect to the efficiency parameter (F(t_n_i)) determined during said first time interval (Δί_η_₂) and the rotation speed (Cs) or the power (Cp) of said compressor (24) was increased in said second time interval (Δί_η_ι) with respect to said first time interval (Δί_η_₂).

6. The method according to any preceding claim, characterized in that said step c) of adjusting the rotation speed (Cs) or the power (Cp) comprises decreasing the rotation speed (Cs) or the power (Cp) of said compressor (24) in said third time interval (Δί_η) if the comparison performed in said step b) indicates that the efficiency parameter (F(t_n)) determined during said second time interval (Δί_η_ι) increases with respect to the efficiency parameter (F(t_n_i)) determined during said first time interval (Δί_η_₂) and the rotation speed (Cs) or the power (Cp) of said compressor (24) was decreased in said second time interval (Δί_η_ι) with respect to said first time interval (Δί_η_₂).

7. The method according to any preceding claim, characterized in that said step c) of adjusting the rotation speed (Cs) or the power (Cp) comprises increasing the rotation speed (Cs) or the power (Cp) of said compressor (24) in said third time interval (Δί_η) if the comparison performed in said step b) indicates that the efficiency parameter (F(t_n)) determined during said second time interval (Δί_η_ι) decreases with respect to the efficiency parameter (F(t_n_i)) determined during said first time interval (Δί_η_₂) and the rotation speed (Cs) or the power (Cp) of said compressor (24) was decreased in said second time interval (Δί_η_ι) with respect to said first time interval (Δί_η_₂).

8. The method according to any preceding claim, characterized in that said step c) of adjusting the rotation speed (Cs) or the power (Cp) comprises decreasing the rotation speed (Cs) or the power (Cp) of said compressor (24) in said third time interval (Δί_η) if the comparison performed in said step b) indicates that the efficiency parameter (F(t_n)) determined during said second time interval (Δί_η_ι) decreases with respect to the efficiency parameter (F(t_n_i)) determined during said first time interval (Δί_η_₂) and the rotation speed (Cs) or the power (Cp) of said compressor (24) was increased in said second time interval (Δί_η_ι) with respect to said first time interval (Δί_η_₂).

9. The method according to any preceding claim, characterized in that said step al) takes place at the end of said first time interval (Δί_η_₂).

10. The method according to any preceding claim, characterized in that said step a2) takes place at the end of said second time interval (Δί_η_ι).

11. The method according to any preceding claim, characterized in that said step c) takes place at the beginning of said third time interval (Δί_η).

12. The method according to any preceding claim, characterized in that said efficiency parameter (F(t_n)) is calculated by:

bl) determining a first parameter (rh(t_n)) related to the amount of water removed from said laundry during said time interval (Δί_η_ι);

b2) determining the power consumption (Pcompressor(t_D)) of said compressor (24) during said time interval (Δί_η_ι);

b3) determining a relation between said first parameter (rh(t_n)) and said power consumption (Pcompressor(t_D)) of said compressor (24).

13. The method according to claim 12, characterized in that said step b3) comprises the step of calculating the ratio between said first parameter (rh(t_n)) and said power consumption (Pcompressor(t_D)) of said compressor (24).

14. The method according to any of the preceding claims from 1 to 11, characterized in that said efficiency parameter (F(t_n)) is calculated by:

b4) determining the sum of the power consumption (Pcompressor(t_D)) of said compressor (24) and the power consumption (P/an(t_n); Pdrum(t_n)) of at least one other electric component (45; 27) of said laundry drying machine (1) during said time interval (Δί_η_ι);

b5) determining a relation between said first parameter (rh(t_n)) and said sum of power consumptions (Pcompressor(t_n); Pfan(t_n); Pdrum(t_n)).

15. The method according to claim 14, characterized in that said step b5) comprises the step of calculating the ratio between said first parameter (rh(t_n)) and said sum of power consumptions (Pcompressor(t_D); Pfan(t_D); Pdrum(t_D)).

16. The method according to claim 14 or 15, characterized by further comprising a step d) of adjusting the rotation speed (Fs; Ds) or the power (Fp;

Dp) of said at least one other electric component (45; 27) in said third time interval (Δί_η) according to the result of said comparison performed in said step b), wherein said at least one other electric component (45; 27) is an electric motor (45; 27).

17. The method according to claim 16, characterized in that said step d) of adjusting the rotation speed (Fs; Ds) or the power (Fp; Dp) of said at least one other electric component (45; 27) in said third time interval (Δί_η) according to the result of said comparison performed in said step b), comprises increasing or decreasing the rotation speed (Fs; Ds) or the power (Fp; Dp) of said of said at least one other electric component (45; 27) in said third time interval (Δί_η) according to the result of said comparison in said step b) and according to an increasing or a decreasing of the rotation speed (Fs; Ds) or the power (Fp; Dp) of said at least one other electric component (45; 27) in said second time interval (Δί_η_ι) with respect to said first time interval (Δί_η_₂).

18. The method according to any of the preceding claims from 14 to 17, characterized in that said at least one other electric component is a fan motor (45) or a drum motor (27).

19. The method according to claim 14 or 15, characterized by further comprising a step d) of increasing or decreasing the rotation speed (Fs) or the power (Fp) of a fan motor (45) in said third time interval (Δί_η) according to the result of said comparison in said step b) and according to an increasing or a decreasing of the rotation speed (Fs) or the power (Fp) of said fan motor (45) in said second time interval (Δί_η_ι) with respect to said first time interval (Δί_η_₂).

20. The method according to any of the preceding claims from 14 to 19, characterized in that said step b4) of determining the sum of the power consumption (Cp) of said compressor (24) and the power consumption (Pcompressor(t_D); Pfan(t_D); Pdrum(t_D)) of at least one other component is carried out using a weighted sum.

21. The method according to any of the preceding claims from 12 to 20, characterized in that said first parameter (rh(t_n)) is obtained according to one of the following criteria: estimation by a direct measure of the condensed water generated at said second heat exchanger (23); estimation by considering the trend of the power consumption (Dp) of a drum motor (27); estimation by considering the trend of the electrical current and/or of the torque of the drum motor (27); estimation by measuring the weight variations of the laundry in said laundry drum (9); estimation by considering the number of switch on/off of a draining pump of a draining pump associated to the condensed water generated at said second heat exchanger (23); estimation by detecting the signal of a water level or flow sensor associated to a water collecting container ( 14); estimation by considering the trend of the temperature and/or the humidity level of the moist air leaving said laundry drum (9); estimation by considering the trend of the temperature and/or the pressure level of said refrigerant in said refrigerant circuit (30); estimation by the difference of the drying air temperature at the second heat exchanger outlet and the drying air temperature at the second heat exchanger inlet; estimation by the difference between two refrigerant temperature levels at the second heat exchanger; estimation by the difference between the drying air humidity at the second heat exchanger outlet and the drying air humidity at the second heat exchanger inlet.

22. The method according to any preceding claim, characterized in that said optimization procedure is performed after an initial transitional phase of said drying cycle.

23. The method according to any preceding claim, characterized in that said optimization procedure is performed after a prefixed period of time from the beginning of the drying cycle.

24. The method according to any preceding claim, characterized in that said optimization procedure is performed when said drying cycle reaches a steady- state condition.

25. The method according to claim 24, characterized in that said steady-state condition occurs after an initial transitional phase and when the condensation temperature and the evaporation temperature of the refrigerant in said first heat exchanger (21) and in said second heat exchanger (23) remain within a respective prefixed temperature range.

26. The method according to any preceding claim, characterized in that said optimization procedure is carried out in said laundry drying machine (1) by means of a central processing unit.

27. A laundry drying machine (1) suited to implement a method according to any of the preceding claims.